rs11031006 — FSHB

FSHB rs11031006 — The FSH Gonadotropin Locus Governing Reproductive Timing and Twinning

Follicle-stimulating hormone (FSH) is the master regulator of follicle development in women and spermatogenesis in men. It is produced by the pituitary gland when the beta-subunit gene FSHB¹¹ beta-subunit gene FSHB
Located on chromosome 11p14.1, encoding the hormone-specific subunit that confers biological activity is transcribed and translated. rs11031006 is a G-to-A variant located approximately 26 kilobases upstream of the FSHB transcription start site, sitting within a conserved regulatory enhancer rather than the coding sequence of FSHB itself. This locus has emerged as one of the most robustly replicated genetic determinants of circulating FSH levels, female reproductive timing, dizygotic twinning propensity, and PCOS susceptibility — and it also influences male spermatogenic function through its effect on FSH production.

Note on variant identity: rs11031006 is a GWAS lead SNP at the FSHB locus; it is distinct from rs10835638, the -211G>T proximal promoter variant studied in many clinical male infertility trials. Both variants affect FSHB regulation, but through different mechanisms and at different distances from the gene. The two are not in strong linkage disequilibrium. Studies of male infertility citing "FSHB c.-211G>T" refer to rs10835638, while studies citing the 11p14.1 GWAS locus and twinning associations predominantly discuss rs11031006.

The Mechanism

The rs11031006 variant sits within a ~450 base-pair region that is highly conserved across placental mammals, a hallmark of functional regulatory elements. In vitro luciferase assays demonstrate that this region acts as a transcriptional enhancer of FSHB²² In vitro luciferase assays demonstrate that this region acts as a transcriptional enhancer of FSHB
The enhancer augments activin- and GnRH-stimulated FSHB transcription in gonadotrope cell models. The minor A allele creates a stronger binding site for Steroidogenic Factor 1 (SF1)³³ Steroidogenic Factor 1 (SF1)
A nuclear receptor transcription factor essential for gonadotrope cell identity and FSH gene expression, increasing enhancer activity approximately 1.5-fold compared to the major G allele in cell culture experiments.

This in vitro finding presents a mechanistic paradox: the A allele increases FSHB transcription experimentally, yet population data consistently show that individuals carrying the A allele have lower circulating FSH levels, higher LH/FSH ratios, and altered reproductive phenotypes compared to G-allele carriers. The discrepancy likely reflects the complexity of pituitary negative feedback regulation in vivo — the G allele may be associated with higher FSH in part because its carriers have faster hypothalamic-pituitary-gonadal axis dynamics overall. Mouse models partially reconcile this: female mice homozygous for the A-equivalent mutation show fewer litters and abnormal estrous cycling⁴⁴ fewer litters and abnormal estrous cycling
Despite no reduction in baseline FSH measured in the deletion model, the point mutation itself disrupts reproductive cycling, suggesting the locus affects reproductive cycling through mechanisms beyond steady-state FSH levels.

The Evidence

The most robust evidence comes from multiple GWAS. A 2016 study of mothers of spontaneous dizygotic twins (n~95,000 births in Iceland plus replication cohorts)⁵⁵ A 2016 study of mothers of spontaneous dizygotic twins (n~95,000 births in Iceland plus replication cohorts)
Mbarek et al., American Journal of Human Genetics identified rs11031006-G as a genome-wide significant twinning variant (p=1.54×10⁻⁹), with each copy of the G allele increasing the likelihood of a mother delivering fraternal twins by approximately 18% (OR 1.18 per copy). The same G allele was associated with higher serum FSH levels, earlier age at menarche, earlier age at first child, higher lifetime parity, lower PCOS risk, and earlier age at natural menopause — a constellation that collectively points to a more "fast-cycling" reproductive phenotype.

In polycystic ovary syndrome genetics, the 11p14.1 locus containing rs11031006 was identified in European PCOS GWAS as associated with altered LH levels and LH/FSH ratio. Women carrying copies of the A allele show higher LH/FSH ratios⁶⁶ Women carrying copies of the A allele show higher LH/FSH ratios
Consistent with the gonadotropin imbalance characteristic of PCOS, a pattern distinct from G-allele carriers who tend to have higher FSH relative to LH.

The male fertility significance was established in a 2022 GWAS of 760 idiopathic infertile men (validated in 1,140)⁷⁷ 2022 GWAS of 760 idiopathic infertile men (validated in 1,140)
Schubert et al., Journal of Clinical Endocrinology & Metabolism. The 11p14.1 locus (represented by rs11031005, in high LD with rs11031006) was the top genome-wide significant hit for serum FSH levels, explaining 4.65% of FSH variance overall and 6.95% of variance in the oligozoospermic subgroup specifically — a larger effect than the well-studied proximal promoter variant rs10835638 (which explains ~3.6% of FSH variance). Lower FSH in men impairs Sertoli cell function and reduces sperm production; the FSHB locus was identified as a potential etiologic factor in approximately 28% of men with idiopathic infertility.

Practical Implications

The practical implications of this variant differ by sex. In women, the A allele (associated with lower FSH and higher LH/FSH ratio) may contribute to longer menstrual cycles, slightly delayed folliculogenesis, and a reproductive axis phenotype that overlaps with some PCOS features — although rs11031006 alone is not diagnostic of PCOS. Women carrying the AA genotype (approximately 2% of European-ancestry individuals) may wish to discuss FSH and LH panel interpretation with a reproductive endocrinologist if experiencing irregular cycles, delayed conception, or unexplained subfertility.

In men, the A allele is associated with measurably lower FSH levels at the population level, which may impair spermatogenesis. Men with low-normal FSH and idiopathic infertility who carry variants at this locus represent a distinct etiologic subgroup⁸⁸ Men with low-normal FSH and idiopathic infertility who carry variants at this locus represent a distinct etiologic subgroup
Defined as functional secondary hypogonadism with isolated FSH deficiency, this group responds to exogenous FSH treatment with improved sperm parameters. Semen analysis combined with FSH measurement is the key initial investigation for male carriers, particularly for the AA homozygous genotype.

Interactions

rs10835638 (FSHB -211G>T, proximal promoter): This is a separate variant located 211 bp upstream of the FSHB mRNA transcription start site. Both rs11031006 and rs10835638 affect FSHB expression but at different positions and through distinct mechanisms. In men, rs10835638 T allele has been extensively studied and reduces FSH by ~0.51 IU/L per allele and testicular volume by ~3.2 ml. These two variants are not in strong LD and may have partially independent effects; their combined impact on FSH levels in men with idiopathic infertility is additive and warrants separate genotyping.

rs6166 (FSHR N680S): The FSH receptor sensitivity variant interacts functionally with FSHB variants. In men, the effect of FSHB locus variants on FSH-driven spermatogenesis is modulated by FSHR genotype — men with lower FSH production (FSHB A allele) and reduced FSH receptor sensitivity (FSHR GG) have a compounded spermatogenic disadvantage. This interaction has been documented for the proximal FSHB variant, and the same pathway logic applies to rs11031006. A compound action may be warranted when both unfavorable genotypes co-occur.