Genetic determinants of polycystic ovary syndrome: progress and future directions

The field of the genetics of polycystic ovary syndrome (PCOS) has relatively recently moved into the era of genome-wide association studies. This has led to the discovery of 16 robust loci for PCOS. Some loci contain genes with clear roles in reproductive (LHCGR, FSHR, and FSHB) and metabolic (INSR and HMGA2) dysfunction in the syndrome. The next challenge facing the field is the identification of causal variants and genes and the role they play in PCOS pathophysiology. The potential for gene discovery to improve diagnosis and treatment of PCOS is promising, though there is much to be done in the field before the current findings can be translated to the clinic. The field of the genetics of polycystic ovary syndrome (PCOS) has relatively recently moved into the era of genome-wide association studies. This has led to the discovery of 16 robust loci for PCOS. Some loci contain genes with clear roles in reproductive (LHCGR, FSHR, and FSHB) and metabolic (INSR and HMGA2) dysfunction in the syndrome. The next challenge facing the field is the identification of causal variants and genes and the role they play in PCOS pathophysiology. The potential for gene discovery to improve diagnosis and treatment of PCOS is promising, though there is much to be done in the field before the current findings can be translated to the clinic. Discuss: You can discuss this article with its authors and with other ASRM members at http://fertstertforum.com/jonesm-genetics-pcos/ Discuss: You can discuss this article with its authors and with other ASRM members at http://fertstertforum.com/jonesm-genetics-pcos/ Polycystic ovary syndrome (PCOS) is a heterogeneous reproductive and metabolic endocrine disorder. The primary features of PCOS are hyperandrogenism and oligo- or anovulation. Elevated circulating testosterone levels manifest clinically as hirsutism and acne, and dysfunction in gonadotropin pathways contribute to oligo-ovulation. Polycystic ovaries featuring numerous immature antral follicles can be seen with the use of ultrasound (1Goodarzi M.O. Dumesic D.A. Chazenbalk G. Azziz R. Polycystic ovary syndrome: etiology, pathogenesis and diagnosis.Nat Rev Endocrinol. 2011; 7: 219-231Crossref PubMed Scopus (926) Google Scholar). Two sets of diagnostic criteria developed by the National Institutes of Health (NIH)/National Institute of Child Health and Human Development (2Zawadzki J.K. Dunaif A. Diagnostic criteria for polycystic ovary syndrome: toward a rational approach.in: Dunaif A. Givens J.R. Haseltine F. Merriam G.R. Polycystic ovary syndrome. Blackwell Scientific Publications, Cambridge1992: 377-384Google Scholar) and European Society for Human Reproduction and Embryology/American Society for Reproductive Medicine (known as the Rotterdam criteria) (3Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus Workshop GroupRevised 2003 consensus on diagnostic criteria and long-term health risks related to polycystic ovary syndrome.Fertil Steril. 2004; 81: 19-25Abstract Full Text Full Text PDF Scopus (4723) Google Scholar) have been used and created some controversy in the field (4Fauser B.C. Tarlatzis B.C. Rebar R.W. Legro R.S. Balen A.H. Lobo R. et al.Consensus on women’s health aspects of polycystic ovary syndrome (PCOS): the Amsterdam ESHRE/ASRM–Sponsored 3rd PCOS Consensus Workshop Group.Fertil Steril. 2012; 97: 28-38.e25Abstract Full Text Full Text PDF PubMed Scopus (1061) Google Scholar). The NIH criteria requires both oligo- or anovulation and either clinical or biochemical signs of hyperandrogenism, whereas the Rotterdam criteria also considers polycystic ovaries seen with the use of ultrasound and requires two of three signs (hyperandrogenism, oligo-ovulation, polycystic ovarian morphology). The multifactorial etiology of PCOS is underpinned by a complex genetic architecture that has only recently begun to be elucidated. The use of candidate gene analysis has provided several promising genes as PCOS risk loci or as genetic modifiers of component phenotypes of PCOS. Genome-wide association studies (GWAS) in very large case-control cohorts, made possible through the development of high-throughput genotyping methods, have proved to be successful in PCOS, as in other common complex diseases. The growing list of PCOS susceptibility genes contributes to our understanding of pathways and processes implicated in the syndrome's etiology and have revealed relatively homogenous genetic underpinnings of PCOS. The first evidence for the genetic basis of PCOS was reported in 1968 by Cooper et al. (5Cooper H.E. Spellacy W.E. Prem K.A. Cohen W.D. Hereditary factors in Stein-Leventhal syndrome.Am J Obstet Gynecol. 1968; 100: 371-382Abstract Full Text PDF PubMed Scopus (165) Google Scholar). Early studies reported increased prevalence of PCOS-related traits in the siblings of PCOS probands, with many indications for an autosomal dominant model of inheritance (6Ferriman D. Purdie A.W. The inheritance of PCO and possible relationship to premature balding.Clin Endocrinol. 1979; 11: 291-300Crossref PubMed Scopus (139) Google Scholar, 7Hague W.H. Adams J. Reeders S.T. Peto T.E. Jacobs H.S. Familial polycystic ovaries: a genetic disease?.Clin Endocrinol. 1988; 29: 593-605Crossref PubMed Scopus (155) Google Scholar). Several small family studies supported this hypothesis, reporting prevalence of PCOS ranging from 51% to 66% in first-degree relatives of probands (8Govind A. Obhrai M.S. Clayton R.N. Polycystic ovaries are inherited as an autosomal dominant trait: analysis of 29 polycystic ovary syndrome and 10 control families.J Clin Endocrinol Metab. 1999; 84: 38-43Crossref PubMed Scopus (149) Google Scholar, 9Carey A.H. Chan K.L. Short F. White D. Williamson R. Franks S. Evidence for a single gene effect causing polycystic ovaries and male pattern baldness.Clin Endocrinol (Oxf). 1993; 38: 653-658Crossref PubMed Scopus (258) Google Scholar). The rate of polycystic ovarian morphology or male-pattern baldness in first-degree relatives of PCOS subjects was reported to be 51%, and it was subsequently suggested that a single gene might be responsible for oligomenorrhea and hirsutism in PCOS women and male-pattern baldness in male family members of PCOS subjects (9Carey A.H. Chan K.L. Short F. White D. Williamson R. Franks S. Evidence for a single gene effect causing polycystic ovaries and male pattern baldness.Clin Endocrinol (Oxf). 1993; 38: 653-658Crossref PubMed Scopus (258) Google Scholar). A large study of 250 consecutive PCOS probands and their families found that 75% of probands reported at least one family member with either hirsutism alone or PCOS including hirsutism (10Kahsar-Miller M. Azziz R. Heritability and the risk of developing androgen excess.J Steroid Biochem Mol Biol. 1999; 69: 261-268Crossref PubMed Scopus (35) Google Scholar). A strong familial aggregation for PCOS and its component traits was estimated in this study, with 50% of mothers or sisters, 25% of aunts, and 20% of grandmothers having hirsutism or hirsutism with oligomennorhea (10Kahsar-Miller M. Azziz R. Heritability and the risk of developing androgen excess.J Steroid Biochem Mol Biol. 1999; 69: 261-268Crossref PubMed Scopus (35) Google Scholar). This finding was supported by subsequent reports that established prevalences of PCOS in premenopausal mothers and sisters of probands of 35% and 40%, respectively (11Kahsar-Miller M.D. Nixon C. Boots L.R. Go R.C. Azziz R. Prevalence of polycystic ovary syndrome (PCOS) in first-degree relatives of patients with PCOS.Fertil Steril. 2001; 75: 53-58Abstract Full Text Full Text PDF PubMed Scopus (279) Google Scholar). A study of hyperandrogenemia in sisters of PCOS probands found that although 22% of sisters had PCOS themselves, another 24% of sisters had hyperandrogenemia with regular menstrual cycles; the bimodal distribution of testosterone levels in the sisters led the authors to propose control by a single autosomal locus (12Legro R.S. Driscoll D. Strauss J.F. Fox J. Dunaif A. Evidence for a genetic basis for hyperandrogenemia in polycystic ovary syndrome.Proc Natl Acad Sci U S A. 1998; 95: 14956-14960Crossref PubMed Scopus (549) Google Scholar). The presence of a single-component phenotype in the absence of a diagnosis of PCOS suggested that phenotypic heterogeneity between patients may have a genetic basis, demonstrated by the fact that sisters of PCOS probands had inherited genetic risk factors for some but not all traits of PCOS, resulting in a partial phenotype. Examination of the rate of PCOS in a small cohort of both monozygotic and dizygotic twin pairs initially suggested that PCOS was unlikely to be an autosomal dominant disorder, owing to the high rate of discordance in phenotype, but rather a polygenic or X-linked disorder (13Jahanfar S. Eden J.A. Warren P. Seppala M. Nguyen T.V. A twin study of polycystic ovary syndrome.Fertil Steril. 1995; 63: 478-486Abstract Full Text PDF PubMed Google Scholar, 14Jahanfar S. Eden J.A. Nguyen T. Wang X.L. Wilcken D.E. A twin study of polycystic ovary syndrome and lipids.Gynecol Endocrinol. 1997; 11: 111-117Crossref PubMed Scopus (29) Google Scholar). A subsequent very large twin study of more than 3,100 Dutch twins identified 92 subjects with PCOS with the use of a questionnaire (prevalence of 2.97%) (15Vink J.M. Sadrzadeh S. Lambalk C.B. Boomsma D.I. Heritability of polycystic ovary syndrome in a Dutch twin-family study.J Clin Endocrinol Metab. 2006; 91: 2100-2104Crossref PubMed Scopus (394) Google Scholar), a lower prevalence than typically reported in population-based studies. This large twin study reported a monozygotic correlation for PCOS (r2) of 0.72, and a dizygotic correlation for PCOS of 0.38. With the use of this twin approach, the authors were able to estimate that 72% of variance in risk of PCOS has a genetic basis (narrow-sense heritability calculated with the use of an additive genetic model), demonstrating that there is a significant genetic component to the disease (15Vink J.M. Sadrzadeh S. Lambalk C.B. Boomsma D.I. Heritability of polycystic ovary syndrome in a Dutch twin-family study.J Clin Endocrinol Metab. 2006; 91: 2100-2104Crossref PubMed Scopus (394) Google Scholar). Androgen exposure in opposite-sex twin pairs has been proposed as a model for prenatal programming; however, increased risk to the female twin has not been consistently demonstrated, suggesting that the genetic risk for PCOS outweighs the environmental risk from prenatal androgen exposure in opposite-sex twin pairs (16Kuijper E.A. Vink J.M. Lambalk C.B. Boomsma D.I. Prevalence of polycystic ovary syndrome in women from opposite-sex twin pairs.J Clin Endocrinol Metab. 2009; 94: 1987-1990Crossref PubMed Scopus (34) Google Scholar). The genetics of PCOS was widely pursued by means of the candidate gene approach, which focuses on a gene of interest selected by its hypothesized role in the disease under study. This approach has been generally unsuccessful in complex disease, in part owing to incomplete understanding of disease pathophysiology and the inclusion of a single or very few markers in a gene. Another challenge of gene discovery in complex disease is inherent to the genetic architecture of multigenic diseases. Many variants each contribute a small amount of susceptibility risk to the disease, meaning that thousands or tens of thousands of samples need to be included to detect the effect of individual single-nucleotide polymorphisms (SNPs). Sample sizes of this magnitude have not been available in PCOS studies until very recently. More than 100 candidate genes have been examined in PCOS, but only a single candidate gene, the insulin receptor (INSR), went on to be validated as a risk locus in large well designed GWAS (17Shi Y. Zhao H. Cao Y. Yang D. Li Z. Zhang B. et al.Genome-wide association study identifies eight new risk loci for polycystic ovary syndrome.Nat Genet. 2012; 44: 1020-1025Crossref PubMed Scopus (415) Google Scholar). Initial studies of the INSR locus described both linkage and association between the dinucleotide repeat microsatellite marker D19S884 in intron 55 of the fibrillin-3 gene (18Urbanek M. Legro R.S. Driscoll D.A. Azziz R. Ehrmann D.A. Norman R.J. et al.Thirty-seven candidate genes for polycystic ovary syndrome: strongest evidence for linkage is with follistatin.Proc Natl Acad Sci U S A. 1999; 96: 8573-8578Crossref PubMed Scopus (426) Google Scholar), 1.2 cM from INSR, the intended candidate gene (18Urbanek M. Legro R.S. Driscoll D.A. Azziz R. Ehrmann D.A. Norman R.J. et al.Thirty-seven candidate genes for polycystic ovary syndrome: strongest evidence for linkage is with follistatin.Proc Natl Acad Sci U S A. 1999; 96: 8573-8578Crossref PubMed Scopus (426) Google Scholar). Replication of association between D19S884 and PCOS in independent cohorts was subsequently reported by the original authors, with larger sample sizes gleaning increasingly significant P values (19Urbanek M. Woodroffe A. Ewens K.G. Diamanti-Kandarakis E. Legro R.S. Strauss J.F. et al.Candidate gene region for polycystic ovary syndrome on chromosome 19p13.2.J Clin Endocrinol Metab. 2005; 90: 6623-6629Crossref PubMed Scopus (126) Google Scholar, 20Stewart D.R. Dombroski B.A. Urbanek M. Ankener W. Ewens K.G. Wood J.R. et al.Fine mapping of genetic susceptibility to polycystic ovary syndrome on chromosome 19p13.2 and tests for regulatory activity.J Clin Endocrinol Metab. 2006; 91: 4112-4117Crossref PubMed Scopus (83) Google Scholar, 21Ewens K.G. Stewart D.R. Ankener W. Urbanek M. McAllister J.M. Chen C. et al.Family-based analysis of candidate genes for polycystic ovary syndrome.J Clin Endocrinol Metab. 2010; 95: 2306-2315Crossref PubMed Scopus (105) Google Scholar). The function of this intronic microsatellite marker is unknown; however, promoter activity has been detected at the sequence encompassing D19S884 (20Stewart D.R. Dombroski B.A. Urbanek M. Ankener W. Ewens K.G. Wood J.R. et al.Fine mapping of genetic susceptibility to polycystic ovary syndrome on chromosome 19p13.2 and tests for regulatory activity.J Clin Endocrinol Metab. 2006; 91: 4112-4117Crossref PubMed Scopus (83) Google Scholar). The expression level of the FBN3 gene itself has been reported to be extremely low in a number of ovarian cell types (22Prodoehl M.J. Hatzirodos N. Irving-Rodgers H.F. Zhao Z.Z. Painter J.N. Hickey T.E. et al.Genetic and gene expression analyses of the polycystic ovary syndrome candidate gene fibrillin-3 and other fibrillin family members in human ovaries.Mol Hum Reprod. 2009; 15: 829-841Crossref PubMed Scopus (42) Google Scholar), leading some to conclude that the D19S884 association signal is likely a proxy signal for a causal variants in other genes elsewhere in the region (22Prodoehl M.J. Hatzirodos N. Irving-Rodgers H.F. Zhao Z.Z. Painter J.N. Hickey T.E. et al.Genetic and gene expression analyses of the polycystic ovary syndrome candidate gene fibrillin-3 and other fibrillin family members in human ovaries.Mol Hum Reprod. 2009; 15: 829-841Crossref PubMed Scopus (42) Google Scholar). FBN3 is expressed in the pituitary; however, its role there remains to be explored. After the initial reports of both linkage and association at D19S884, the INSR gene became a target for additional studies. Seven studies were able to find associations between SNPs in the INSR gene and PCOS risk, many of which are novel exonic SNPs (23Tucci S. Futterweit W. Concepcion E.S. Greenberg D.A. Villanueva R. Davies T.F. et al.Evidence for association of polycystic ovary syndrome in caucasian women with a marker at the insulin receptor gene locus.J Clin Endocrinol Metab. 2001; 86: 446-449Crossref PubMed Scopus (119) Google Scholar, 24Siegel S. Futterweit W. Davies T.F. Concepcion E.S. Greenberg D.A. 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Zhao Y. et al.Family association study between INSR gene polymorphisms and PCOS in Han Chinese.Reprod Biol Endocrinol. 2011; 9: 76Crossref PubMed Scopus (21) Google Scholar). There have also been three reports failing to find association between this gene and PCOS risk; however, those studies were hampered by small sample sizes (31Xu X. Zhao H. Shi Y. You L. Bian Y. Zhao Y. et al.Family association study between INSR gene polymorphisms and PCOS in Han Chinese.Reprod Biol Endocrinol. 2011; 9: 76Crossref PubMed Scopus (21) Google Scholar, 32Unsal T. Konac E. Yesilkaya E. Yilmaz A. Bideci A. Ilke Onen H. et al.Genetic polymorphisms of FSHR, CYP17, CYP1A1, CAPN10, INSR, SERPINE1 genes in adolescent girls with polycystic ovary syndrome.J Assist Reprod Genet. 2009; 26: 205-216Crossref PubMed Scopus (91) Google Scholar) or included only a single marker (27Lee E.J. Oh B. Lee J.Y. Kimm K. Lee S.H. Baek K.H. A novel single nucleotide polymorphism of INSR gene for polycystic ovary syndrome.Fertil Steril. 2008; 89: 1213-1220Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar). To date, the most extensive investigation of the INSR gene in PCOS interrogated the entire gene with a tagging approach (29Goodarzi M.O. Louwers Y.V. Taylor K.D. Jones M.R. Cui J. Kwon S. et al.Replication of association of a novel insulin receptor gene polymorphism with polycystic ovary syndrome.Fertil Steril. 2011; 95: 1736-1741Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar). In the discovery cohort, five out of 30 SNPs across the INSR gene were associated with PCOS, four of which were carried forward to a replication study where association between rs2252673 and PCOS was successfully confirmed. This variant is located in intron 11 of the INSR gene, so it is unclear how this SNP might affect INSR expression or function to influence the PCOS phenotype. As with D19S884, rs2252673 may be a proxy marker for other functional causal variants in the region. The most important validation of the INSR locus as a PCOS risk locus is the discovery of an association signal at the INSR gene in a very large well designed case-control GWAS (17Shi Y. Zhao H. Cao Y. Yang D. Li Z. Zhang B. et al.Genome-wide association study identifies eight new risk loci for polycystic ovary syndrome.Nat Genet. 2012; 44: 1020-1025Crossref PubMed Scopus (415) Google Scholar). The role of body mass index (BMI)–associated variants has been widely studied in PCOS. Initial reports looking at the role of BMI loci in PCOS found little evidence to indicate shared risk between BMI and PCOS at these variants in early studies (33Louwers Y.V. Stolk L. Uitterlinden A.G. Laven J.S. Cross-ethnic meta-analysis of genetic variants for polycystic ovary syndrome.J Clin Endocrinol Metab. 2013; 98: E2006-E2012Crossref PubMed Scopus (93) Google Scholar, 34Ewens K.G. Jones M.R. Ankener W. 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Sluijs I. et al.FTO genetic variants, dietary intake and body mass index: insights from 177,330 individuals.Hum Mol Genet. 2014; 23: 6961-6972Crossref PubMed Scopus (120) Google Scholar) with BMI in PCOS have yielded mixed results, and a meta-analysis found that FTO is not a PCOS susceptibility locus despite its increased effect on BMI in PCOS women compared with the general population (37Wojciechowski P. Lipowska A. Rys P. Ewens K.G. Franks S. Tan S. et al.Impact of FTO genotypes on BMI and weight in polycystic ovary syndrome: a systematic review and meta-analysis.Diabetologia. 2012; 55: 2636-2645Crossref PubMed Scopus (78) Google Scholar). It has recently been shown that the causal genes at this locus are IRX3 and IRX5, which affect the browning of adipose tissue in early adipocyte development (38Claussnitzer M. Dankel S.N. Kim K.H. Quon G. Meuleman W. Haugen C. et al.FTO obesity variant circuitry and adipocyte browning in humans.N Engl J Med. 2015; 373: 895-907Crossref PubMed Scopus (799) Google Scholar). Another plausible candidate gene, the type 2 diabetes locus with the greatest odds for diabetes, TCF7L2 (39Mahajan A. Go M.J. Zhang W. Below J.E. Gaulton K.J. Ferreira T. et al.Genome-wide trans-ancestry meta-analysis provides insight into the genetic architecture of type 2 diabetes susceptibility.Nat Genet. 2014; 46: 234-244Crossref PubMed Scopus (754) Google Scholar), has also been widely investigated in PCOS. Index SNPs associated with type 2 diabetes (rs7903146 and rs12255372) were not associated with PCOS (40Barber T.M. Bennett A.J. Groves C.J. Sovio U. Ruokonen A. 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Association between rs7903146 and rs12255372 polymorphisms of transcription factor 7–like 2 gene and polycystic ovary syndrome: a systematic review and meta-analysis.Endocrine. 2015; 49: 635-642Crossref PubMed Scopus (9) Google Scholar). These findings are in contrast with the results of a third meta-analysis, which reported a significant association between rs7903146 and PCOS in European and Asian PCOS subjects, but not in African PCOS subjects (53Shen W.J. Li T.R. Hu Y.J. Liu H.B. Song M. Relationships between TCF7L2 genetic polymorphisms and polycystic ovary syndrome risk: a meta-analysis.Metab Syndr Relat Disord. 2014; 12: 210-219Crossref PubMed Scopus (11) Google Scholar). Although it seems unlikely that the TCF7L2 type 2 diabetes variants are also risk variants for PCOS, fine mapping of the region identified two distal SNPs (>100 kb from the index signals) associated with PCOS (42Biyasheva A. Legro R.S. Dunaif A. Urbanek M. Evidence for association between polycystic ovary syndrome (PCOS) and TCF7L2 and glucose intolerance in women with PCOS and TCF7L2.J Clin Endocrinol Metab. 2009; 94: 2617-2625Crossref PubMed Scopus (56) Google Scholar), a finding that requires replication in a large well powered cohort. With the introduction of genotyping platforms capable of assaying hundreds of thousands of SNPs in a single experiment, a new case-control association approach has been applied to many common multigenic diseases with great success. Completion of the 1000 Genomes Project made available complete reference genomes in more than 1,000 individuals; therefore, genotypes can now be imputed for millions of SNPs by the use of linkage disequilibrium to predict genotype at SNPs not directly assayed. These platforms allow a systematic scanning of variation to assay the majority of the genes in the genome without knowing their biologic function or relevance to the disease being studied. This is a natural extension of the earlier application of genome-wide linkage used in family-based studies, which was so successful for gene discovery in mendelian diseases (54Hirschhorn J.N. Daly M.J. Genome-wide association studies for common diseases and complex traits.Nat Rev Genet. 2005; 6: 95-108Crossref PubMed Scopus (2150) Google Scholar). The first reported GWAS in PCOS, published by Chen et al. in early 2011, reported three susceptibility loci identified in a large case-control cohort of PCOS subjects of Chinese descent (4,082 cases diagnosed according to the Rotterdam criteria and 6,687 control subjects) (55Chen Z.J. Zhao H. He L. Shi Y. Qin Y. Li Z. et al.Genome-wide association study identifies susceptibility loci for polycystic ovary syndrome on chromosome 2p16.3, 2p21 and 9q33.3.Nat Genet. 2011; 43: 55-59Crossref PubMed Scopus (511) Google Scholar). The study design used a modest-size discovery cohort (744 PCOS cases and 895 control subjects) with replication of suggestive loci in a very large replication cohort (cohort I: 2,840 PCOS case and 5,012 control subjects; cohort II: 498 PCOS case and 780 control subjects) to provide confirmation of results found in the initial GWAS. Three distinct loci were identified by this group: 2p16.3, 2p21, and 9q33.3, each containing promising candidates for PCOS risk genes: LHCGR, THADA, and DENND1A, respectively. A second GWAS in Chinese individuals was published shortly thereafter in an expanded cohort of subjects and added eight novel risk loci: 2p16.3 (an signal independent from that reported in the initial study), 9q22.32, 11q22.1, 12q13.2, 12q14.3, 16q12.1, 19p13.3, and 20q13.2 (Table 1) (17Shi Y. Zhao H. Cao Y. Yang D. Li Z. Zhang B. et al.Genome-wide association study identifies eight new risk loci for polycystic ovary syndrome.Nat Genet. 201