Genetic Research and Women’s Heart Disease: a Primer
Curr Atheroscler Rep
Genetic Research and Women's Heart Disease: a Primer
Maryam Kavousi 0
Lawrence F. Bielak 0
Patricia A. Peyser 0
0 Department of Epidemiology, School of Public Health, University of Michigan , 1415 Washington Heights, Ann Arbor, MI 48109-2029 , USA
Purpose of Review This review provides a brief synopsis of sexual dimorphism in atherosclerosis with an emphasis on genetic studies aimed to better understand the atherosclerotic process and clinical outcomes in women. Such studies are warranted because development of atherosclerosis, impact of several traditional risk factors, and burden of coronary heart disease (CHD) differ between women and men. Recent Findings While most candidate gene studies pool women and men and adjust for sex, some sex-specific studies provide evidence of association between candidate genes and prevalent and incident CHD in women. So far, most genomewide association studies (GWAS) also failed to consider sexspecific associations. The few GWAS focused on women tended to have small sample sizes and insufficient power to reject the null hypothesis of no association even if associations exist. Summary Few studies consider that sex can modify the effect of gene variants on CHD. Sufficiently large-scale genetic studies in women of different race/ethnic groups, taking into account possible gene-gene and gene-environment interactions as well as hormone-mediated epigenetic mechanisms, are needed. Using the same disease definition for women
Cardiovascular disease; Coronary heart disease; Atherosclerosis; Women; Genes; Sex-differences
and men might not be appropriate. Accurate phenotyping
and inclusion of relevant outcomes in women, together with
targeting the entire spectrum of atherosclerosis, could help
address the contribution of genes to sexual dimorphism in
atherosclerosis. Discovered genetic loci should be taken
forward for replication and functional studies to elucidate the
plausible underlying biological mechanisms. A better
understanding of the etiology of atherosclerosis in women would
facilitate future prevention efforts and interventions.
Cardiovascular disease (CVD) remains the leading cause of
mortality among women and men [1, 2]. Although overall
CVD mortality rates have declined, the annual mortality rate
for women remains higher than men . Greater life
expectancy for women, together with improvements in primary and
secondary prevention of CVD, will lead to a larger proportion
of women living with CVD . Substantial sex differences in
the burden of different manifestations of CVD, including
coronary heart disease (CHD), stroke, heart failure, and
peripheral artery disease, are widely recognized [4–7]. Despite the
excess CHD incidence and prevalence in men compared to
women, CHD remains the leading contributor to CVD
morbidity and mortality among both women and men [1, 2].
In spite of statistics that show CHD develops on average 7–
10 years later in women compared with men, adverse trends in
many risk factors among women are of growing concern [3,
8]. Additionally, while the decrease in CHD mortality among
women is well documented, the decline still lags behind that
of men, with an alarming tendency towards an increase
mortality rate among younger women [9, 10 ]. Recent data
report substantial declines in sudden cardiac death in men
while no changes are observed among women .
We provide a review of sex differences in atherosclerosis, the
contribution of genetic research to explaining sex differences in
atherosclerosis, possible sex hormone-mediated epigenetic
mechanisms, and use of subclinical measures of atherosclerosis
for genetic studies. Next, we discuss challenges in accounting for
sex differences in genetic studies, importance of the proper
definition of outcomes, the need to include multiple race/ethnic
groups in genetic studies of atherosclerosis in women, and how
this genetic information can contribute to efforts in precision
medicine for CHD in women. We conclude with directions for
future research, limitations of this review, and conclusions. Here,
sex refers to biological differences between women and men
(i.e., anatomical and physiological differences, genetic
differences in the X and Y chromosomes, and levels and types of
hormones). Gender is Bthe socially constructed characteristics
of women and men—such as norms, roles and relationships of
and between groups of women and men. It varies from society to
society and can be changed^
Sex Differences in Atherosclerosis
CHD is mainly characterized by atherosclerosis in the
epicardial coronary arteries. Atherosclerosis is a systemic
progressive pathologic condition involving atherosclerotic plaque
formation typified by accumulation of cholesterol, infiltration of
macrophages, proliferation of smooth muscle cells,
accumulation of connective tissue components, and formation of
thrombus . Atherosclerosis is considered a complex trait
involving multiple genes and their interactions with
behavioral and environmental factors. Complex traits do not follow
predictable patterns of inheritance. Although women and
men share many similarities in core processes underlying
atherosclerosis, recent evidence points towards some inherent
differences in the development of the disease. Sex differences
in the association of traditional risk factors with CHD are
established. In particular, smoking and diabetes seem to be
stronger risk factors in women [13, 14]. Sex differences,
however, cannot be entirely explained by the differential
distribution of traditional risk factors between women and men.
Furthermore, women tend to show a more Bdiffuse
atherosclerosis^ pattern, as opposed to discrete Bfocal atherosclerosis^
that obstructs the lumen .Compared to men, women tend
to have a higher prevalence of microvascular dysfunction [15,
16]. Plaque characteristics relative to calcification and lipid
accumulation may also differ between women and men, and
transitions toward vulnerable plaques seem to be slower in
females . Sex differences in inflammatory, coagulation,
and thrombotic pathways may contribute to this sexual
dimorphism [15–17]. The exact pathways associated with
Contribution of Genetic Research to Explaining Sex
Differences in Atherosclerosis
The heritability of CHD is approximately 40 % . Several
studies have shown that a family history of CHD, especially
when disease occurs before age 60 years, is more important
for women than men . Understanding the genetic basis of
sexual dimorphism in atherogenesis may provide novel
additions to existing knowledge.
I. Candidate Genes for Atherosclerosis
A gene whose function or location indicates it is likely to be
responsible for a particular disease or trait level is referred to
as a candidate gene. Detailed information about specific genes
can be found at http://www.ncbi.nlm.nih.gov/gene. Most
candidate gene studies pool women and men and use sex as
a covariate for adjustment. In the selected studies described
below, men and women were considered separately. While
some studies found associations in men, but not women, we
only report those where an association only occurred in
One of the most studied candidate genes for CHD is
apolipoprotein E (APOE) . APOE produces a protein involved in
metabolism of cholesterol and triglycerides by binding to
receptors in the liver to promote clearance of chylomicrons and
very low-density lipoproteins from circulating blood . The
major alleles of APOE are ε2, ε3, and ε4. The most common
genotypes are ε3 homozygotes, ε3/ε4 heterozygotes, and ε2/
ε3 heterozygotes in most populations. ε2/ε3 heterozygotes
have higher high-density lipoprotein cholesterol (HDL-C)
levels while ε3/ε4 heterozygotes have higher low-density
lipoprotein cholesterol (LDL-C) levels compared to ε3
homozygotes . ε3/ε4 heterozygotes have higher risk of CHD
compared to ε3 homozygotes and ε2/ε3 heterozygotes, while
there is no significant difference in CHD risk between ε3
homozygotes and ε2/ε3 heterozygotes .
The Framingham Offspring Study, one of the earliest
studies to demonstrate a gender-specific association between ε4
and prevalent CHD, found a significant positive association in
women, but not men, after adjusting for age and traditional
CHD risk factors . No protective effect of ε2 was found in
either women or men in this study of 1034 men and 916
women. More recently, a large prospective study of 10,035
men and 12,134 women from the Norfolk, England, arm of
the European Prospective Investigation into Cancer and
Nutrition study (EPIC Norfolk study), reported no association
between CHD risk and APOE in either men or women after
adjustment for multiple traditional risk factors . While
both the Framingham Study and the EPIC Norfolk study
adjusted for LDL-C and HDL-C levels, the Framingham Study
assessed prevalence of CHD while the EPIC Norfolk study
assessed incident CHD over 11 years. Finally, the EPIC
Norfolk study also included alcohol use and physical activity
as risk factors. The large, prospective EPIC Norfolk study
supports a limited role for APOE in risk of CHD for either
sex after accounting for many CHD risk factors .
Importantly, these studies suggest that genes for prevalent
CHD may differ from genes for incident CHD.
Selected Other Candidate Genes
In two large Finnish cohorts, 46 candidate genes were studied
for association with CHD . No variants in any candidate
gene were associated with incident CHD in men. Variants in
three genes, however, showed an association with incident
CHD in women: upstream stimulatory factor 1 (UFS1),
coagulation factor XIII A (F13A1), and carboxypeptidase B2
(CPB2). UFS1, which was also associated with prevalent
CHD in women, is a ubiquitously expressed transcription
factor that regulates several genes of glucose and lipid
metabolism. F13A1 is involved in the blood coagulation cascade
while CPB2 is involved in fibrinolysis. Additionally,
mutations in either of two polymorphisms in the hemochromatosis
(HFE) gene in the Rotterdam Study were significantly
associated with incident CHD in women, but not men . HFE
regulates circulatory iron uptake. All the candidate gene
studies described above included only individuals of European
The candidate gene approach has been criticized primarily
because many results could not be replicated and this
approach fails to include all possible causative genes and
polymorphisms . These criticisms spurred new approaches
such as genome-wide association studies (GWAS) to identify
genes for complex diseases and traits
II. Genome-Wide Association Studies
GWAS typically consider millions of single-nucleotide
polymorphisms (SNPs) (or other genetic variants such as copy
number polymorphisms). Genetic variants are measured with
microarrays, and the measures from the microarrays are often
combined with publically available data, such as the 1000
Genomes Project, to impute additional genotypes that were
not directly measured. The 1000 Genomes Project has
provided genomic sequence data on more than 2500 individuals
from 26 globally diverse populations .
In contrast to candidate gene studies, there are no specific a
priori hypotheses in GWAS with the exception of replication
of already identified genes. These studies typically
metaanalyze results from multiple cohorts to increase power.
There is usually a discovery phase that includes the initial
cohorts and then a replication phase with additional new
cohorts. Cohorts from the discovery and replication phases may
be meta-analyzed together. Depending on the number of SNPs
considered, the p value is set to meet a Bonferroni correction.
Many GWAS of traditional CHD risk factors explore whether
any identified genes are also associated with clinical
outcomes. A catalogue, continually updated, of GWAS findings
is available [27 ].
The first GWAS papers on CHD were published in 2007
[28–32]. The studies pooled women and men of European
ancestry and adjusted for sex as a covariate. The most
consistent finding in these studies was an association with variants
on the short arm of chromosome 9 (9p21).
Recent GWAS have included more genetic variants and
larger sample sizes. The most recent GWAS included 60,801
CHD cases and 123,504 controls from 48 different cohorts
[33 ]. While most of the cases and controls were of
European ancestry, some were of other ancestries. The study
interrogated 9.4 million variants across the genome. Ten new
loci were identified bringing the total number of loci
associated with CHD to 58 [33 , 34 ].
Sex-specific associations for CHD were conducted in the
Wellcome Trust Case Control Consortium. In women, there
were 399 cases for CHD and 1492 controls while in men there
were 1527 cases and 1446 controls. Variants considered were
selected from results of other GWAS for CHD. No variants
were significant in women (or in men) . Power was low in
In a recent sex-stratified study, an SNP in SCARB1, a
plasma membrane receptor for HDL, was associated with
angiographic CHD in women, but not men [36 ]. A GWAS of
betaine levels, a novel risk factor for atherosclerosis, found
an association with variation in a SNP in
carbamoylphosphate synthase 1 (CPS1) [37 ]. CPS1 encodes a
mitochondrial enzyme that catalyzes the first committed reaction
and rate-limiting step in the urea cycle. This SNP was weakly
associated (p = 0.01) with CHD in approximately 54,000
individuals from the CARDIoGRAM Consortium . The
SNP was not associated with CHD in men; however, it was
significantly associated with CHD in women [37 ].
All CHD GWAS so far focused exclusively on the
autosomes even though the X chromosome is included on all the
current microarrays . The X chromosome contains 1973
known genes. Even with the challenges of including the X
chromosome in GWAS because women have two X
chromosomes while men have one and because of X-inactivation, this
chromosome may provide important information regarding
differences in atherosclerosis and its risk factors between
women and men. The one exception is a recent GWAS of
nonobstructive coronary artery disease in women that
included 52,371 variants important in metabolic traits and CVD [40,
41]. Ninety variants were on the X chromosome. The 332
European ancestry cases came from a cohort of women with
chest pain and/or suspected myocardial ischemia with <50 %
stenosis in any coronary artery on angiography. The 1003
European ancestry controls came from a cohort of women
without known CHD. While there were no associations with
any SNPs on the X chromosome, SNPs at two autosomal
genes showed association at nominal significance levels. In
a candidate gene study of 5HTR2C on the X chromosome,
men who had one copy of the high-risk allele and women with
two copies of the high-risk allele were at significantly
increased risk for death or nonfatal myocardial infarction .
Sex Hormones and Epigenetics
CHD in women tends to manifest during and after the
menopausal transition, indicating that sex hormones play a critical
role in disease development. The major impact of sex
hormones on atherosclerosis, either directly affecting the function
of the heart and vessels or indirectly through other CHD risk
factors, has long been investigated . A recent
understanding of the interaction of genes with the environment has
revealed the importance of sex hormones on pathogenesis of
atherosclerosis through epigenetic mechanisms.
Epigenetics refers to heritable changes in gene activity and
expression that do not entail an alteration in DNA sequence. In
other words, epigenetics involves the molecular pathways that
modulate the expression of a genotype into a particular
phenotype . Epigenetic modifications often investigated
include DNA methylation, histone variants, and histone
modifications as well as nucleosome positioning . Epigenetic
mechanisms are reversible and can be modulated by
environmental factors . Thus, epigenetics has emerged as a
promising tool to address knowledge gaps in atherosclerosis.
CHD risk factors, such as nutrition, smoking, pollution,
stress, and the circadian rhythm, have been associated with
epigenetic modifications [47 ]. Additionally, sex hormones
are uniquely poised to exert epigenetic effects through
hormone-induced DNA methylation and histone modification
at specific gene regulatory regions . Any evidence
regarding a sex-specific association of a particular genetic locus with
atherosclerosis could indicate underlying epigenetic
mechanisms mediated through sex hormones.
Subclinical Measures of Atherosclerosis
The distribution of atherosclerosis in different vascular beds is
variable and sex- and race/ethnicity-specific [49–52].
Differences in atherosclerosis between men and women have
been suggested to be larger in the coronary arteries than in
other vascular beds . Calcification in the coronary arteries
is a surrogate marker of overall plaque burden and is
considered the hallmark of atherosclerosis. While women have
overall lower presence and quantity of coronary artery
calcification (CAC) than men, CAC carries a higher mortality risk
for women . The quantity of CAC is heritable . So far,
none of the GWAS for CAC have considered men and women
separately [55 , 56]. The GWAS for CAC in those of
European ancestry identified associations with SNPs in the
9p21 region and in PHACTR1 on chromosome 6 that
replicated for myocardial infarction [55 ]. A GWAS for CAC in those
of African ancestry failed to identify any genome-wide
significant associations .
Few candidate gene studies have evaluated genetic
associations for CAC in men and women separately. A polymorphism
in E-selectin, a gene involved in cellular adhesion, was
associated with presence of CAC only in women age 50 years or
younger after adjustment for CHD risk factors. There was no
association with CAC presence in men of any age or in women
over age 50 after adjustment for CHD risk factors . A
promoter polymorphism of leukotriene C4 synthase (LTC4S), the
rate-limiting enzyme in the production of the potent
proinflammatory cysteinyl leukotriene metabolites of arachidonic acid,
was studied in women ages 29–43 years and men ages 29–
37 years . Risk for having CAC was significantly
associated with this polymorphism in women, but not men.
Increased common carotid intima-media thickness (CIMT)
is considered a marker of early atherosclerosis. Sex
differences in CIMT seem to be pertinent only in younger and
middle-aged individuals, becoming progressively irrelevant
at older age . In a longitudinal study of CIMT, progression
of CIMT was associated with parental history of stroke
especially among young women . CIMT is heritable . A
GWAS in those of European ancestry identified variants near
ZHX2, ACPOC1, and PINX1 associated with CIMT and did
not consider women and men separately [62 ]. A recent
GWAS in a study that included individuals from multiple
race/ethnic groups identified 14 genes with evidence for an
association with CIMT [63 ]. An SNP-by-sex interaction was
found for a SNP in LEKR1 and an SNP in GALNT10. Both of
these loci have been associated with adiposity and weight
control [63 ]. Similar to the association with CAC described
above, LTC4S was associated with CIMT in women but not in
men . Others have identified a locus on chromosome 16
that is associated with CIMT, which contains the BCAR1,
CFDP1, and TMEM170A genes . More recently,
Boardman-Pretty and colleagues [65 ] studied the lead SNP
in BCAR1, identified its function, and showed it to be
associated with progression of CIMT in women, but not men.
Endothelial function, a measure of physiological functions
of the vascular endothelium, is a marker representing the
effects of CHD risk factors on the arterial wall [66, 67].
Functional and structural damages to the arterial wall precede
and accompany atherosclerosis process and its associated
obstructive and thrombotic events . Sex has been suggested
as an independent factor contributing to endothelial
dysfunction, and the effect of cardiovascular risk factors on
endothelium-dependent dilation has been shown to be
sexspecific [68, 69]. Endothelial function is heritable . An
early GWAS did not find any genome-wide significant
associations but found some suggestive associations for
endothelial function . A recent study investigated the association
between almost 1300 SNPs previously associated with
vasoreactivity, angiogenesis, inflammation, artery
calcification, atherosclerotic risk factors, insulin resistance, hormone
levels, blood coagulability, or CHD with coronary endothelial
dysfunction . SNPs in LPA, MYBPH, ADORA3, and
PON1 were significantly associated in the 426 women, but
not in the 217 men.
Analyzing sex differences in genetic associations is
complicated, particularly for a disease such as CHD where the typical
age of onset is gender-dependent. Sufficiently powered
studies are an essential requirement in both candidate gene and
GWAS studies. Lack of adequate statistical power has
hampered meaningful comparisons between women and men in
most of the numerous published GWAS for CHD so far. The
sex-specific GWAS in the Wellcome Trust Case Control
Consortium  and the study of nonobstructive coronary
disease in women  illustrate the challenges of smaller
samples and the need for large-scale studies in women.
Importantly, when women and men are pooled in a study,
the analysis of any variant, but especially rare genetic variants,
might be prone to bias resulting from the disproportionate
blend of women and men in case and control samples. Other
genetic or environmental factors can modify or mediate the
effect of a particular genetic variant to increase or decrease the
risk for CHD in women. For example, a study with a large
sample of men and women found an interaction between
smoking and APOE on CHD risk . Women, but not
men, had an increased smoking-related risk in ε4 allele
carriers. Differences between men and women can result from the
joint effects of genetic variants with other biological and/or
While investigating sex differences in genetic studies of
atherosclerosis are of paramount importance, researchers should
be aware not to overstate sex differences in their studies, as
spurious claims of sex differences in genetic studies might be
asserted in the absence of sufficient data, proper data analysis,
or good internal and external validity [74 ]. Bias against
reporting negative or null results can lead to more publications
conveying findings of sex differences compared to studies with
no sex differences appearing in the literature .
Since the sex-specific differences may be larger for causal
variants than for their linked markers that might be studied,
systematic assessment should be aimed at targeting the true
causal variants associated with atherosclerosis . Candidate
gene studies should be based on a priori, clearly defined
hypotheses. Any claim of sex-specific genetic association should
be accompanied by appropriate examination of subgroup
comparisons or interaction tests. Likewise, gene-gene and
gene-environment interactions should be interpreted with
caution and viewed in the context of additional prior/external
evidence. The identified interactions should be interpreted as
hypothesis-generating, followed by further replication in other
studies as well as meta-analyses [76, 77].
Proper Definition of Outcome in Sex-specific Genetic
Studies of Atherosclerosis
In studies of complex disease, defining a trait with insufficient
specificity, i.e., trait heterogeneity, is viewed as a confounding
factor . The pathophysiology of CHD involves more
severe structural and functional abnormalities in epicardial
coronary arteries in men and more microvascular coronary
dysfunction in women . Thus, using the same definition for
the CHD phenotype for both women and men might not be
appropriate and could lead to differential misclassification of
cases and controls between women and men. For example, in
the most recent GWAS for CHD, case status was defined by
an inclusive CAD diagnosis including myocardial infarction,
acute coronary syndrome, chronic stable angina, or coronary
stenosis of >50 % [33 ]. Accordingly, the term ischemic heart
disease (IHD) is more appropriate for women than CHD.
Abnormal coronary reactivity, microvascular dysfunction,
and plaque erosion/distal microembolization contribute to a
female-specific IHD pathophysiology. Using IHD, rather than
obstructive CHD, covers the whole spectrum of the disease in
women and could allow further identification of genes in
studies focusing on women .
Another issue is whether studies have included prevalent or
incident CHD cases. A recent GWAS was the first to
investigate incident cases [80 ]. Importantly, a new gene was
identified that was not identified in prior GWAS of prevalent cases
while the 9p21 region and other genes identified in studies
with prevalent cases were only marginally associated or not
associated with incident disease. One explanation is that SNPs
in the 9p21 region are associated with increased risk as well as
survival after onset of CHD.
The majority of genetic studies include only those of
European ancestry. The differences among the various race/
ethnic groups in CHD morbidity and mortality are well
documented . The importance of sex and race/ethnicity in
genetic studies of CVD has been recognized . One GWAS of
incident CHD was conducted in African Americans . The
discovery cohort included both women and men, but the
replication cohort included only women. One SNP near
PFTAIRE-1, which is involved in cell proliferation, was
genome-wide significant and replicated. The region was not
implicated in the incident CHD GWAS that pooled men and
women [80 ].
A recent study was designed to understand how to facilitate
inclusion of minority individuals in genetic studies .
Starting with a large random sample from a community, they
performed telephone screening interviews, determined
eligibility, and then recruited participants for a genetic study on
dependence on cigarettes and nicotine. In zip codes with a
high proportion of African Americans, compared to those with
very low proportions of African Americans, there was a
significantly higher proportion of individuals with incorrect
telephone numbers or addresses but a lower proportion of
individuals who did not answer the telephone or refused the
interview. Importantly, a significantly higher proportion of eligible
African Americans participated in the genetic study compared
to eligible European Americans (71 % versus 54 %). Results
suggest that increasing the number of African Americans in
genetic studies and registries may be achieved by increasing
efforts to locate and contact them. This study did not address
some other issues regarding possible lack of participation of
minorities in research studies because of mistrust or limited
access to health care.
Genetics of Women’s Heart Disease: the Biological Basis
for Precision Medicine
Contrary to the traditional approach of Bone size fits all,^
precision medicine aims to tailor disease prevention,
treatment, and prognosis regimens [84 ]. Sex-specific CHD
research will lead to a new understanding of the
pathophysiology of CHD in women, allowing for a comprehensive disease
definition and classification. Taking into account individual
variability in the role of genetic, environmental, and social
factors as well as their interaction in the pathophysiology of
atherosclerosis would serve to implement effective preventive
or therapeutic interventions. As gene therapy becomes a
promising new addition to conventional therapies, the
sexspecific as well as the race/ethnic-specific genetic basis of
the atherosclerosis assumes more significance.
Table 1 Accounting for sex
differences in genetic studies of
atherosclerosis: challenges and
Lack of meaningful comparisons between women
and men due to inadequate statistical power
Bias resulting from disproportionate blend of women
and men in case and control samples
Differential misclassification of cases and controls
between women and men due to suboptimal
accuracy in phenotyping (i.e., trait heterogeneity)
Inconsistent results due to race/ethnic differences in
CHD morbidity and mortality
Spurious findings due to the interaction of genes with
other genetic and/or environmental factors
Lack of comprehensive evidence supporting the
biological relevance of findings
Design sufficiently powered studies
Directions for Future Research
CHD is increasingly recognized as a pathophysiological
continuum , emphasizing the notion that intervention at any
point along this path can modify disease progression. Disease
progression does not occur as a sequence of discrete, tandem
incidents but instead the phases of disease progression
overlap. Therefore, sex-specific studies across the entire spectrum
of atherosclerosis are needed. Studying intermediate
phenotypes in the continuum, including CAC, CIMT, or endothelial
function, may help disentangle sex differences in disease
susceptibility and progression. Moreover, focusing on
quantitative atherosclerosis markers might overcome some of the
limitations regarding the heterogeneity in discrete CHD event
ascertainment for women and men. In GWAS, the imprecision
involved in measurement of quantitative phenotypes may not
have large systematic effects on location of significant
associations, but assessing repeatability of the phenotype is
The vast majority of identified SNPs do not result in protein
changes. Instead, they could be, for example, long noncoding
RNA, microRNA (small noncoding RNA that functions in
RNA silencing and post-transcriptional regulation), or some
transcribed or regulatory element. Thus, understanding the
function of the identified SNPs is crucial for understanding
their role in the disease process [87 ]. One approach is to use
available databases such as Encyclopedia of DNA elements
(ENCODE) to identify the function of specific SNPs. Another
is to utilize the data collected by the Genotype-Tissue
Expression (GTEx) project, which provides a database and
associated tissue bank to study the relationship between
genetic variation and gene expression in human tissues . A
complete list of available databases is updated each year [89 ].
Another approach is to study the function of a specific SNP
and genomic region. For example, Yang and colleagues [90 ]
focused on an SNP in the COL4A2 gene that has been shown
to be associated with CHD in GWAS. They studied the
functional effect of this variant in primary cultures of vascular
smooth cells and endothelial cells from individuals with
different genotypes at this SNP. They found differences in gene
expression levels of COL4A2 and COL4A1 (COL4A1 and
COL4A2 reside next to each other and share common
transcriptional regulatory sequences). Additional
immunohistochemical and histological studies of ex vivo atherosclerotic
coronary arteries identified plaque differences dependent on
genotype. Together, these studies, as well as their research in
patients with angiographically documented disease, provide a
mechanistic explanation for the association between the
genetic variants and CHD [90 ]. One recent study used reporter
gene assays, computational predictions, and epigenomic
marks to assess activity of an enhancer region active in
multiple human tissues  while another study used integrative
genomic, epigenomic, and transcriptomic profiling of
perturbed human coronary artery smooth muscle cells and
tissues to begin to identify causal regulatory variation and
mechanisms responsible for CHD associations [92 ].
More recent studies are now examining exomes [93, 94 ].
In addition, there are large-scale whole-genome sequencing
projects in progress. The National Heart Lung and Blood
Institute at the National Institutes of Health in the USA is
sequencing more than 62,000 individuals from more than 30
studies, many with measures of CHD and risk factors through
the Trans-Omics for Precision Medicine (TOPMed) Program
(http://www.nhlbi.nih.gov/research/resources/nhlbiprecision-medicine-initiative/topmed). Hopefully, these
various approaches will result in large samples of women for
genomic studies of CHD.
There are many articles in the literature on the genetics of
CHD and its risk factors that could not be included. We did
not consider other manifestations of CVD that are relevant for
women such as stroke, heart failure, and peripheral artery
disease. We included only candidate gene and GWAS studies
based on microarray data and did not include any GWAS
based on exome sequencing or whole genome sequencing
data that are just becoming available.
Sex-based differences in the heart, in vessel function, and in
major manifestations of CVD have been recognized. The
influence of sex on CHD is increasingly being explored at
cellular, molecular, and genetic levels. Challenges and
recommendations to account for sex differences in genetic studies
of atherosclerosis are summarized in Table 1. So far,
sexspecific genetic studies of atherosclerosis are sparse, mostly
due to lack of power or lack of appreciation for sex
differences. Sufficiently large hypothesis-free GWAS or candidate
gene studies with a priori, clearly defined hypotheses are
needed. Accurate phenotyping and inclusion of relevant
outcomes in women, consideration of possible gene-gene and
gene-environment interactions, and sex hormone-mediated
epigenetic mechanisms are of importance. Finally, discovered
genetic loci should be taken forward for replication and
functional studies to elucidate plausible underlying biological
Compliance with Ethical Standards
Conflict of Interest Maryam Kavousi is supported by the VENI grant
(91616079) from The Netherlands Organization for Health Research and
Lawrence F. Bielak and Patricia A. Peyser declare that they have no
conflict of interest.
Human and Animal Rights and Informed Consent This article does
not contain any studies with human or animal subjects performed by any
of the authors.
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