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Sex Differences in Alzheimer’s Disease: Insights From the Multiomics Landscape

  • Lei Guo
    Affiliations
    Department of Genetics and Genomic Sciences, Icahn School of Medicine at Mount Sinai, New York, New York

    Mount Sinai Center for Transformative Disease Modeling, Icahn School of Medicine at Mount Sinai, New York, New York
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  • Margaret B. Zhong
    Affiliations
    Department of Neuroscience, Barnard College of Columbia University, New York, New York
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  • Larry Zhang
    Affiliations
    Department of Neurology, Icahn School of Medicine at Mount Sinai, New York, New York

    Research and Development Service, James J. Peters VA Medical Center, Bronx, New York
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  • Bin Zhang
    Correspondence
    Bin Zhang, Ph.D.
    Affiliations
    Department of Genetics and Genomic Sciences, Icahn School of Medicine at Mount Sinai, New York, New York

    Mount Sinai Center for Transformative Disease Modeling, Icahn School of Medicine at Mount Sinai, New York, New York
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  • Dongming Cai
    Correspondence
    Address correspondence to Dongming Cai, M.D., Ph.D.
    Affiliations
    Department of Neurology, Icahn School of Medicine at Mount Sinai, New York, New York

    Alzheimer Disease Research Center, Icahn School of Medicine at Mount Sinai, New York, New York

    Ronald M. Loeb Center for Alzheimer’s Disease, Icahn School of Medicine at Mount Sinai, New York, New York

    Research and Development Service, James J. Peters VA Medical Center, Bronx, New York
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Open AccessPublished:March 02, 2021DOI:https://doi.org/10.1016/j.biopsych.2021.02.968

      Abstract

      Alzheimer’s disease (AD) has complex etiologies, and the impact of sex on AD varies over the course of disease development. The literature provides some evidence of sex-specific contributions to AD. However, molecular mechanisms of sex-biased differences in AD remain elusive. Multiomics data in tandem with systems biology approaches offer a new avenue to dissect sex-stratified molecular mechanisms of AD and to develop sex-specific diagnostic and therapeutic strategies for AD. Single-cell transcriptomic datasets and cell deconvolution of bulk tissue transcriptomic data provide additional insights into brain cell type–specific impact on sex-biased differences in AD. In this review, we summarize the impact of sex chromosomes and sex hormones on AD, the impact of sex-biased differences during AD development, and the interplay between sex and a major AD genetic risk factor, the APOE ε4 genotype, through the multiomics landscape. Several sex-biased molecular pathways such as neuroinflammation and bioenergetic metabolism have been identified. The importance of sex chromosome and sex hormones, as well as the associated pathways in AD pathogenesis, is further strengthened by findings from omics studies. Future research efforts should integrate the multiomics data from different brain regions and different cell types using systems biology approaches, and leverage the knowledge into a holistic examination of sex differences in AD. Advances in systems biology technologies and increasingly available large-scale multiomics datasets will facilitate future studies dissecting such complex signaling mechanisms to better understand AD pathogenesis in both sexes, with the ultimate goals of developing efficacious sex- and APOE-stratified preventive and therapeutic interventions for AD.

      Keywords

      Alzheimer’s disease (AD) is a multifactorial disorder with heterogeneous etiologies. Evidence from basic and clinical studies support sex-specific differences contributing to its complexity. Approximately two-thirds of patients with AD are female (
      Alzheimer’s Association
      2020 Alzheimer's disease facts and figures.
      ). Higher AD prevalence in females could be secondary to longer life expectancy or higher dementia incidence in females than in males. Literature reports regarding AD incidents among males and females are conflicting, with some suggesting no differences (
      • Barnes L.L.
      • Wilson R.S.
      • Schneider J.A.
      • Bienias J.L.
      • Evans D.A.
      • Bennett D.A.
      Gender, cognitive decline, and risk of AD in older persons.
      ). On the other hand, sex differences in AD have been observed in clinical, neuroimaging, and pathology studies (Supplement). Surprisingly, sex-specific analyses were only performed occasionally during AD clinical trial phases, and the importance of considering sex as a critical modulator of patient responses to treatments was vastly underappreciated (
      • Henley D.B.
      • May P.C.
      • Dean R.A.
      • Siemers E.R.
      Development of semagacestat (LY450139), a functional gamma-secretase inhibitor, for the treatment of Alzheimer's disease.
      ,
      • Doody R.S.
      • Raman R.
      • Farlow M.
      • Iwatsubo T.
      • Vellas B.
      • Joffe S.
      • et al.
      A phase 3 trial of semagacestat for treatment of Alzheimer's disease.
      ,
      • Doody R.S.
      • Thomas R.G.
      • Farlow M.
      • Iwatsubo T.
      • Vellas B.
      • Kieburtz K.
      • et al.
      Phase 3 trials of solanezumab for mild-to-moderate Alzheimer's disease.
      ,
      • Farlow M.
      • Arnold S.E.
      • van Dyck C.H.
      • Aisen P.S.
      • Snider B.J.
      • Porsteinsson A.P.
      • et al.
      Safety and biomarker effects of solanezumab in patients with Alzheimer's disease.
      ). While it is critical to take into consideration the sex differences in clinical trial design, current challenges reside in the limited knowledge of molecular mechanisms underlying sex-biased differences in AD.
      Several mechanisms have been proposed for sex-biased differences in AD, including physiological differences during development, and sex-specific changes during aging and AD development. In recent years, large-scale omics analysis in tandem with systems biology studies has improved our understanding of molecular mechanisms of AD stratified by sex. Multiomics analysis of human AD samples, AD mouse models, and human brain cells derived from inducible pluripotent stem cells of subjects with AD allows researchers to systematically unmask the complexity of sex-mixed samples and reveal sex-biased genomic, genetic, and epigenetic landscapes in AD. Unlike traditional hypothesis-driven studies, multiomics studies enable discovery of novel molecular changes, pathways, and targets common or specific to each sex group in AD development and progression. More importantly, recent advances in single-cell omics studies facilitate in-depth dissection of cell type–specific contributions to disease mechanisms.
      While several aspects of sex differences in AD have been discussed extensively in the literature (
      • Cavedo E.
      • Chiesa P.A.
      • Houot M.
      • Ferretti M.T.
      • Grothe M.J.
      • Teipel S.J.
      • et al.
      Sex differences in functional and molecular neuroimaging biomarkers of Alzheimer's disease in cognitively normal older adults with subjective memory complaints.
      ,
      • Ferretti M.T.
      • Iulita M.F.
      • Cavedo E.
      • Chiesa P.A.
      • Schumacher Dimech A.
      • Santuccione Chadha A.
      • et al.
      Sex differences in Alzheimer disease - the gateway to precision medicine.
      ,
      • Mielke M.M.
      • Vemuri P.
      • Rocca W.A.
      Clinical epidemiology of Alzheimer's disease: Assessing sex and gender differences.
      ,
      • Nebel R.A.
      • Aggarwal N.T.
      • Barnes L.L.
      • Gallagher A.
      • Goldstein J.M.
      • Kantarci K.
      • et al.
      Understanding the impact of sex and gender in Alzheimer's disease: A call to action.
      ,
      • Toro C.A.
      • Zhang L.
      • Cao J.
      • Cai D.
      Sex differences in Alzheimer's disease: Understanding the molecular impact.
      ), we will discuss current studies of sex chromosomes and sex hormones, multiomics analyses of sex-biased differences in AD, and the molecular architecture underlying the interplay between sex and genetic risk factors of AD.

      Impact of Sex Chromosomes in AD

      Mounting evidence has suggested that sex chromosomes contribute to the heterogeneity in AD. For example, the molecular cytogenetic analysis showed that somatically acquired X chromosome aneuploidy may contribute to brain aging and neurodegenerative processes. The frequency of X chromosome loss is over 100 times higher in older women (>65 years of age) than in younger women (<16 years of age) (
      • Russell L.M.
      • Strike P.
      • Browne C.E.
      • Jacobs P.A.
      X chromosome loss and ageing.
      ). The rates of X chromosome aneuploidy were increased twofold in neural cells of the hippocampus and cerebrum of subjects with AD (
      • Yurov Y.B.
      • Vorsanova S.G.
      • Liehr T.
      • Kolotii A.D.
      • Iourov I.Y.
      X chromosome aneuploidy in the Alzheimer’s disease brain.
      ). Furthermore, premature centromere division (PCD), a genetic mechanism associated with increased aneuploidy, is found to be tightly correlated with aging and AD. The average frequency of PCD on the X chromosome in frontal cortical neurons of patients with AD is almost three times higher than that of control subjects (
      • Spremo-Potparević B.
      • Živković L.
      • Djelić N.
      • Plećaš-Solarović B.
      • Smith M.A.
      • Bajić V.
      Premature centromere division of the X chromosome in neurons in Alzheimer’s disease.
      ). Significantly higher percentages of PCD were observed on chromosomes of peripheral lymphocytes of both men and women of advanced age (
      • Wojda A.
      • Ziętkiewicz E.
      • Mossakowska M.
      • Pawłowski W.
      • Skrzypczak K.
      • Witt M.
      Correlation between the level of cytogenetic aberrations in cultured human lymphocytes and the age and gender of donors.
      ) and on the X chromosome of patients with AD (
      • Spremo-Potparevic B.
      • Zivkovic L.
      • Djelic N.
      • Bajic V.
      Analysis of premature centromere division (PCD) of the X chromosome in Alzheimer patients through the cell cycle.
      ).
      On the other hand, an extra X chromosome may account for protective effects against AD, probably by augmenting the expression of genes that elude X inactivation. Approximately 15% of X-linked genes escape this inactivation, resulting in an increased expression of X-linked genes in females compared with males (
      • Carrel L.
      • Willard H.F.
      X-inactivation profile reveals extensive variability in X-linked gene expression in females.
      ). For example, the loss-of-function mutations in KDM6A, a gene encoding a histone demethylase (
      • Greenfield A.
      • Carrel L.
      • Pennisi D.
      • Philippe C.
      • Quaderi N.
      • Siggers P.
      • et al.
      The UTX gene escapes X inactivation in mice and humans.
      ), are associated with intellectual disability in humans (
      • Miyake N.
      • Mizuno S.
      • Okamoto N.
      • Ohashi H.
      • Shiina M.
      • Ogata K.
      • et al.
      KDM6A point mutations cause Kabuki syndrome.
      ), while deletion of KDM6A in CD4+ T cells ameliorated clinical disease and reduced neuropathology in autoimmune disorders like multiple sclerosis, indicating the cell type–specific roles of KDM6A in human diseases (
      • Itoh Y.
      • Golden L.C.
      • Itoh N.
      • Matsukawa M.A.
      • Ren E.
      • Tse V.
      • et al.
      The X-linked histone demethylase Kdm6a in CD4+ T lymphocytes modulates autoimmunity.
      ). Consistently, a recent mouse study showed that adding a second X chromosome to hAPP mice confers resilience to AD-related vulnerability in mice, probably through elevating KDM6A expression (
      • Davis E.J.
      • Broestl L.
      • Abdulai-Saiku S.
      • Worden K.
      • Bonham L.W.
      • Miñones-Moyano E.
      • et al.
      A second X chromosome contributes to resilience in a mouse model of Alzheimer’s disease.
      ), and that Kdm6a−/− mice manifested synaptic plasticity and memory deficits (
      • Tang G.B.
      • Zeng Y.Q.
      • Liu P.P.
      • Mi T.W.
      • Zhang S.F.
      • Dai S.K.
      • et al.
      The histone H3K27 demethylase UTX regulates synaptic plasticity and cognitive behaviors in mice.
      ).
      Another example of X-inactivation escapees associated with sex differences in developing AD is PCDH11X, a gene involved in cell-cell recognition essential for central nervous system function (
      • Lopes A.M.
      • Ross N.
      • Close J.
      • Dagnall A.
      • Amorim A.
      • Crow T.J.
      Inactivation status of PCDH11X: Sexual dimorphisms in gene expression levels in brain.
      ,
      • O'Leary N.A.
      • Wright M.W.
      • Brister J.R.
      • Ciufo S.
      • Haddad D.
      • McVeigh R.
      • et al.
      Reference sequence (RefSeq) database at NCBI: Current status, taxonomic expansion, and functional annotation.
      ). It was suggested that PCDH11X escapes X inactivation through epigenetic mechanisms (
      • Lopes A.M.
      • Ross N.
      • Close J.
      • Dagnall A.
      • Amorim A.
      • Crow T.J.
      Inactivation status of PCDH11X: Sexual dimorphisms in gene expression levels in brain.
      ). A study investigated 2356 patients with AD and 2384 control subjects and found a single nucleotide polymorphism (rs5984894) in the PCDH11X gene associated with higher risks of developing AD in women (
      • Carrasquillo M.M.
      • Zou F.
      • Pankratz V.S.
      • Wilcox S.L.
      • Ma L.
      • Walker L.P.
      • et al.
      Genetic variation in PCDH11X is associated with susceptibility to late-onset Alzheimer's disease.
      ). However, two subsequent genome-wide association studies failed to confirm the association between PCDH11X polymorphisms and AD (
      • Beecham G.W.
      • Naj A.C.
      • Gilbert J.R.
      • Haines J.L.
      • Buxbaum J.D.
      • Pericak-Vance M.A.
      PCDH11X variation is not associated with late-onset Alzheimer disease susceptibility.
      ,
      • Miar A.
      • Alvarez V.
      • Corao A.I.
      • Alonso B.
      • Díaz M.
      • Menéndez M.
      • et al.
      Lack of association between protocadherin 11-X/Y (PCDH11X and PCDH11Y) polymorphisms and late onset Alzheimer's disease.
      ). Moreover, many genes involved in immune processes are located on the X chromosome (
      • Libert C.
      • Dejager L.
      • Pinheiro I.
      The X chromosome in immune functions: When a chromosome makes the difference.
      ). Compared with males, females demonstrate higher diversity in immune responses due to random inactivation of one of the two X chromosomes and incomplete X inactivation (
      • Spolarics Z.
      • Peña G.
      • Qin Y.
      • Donnelly R.J.
      • Livingston D.H.
      Inherent X-linked genetic variability and cellular mosaicism unique to females contribute to sex-related differences in the innate immune response.
      ). Future studies are needed to understand the regulation of neuroinflammation by X chromosomes.
      Furthermore, loss of chromosome Y (LOY) is the most commonly acquired mutation in aging men (
      • Forsberg L.A.
      • Gisselsson D.
      • Dumanski J.P.
      Mosaicism in health and disease – clones picking up speed.
      ). Dumanski et al. (
      • Dumanski J.P.
      • Lambert J.-C.
      • Rasi C.
      • Giedraitis V.
      • Davies H.
      • Grenier-Boley B.
      • et al.
      Mosaic loss of chromosome Y in blood is associated with Alzheimer disease.
      ) investigated the susceptibility to AD in men with LOY in over 3000 subjects. It was found that male subjects with AD had higher degrees of LOY mosaicism, and men with higher levels of LOY had a greater risk of developing AD. It was speculated that LOY might lead to dysregulated immune system function contributing to AD pathogenesis. Moreover, a study of human inducible pluripotent stem cell–derived neurons from a patient with familial AD with presenilin 1 E280A mutation demonstrated that LOY in these neurons exacerbated toxic effects of Aβ42 leading to impaired neuronal differentiation and premature cell death (
      • Mendivil-Perez M.
      • Velez-Pardo C.
      • Kosik K.S.
      • Lopera F.
      • Jimenez-Del-Rio M.
      IPSCs-derived nerve-like cells from familial Alzheimer’s disease PSEN 1 E280A reveal increased amyloid-beta levels and loss of the Y chromosome.
      ). Consistently, a recent study of five transcriptomic datasets examined the degrees of extreme downregulation of chromosome Y and observed a significant interaction between age and extreme downregulation of chromosome Y associated with AD, supporting the role of extreme downregulation of chromosome Y in age-related increased risks of AD in men (
      • Caceres A.
      • Jene A.
      • Esko T.
      • Perez-Jurado L.A.
      • Gonzalez J.R.
      Extreme downregulation of chromosome Y and Alzheimer's disease in men.
      ).
      Taken together, these lines of studies support the importance of sex chromosomes in contributing to sex-specific vulnerabilities and/or resilience in brain aging and neurodegenerative processes. The long-overlooked roles of sex chromosomes and linked genes in AD pathogenesis may indicate new directions of future studies to understand molecular mechanisms of sex differences in AD.

      Impact of Sex Hormones in AD

      The roles of sex hormones in brain development as well as in aging and AD processes have been well recognized (
      • Gurvich C.
      • Hoy K.
      • Thomas N.
      • Kulkarni J.
      Sex differences and the influence of sex hormones on cognition through adulthood and the aging process.
      ,
      • McCarthy M.M.
      Multifaceted origins of sex differences in the brain.
      ,
      • Pike C.J.
      Sex and the development of Alzheimer's disease.
      ,
      • Rahman A.
      • Jackson H.
      • Hristov H.
      • Isaacson R.S.
      • Saif N.
      • Shetty T.
      • et al.
      Sex and gender driven modifiers of Alzheimer's: The role for estrogenic control across age, race, medical, and lifestyle risks.
      ). Evidence from animal and human studies support functional roles of sex hormones such as estrogens, progesterone, and androgens in cognition and behavior (
      • Gurvich C.
      • Hoy K.
      • Thomas N.
      • Kulkarni J.
      Sex differences and the influence of sex hormones on cognition through adulthood and the aging process.
      ,
      • Gurvich C.
      • Thomas N.
      • Kulkarni J.
      Sex differences in cognition and aging and the influence of sex hormones.
      ). With many neuroprotective effects implicated, age-related decreases in sex hormone levels were associated with increased risks of cognitive decline and AD (
      • Gurvich C.
      • Hoy K.
      • Thomas N.
      • Kulkarni J.
      Sex differences and the influence of sex hormones on cognition through adulthood and the aging process.
      ,
      • Rahman A.
      • Jackson H.
      • Hristov H.
      • Isaacson R.S.
      • Saif N.
      • Shetty T.
      • et al.
      Sex and gender driven modifiers of Alzheimer's: The role for estrogenic control across age, race, medical, and lifestyle risks.
      ). For example, reduced exposure to estrogens across the lifetime is associated with increased risks of developing AD in females, whereas age-related decline in both peripheral and brain testosterone levels is associated with increased vulnerabilities of developing AD in males (
      • Pike C.J.
      Sex and the development of Alzheimer's disease.
      ). Moreover, changes in sex hormone receptors and downstream signaling pathways during aging have been reported (
      • Rahman A.
      • Jackson H.
      • Hristov H.
      • Isaacson R.S.
      • Saif N.
      • Shetty T.
      • et al.
      Sex and gender driven modifiers of Alzheimer's: The role for estrogenic control across age, race, medical, and lifestyle risks.
      ). For example, the expression of nonfunctional splicing variants of estrogen receptor alpha in the hippocampus was increased during aging and AD (
      • Ishunina T.A.
      • Fischer D.F.
      • Swaab D.F.
      Estrogen receptor alpha and its splice variants in the hippocampus in aging and Alzheimer's disease.
      ), with higher expression levels in female elderly subjects than in males (
      • Foster T.C.
      Role of estrogen receptor alpha and beta expression and signaling on cognitive function during aging.
      ,
      • Rettberg J.R.
      • Yao J.
      • Brinton R.D.
      Estrogen: A master regulator of bioenergetic systems in the brain and body.
      ). In addition, studies identified polymorphisms of estrogen receptors associated with cognitive decline (
      • Yaffe K.
      • Lindquist K.
      • Sen S.
      • Cauley J.
      • Ferrell R.
      • Penninx B.
      • et al.
      Estrogen receptor genotype and risk of cognitive impairment in elders: Findings from the Health ABC study.
      ) and AD development in females, particularly in APOE ε4 (APOE4) carriers (
      • Ryan J.
      • Carriere I.
      • Carcaillon L.
      • Dartigues J.F.
      • Auriacombe S.
      • Rouaud O.
      • et al.
      Estrogen receptor polymorphisms and incident dementia: The prospective 3C study.
      ). These data suggest decreased brain responsiveness to sex hormones during aging and AD development.
      On the other hand, clinical trial results of sex hormone therapy in AD are rather controversial (
      • Gurvich C.
      • Hoy K.
      • Thomas N.
      • Kulkarni J.
      Sex differences and the influence of sex hormones on cognition through adulthood and the aging process.
      ,
      • Gleason C.E.
      • Dowling N.M.
      • Wharton W.
      • Manson J.E.
      • Miller V.M.
      • Atwood C.S.
      • et al.
      Effects of hormone therapy on cognition and mood in recently postmenopausal women: Findings from the randomized, controlled KEEPS-Cognitive and Affective Study.
      ,
      • Henderson V.W.
      • St John J.A.
      • Hodis H.N.
      • McCleary C.A.
      • Stanczyk F.Z.
      • Shoupe D.
      • et al.
      Cognitive effects of estradiol after menopause: A randomized trial of the timing hypothesis.
      ,
      • Hodis H.N.
      • Mack W.J.
      • Shoupe D.
      • Azen S.P.
      • Stanczyk F.Z.
      • Hwang-Levine J.
      • et al.
      Methods and baseline cardiovascular data from the Early versus Late Intervention Trial with Estradiol testing the menopausal hormone timing hypothesis.
      ,
      • Maki P.M.
      Critical window hypothesis of hormone therapy and cognition: A scientific update on clinical studies.
      ,
      • Shumaker S.A.
      • Legault C.
      • Kuller L.
      • Rapp S.R.
      • Thal L.
      • Lane D.S.
      • et al.
      Conjugated equine estrogens and incidence of probable dementia and mild cognitive impairment in postmenopausal women: Women's Health Initiative Memory Study.
      ,
      • Shumaker S.A.
      • Legault C.
      • Rapp S.R.
      • Thal L.
      • Wallace R.B.
      • Ockene J.K.
      • et al.
      Estrogen plus progestin and the incidence of dementia and mild cognitive impairment in postmenopausal women: The Women's Health Initiative Memory Study: A randomized controlled trial.
      ,
      • Zandi P.P.
      • Carlson M.C.
      • Plassman B.L.
      • Welsh-Bohmer K.A.
      • Mayer L.S.
      • Steffens D.C.
      • et al.
      Hormone replacement therapy and incidence of Alzheimer disease in older women: The Cache County Study.
      ). Despite earlier studies implicating protective effects of estrogen replacement against AD in females, large clinical studies failed to demonstrate any beneficial effects (
      • Barron A.M.
      • Pike C.J.
      Sex hormones, aging, and Alzheimer's disease.
      ). It was proposed that initiation of hormone replacement in the critical window of perimenopause may lessen the risks of dementia, whereas it may elevate the risks if initiated years after menopause (
      • Whitmer R.A.
      • Quesenberry C.P.
      • Zhou J.
      • Yaffe K.
      Timing of hormone therapy and dementia: The critical window theory revisited.
      ). Besides treatment timing, decreased responsiveness at brain receptors and downstream signaling pathways may contribute to the ineffectiveness of hormone therapy. Together, these studies suggest the complexity of sex hormones’ involvement in AD.

      Bulk Tissue Omics Studies of Sex Differences in AD

      Large-scale bulk tissue omics studies in the recent years have advanced our knowledge of the molecular architecture of sex differences in AD. As shown in Figure 1, we searched published studies of sex differences in AD on PubMed and selected 134 publications for further analysis. Among them, gene association, gene expression, and protein expression studies account for the majority of selected studies showing female-specific changes at molecular levels in the brain, blood, or cerebrospinal fluid (CSF) samples, with only about 20% of studies demonstrating male-specific changes. The reviewed and discussed omics studies in the following section are summarized in Table 1.
      Figure thumbnail gr1
      Figure 1A summary of multiomics studies published in the literature. To explore the studies focused on sex differences in Alzheimer’s disease, we searched on PubMed with the key words Alzheimer, sex, gender, men, women, male, and female. The search resulted in over 800 publications. We then manually scanned these studies and selected 134 publications covering multiple areas for further analysis. Among them, gene association studies, gene expression studies, and protein expression studies are in the majority (82%) (). Over 75% of the selected omics studies showed female-specific changes at molecular levels in the brain, blood, or cerebrospinal fluid (CSF), while only about 20% showed male-specific changes.
      Table 1Detailed Information of Large-Scale Omics Studies Cited, Discussed, or Reviewed in the Article
      StudyYearOrganismTissueExperimentSample SizeFocus
      Impact of Sex Chromosomes in AD
      Yurov et al. (
      • Yurov Y.B.
      • Vorsanova S.G.
      • Liehr T.
      • Kolotii A.D.
      • Iourov I.Y.
      X chromosome aneuploidy in the Alzheimer’s disease brain.
      )
      2014HumanBrainGenetic abnormalities20Sex chromosome
      Spremo-Potparević et al. (
      • Spremo-Potparević B.
      • Živković L.
      • Djelić N.
      • Plećaš-Solarović B.
      • Smith M.A.
      • Bajić V.
      Premature centromere division of the X chromosome in neurons in Alzheimer’s disease.
      )
      2008HumanChromosomeGenetic abnormalities10Sex chromosome
      Wojda et al. (
      • Wojda A.
      • Ziętkiewicz E.
      • Mossakowska M.
      • Pawłowski W.
      • Skrzypczak K.
      • Witt M.
      Correlation between the level of cytogenetic aberrations in cultured human lymphocytes and the age and gender of donors.
      )
      2006HumanBloodGenetic abnormalities123Sex chromosome
      Spremo-Potparevic et al. (
      • Spremo-Potparevic B.
      • Zivkovic L.
      • Djelic N.
      • Bajic V.
      Analysis of premature centromere division (PCD) of the X chromosome in Alzheimer patients through the cell cycle.
      )
      2004HumanChromosomeCytogenetic analysis23Sex chromosome
      Carrel and Willard (
      • Carrel L.
      • Willard H.F.
      X-inactivation profile reveals extensive variability in X-linked gene expression in females.
      )
      2005HumanHuman cellsChromosomal expression40 fibroblast lines with 931 X-linked transcriptsSex chromosome
      Greenfield et al. (
      • Greenfield A.
      • Carrel L.
      • Pennisi D.
      • Philippe C.
      • Quaderi N.
      • Siggers P.
      • et al.
      The UTX gene escapes X inactivation in mice and humans.
      )
      1998Human

      Mice
      BrainChromosomal expressionHuman and mouse cDNA clonesSex chromosome
      Miyake et al. (
      • Miyake N.
      • Mizuno S.
      • Okamoto N.
      • Ohashi H.
      • Shiina M.
      • Ogata K.
      • et al.
      KDM6A point mutations cause Kabuki syndrome.
      )
      2013HumanBrainGenetic variation32Genomic (KDM6A)
      Itoh et al. (
      • Itoh Y.
      • Golden L.C.
      • Itoh N.
      • Matsukawa M.A.
      • Ren E.
      • Tse V.
      • et al.
      The X-linked histone demethylase Kdm6a in CD4+ T lymphocytes modulates autoimmunity.
      )
      2019MiceBrainGene expression34Histone demethylase (KDM6A)
      Davis et al. (
      • Davis E.J.
      • Broestl L.
      • Abdulai-Saiku S.
      • Worden K.
      • Bonham L.W.
      • Miñones-Moyano E.
      • et al.
      A second X chromosome contributes to resilience in a mouse model of Alzheimer’s disease.
      )
      2020Human MiceChromosomeChromosomal modification6065 (human)

      10–15 per group (mice)
      Sex chromosome
      Tang et al. (
      • Tang G.B.
      • Zeng Y.Q.
      • Liu P.P.
      • Mi T.W.
      • Zhang S.F.
      • Dai S.K.
      • et al.
      The histone H3K27 demethylase UTX regulates synaptic plasticity and cognitive behaviors in mice.
      )
      2017MiceBrainGene expression70Histone demethylase
      Lopes et al. (
      • Lopes A.M.
      • Ross N.
      • Close J.
      • Dagnall A.
      • Amorim A.
      • Crow T.J.
      Inactivation status of PCDH11X: Sexual dimorphisms in gene expression levels in brain.
      )
      2006HumanBrainGene expression58Gene expression on autism
      Carrasquillo et al. (
      • Carrasquillo M.M.
      • Zou F.
      • Pankratz V.S.
      • Wilcox S.L.
      • Ma L.
      • Walker L.P.
      • et al.
      Genetic variation in PCDH11X is associated with susceptibility to late-onset Alzheimer's disease.
      )
      2009HumanBrainGenetic variation2099Genomic (PCDH11X)
      Beecham et al. (
      • Beecham G.W.
      • Naj A.C.
      • Gilbert J.R.
      • Haines J.L.
      • Buxbaum J.D.
      • Pericak-Vance M.A.
      PCDH11X variation is not associated with late-onset Alzheimer disease susceptibility.
      )
      2010HumanBrainGene expression1739PCDH11X gene on AD
      Miar et al. (
      • Miar A.
      • Alvarez V.
      • Corao A.I.
      • Alonso B.
      • Díaz M.
      • Menéndez M.
      • et al.
      Lack of association between protocadherin 11-X/Y (PCDH11X and PCDH11Y) polymorphisms and late onset Alzheimer's disease.
      )
      2011HumanBrainGene expression770SNP and PCDH11X on AD
      Dumanski et al. (
      • Dumanski J.P.
      • Lambert J.-C.
      • Rasi C.
      • Giedraitis V.
      • Davies H.
      • Grenier-Boley B.
      • et al.
      Mosaic loss of chromosome Y in blood is associated with Alzheimer disease.
      )
      2016HumanBrainChromosomal expression1611Sex chromosome
      Mendevil-Perez et al. (
      • Mendivil-Perez M.
      • Velez-Pardo C.
      • Kosik K.S.
      • Lopera F.
      • Jimenez-Del-Rio M.
      IPSCs-derived nerve-like cells from familial Alzheimer’s disease PSEN 1 E280A reveal increased amyloid-beta levels and loss of the Y chromosome.
      )
      2019HumanStem cellsChromosomal modification2Sex chromosome
      Caceres et al. (
      • Caceres A.
      • Jene A.
      • Esko T.
      • Perez-Jurado L.A.
      • Gonzalez J.R.
      Extreme downregulation of chromosome Y and Alzheimer's disease in men.
      )
      2020HumanBrain/bloodChromosomal expression1018Sex chromosome
      Impact of Sex Hormones in AD
      Ishunina et al. (
      • Ishunina T.A.
      • Fischer D.F.
      • Swaab D.F.
      Estrogen receptor alpha and its splice variants in the hippocampus in aging and Alzheimer's disease.
      )
      2007HumanHippocampal tissueGene expression27ER and splicing variants
      Yaffe et al. (
      • Yaffe K.
      • Lindquist K.
      • Sen S.
      • Cauley J.
      • Ferrell R.
      • Penninx B.
      • et al.
      Estrogen receptor genotype and risk of cognitive impairment in elders: Findings from the Health ABC study.
      )
      2009HumanBloodSNP1343 females

      1184 males
      ER polymorphisms
      Ryan et al. (
      • Ryan J.
      • Carriere I.
      • Carcaillon L.
      • Dartigues J.F.
      • Auriacombe S.
      • Rouaud O.
      • et al.
      Estrogen receptor polymorphisms and incident dementia: The prospective 3C study.
      )
      2014HumanBloodSNP6959ER polymorphisms APOE4 association
      Gleason et al. (
      • Gleason C.E.
      • Dowling N.M.
      • Wharton W.
      • Manson J.E.
      • Miller V.M.
      • Atwood C.S.
      • et al.
      Effects of hormone therapy on cognition and mood in recently postmenopausal women: Findings from the randomized, controlled KEEPS-Cognitive and Affective Study.
      )
      2015HumanHormone therapy

      Clinical trial
      727Efficacy
      Henderson et al. (
      • Henderson V.W.
      • St John J.A.
      • Hodis H.N.
      • McCleary C.A.
      • Stanczyk F.Z.
      • Shoupe D.
      • et al.
      Cognitive effects of estradiol after menopause: A randomized trial of the timing hypothesis.
      )
      2016HumanHormone therapy

      Clinical trial
      567Efficacy regarding timing

      Perimenopausal vs after menopause
      Shumaker et al. (
      • Shumaker S.A.
      • Legault C.
      • Kuller L.
      • Rapp S.R.
      • Thal L.
      • Lane D.S.
      • et al.
      Conjugated equine estrogens and incidence of probable dementia and mild cognitive impairment in postmenopausal women: Women's Health Initiative Memory Study.
      )
      2004HumanHormone therapy

      Clinical trial
      1464 estrogen vs. 1483 placebo; 3693 estrogen + progesterone vs. 3786 placeboEfficacy
      Shumaker et al. (
      • Shumaker S.A.
      • Legault C.
      • Rapp S.R.
      • Thal L.
      • Wallace R.B.
      • Ockene J.K.
      • et al.
      Estrogen plus progestin and the incidence of dementia and mild cognitive impairment in postmenopausal women: The Women's Health Initiative Memory Study: A randomized controlled trial.
      )
      2003HumanHormone therapy

      Clinical trial
      2229 estrogen + progesterone vs. 2303 placeboEfficacy
      Zandi et al. (
      • Zandi P.P.
      • Carlson M.C.
      • Plassman B.L.
      • Welsh-Bohmer K.A.
      • Mayer L.S.
      • Steffens D.C.
      • et al.
      Hormone replacement therapy and incidence of Alzheimer disease in older women: The Cache County Study.
      )
      2002HumanProspective clinical studies1357 men

      1889 women
      HRT

      Incidence of AD
      Bulk Tissue Omics Studies
      Sun et al. (
      • Sun L.-L.
      • Yang S.-L.
      • Sun H.
      • Li W.-D.
      • Duan S.-R.
      Molecular differences in Alzheimer's disease between male and female patients determined by integrative network analysis.
      )
      2019HumanBrainTranscriptomics1053

      19 brain regions

      125 subjects
      Sex-biased gene topology in different brain regions
      Winkler and Fox (
      • Winkler J.M.
      • Fox H.S.
      Transcriptome meta-analysis reveals a central role for sex steroids in the degeneration of hippocampal neurons in Alzheimer’s disease.
      )
      2013HumanHippocampal neuronsTranscriptomics17 AD and 21 control casesSex steroid pathways
      Paranjpe et al. (
      • Paranjpe M.D.
      • Belonwu S.
      • Wang J.K.
      • Oskotsky T.
      • Gupta A.
      • Taubes A.
      • et al.
      Sex-specific cross tissue meta-analysis identifies immune dysregulation in women with Alzheimer’s disease.
      )
      2020HumanBrain/bloodTranscriptomics1084 (brain)

      645 (blood)
      Female-specific immune signature
      Brooks and Mias (
      • Brooks L.R.K.
      • Mias G.I.
      Data-driven analysis of age, sex, and tissue effects on gene expression variability in Alzheimer's disease.
      )
      2019HumanBrain/bloodMicroarray2088Female-specific increase in CXCR4
      Sanfilippo et al. (
      • Sanfilippo C.
      • Castrogiovanni P.
      • Imbesi R.
      • Kazakowa M.
      • Musumeci G.
      • Blennow K.
      • et al.
      Sex difference in CHI3L1 expression levels in human brain aging and in Alzheimer’s disease.
      )
      2019HumanBrainMicroarray992 subjects with AD and 1290 control subjectsFemale-specific increase in CHI3L1
      Deming et al. (
      • Deming Y.
      • Dumitrescu L.
      • Barnes L.L.
      • Thambisetty M.
      • Kunkle B.
      • Gifford K.A.
      • et al.
      Sex-specific genetic predictors of Alzheimer's disease biomarkers.
      )
      2018HumanBrain/CSFGWAS3036Sex-specific GWAS loci in AD
      Cáceres and González (
      • Cáceres A.
      • González J.R.
      Female-specific risk of Alzheimer's disease is associated with tau phosphorylation processes: A transcriptome-wide interaction analysis.
      )
      2020HumanBrainTranscriptomics785Sex-biased tau phosphorylation
      Fukumoto et al. (
      • Fukumoto N.
      • Fujii T.
      • Combarros O.
      • Kamboh M.I.
      • Tsai S.J.
      • Matsushita S.
      • et al.
      Sexually dimorphic effect of the Val66Met polymorphism of BDNF on susceptibility to Alzheimer's disease: New data and meta-analysis.
      )
      2010HumanTissueGene expression1182BDNF SNPs
      Bangasser et al. (
      • Bangasser D.A.
      • Dong H.
      • Carroll J.
      • Plona Z.
      • Ding H.
      • Rodriguez L.
      • et al.
      Corticotropin-releasing factor overexpression gives rise to sex differences in Alzheimer's disease-related signaling.
      )
      2017RatBrain tissueProtein receptor expression40CRF expression on AD
      Zhao et al. (
      • Zhao L.
      • Mao Z.
      • Woody S.K.
      • Brinton R.D.
      Sex differences in metabolic aging of the brain: Insights into female susceptibility to Alzheimer's disease.
      )
      2016MouseBrainTargeted array of qPCR of 182 genes40Hypometabolism phenotypes in AD
      Aberg et al. (
      • Aberg D.
      • Johansson P.
      • Isgaard J.
      • Wallin A.
      • Johansson J.O.
      • Andreasson U.
      • et al.
      Increased cerebrospinal fluid level of insulin-like growth factor-II in male patients with Alzheimer's disease.
      )
      2015HumanBrain/CSFProtein expression80Insulin growth factor in CSF AD
      Piccio et al. (
      • Piccio L.
      • Deming Y.
      • Del-Aguila J.L.
      • Ghezzi L.
      • Holtzman D.M.
      • Fagan A.M.
      • et al.
      Cerebrospinal fluid soluble TREM2 is higher in Alzheimer disease and associated with mutation status.
      )
      2016HumanBrain/CSFProtein expression107sTREM2 in AD CSF
      Viswanathan et al. (
      • Viswanathan J.
      • Makinen P.
      • Helisalmi S.
      • Haapasalo A.
      • Soininen H.
      • Hiltunen M.
      An association study between granulin gene polymorphisms and Alzheimer's disease in Finnish population.
      )
      2009HumanBloodSNPs1161GRN polymorphisms in AD
      Boks et al. (
      • Boks M.P.
      • Derks E.M.
      • Weisenberger D.J.
      • Strengman E.
      • Janson E.
      • Sommer I.E.
      • et al.
      The relationship of DNA methylation with age, gender and genotype in twins and healthy controls.
      )
      2009HumanBloodDNA methylation188Methylation modified by age and sex
      El-Maarri et al. (
      • El-Maarri O.
      • Becker T.
      • Junen J.
      • Manzoor S.S.
      • Diaz-Lacava A.
      • Schwaab R.
      • et al.
      Gender specific differences in levels of DNA methylation at selected loci from human total blood: A tendency toward higher methylation levels in males.
      )
      2007HumanBrainEpigenomics192Sex-biased epigenetics in AD
      Mano et al. (
      • Mano T.
      • Nagata K.
      • Nonaka T.
      • Tarutani A.
      • Imamura T.
      • Hashimoto T.
      • et al.
      Neuron-specific methylome analysis reveals epigenetic regulation and tau-related dysfunction of BRCA1 in Alzheimer's disease.
      )
      2017HumanBrain/tissueMethylation microarray60Epigenetics in tau and AD
      Mahady et al. (
      • Mahady L.
      • Nadeem M.
      • Malek-Ahmadi M.
      • Chen K.
      • Perez S.E.
      • Mufson E.J.
      HDAC2 dysregulation in the nucleus basalis of Meynert during the progression of Alzheimer's disease.
      )
      2019HumanBrain/tissueProtein, IHC61HDAC2 dysregulation in AD brain
      Cao et al. (
      • Cao M.
      • Li H.
      • Zhao J.
      • Cui J.
      • Hu G.
      Identification of age- and gender-associated long noncoding RNAs in the human brain with Alzheimer's disease.
      )
      2019HumanBrainMicroarray214Long ncRNAs in AD
      Single-Cell Omics Studies
      Mathys et al. (
      • Mathys H.
      • Davila-Velderrain J.
      • Peng Z.
      • Gao F.
      • Mohammadi S.
      • Young J.Z.
      • et al.
      Single-cell transcriptomic analysis of Alzheimer's disease.
      )
      2019Human/miceCell/tissueTranscriptomics48Brain cell types in AD
      Pinheiro et al. (
      • Pinheiro I.
      • Dejager L.
      • Libert C.
      X-chromosome-located microRNAs in immunity: Might they explain male/female differences? The X chromosome-genomic context may affect X-located miRNAs and downstream signaling, thereby contributing to the enhanced immune response of females.
      )
      2011Human/miceBrainTranscriptomics48MicroRNA and X chromosome
      Villa et al. (
      • Villa A.
      • Gelosa P.
      • Castiglioni L.
      • Cimino M.
      • Rizzi N.
      • Pepe G.
      • et al.
      Sex-specific features of microglia from adult mice.
      )
      2018MiceBrain/tissueTranscriptomics24Sex features on microglia in mice
      Sarvari et al. (
      • Sarvari M.
      • Hrabovszky E.
      • Kallo I.
      • Solymosi N.
      • Liko I.
      • Berchtold N.
      • et al.
      Menopause leads to elevated expression of macrophage-associated genes in the aging frontal cortex: Rat and human studies identify strikingly similar changes.
      )
      2012Human/ratBrainTranscriptomics39Menopause and macrophage
      Guneykaya et al. (
      • Guneykaya D.
      • Ivanov A.
      • Hernandez D.P.
      • Haage V.
      • Wojtas B.
      • Meyer N.
      • et al.
      Transcriptional and translational differences of microglia from male and female brains.
      )
      2018MiceBrainTranscriptomics6Microglia in male female brains
      Hanamsagar et al. (
      • Hanamsagar R.
      • Alter M.D.
      • Block C.S.
      • Sullivan H.
      • Bolton J.L.
      • Bilbo S.D.
      Generation of a microglial developmental index in mice and in humans reveals a sex difference in maturation and immune reactivity.
      )
      2018MiceBrainTranscriptomics20Microglia on AD sex differences
      Keren-Shaul et al. (
      • Keren-Shaul H.
      • Spinrad A.
      • Weiner A.
      • Matcovitch-Natan O.
      • Dvir-Szternfeld R.
      • Ulland T.K.
      • et al.
      A unique microglia type associated with restricting development of Alzheimer's disease.
      )
      2017MiceBrain/tissueTranscriptomics12Microglia in AD
      Stephen et al. (
      • Stephen T.L.
      • Cacciottolo M.
      • Balu D.
      • Morgan T.E.
      • LaDu M.J.
      • Finch C.E.
      • et al.
      APOE genotype and sex affect microglial interactions with plaques in Alzheimer's disease mice.
      )
      2019MiceBrain/tissueTranscriptomics25Sex and AD with microglia
      Kodama L et al. (
      • Kodama L.
      • Guzman E.
      • Etchegaray J.I.
      • Li Y.
      • Sayed F.A.
      • Zhou L.
      • et al.
      Microglial microRNAs mediate sex-specific responses to tau pathology.
      )
      2020MiceBrain/tissueTranscriptomics87Sex effect on microglia and AD
      The Interplay of Sex and Genetic Risk Factors in AD
      Hsu et al. (
      • Hsu M.
      • Dedhia M.
      • Crusio W.E.
      • Delprato A.
      Sex differences in gene expression patterns associated with the APOE4 allele.
      )
      2019HumanBrainTranscription factors100APOE and conversion of MCI to AD
      Shang et al. (
      • Shang Y.
      • Mishra A.
      • Wang T.
      • Wang Y.
      • Desai M.
      • Chen S.
      • et al.
      Evidence in support of chromosomal sex influencing plasma based metabolome vs APOE genotype influencing brain metabolome profile in humanized APOE male and female mice.
      )
      2020MiceBrain/bloodTranscriptomics

      Metabolomics
      40APOE on metabolome
      AD, Alzheimer’s disease; APOE4, APOE ε4; cDNA, complementary DNA; CRF, corticotropin-releasing factor; CSF, cerebrospinal fluid; ER, estrogen receptor; GWAS, genome-wide association study; HRT, hormone replacement therapy; IHC, immunohistochemistry; MCI, mild cognitive impairment; ncRNA, noncoding RNA; qPCR, quantitative polymerase chain reaction; SNP, single nucleotide polymorphism.
      Transcriptome-wide sex differences in AD have been characterized in multiple studies. A recent study of 1053 postmortem brain samples across 19 brain regions constructed sex-specific gene coexpression networks for each brain region and found three regions with the most prominent differences between females and males in terms of network topology: the superior parietal lobule, dorsolateral prefrontal cortex, and occipital visual cortex. There were very few commonly shared differentially expressed genes (DEGs) (AD vs. control), except sex hormone genes such as estrogen-related receptor beta differentially expressed in four brain regions (
      • Sun L.-L.
      • Yang S.-L.
      • Sun H.
      • Li W.-D.
      • Duan S.-R.
      Molecular differences in Alzheimer's disease between male and female patients determined by integrative network analysis.
      ). A meta-analysis study of protein-protein interaction network revealed important roles for sex steroids in hippocampal neuronal degeneration with androgen and estrogen receptors identified as key drivers of disrupted homeostatic processes of AD neurons (
      • Winkler J.M.
      • Fox H.S.
      Transcriptome meta-analysis reveals a central role for sex steroids in the degeneration of hippocampal neurons in Alzheimer’s disease.
      ). The identification of sex hormones through omics studies further strengthens their importance in sex-biased differences in AD. Future studies with multiomics analyses will facilitate a better understanding of the effects of sex hormones and associated pathways on sex-biased vulnerability and treatment responsiveness in AD.
      Besides sex hormones, many female-specific pathways have been identified as contributing to AD susceptibilities. A cross-tissue meta-analysis revealed a female-specific immune signature in both brain and blood (
      • Paranjpe M.D.
      • Belonwu S.
      • Wang J.K.
      • Oskotsky T.
      • Gupta A.
      • Taubes A.
      • et al.
      Sex-specific cross tissue meta-analysis identifies immune dysregulation in women with Alzheimer’s disease.
      ). With network-based analysis and cell type deconvolution approaches, this study analyzed gene expression profiles derived from brain samples of 1084 patients with AD and age-matched control subjects, and whole blood samples of 645 patients with AD and control subjects. More DEGs and gene coexpression modules were identified in women than in men. Many upregulated DEGs in females were enriched in innate and adaptive immune systems, whereas downregulated DEGs were enriched in neurological signaling pathways such as synaptic vehicle exocytosis and autophagy. The blood sample analysis discovered female-specific immune cell-type changes in AD, i.e., increases in neutrophils and naïve B cells and decreases in M2 macrophages, memory B cells, and CD8+ T cells. This study also showed that machine learning predictive models performed better with female-only data, suggesting that molecular changes in females might better model AD-related changes than might those in males. Another meta-analysis examined gene expression changes in AD with eight publicly available microarray datasets from 2088 brain and blood samples and identified a female-specific increase of CXCR4 in AD samples compared with control samples (
      • Brooks L.R.K.
      • Mias G.I.
      Data-driven analysis of age, sex, and tissue effects on gene expression variability in Alzheimer's disease.
      ). CXCR4 is a chemokine receptor involved in inflammatory signaling pathways, the dysregulation of which is associated with neurodegenerative diseases (
      • Bonham L.W.
      • Karch C.M.
      • Fan C.C.
      • Tan C.
      • Geier E.G.
      • Wang Y.
      • et al.
      CXCR4 involvement in neurodegenerative diseases.
      ). Female-specific neuroinflammation in AD was also embodied in the female-specific expression pattern of chitinase-3-like 1, a protein related to inflammatory processes and neurodegeneration (
      • Sanfilippo C.
      • Castrogiovanni P.
      • Imbesi R.
      • Kazakowa M.
      • Musumeci G.
      • Blennow K.
      • et al.
      Sex difference in CHI3L1 expression levels in human brain aging and in Alzheimer’s disease.
      ).
      Female-biased changes in AD have also been observed in many other pathways associated with the development of amyloid and tau pathology. A genome-wide association study of 1527 male and 1509 female brain and CSF samples from the Religious Orders Study and Rush Memory and Aging Project (ROSMAP) cohort identified a strong association of serpin family B members SERPINB1, SERPINB6, and SERPINB9 with amyloidosis, and OSTN and CLDN16 with tau pathology in females but not in males (
      • Deming Y.
      • Dumitrescu L.
      • Barnes L.L.
      • Thambisetty M.
      • Kunkle B.
      • Gifford K.A.
      • et al.
      Sex-specific genetic predictors of Alzheimer's disease biomarkers.
      ). A transcriptome-wide meta-analysis of cerebral cortex samples discovered two female-specific AD risk genes associated with tau phosphorylation processes (
      • Cáceres A.
      • González J.R.
      Female-specific risk of Alzheimer's disease is associated with tau phosphorylation processes: A transcriptome-wide interaction analysis.
      ), NCL and KIF2A. Their expression levels were upregulated in females with AD when compared with control females but downregulated in males with AD in contrast to control males. Other female-specific genetic associations in AD such as neurotrophin signaling pathway and polymorphisms in SERPINB1 have been observed (
      • Deming Y.
      • Dumitrescu L.
      • Barnes L.L.
      • Thambisetty M.
      • Kunkle B.
      • Gifford K.A.
      • et al.
      Sex-specific genetic predictors of Alzheimer's disease biomarkers.
      ,
      • Aguirre C.C.
      • Baudry M.
      Progesterone reverses 17beta-estradiol-mediated neuroprotection and BDNF induction in cultured hippocampal slices.
      ,
      • Fisher D.W.
      • Bennett D.A.
      • Dong H.
      Sexual dimorphism in predisposition to Alzheimer's disease.
      ). A meta-analysis of 657 patients with AD and 525 control subjects in a Japanese population identified a significant allelic association between Val66Met of the brain-derived neurotrophic factor gene (BDNF) and AD in women (
      • Fukumoto N.
      • Fujii T.
      • Combarros O.
      • Kamboh M.I.
      • Tsai S.J.
      • Matsushita S.
      • et al.
      Sexually dimorphic effect of the Val66Met polymorphism of BDNF on susceptibility to Alzheimer's disease: New data and meta-analysis.
      ). A follow-up study of 4711 patients with AD and 4537 control subjects from 16 research centers around the world further confirmed the sex differences in BDNF allelic association with the Met66 allele conferring female-specific AD susceptibility.
      Consistently, many female-biased changes in AD human studies as described above have been recapitulated in animal studies. A mouse study shows that expression changes of inflammatory mediators (5xFAD vs. control) in the brain were more robust in females than males, with significant expression changes of cytokines (interleukin 1β, interleukin 6) and chemokines (CCL2 and CXCL10) only observed in female brains when compared with control brains (
      • Manji Z.
      • Rojas A.
      • Wang W.
      • Dingledine R.
      • Varvel N.H.
      • Ganesh T.
      5xFAD mice display sex-dependent inflammatory gene induction during the prodromal stage of Alzheimer's disease.
      ). Moreover, a study of corticotropin-releasing factor overexpression mice using phosphoproteomic approaches found that cortical phosphopeptides of female corticotropin-releasing factor overexpression mice were overrepresented in tau pathology–related signaling pathways, suggesting a sex-biased corticotropin-releasing factor signaling in tau phosphorylation of AD (
      • Bangasser D.A.
      • Dong H.
      • Carroll J.
      • Plona Z.
      • Ding H.
      • Rodriguez L.
      • et al.
      Corticotropin-releasing factor overexpression gives rise to sex differences in Alzheimer's disease-related signaling.
      ). A mouse study of brain aging also revealed an overall decreased bioenergetic metabolism during early disease transition in female brains, suggesting hypometabolic phenotypes at the onset of AD (
      • Zhao L.
      • Mao Z.
      • Woody S.K.
      • Brinton R.D.
      Sex differences in metabolic aging of the brain: Insights into female susceptibility to Alzheimer's disease.
      ).
      Interestingly, some male-specific risk genes are associated with AD, such as GRN, TREM2, and IGF-2 (
      • Aberg D.
      • Johansson P.
      • Isgaard J.
      • Wallin A.
      • Johansson J.O.
      • Andreasson U.
      • et al.
      Increased cerebrospinal fluid level of insulin-like growth factor-II in male patients with Alzheimer's disease.
      ,
      • Piccio L.
      • Deming Y.
      • Del-Aguila J.L.
      • Ghezzi L.
      • Holtzman D.M.
      • Fagan A.M.
      • et al.
      Cerebrospinal fluid soluble TREM2 is higher in Alzheimer disease and associated with mutation status.
      ,
      • Viswanathan J.
      • Makinen P.
      • Helisalmi S.
      • Haapasalo A.
      • Soininen H.
      • Hiltunen M.
      An association study between granulin gene polymorphisms and Alzheimer's disease in Finnish population.
      ). For example, a Finnish study of 512 patients with AD and 649 control subjects identified male-specific single nucleotide polymorphisms of GRN associated with AD (
      • Viswanathan J.
      • Makinen P.
      • Helisalmi S.
      • Haapasalo A.
      • Soininen H.
      • Hiltunen M.
      An association study between granulin gene polymorphisms and Alzheimer's disease in Finnish population.
      ). Moreover, in a small cohort of 60 participants, CSF IGF-2 and IGFBP-2 levels were found higher in males with AD when compared with levels of mild cognitive impairment and/or control male counterparts, with no differences seen between groups of female subjects (
      • Aberg D.
      • Johansson P.
      • Isgaard J.
      • Wallin A.
      • Johansson J.O.
      • Andreasson U.
      • et al.
      Increased cerebrospinal fluid level of insulin-like growth factor-II in male patients with Alzheimer's disease.
      ). A significant correlation of CSF IGF-2 levels with CSF phosphorylated tau and total tau levels was identified in male subjects as well (
      • Aberg D.
      • Johansson P.
      • Isgaard J.
      • Wallin A.
      • Johansson J.O.
      • Andreasson U.
      • et al.
      Increased cerebrospinal fluid level of insulin-like growth factor-II in male patients with Alzheimer's disease.
      ). In addition, significantly higher levels of soluble TREM2 in CSF have been reported in subjects with AD and subjects with frontotemporal dementia than those in normal aging control subjects in a cohort of 180 samples (
      • Piccio L.
      • Deming Y.
      • Del-Aguila J.L.
      • Ghezzi L.
      • Holtzman D.M.
      • Fagan A.M.
      • et al.
      Cerebrospinal fluid soluble TREM2 is higher in Alzheimer disease and associated with mutation status.
      ). Intriguingly, levels of CSF soluble TREM2 were higher in male than in female participants, suggesting a possibility of sex-biased effects of TREM2 on microglial activation and neurodegeneration in AD (
      • Piccio L.
      • Deming Y.
      • Del-Aguila J.L.
      • Ghezzi L.
      • Holtzman D.M.
      • Fagan A.M.
      • et al.
      Cerebrospinal fluid soluble TREM2 is higher in Alzheimer disease and associated with mutation status.
      ).
      Evidence also indicates the impact of sex on epigenomic signatures of AD (
      • Boks M.P.
      • Derks E.M.
      • Weisenberger D.J.
      • Strengman E.
      • Janson E.
      • Sommer I.E.
      • et al.
      The relationship of DNA methylation with age, gender and genotype in twins and healthy controls.
      ,
      • El-Maarri O.
      • Becker T.
      • Junen J.
      • Manzoor S.S.
      • Diaz-Lacava A.
      • Schwaab R.
      • et al.
      Gender specific differences in levels of DNA methylation at selected loci from human total blood: A tendency toward higher methylation levels in males.
      ). For example, Mano et al. (
      • Mano T.
      • Nagata K.
      • Nonaka T.
      • Tarutani A.
      • Imamura T.
      • Hashimoto T.
      • et al.
      Neuron-specific methylome analysis reveals epigenetic regulation and tau-related dysfunction of BRCA1 in Alzheimer's disease.
      ) showed hypomethylated CpG islands in the promoter region of a gene aurora kinase C in male subjects with AD, particularly in APOE4 carriers, but the same CpG islands were hypermethylated in female subjects with AD. In addition, studies demonstrated reduced histone deacetylase 2 levels in female subjects with normal aging, female subjects with mild cognitive impairment, and female subjects with AD when compared with their male counterparts (
      • Mahady L.
      • Nadeem M.
      • Malek-Ahmadi M.
      • Chen K.
      • Perez S.E.
      • Mufson E.J.
      HDAC2 dysregulation in the nucleus basalis of Meynert during the progression of Alzheimer's disease.
      ). Moreover, the expression of long noncoding RNAs was found to be different between male and female subjects with AD (
      • Cao M.
      • Li H.
      • Zhao J.
      • Cui J.
      • Hu G.
      Identification of age- and gender-associated long noncoding RNAs in the human brain with Alzheimer's disease.
      ).
      While most current studies focus on one or two modalities of omics analysis, little effort was devoted to integrating these studies into a holistic examination at multiscale levels, partially owing to limited data availability as well as technical challenges in developing more integrative systems biology approaches. With increasingly available large-scale omics data and fast-paced advances in integrative multiomics approaches, we will gain more mechanistic insights into AD pathogenesis and sex-biased differences in AD, which will guide future development of sex-stratified preventive and therapeutic interventions.

      Single-Cell Omics Studies of Cell Type–Specific Contributions to Sex Differences in AD

      It has been increasingly recognized that the complexity of AD pathogenesis may reside in the heterogeneity of disease-associated changes in different brain cell types. While bulk tissue multiomics profiling highlights downregulation of neuronal activities and upregulation of immune responses in AD, the single-cell RNA sequencing enables in-depth dissection of cell type–specific contributions to disease mechanisms during AD development and progression.
      A recent seminal study by Mathys et al. (
      • Mathys H.
      • Davila-Velderrain J.
      • Peng Z.
      • Gao F.
      • Mohammadi S.
      • Young J.Z.
      • et al.
      Single-cell transcriptomic analysis of Alzheimer's disease.
      ) performed a single-nucleus transcriptomic analysis of the prefrontal cortex of 48 subjects with AD of the ROSMAP (Religious Orders Study and Rush Memory and Aging Project) cohort. It was found that DEGs in neurons were downregulated, whereas most DEGs in non-neuronal cells were upregulated. Cell type–specific DEGs were mostly seen in early AD stages, whereas upregulated DEGs in late AD stage were commonly shared across cell types. More importantly, robust sex-specific differences in association with AD were identified among female and male brain cells. It was demonstrated that AD pathology–associated cell subpopulations were enriched in female cells with higher levels of marker gene expression, whereas no-pathology subpopulations were enriched in male cells. The most dramatic differences in sex-specific transcriptional responses were observed in neurons and oligodendrocytes, with global transcriptional activation in male oligodendrocytes and global downregulation of gene activities in female excitatory and inhibitory neurons that were associated with increased AD pathology, respectively (
      • Mathys H.
      • Davila-Velderrain J.
      • Peng Z.
      • Gao F.
      • Mohammadi S.
      • Young J.Z.
      • et al.
      Single-cell transcriptomic analysis of Alzheimer's disease.
      ). This study provides evidence of sex-dimorphic changes in AD, suggesting greater transcriptional dysregulation accounting for higher disease burden in female subjects with AD.
      The importance of microglia in sex-biased differences in AD has also been increasingly recognized. Understanding sex differences in microglial responses to disease states may help unveil sex-biased AD susceptibility. It is well known that the X chromosome contains the largest numbers of immune-related genes and micro-RNAs involved in immune regulation (
      • Klein S.L.
      • Flanagan K.L.
      Sex differences in immune responses.
      ,
      • Pinheiro I.
      • Dejager L.
      • Libert C.
      X-chromosome-located microRNAs in immunity: Might they explain male/female differences? The X chromosome-genomic context may affect X-located miRNAs and downstream signaling, thereby contributing to the enhanced immune response of females.
      ,
      • Fish E.N.
      The X-files in immunity: Sex-based differences predispose immune responses.
      ). In addition, microglia express steroid hormone receptors that are important in microglial sexual differentiation. Once differentiated, microglia maintain transcriptional profiles even in the absence of hormones during adulthood (
      • Villa A.
      • Gelosa P.
      • Castiglioni L.
      • Cimino M.
      • Rizzi N.
      • Pepe G.
      • et al.
      Sex-specific features of microglia from adult mice.
      ). Hormone depletion in female aging brains led to profound transcriptomic changes with heightened inflammatory responses (
      • Benedusi V.
      • Meda C.
      • Della Torre S.
      • Monteleone G.
      • Vegeto E.
      • Maggi A.
      A lack of ovarian function increases neuroinflammation in aged mice.
      ,
      • Sarvari M.
      • Hrabovszky E.
      • Kallo I.
      • Solymosi N.
      • Liko I.
      • Berchtold N.
      • et al.
      Menopause leads to elevated expression of macrophage-associated genes in the aging frontal cortex: Rat and human studies identify strikingly similar changes.
      ), which may contribute to sex differences in developing AD.
      A recent study performed transcriptomic and proteomic analyses of microglia from five different brain regions of male and female C57BL/6J mice (
      • Guneykaya D.
      • Ivanov A.
      • Hernandez D.P.
      • Haage V.
      • Wojtas B.
      • Meyer N.
      • et al.
      Transcriptional and translational differences of microglia from male and female brains.
      ). The baseline phagocytosis was similar, but the expression of major histocompatibility class I and II genes was higher in male cortical microglia than in females, suggesting a higher antigen-presenting capacity. More pronounced differences were seen in the hippocampus, with 1109 DEGs identified between male and female microglia, with only 55 DEGs in cortical microglia. The proteomic analysis indicated that proteins highly expressed in male microglia were involved in toll-like receptor pathways (immune responses after challenges), whereas proteins highly expressed in female microglia were involved in interferon-related processes (
      • Guneykaya D.
      • Ivanov A.
      • Hernandez D.P.
      • Haage V.
      • Wojtas B.
      • Meyer N.
      • et al.
      Transcriptional and translational differences of microglia from male and female brains.
      ). Another study of transcriptomic profiles of purified male and female hippocampal microglia from embryonic day 18 to postnatal day 60 mice reported that the increases of microglial genes occurred later in males than females, and that microglial maturation accelerated by acute immune stimuli were observed only in males (
      • Hanamsagar R.
      • Alter M.D.
      • Block C.S.
      • Sullivan H.
      • Bolton J.L.
      • Bilbo S.D.
      Generation of a microglial developmental index in mice and in humans reveals a sex difference in maturation and immune reactivity.
      ).
      A study of an AD mouse model identified a stepwise microglia activation with downregulation of microglia checkpoints followed by TREM2-dependent activation (
      • Keren-Shaul H.
      • Spinrad A.
      • Weiner A.
      • Matcovitch-Natan O.
      • Dvir-Szternfeld R.
      • Ulland T.K.
      • et al.
      A unique microglia type associated with restricting development of Alzheimer's disease.
      ). However, how sex and APOE genotype impact these processes remains to be elucidated. A recent study examined microglial interaction with amyloid plaques in EFAD mouse models and reported significantly lower microglial coverage of amyloid plaques in female mouse brains with the APOE4 genotype (
      • Stephen T.L.
      • Cacciottolo M.
      • Balu D.
      • Morgan T.E.
      • LaDu M.J.
      • Finch C.E.
      • et al.
      APOE genotype and sex affect microglial interactions with plaques in Alzheimer's disease mice.
      ). The microglial expression of TREM2 around amyloid plaques was also significantly reduced by APOE4 genotype and female sex. These studies support the regulatory roles of APOE and sex in a TREM2-dependent microglial activation (
      • Stephen T.L.
      • Cacciottolo M.
      • Balu D.
      • Morgan T.E.
      • LaDu M.J.
      • Finch C.E.
      • et al.
      APOE genotype and sex affect microglial interactions with plaques in Alzheimer's disease mice.
      ).
      Furthermore, a study of a microRNA (miRNA) sequencing dataset of microglia identified a unique role of microglial miRNAs in mediating sex-specific responses to tau pathology (
      • Kodama L.
      • Guzman E.
      • Etchegaray J.I.
      • Li Y.
      • Sayed F.A.
      • Zhou L.
      • et al.
      Microglial microRNAs mediate sex-specific responses to tau pathology.
      ). Using microglia-specific knockout mice of Dicer with the depletion of the miRNA-processing enzyme and Dicer-dependent miRNAs, it was reported that the loss of mature miRNAs led to more dramatic changes in the male microglial transcriptome than in female counterparts with DEGs enriched in immune system pathways suggestive of a heightened inflammatory state. The studies of miRNA and messenger RNA profiles of male and female PS19 mouse models further noted greater changes induced by tau pathology in male microglia than female cells. The male microglia were enriched with genes involved in immune modulation and phagocytosis, characteristic of disease-associated microglia signature, whereas female microglia were enriched with homeostatic microglial genes and human AD genes (
      • Kodama L.
      • Guzman E.
      • Etchegaray J.I.
      • Li Y.
      • Sayed F.A.
      • Zhou L.
      • et al.
      Microglial microRNAs mediate sex-specific responses to tau pathology.
      ). This study suggests that loss of miRNAs led to sex-dimorphic changes in microglial transcriptome and tauopathy-related phenotypes.
      While single-cell or single-nucleus omics studies enable investigations of cell type–specific contributions to AD, future efforts are needed to integrate single-cell and bulk tissue omics data to better understand sex differences in AD and to determine the interactions among different brain cell types during AD development and progression.

      The Interplay of Sex and Genetic Risk Factors in AD

      Stratifying genetic association analysis by sex enables the identification of novel AD genetic risk loci and the determination of sex-dependent polygenic risks (
      • Gamache J.
      • Yun Y.
      • Chiba-Falek O.
      Sex-dependent effect of APOE on Alzheimer's disease and other age-related neurodegenerative disorders.
      ). For example, BIN1 and MS4A6A were found to contribute to AD progression significantly more in females than in males (
      • Fan C.C.
      • Banks S.J.
      • Thompson W.K.
      • Chen C.H.
      • McEvoy L.K.
      • Tan C.H.
      • et al.
      Sex-dependent autosomal effects on clinical progression of Alzheimer's disease.
      ). Among many genetic loci that manifest sex-specific effects on AD, the interplay between sex and the APOE allele in AD has been extensively explored (
      • Farrer L.A.
      • Cupples L.A.
      • Haines J.L.
      • Hyman B.
      • Kukull W.A.
      • Mayeux R.
      • et al.
      Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease. A meta-analysis. APOE and Alzheimer Disease Meta Analysis Consortium.
      ,
      • Neu S.C.
      • Pa J.
      • Kukull W.
      • Beekly D.
      • Kuzma A.
      • Gangadharan P.
      • et al.
      Apolipoprotein E genotype and sex risk factors for Alzheimer disease: A meta-analysis.
      ). However, only limited multiomics studies have been performed to understand the interplay between APOE and sex in AD.
      A small cohort of 100 RNA sequencing data derived from four human brain regions examined APOE4 genotype–associated gene expression patterns. Many more transcription factors differentially expressed in APOE4+ females versus APOE3+ females than those in APOE4+ males versus APOE3+ males, with PSEN2 expressed highest in the temporal cortex of APOE4+ females and CNTNAP2 in APOE4+ males (
      • Hsu M.
      • Dedhia M.
      • Crusio W.E.
      • Delprato A.
      Sex differences in gene expression patterns associated with the APOE4 allele.
      ). A separate study performed transcriptomic and metabolomic analyses of 16-month-old human APOE mice (
      • Shang Y.
      • Mishra A.
      • Wang T.
      • Wang Y.
      • Desai M.
      • Chen S.
      • et al.
      Evidence in support of chromosomal sex influencing plasma based metabolome vs APOE genotype influencing brain metabolome profile in humanized APOE male and female mice.
      ). There were more DEGs between sexes identified with the APOE4 genotype when compared with the APOE3 genotype. Metabolomic analysis showed elevated circulating acylcarnitine levels in APOE4 females. The study also demonstrated that sex and APOE genotype in combination modulate metabolism, bioenergetics, and neuroinflammation processes, and APOE4 females present with robust gene expression involved in lipid metabolism, antigen presentation, and interferon response genes (
      • Shang Y.
      • Mishra A.
      • Wang T.
      • Wang Y.
      • Desai M.
      • Chen S.
      • et al.
      Evidence in support of chromosomal sex influencing plasma based metabolome vs APOE genotype influencing brain metabolome profile in humanized APOE male and female mice.
      ). These observations need to be validated in human samples.
      Interestingly, a hypothetic framework proposed by Dumitrescu et al. (
      • Dumitrescu L.
      • Mayeda E.R.
      • Sharman K.
      • Moore A.M.
      • Hohman T.J.
      Sex differences in the genetic architecture of Alzheimer's disease.
      ) suggests that during the development and progression of AD, genetic regulators of amyloid processes may be largely shared in both sexes, but among amyloid-positive subjects, a sex-specific genetic architecture may emerge with substantial APOE contributions to AD in females, including the sex-biased association between APOE and tau (
      • Altmann A.
      • Tian L.
      • Henderson V.W.
      • Greicius M.D.
      Alzheimer's Disease Neuroimaging Initiative Investigators
      Sex modifies the APOE-related risk of developing Alzheimer disease.
      ,
      • Hohman T.J.
      • Dumitrescu L.
      • Barnes L.L.
      • Thambisetty M.
      • Beecham G.
      • Kunkle B.
      • et al.
      Sex-specific association of apolipoprotein E with cerebrospinal fluid levels of tau.
      ). This hypothesis needs to be further tested.

      Summary

      To better understand the molecular and cellular mechanisms underlying sex-biased differences in AD, multiomics data have been made publicly available and systems biology approaches have been employed to model disease processes. Advances in single-cell and single-nucleus transcriptomic studies enable a better understanding of cell type–specific impact on sex-biased differences in AD. In this review, we discussed current studies of sex chromosomes and sex hormones, as well as multiomics analyses of sex-biased differences in AD processes. It should be noted that to better understand the varying susceptibilities of AD between males and females, it is important to examine sex-biased neurological changes in gene expression profiles and underlying molecular pathways during development and aging (Supplement). Leveraging the knowledge from these processes enables a better understanding of sex-biased differences in neurodegenerative processes.
      Several sex-biased signatures in AD have been identified through omics studies, e.g., genes and pathways involved in neuroinflammation and bioenergetic metabolism. The importance of genes and pathways associated with sex chromosomes and sex hormones in AD have been further strengthened by omics studies. The major limitations are that most current analyses focused on one or two modalities of omics data, with little effort devoted to integrating multiomics and multiple studies into a holistic picture of AD. While single-cell or single-nucleus omics studies facilitate in-depth understanding of cell type–specific contributions to AD, future efforts are needed to integrate single-cell and bulk tissue omics data to better understand sex differences in AD and to determine the interactions among different brain cell types during AD development. More importantly, there are many overlooked aspects in studying sex-biased differences in AD such as the roles of sex chromosomes, and the intrinsic differences in male and female brains during development and aging. As summarized in Figure 2, future research should integrate the multiomics data from different brain regions and different cell types into multiscale network analysis and leverage the knowledge from holistic models of sex-biased differences in development and adulthood, aging, and AD. Advances in systems biology technologies and increasingly available large-scale multiomics data will facilitate future studies’ dissecting such complex signaling mechanisms to better understand AD pathogenesis in both sexes, with the ultimate goals of developing efficacious sex- and APOE-stratified preventive and therapeutic interventions for AD.
      Figure thumbnail gr2
      Figure 2A holistic overview of sex differences in Alzheimer’s disease (AD). Future studies need to take into considerations of different aspects of sex differences in AD including prevalence/incidence, clinical manifestations, neuroimaging studies, and pathology studies as well as treatment responsiveness. The integration of multiomics studies such as transcriptomics, proteomics, metabolomics, epigenomics, and genomics data will facilitate a better understanding of sex differences in AD. More importantly, leveraging the knowledge from studies of sex chromosomes, intrinsic differences in development and adulthood such as sex hormones, aging processes, and the interplay between sex and other genetic rick factors like APOE will enable a holistic examination of sex differences in AD, guiding future development of efficacious sex- and APOE-stratified preventive and therapeutic interventions for AD.

      Acknowledgments and Disclosures

      This work was supported in part by funding from the National Institutes of Health (Grant Nos. 1R01AG048923 [to DC], RF1AG054014 [to DC and BZ], RO1AG068030 [to DC and BZ], R56AG058655 [to DC and BZ], UO1AG046170 [to BZ], RF1AG057440 [to BZ], RO1AG057907 [to BZ], and UO1AG052411 [to BZ]); and the Department of Veteran Affairs Biomedical Laboratory Research and Development (Grant No. I01BX003380) [to DC], and Rehabilitation Research and Development (Grant No. I01RX002290 [to DC]).
      The authors report no biomedical financial interests or potential conflicts of interest.

      Supplementary Material

      References

        • Alzheimer’s Association
        2020 Alzheimer's disease facts and figures.
        Alzheimers Dement. 2020; 16: 391-460
        • Barnes L.L.
        • Wilson R.S.
        • Schneider J.A.
        • Bienias J.L.
        • Evans D.A.
        • Bennett D.A.
        Gender, cognitive decline, and risk of AD in older persons.
        Neurology. 2003; 60: 1777-1781
        • Henley D.B.
        • May P.C.
        • Dean R.A.
        • Siemers E.R.
        Development of semagacestat (LY450139), a functional gamma-secretase inhibitor, for the treatment of Alzheimer's disease.
        Expert Opin Pharmacother. 2009; 10: 1657-1664
        • Doody R.S.
        • Raman R.
        • Farlow M.
        • Iwatsubo T.
        • Vellas B.
        • Joffe S.
        • et al.
        A phase 3 trial of semagacestat for treatment of Alzheimer's disease.
        N Engl J Med. 2013; 369: 341-350
        • Doody R.S.
        • Thomas R.G.
        • Farlow M.
        • Iwatsubo T.
        • Vellas B.
        • Kieburtz K.
        • et al.
        Phase 3 trials of solanezumab for mild-to-moderate Alzheimer's disease.
        N Engl J Med. 2014; 370: 311-321
        • Farlow M.
        • Arnold S.E.
        • van Dyck C.H.
        • Aisen P.S.
        • Snider B.J.
        • Porsteinsson A.P.
        • et al.
        Safety and biomarker effects of solanezumab in patients with Alzheimer's disease.
        Alzheimers Dement. 2012; 8: 261-271
        • Cavedo E.
        • Chiesa P.A.
        • Houot M.
        • Ferretti M.T.
        • Grothe M.J.
        • Teipel S.J.
        • et al.
        Sex differences in functional and molecular neuroimaging biomarkers of Alzheimer's disease in cognitively normal older adults with subjective memory complaints.
        Alzheimers Dement. 2018; 14: 1204-1215
        • Ferretti M.T.
        • Iulita M.F.
        • Cavedo E.
        • Chiesa P.A.
        • Schumacher Dimech A.
        • Santuccione Chadha A.
        • et al.
        Sex differences in Alzheimer disease - the gateway to precision medicine.
        Nat Rev Neurol. 2018; 14: 457-469
        • Mielke M.M.
        • Vemuri P.
        • Rocca W.A.
        Clinical epidemiology of Alzheimer's disease: Assessing sex and gender differences.
        Clin Epidemiol. 2014; 6: 37-48
        • Nebel R.A.
        • Aggarwal N.T.
        • Barnes L.L.
        • Gallagher A.
        • Goldstein J.M.
        • Kantarci K.
        • et al.
        Understanding the impact of sex and gender in Alzheimer's disease: A call to action.
        Alzheimers Dement. 2018; 14: 1171-1183
        • Toro C.A.
        • Zhang L.
        • Cao J.
        • Cai D.
        Sex differences in Alzheimer's disease: Understanding the molecular impact.
        Brain Res. 2019; 1719: 194-207
        • Russell L.M.
        • Strike P.
        • Browne C.E.
        • Jacobs P.A.
        X chromosome loss and ageing.
        Cytogenet Genome Res. 2007; 116: 181-185
        • Yurov Y.B.
        • Vorsanova S.G.
        • Liehr T.
        • Kolotii A.D.
        • Iourov I.Y.
        X chromosome aneuploidy in the Alzheimer’s disease brain.
        Mol Cytogenet. 2014; 7: 20
        • Spremo-Potparević B.
        • Živković L.
        • Djelić N.
        • Plećaš-Solarović B.
        • Smith M.A.
        • Bajić V.
        Premature centromere division of the X chromosome in neurons in Alzheimer’s disease.
        J Neurochem. 2008; 106: 2218-2223
        • Wojda A.
        • Ziętkiewicz E.
        • Mossakowska M.
        • Pawłowski W.
        • Skrzypczak K.
        • Witt M.
        Correlation between the level of cytogenetic aberrations in cultured human lymphocytes and the age and gender of donors.
        J Gerontol A Biol Sci Med Sci. 2006; 61: 763-772
        • Spremo-Potparevic B.
        • Zivkovic L.
        • Djelic N.
        • Bajic V.
        Analysis of premature centromere division (PCD) of the X chromosome in Alzheimer patients through the cell cycle.
        Exp Gerontol. 2004; 39: 849-854
        • Carrel L.
        • Willard H.F.
        X-inactivation profile reveals extensive variability in X-linked gene expression in females.
        Nature. 2005; 434: 400-404
        • Greenfield A.
        • Carrel L.
        • Pennisi D.
        • Philippe C.
        • Quaderi N.
        • Siggers P.
        • et al.
        The UTX gene escapes X inactivation in mice and humans.
        Hum Mol Genet. 1998; 7: 737-742
        • Miyake N.
        • Mizuno S.
        • Okamoto N.
        • Ohashi H.
        • Shiina M.
        • Ogata K.
        • et al.
        KDM6A point mutations cause Kabuki syndrome.
        Hum Mutat. 2013; 34: 108-110
        • Itoh Y.
        • Golden L.C.
        • Itoh N.
        • Matsukawa M.A.
        • Ren E.
        • Tse V.
        • et al.
        The X-linked histone demethylase Kdm6a in CD4+ T lymphocytes modulates autoimmunity.
        J Clin Invest. 2019; 129: 3852-3863
        • Davis E.J.
        • Broestl L.
        • Abdulai-Saiku S.
        • Worden K.
        • Bonham L.W.
        • Miñones-Moyano E.
        • et al.
        A second X chromosome contributes to resilience in a mouse model of Alzheimer’s disease.
        Sci Transl Med. 2020; 12eaaz5677
        • Tang G.B.
        • Zeng Y.Q.
        • Liu P.P.
        • Mi T.W.
        • Zhang S.F.
        • Dai S.K.
        • et al.
        The histone H3K27 demethylase UTX regulates synaptic plasticity and cognitive behaviors in mice.
        Front Mol Neurosci. 2017; 10: 267
        • Lopes A.M.
        • Ross N.
        • Close J.
        • Dagnall A.
        • Amorim A.
        • Crow T.J.
        Inactivation status of PCDH11X: Sexual dimorphisms in gene expression levels in brain.
        Hum Genet. 2006; 119: 267-275
        • O'Leary N.A.
        • Wright M.W.
        • Brister J.R.
        • Ciufo S.
        • Haddad D.
        • McVeigh R.
        • et al.
        Reference sequence (RefSeq) database at NCBI: Current status, taxonomic expansion, and functional annotation.
        Nucleic Acids Res. 2016; 44: D733-D745
        • Carrasquillo M.M.
        • Zou F.
        • Pankratz V.S.
        • Wilcox S.L.
        • Ma L.
        • Walker L.P.
        • et al.
        Genetic variation in PCDH11X is associated with susceptibility to late-onset Alzheimer's disease.
        Nat Genet. 2009; 41: 192-198
        • Beecham G.W.
        • Naj A.C.
        • Gilbert J.R.
        • Haines J.L.
        • Buxbaum J.D.
        • Pericak-Vance M.A.
        PCDH11X variation is not associated with late-onset Alzheimer disease susceptibility.
        Psychiatr Genet. 2010; 20: 321-324
        • Miar A.
        • Alvarez V.
        • Corao A.I.
        • Alonso B.
        • Díaz M.
        • Menéndez M.
        • et al.
        Lack of association between protocadherin 11-X/Y (PCDH11X and PCDH11Y) polymorphisms and late onset Alzheimer's disease.
        Brain Res. 2011; 1383: 252-256
        • Libert C.
        • Dejager L.
        • Pinheiro I.
        The X chromosome in immune functions: When a chromosome makes the difference.
        Nat Rev Immunol. 2010; 10: 594-604
        • Spolarics Z.
        • Peña G.
        • Qin Y.
        • Donnelly R.J.
        • Livingston D.H.
        Inherent X-linked genetic variability and cellular mosaicism unique to females contribute to sex-related differences in the innate immune response.
        Front Immunol. 2017; 8: 1455
        • Forsberg L.A.
        • Gisselsson D.
        • Dumanski J.P.
        Mosaicism in health and disease – clones picking up speed.
        Nat Rev Genet. 2017; 18: 128-142
        • Dumanski J.P.
        • Lambert J.-C.
        • Rasi C.
        • Giedraitis V.
        • Davies H.
        • Grenier-Boley B.
        • et al.
        Mosaic loss of chromosome Y in blood is associated with Alzheimer disease.
        Am J Hum Genet. 2016; 98: 1208-1219
        • Mendivil-Perez M.
        • Velez-Pardo C.
        • Kosik K.S.
        • Lopera F.
        • Jimenez-Del-Rio M.
        IPSCs-derived nerve-like cells from familial Alzheimer’s disease PSEN 1 E280A reveal increased amyloid-beta levels and loss of the Y chromosome.
        Neurosci Lett. 2019; 703: 111-118
        • Caceres A.
        • Jene A.
        • Esko T.
        • Perez-Jurado L.A.
        • Gonzalez J.R.
        Extreme downregulation of chromosome Y and Alzheimer's disease in men.
        Neurobiol Aging. 2020; 90: 150.e1-150.e4
        • Gurvich C.
        • Hoy K.
        • Thomas N.
        • Kulkarni J.
        Sex differences and the influence of sex hormones on cognition through adulthood and the aging process.
        Brain Sci. 2018; 8: 163
        • McCarthy M.M.
        Multifaceted origins of sex differences in the brain.
        Philos Trans R Soc Lond B Biol Sci. 2016; 371: 20150106
        • Pike C.J.
        Sex and the development of Alzheimer's disease.
        J Neurosci Res. 2017; 95: 671-680
        • Rahman A.
        • Jackson H.
        • Hristov H.
        • Isaacson R.S.
        • Saif N.
        • Shetty T.
        • et al.
        Sex and gender driven modifiers of Alzheimer's: The role for estrogenic control across age, race, medical, and lifestyle risks.
        Front Aging Neurosci. 2019; 11: 315
        • Gurvich C.
        • Thomas N.
        • Kulkarni J.
        Sex differences in cognition and aging and the influence of sex hormones.
        Handb Clin Neurol. 2020; 175: 103-115
        • Ishunina T.A.
        • Fischer D.F.
        • Swaab D.F.
        Estrogen receptor alpha and its splice variants in the hippocampus in aging and Alzheimer's disease.
        Neurobiol Aging. 2007; 28: 1670-1681
        • Foster T.C.
        Role of estrogen receptor alpha and beta expression and signaling on cognitive function during aging.
        Hippocampus. 2012; 22: 656-669
        • Rettberg J.R.
        • Yao J.
        • Brinton R.D.
        Estrogen: A master regulator of bioenergetic systems in the brain and body.
        Front Neuroendocrinol. 2014; 35: 8-30
        • Yaffe K.
        • Lindquist K.
        • Sen S.
        • Cauley J.
        • Ferrell R.
        • Penninx B.
        • et al.
        Estrogen receptor genotype and risk of cognitive impairment in elders: Findings from the Health ABC study.
        Neurobiol Aging. 2009; 30: 607-614
        • Ryan J.
        • Carriere I.
        • Carcaillon L.
        • Dartigues J.F.
        • Auriacombe S.
        • Rouaud O.
        • et al.
        Estrogen receptor polymorphisms and incident dementia: The prospective 3C study.
        Alzheimers Dement. 2014; 10: 27-35
        • Gleason C.E.
        • Dowling N.M.
        • Wharton W.
        • Manson J.E.
        • Miller V.M.
        • Atwood C.S.
        • et al.
        Effects of hormone therapy on cognition and mood in recently postmenopausal women: Findings from the randomized, controlled KEEPS-Cognitive and Affective Study.
        PLoS Med. 2015; 12e1001833
        • Henderson V.W.
        • St John J.A.
        • Hodis H.N.
        • McCleary C.A.
        • Stanczyk F.Z.
        • Shoupe D.
        • et al.
        Cognitive effects of estradiol after menopause: A randomized trial of the timing hypothesis.
        Neurology. 2016; 87: 699-708
        • Hodis H.N.
        • Mack W.J.
        • Shoupe D.
        • Azen S.P.
        • Stanczyk F.Z.
        • Hwang-Levine J.
        • et al.
        Methods and baseline cardiovascular data from the Early versus Late Intervention Trial with Estradiol testing the menopausal hormone timing hypothesis.
        Menopause. 2015; 22: 391-401
        • Maki P.M.
        Critical window hypothesis of hormone therapy and cognition: A scientific update on clinical studies.
        Menopause. 2013; 20: 695-709
        • Shumaker S.A.
        • Legault C.
        • Kuller L.
        • Rapp S.R.
        • Thal L.
        • Lane D.S.
        • et al.
        Conjugated equine estrogens and incidence of probable dementia and mild cognitive impairment in postmenopausal women: Women's Health Initiative Memory Study.
        JAMA. 2004; 291: 2947-2958
        • Shumaker S.A.
        • Legault C.
        • Rapp S.R.
        • Thal L.
        • Wallace R.B.
        • Ockene J.K.
        • et al.
        Estrogen plus progestin and the incidence of dementia and mild cognitive impairment in postmenopausal women: The Women's Health Initiative Memory Study: A randomized controlled trial.
        JAMA. 2003; 289: 2651-2662
        • Zandi P.P.
        • Carlson M.C.
        • Plassman B.L.
        • Welsh-Bohmer K.A.
        • Mayer L.S.
        • Steffens D.C.
        • et al.
        Hormone replacement therapy and incidence of Alzheimer disease in older women: The Cache County Study.
        JAMA. 2002; 288: 2123-2129
        • Barron A.M.
        • Pike C.J.
        Sex hormones, aging, and Alzheimer's disease.
        Front Biosci (Elite Ed). 2012; 4: 976-997
        • Whitmer R.A.
        • Quesenberry C.P.
        • Zhou J.
        • Yaffe K.
        Timing of hormone therapy and dementia: The critical window theory revisited.
        Ann Neurol. 2011; 69: 163-169
        • Sun L.-L.
        • Yang S.-L.
        • Sun H.
        • Li W.-D.
        • Duan S.-R.
        Molecular differences in Alzheimer's disease between male and female patients determined by integrative network analysis.
        J Cell Mol Med. 2019; 23: 47-58
        • Winkler J.M.
        • Fox H.S.
        Transcriptome meta-analysis reveals a central role for sex steroids in the degeneration of hippocampal neurons in Alzheimer’s disease.
        BMC Syst Biol. 2013; 7: 51
        • Paranjpe M.D.
        • Belonwu S.
        • Wang J.K.
        • Oskotsky T.
        • Gupta A.
        • Taubes A.
        • et al.
        Sex-specific cross tissue meta-analysis identifies immune dysregulation in women with Alzheimer’s disease.
        Front Aging Neurosci. 2021; 13: 735611
        • Brooks L.R.K.
        • Mias G.I.
        Data-driven analysis of age, sex, and tissue effects on gene expression variability in Alzheimer's disease.
        Front Neurosci. 2019; 13 (392-392)
        • Bonham L.W.
        • Karch C.M.
        • Fan C.C.
        • Tan C.
        • Geier E.G.
        • Wang Y.
        • et al.
        CXCR4 involvement in neurodegenerative diseases.
        Transl Psychiatry. 2018; 8: 73
        • Sanfilippo C.
        • Castrogiovanni P.
        • Imbesi R.
        • Kazakowa M.
        • Musumeci G.
        • Blennow K.
        • et al.
        Sex difference in CHI3L1 expression levels in human brain aging and in Alzheimer’s disease.
        Brain Res. 2019; 1720: 146305
        • Deming Y.
        • Dumitrescu L.
        • Barnes L.L.
        • Thambisetty M.
        • Kunkle B.
        • Gifford K.A.
        • et al.
        Sex-specific genetic predictors of Alzheimer's disease biomarkers.
        Acta Neuropathol. 2018; 136: 857-872
        • Cáceres A.
        • González J.R.
        Female-specific risk of Alzheimer's disease is associated with tau phosphorylation processes: A transcriptome-wide interaction analysis.
        Neurobiol Aging. 2020; 96: 104-108
        • Aguirre C.C.
        • Baudry M.
        Progesterone reverses 17beta-estradiol-mediated neuroprotection and BDNF induction in cultured hippocampal slices.
        Eur J Neurosci. 2009; 29: 447-454
        • Fisher D.W.
        • Bennett D.A.
        • Dong H.
        Sexual dimorphism in predisposition to Alzheimer's disease.
        Neurobiol Aging. 2018; 70: 308-324
        • Fukumoto N.
        • Fujii T.
        • Combarros O.
        • Kamboh M.I.
        • Tsai S.J.
        • Matsushita S.
        • et al.
        Sexually dimorphic effect of the Val66Met polymorphism of BDNF on susceptibility to Alzheimer's disease: New data and meta-analysis.
        Am J Med Genet B Neuropsychiatr Genet. 2010; 153B: 235-242
        • Manji Z.
        • Rojas A.
        • Wang W.
        • Dingledine R.
        • Varvel N.H.
        • Ganesh T.
        5xFAD mice display sex-dependent inflammatory gene induction during the prodromal stage of Alzheimer's disease.
        J Alzheimers Dis. 2019; 70: 1259-1274
        • Bangasser D.A.
        • Dong H.
        • Carroll J.
        • Plona Z.
        • Ding H.
        • Rodriguez L.
        • et al.
        Corticotropin-releasing factor overexpression gives rise to sex differences in Alzheimer's disease-related signaling.
        Mol Psychiatry. 2017; 22: 1126-1133
        • Zhao L.
        • Mao Z.
        • Woody S.K.
        • Brinton R.D.
        Sex differences in metabolic aging of the brain: Insights into female susceptibility to Alzheimer's disease.
        Neurobiol Aging. 2016; 42: 69-79
        • Aberg D.
        • Johansson P.
        • Isgaard J.
        • Wallin A.
        • Johansson J.O.
        • Andreasson U.
        • et al.
        Increased cerebrospinal fluid level of insulin-like growth factor-II in male patients with Alzheimer's disease.
        J Alzheimers Dis. 2015; 48: 637-646
        • Piccio L.
        • Deming Y.
        • Del-Aguila J.L.
        • Ghezzi L.
        • Holtzman D.M.
        • Fagan A.M.
        • et al.
        Cerebrospinal fluid soluble TREM2 is higher in Alzheimer disease and associated with mutation status.
        Acta Neuropathol. 2016; 131: 925-933
        • Viswanathan J.
        • Makinen P.
        • Helisalmi S.
        • Haapasalo A.
        • Soininen H.
        • Hiltunen M.
        An association study between granulin gene polymorphisms and Alzheimer's disease in Finnish population.
        Am J Med Genet B Neuropsychiatr Genet. 2009; 150B: 747-750
        • Boks M.P.
        • Derks E.M.
        • Weisenberger D.J.
        • Strengman E.
        • Janson E.
        • Sommer I.E.
        • et al.
        The relationship of DNA methylation with age, gender and genotype in twins and healthy controls.
        PLoS One. 2009; 4e6767
        • El-Maarri O.
        • Becker T.
        • Junen J.
        • Manzoor S.S.
        • Diaz-Lacava A.
        • Schwaab R.
        • et al.
        Gender specific differences in levels of DNA methylation at selected loci from human total blood: A tendency toward higher methylation levels in males.
        Hum Genet. 2007; 122: 505-514
        • Mano T.
        • Nagata K.
        • Nonaka T.
        • Tarutani A.
        • Imamura T.
        • Hashimoto T.
        • et al.
        Neuron-specific methylome analysis reveals epigenetic regulation and tau-related dysfunction of BRCA1 in Alzheimer's disease.
        Proc Natl Acad Sci U S A. 2017; 114: E9645-E9654
        • Mahady L.
        • Nadeem M.
        • Malek-Ahmadi M.
        • Chen K.
        • Perez S.E.
        • Mufson E.J.
        HDAC2 dysregulation in the nucleus basalis of Meynert during the progression of Alzheimer's disease.
        Neuropathol Appl Neurobiol. 2019; 45: 380-397
        • Cao M.
        • Li H.
        • Zhao J.
        • Cui J.
        • Hu G.
        Identification of age- and gender-associated long noncoding RNAs in the human brain with Alzheimer's disease.
        Neurobiol Aging. 2019; 81: 116-126
        • Mathys H.
        • Davila-Velderrain J.
        • Peng Z.
        • Gao F.
        • Mohammadi S.
        • Young J.Z.
        • et al.
        Single-cell transcriptomic analysis of Alzheimer's disease.
        Nature. 2019; 570: 332-337
        • Klein S.L.
        • Flanagan K.L.
        Sex differences in immune responses.
        Nat Rev Immunol. 2016; 16: 626-638
        • Pinheiro I.
        • Dejager L.
        • Libert C.
        X-chromosome-located microRNAs in immunity: Might they explain male/female differences? The X chromosome-genomic context may affect X-located miRNAs and downstream signaling, thereby contributing to the enhanced immune response of females.
        Bioessays. 2011; 33: 791-802
        • Fish E.N.
        The X-files in immunity: Sex-based differences predispose immune responses.
        Nat Rev Immunol. 2008; 8: 737-744
        • Villa A.
        • Gelosa P.
        • Castiglioni L.
        • Cimino M.
        • Rizzi N.
        • Pepe G.
        • et al.
        Sex-specific features of microglia from adult mice.
        Cell Rep. 2018; 23: 3501-3511
        • Benedusi V.
        • Meda C.
        • Della Torre S.
        • Monteleone G.
        • Vegeto E.
        • Maggi A.
        A lack of ovarian function increases neuroinflammation in aged mice.
        Endocrinology. 2012; 153: 2777-2788
        • Sarvari M.
        • Hrabovszky E.
        • Kallo I.
        • Solymosi N.
        • Liko I.
        • Berchtold N.
        • et al.
        Menopause leads to elevated expression of macrophage-associated genes in the aging frontal cortex: Rat and human studies identify strikingly similar changes.
        J Neuroinflammation. 2012; 9: 264
        • Guneykaya D.
        • Ivanov A.
        • Hernandez D.P.
        • Haage V.
        • Wojtas B.
        • Meyer N.
        • et al.
        Transcriptional and translational differences of microglia from male and female brains.
        Cell Rep. 2018; 24: 2773-2783.e6
        • Hanamsagar R.
        • Alter M.D.
        • Block C.S.
        • Sullivan H.
        • Bolton J.L.
        • Bilbo S.D.
        Generation of a microglial developmental index in mice and in humans reveals a sex difference in maturation and immune reactivity.
        Glia. 2018; 66: 460
        • Keren-Shaul H.
        • Spinrad A.
        • Weiner A.
        • Matcovitch-Natan O.
        • Dvir-Szternfeld R.
        • Ulland T.K.
        • et al.
        A unique microglia type associated with restricting development of Alzheimer's disease.
        Cell. 2017; 169: 1276-1290.e17
        • Stephen T.L.
        • Cacciottolo M.
        • Balu D.
        • Morgan T.E.
        • LaDu M.J.
        • Finch C.E.
        • et al.
        APOE genotype and sex affect microglial interactions with plaques in Alzheimer's disease mice.
        Acta Neuropathol Commun. 2019; 7: 82
        • Kodama L.
        • Guzman E.
        • Etchegaray J.I.
        • Li Y.
        • Sayed F.A.
        • Zhou L.
        • et al.
        Microglial microRNAs mediate sex-specific responses to tau pathology.
        Nat Neurosci. 2020; 23: 167-171
        • Gamache J.
        • Yun Y.
        • Chiba-Falek O.
        Sex-dependent effect of APOE on Alzheimer's disease and other age-related neurodegenerative disorders.
        Dis Model Mech. 2020; 13dmm045211
        • Fan C.C.
        • Banks S.J.
        • Thompson W.K.
        • Chen C.H.
        • McEvoy L.K.
        • Tan C.H.
        • et al.
        Sex-dependent autosomal effects on clinical progression of Alzheimer's disease.
        Brain. 2020; 143: 2272-2280
        • Farrer L.A.
        • Cupples L.A.
        • Haines J.L.
        • Hyman B.
        • Kukull W.A.
        • Mayeux R.
        • et al.
        Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease. A meta-analysis. APOE and Alzheimer Disease Meta Analysis Consortium.
        JAMA. 1997; 278: 1349-1356
        • Neu S.C.
        • Pa J.
        • Kukull W.
        • Beekly D.
        • Kuzma A.
        • Gangadharan P.
        • et al.
        Apolipoprotein E genotype and sex risk factors for Alzheimer disease: A meta-analysis.
        JAMA Neurol. 2017; 74: 1178-1189
        • Hsu M.
        • Dedhia M.
        • Crusio W.E.
        • Delprato A.
        Sex differences in gene expression patterns associated with the APOE4 allele.
        F1000Res. 2019; 8: 387
        • Shang Y.
        • Mishra A.
        • Wang T.
        • Wang Y.
        • Desai M.
        • Chen S.
        • et al.
        Evidence in support of chromosomal sex influencing plasma based metabolome vs APOE genotype influencing brain metabolome profile in humanized APOE male and female mice.
        PLoS One. 2020; 15e0225392
        • Dumitrescu L.
        • Mayeda E.R.
        • Sharman K.
        • Moore A.M.
        • Hohman T.J.
        Sex differences in the genetic architecture of Alzheimer's disease.
        Curr Genet Med Rep. 2019; 7: 13-21
        • Altmann A.
        • Tian L.
        • Henderson V.W.
        • Greicius M.D.
        • Alzheimer's Disease Neuroimaging Initiative Investigators
        Sex modifies the APOE-related risk of developing Alzheimer disease.
        Ann Neurol. 2014; 75: 563-573
        • Hohman T.J.
        • Dumitrescu L.
        • Barnes L.L.
        • Thambisetty M.
        • Beecham G.
        • Kunkle B.
        • et al.
        Sex-specific association of apolipoprotein E with cerebrospinal fluid levels of tau.
        JAMA Neurol. 2018; 75: 989-998