O-037 An update on the genetics of dizygotic twinning
Authors: Boomsma D , Martin N , Hubers N , Van Dongen J , Lambalk C , Venkatesh S , Posthuma D , Schipper M , Mbarek H ,   I. Twinning Consortium
Published: July 2024

Abstract

Study question
Which genes are involved in female fertility defined as giving birth to naturally conceived dizygotic (DZ) twins?

Summary answer
In 2016 we identified FSHB and SMAD3 as first loci for having DZ twins. We now report four additional loci: GNRH1, FSHR, ZFPM1 and IPO8.

What is known already
The propensity to give birth to DZ twins differs around the globe, is positively associated with body size and smoking, and runs in families. Heritability estimates tend to be low.

Study design, size, duration
We conducted a genome-wide association meta-analysis (GWAMA) of mothers of naturally conceived dizygotic (DZ) twins (8,265 cases, 264,567 controls) and of independent DZ twin offspring (26,252 cases, 417,433 controls). Participants were carefully screened to exclude twins born after use of Medically Assisted Reproduction (MAR) technologies.

Participants/materials, setting, methods
Over 700.000 participants from Australia, New Zealand, Europe and USA were included. Genetic association analyses by cohort for being a mother of DZ twins and being a DZ twin were combined by fixed-effects inverse variance methods as implemented in METAL software meta-analysis. To predict the most likely effector gene in a locus, we applied the new FLAMES framework which integrates machine learning with biological data linking SNPs to genes, and GWAS-wide convergence of gene interactions.

Main results and the role of chance
By enlarging sample size from previous efforts, both by including new cohorts and by combining through meta-analyses with independent DZ offspring, we identified 4 genome-wide significant loci, in addition to FSHB (follicle stimulating hormone subunit beta ) and SMAD3 (SMAD family member 3). Two novel loci GNRH1 (gonadotropin releasing hormone 1) and FSHR (follicle stimulating hormone receptor) have well-established roles in female reproduction. The 2 further novel genes were implicated by gene level enrichment analyses. Both ZFPM1 (Zinc Finger Protein, FOG Family Member 1) and IPO8 (Importin 8) have not previously been implicated. Significant genetic correlations were found with multiple aspects of female reproduction and body size as well as with a recent GWAMA of female infertility. The 26 top SNPs from our GWAMA in European-origin participants predicted the crude twinning rates in 47 non-European populations (r = 0.23 between risk score and population prevalence, p=.058) indicating that GWAS are needed in African and Asian populations to explore the causes of their respectively high and low DZ twinning rates. A Phenome-wide association study (PheWAS) based on the GWAMA, linked SNPs predisposing to DZT with reduced risk of both ovarian dysfunction and polycystic ovaries.

Limitations, reasons for caution
So far, our studies only included European ancestry cohorts. Inclusion of data from Africa (the highest world-wide DZ twining rates are observed in Western Africa) and Asia (lowest DZ twinning rate) will further illuminate the etiology 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 still much unknown why they run in families. We hope our results will inform investigations of female fertility and infertility.

Trial registration number
not applicable