The Heritability of Twinning in Seven Large Historic Pedigrees

Polygenic scores for dizygotic twinning: insights into the genetic architecture of female fertility

PURPOSE

Natural dizygotic twinning (DZT) results from hyper-ovulation and is an indicator of female fertility. However, some traits linked to DZ twinning are also associated with infertility. We examined the relationship between DZT and female (in)fertility using recent GWAS findings.

METHODS

We investigated the genetic architecture of DZT and compared polygenic scores (PGS) for DZT between mothers of naturally conceived DZ twin pregnancies and mothers who required fertility treatments (MAR) in the Netherlands Twin Register (NTR) and the Norwegian Mother, Father, and Child Cohort Study (MoBa). We also calculated genetic correlations between DZT and seven fertility related traits.

RESULTS

DZT has a low polygenicity, with only 0.20% of SNPs estimated to have a nonzero effect. The DZT PGS explains 1.6% of variance in DZT liability, and we observe an odds ratio of 2.29 between the first and the tenth PGS deciles. The DZT PGS distinguishes between mothers of naturally conceived pregnancies and mothers who received MAR and is associated with a shorter time to pregnancy in mothers of singletons. The lowest PGSs were observed for mothers who received hormonal ovulation induction, indicating maternal fertility issues. DZT showed genetic correlations with anovulatory infertility (rg =  - 0.698) and PCOS (rg =  - 0.278), and endometriosis (rg = 0.279).

CONCLUSIONS

Female fertility appears to exists on a genetic spectrum, with anovulation/infertility at one end and DZT at the other. Results suggest that the DZT PGS can be of added value to evaluate female fertility and be incorporated in clinical practice in the future.

Polygenic Scores for Dizygotic Twinning: Insights into the Genetic Architecture of Female Fertility

Natural dizygotic twinning (DZT) results from hyper-ovulation and is considered an indicator of female fertility. DZT has low polygenicity, with only 0.20% of SNPs estimated to have a nonzero effect. A polygenic score (PGS) for DZT explains 1.6% of variance in DZT liability and we observe an odds ratio of 2.29 between the 1st and the 10th PGS decile. The PGS is higher in mothers of naturally conceived twins compared to mothers who received fertility (MAR) treatments in both the Netherlands and Norway. The largest differences were observed for mothers who received hormonal ovulation induction, indicating maternal fertility issues. A higher PGS was also linked to shorter time to pregnancy. DZT showed significant negative genetic correlations with anovulatory infertility (rg = -0.698) and PCOS (rg = -0.278), and a positive genetic correlation with endometriosis (rg = 0.279). These findings suggest DZT PGS is an important, yet under-recognized fertility marker.

A gene-culture co-evolutionary perspective on the puzzle of human twinship

Natural selection should favour litter sizes that optimise trade-offs between brood-size and offspring viability. Across the primate order, the modal litter size is one, suggesting a deep history of selection favouring minimal litters in primates. Humans, however - despite having the longest juvenile period and slowest life-history of all primates - still produce twin births at appreciable rates, even though such births are costly. This presents an evolutionary puzzle. Why is twinning still expressed in humans despite its cost? More puzzling still is the discordance between the principal explanations for human twinning and extant empirical data. Such explanations propose that twinning is regulated by phenotypic plasticity in polyovulation, permitting the production of larger sib sets if and when resources are abundant. However, comparative data suggest that twinning rates are actually highest in poorer economies and lowest in richer, more developed economies. We propose that a historical dynamic of gene-culture co-evolution might better explain this geographic patterning. Our explanation distinguishes geminophilous and geminophobic cultural contexts, as those celebrating twins (e.g. through material support) and those hostile to twins (e.g. through sanction of twin-infanticide). Geminophilous institutions, in particular, may buffer the fitness cost associated with twinning, potentially reducing selection pressures against polyovulation. We conclude by synthesising a mathematical and empirical research programme that might test our ideas.

Family History of Twinning and Fertility Traits in Nigerian Mothers of Dizygotic Twins

Familial twinning and fertility traits were investigated in Nigerian mothers of dizygotic (DZ) twins (MoDZT; N = 972) and controls (N = 525) who responded to our person-to-person interview, which included questions on pregnancy history and family history of DZ twinning. Controls were defined as women who are not twins themselves and do not have twins in their first-degree relatives. Over 95% of the participants were Yoruba. We found that Nigerian MoDZT had an average of 4.0 (±2.6) pairs of twins among their relatives, and of these, the prevalence of DZ twins was significantly higher than that of monozygotic (MZ) twins (45.9% vs. 25.8%). Controls had an average of 0.5 (±0.4) pairs, and over 95% of the controls had no twins in their relatives. These results suggest genetic influences on DZ twinning in Nigerians. MoDZT were significantly younger in their mean age at first child, and had higher parity than controls, suggesting increased fertility in MoDZT. As compared to mothers with a single set of twins, mothers (N = 130) with multiple sets had significantly more twins among their relatives (5.4 pairs vs. 3.7 pairs) and had their first twins at a younger age (28.4 vs. 30.7 years), indicating that mothers with multiple sets of twins might have higher genetic propensity for twinning associated with earlier age at twin pregnancy. Our findings argue for genomewide association studies for DZ twinning in Nigerians, and may help to develop intervention strategies to overcome infertility/subfertility problems.

Genome-wide association study meta-analysis of dizygotic twinning illuminates genetic regulation of female fecundity

STUDY QUESTION

Which genetic factors regulate female propensity for giving birth to spontaneous dizygotic (DZ) twins?

SUMMARY ANSWER

We identified four new loci, GNRH1, FSHR, ZFPM1, and IPO8, in addition to previously identified loci, FSHB and SMAD3.

WHAT IS KNOWN ALREADY

The propensity to give birth to DZ twins runs in families. Earlier, we reported that FSHB and SMAD3 as associated with DZ twinning and female fertility measures.

STUDY DESIGN, SIZE, DURATION

We conducted a genome-wide association meta-analysis (GWAMA) of mothers of spontaneous dizygotic (DZ) twins (8265 cases, 264 567 controls) and of independent DZ twin offspring (26 252 cases, 417 433 controls).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Over 700 000 mothers of DZ twins, twin individuals and singletons from large cohorts in Australia/New Zealand, Europe, and the USA were carefully screened to exclude twins born after use of ARTs. Genetic association analyses by cohort were followed by meta-analysis, phenome wide association studies (PheWAS), in silico and in vivo annotations, and Zebrafish functional validation.

MAIN RESULTS AND THE ROLE OF CHANCE

This study enlarges the sample size considerably from previous efforts, finding four genome-wide significant loci, including two novel signals and a further two novel genes that are implicated by gene level enrichment analyses. The novel loci, GNRH1 and FSHR, have well-established roles in female reproduction whereas ZFPM1 and IPO8 have not previously been implicated in female fertility. We found significant genetic correlations with multiple aspects of female reproduction and body size as well as evidence for significant selection against DZ twinning during human evolution. The 26 top single nucleotide polymorphisms (SNPs) from our GWAMA in European-origin participants weakly predicted the crude twinning rates in 47 non-European populations (r = 0.23 between risk score and population prevalence, s.e. 0.11, 1-tail P = 0.058) indicating that genome-wide association studies (GWAS) are needed in African and Asian populations to explore the causes of their respectively high and low DZ twinning rates. In vivo functional tests in zebrafish for IPO8 validated its essential role in female, but not male, fertility. In most regions, risk SNPs linked to known expression quantitative trait loci (eQTLs). Top SNPs were associated with in vivo reproductive hormone levels with the top pathways including hormone ligand binding receptors and the ovulation cycle.

LARGE SCALE DATA

The full DZT GWAS summary statistics will made available after publication through the GWAS catalog (https://www.ebi.ac.uk/gwas/).

LIMITATIONS, REASONS FOR CAUTION

Our study only included European ancestry cohorts. Inclusion of data from Africa (with the highest twining rate) and Asia (with the lowest rate) would illuminate further the biology of twinning and female fertility.

WIDER IMPLICATIONS OF THE FINDINGS

About one in 40 babies born in the world is a twin and there is much speculation on why twinning runs in families. We hope our results will inform investigations of ovarian response in new and existing ARTs and the causes of female infertility.

STUDY FUNDING/COMPETING INTEREST(S)

Support for the Netherlands Twin Register came from the Netherlands Organization for Scientific Research (NWO) and The Netherlands Organization for Health Research and Development (ZonMW) grants, 904-61-193, 480-04-004, 400-05-717, Addiction-31160008, 911-09-032, Biobanking and Biomolecular Resources Research Infrastructure (BBMRI.NL, 184.021.007), Royal Netherlands Academy of Science Professor Award (PAH/6635) to DIB, European Research Council (ERC-230374), Rutgers University Cell and DNA Repository (NIMH U24 MH068457-06), the Avera Institute, Sioux Falls, South Dakota (USA) and the National Institutes of Health (NIH R01 HD042157-01A1) and the Genetic Association Information Network (GAIN) of the Foundation for the National Institutes of Health and Grand Opportunity grants 1RC2 MH089951. The QIMR Berghofer Medical Research Institute (QIMR) study was supported by grants from the National Health and Medical Research Council (NHMRC) of Australia (241944, 339462, 389927, 389875, 389891, 389892, 389938, 443036, 442915, 442981, 496610, 496739, 552485, 552498, 1050208, 1075175). L.Y. is funded by Australian Research Council (Grant number DE200100425). The Minnesota Center for Twin and Family Research (MCTFR) was supported in part by USPHS Grants from the National Institute on Alcohol Abuse and Alcoholism (AA09367 and AA11886) and the National Institute on Drug Abuse (DA05147, DA13240, and DA024417). The Women's Genome Health Study (WGHS) was funded by the National Heart, Lung, and Blood Institute (HL043851 and HL080467) and the National Cancer Institute (CA047988 and UM1CA182913), with support for genotyping provided by Amgen. Data collection in the Finnish Twin Registry has been supported by the Wellcome Trust Sanger Institute, the Broad Institute, ENGAGE-European Network for Genetic and Genomic Epidemiology, FP7-HEALTH-F4-2007, grant agreement number 201413, National Institute of Alcohol Abuse and Alcoholism (grants AA-12502, AA-00145, AA-09203, AA15416, and K02AA018755) and the Academy of Finland (grants 100499, 205585, 118555, 141054, 264146, 308248, 312073 and 336823 to J. Kaprio). TwinsUK is funded by the Wellcome Trust, Medical Research Council, Versus Arthritis, European Union Horizon 2020, Chronic Disease Research Foundation (CDRF), Zoe Ltd and the National Institute for Health Research (NIHR) Clinical Research Network (CRN) and Biomedical Research Centre based at Guy's and St Thomas' NHS Foundation Trust in partnership with King's College London. For NESDA, funding was obtained from the Netherlands Organization for Scientific Research (Geestkracht program grant 10000-1002), the Center for Medical Systems Biology (CSMB, NVVO Genomics), Biobanking and Biomolecular Resources Research Infrastructure (BBMRI-NL), VU University's Institutes for Health and Care Research (EMGO+) and Neuroscience Campus Amsterdam, University Medical Center Groningen, Leiden University Medical Center, National Institutes of Health (NIH, ROI D0042157-01A, MH081802, Grand Opportunity grants 1 RC2 Ml-1089951 and IRC2 MH089995). Part of the genotyping and analyses were funded by the Genetic Association Information Network (GAIN) of the Foundation for the National Institutes of Health. Computing was supported by BiG Grid, the Dutch e-Science Grid, which is financially supported by NWO. Work in the Del Bene lab was supported by the Programme Investissements d'Avenir IHU FOReSIGHT (ANR-18-IAHU-01). C.R. was supported by an EU Horizon 2020 Marie Skłodowska-Curie Action fellowship (H2020-MSCA-IF-2014 #661527). H.S. and K.S. are employees of deCODE Genetics/Amgen. The other authors declare no competing financial interests.

TRIAL REGISTRATION NUMBER

N/A.

Maximizing the value of twin studies in health and behaviour

In the classical twin design, researchers compare trait resemblance in cohorts of identical and non-identical twins to understand how genetic and environmental factors correlate with resemblance in behaviour and other phenotypes. The twin design is also a valuable tool for studying causality, intergenerational transmission, and gene-environment correlation and interaction. Here we review recent developments in twin studies, recent results from twin studies of new phenotypes and recent insights into twinning. We ask whether the results of existing twin studies are representative of the general population and of global diversity, and we conclude that stronger efforts to increase representativeness are needed. We provide an updated overview of twin concordance and discordance for major diseases and mental disorders, which conveys a crucial message: genetic influences are not as deterministic as many believe. This has important implications for public understanding of genetic risk prediction tools, as the accuracy of genetic predictions can never exceed identical twin concordance rates.

Zwillingsschwangerschaften nach natürlicher Konzeption: wie stark ist die erbliche Komponente?

Die Faszination, die von Zwillingen, Zwillingsschwangerschaften und deren Ätiologie ausgeht, ist über die Jahrhunderte unverändert groß geblieben. Beim Menschen sind Zwillinge nach natürlicher Konzeption ein relativ häufiges Ereignis. Aufgrund des steigenden maternalen Alters und der reproduktionsmedizinischen Technologien hat die Anzahl an Zwillingen in der industrialisierten Gesellschaft seit den 1980er-Jahren stetig zugenommen. Dank des besseren Verständnisses und der Fortschritte in der Pränataldiagnostik, klinischen Genetik, Molekulargenetik und Entwicklungsbiologie ist es mittlerweile ersichtlicher geworden, warum und wie Zwillingsschwangerschaften beim Menschen entstehen. Man unterscheidet zwei Arten von Zwillingen: monozygote („eineiige“) und dizygote („zweieiige“) Zwillinge, jede dieser Formen mit unterschiedlichen Inzidenzen, Häufungen und Ursachen. Die Tendenz, dizygote Zwillinge nach natürlicher Konzeption zu empfangen, ist ein komplexes Geschehen, das erheblichen geografischen, ethnischen, saisonalen und familiären Einflüssen unterliegt und eine klare genetische Komponente aufweist. Die Rate an monozygoten Zwillingen ist weltweit relativ konstant, wobei die Ursache noch nicht gänzlich geklärt ist. Wie man jetzt weiß, spielen auch bei dieser Entität erbliche Ursachen eine Rolle.