Human INHBB Gene Variant (c.1079T>C:p.Met360Thr) Alters Testis Germ Cell Content, but Does Not Impact Fertility in Mice

Abstract

Testicular derived inhibin B (α/βB dimers) acts in an endocrine manner to suppress pituitary production of follicle stimulating hormone (FSH), by blocking the actions of activins (βA/B/βA/B dimers). Previously, we identified a homozygous genetic variant (c.1079T>C:p.Met360Thr) arising from uniparental disomy of chromosome 2 in the INHBB gene (βB-subunit of inhibin B and activin B) in a man suffering from infertility (azoospermia). In this study, we aimed to test the causality of the p.Met360Thr variant in INHBB and testis function. Here, we used CRISPR/Cas9 technology to generate Inhbb M364T/M364T mice, where mouse INHBB p.Met364 corresponds with human p.Met360. Surprisingly, we found that the testes of male Inhbb M364T/M364T mutant mice were significantly larger compared with those of aged-matched wildtype littermates at 12 and 24 weeks of age. This was attributed to a significant increase in Sertoli cell and round spermatid number and, consequently, seminiferous tubule area, in Inhbb M364T/M364T males compared to wildtype males. Despite this testis phenotype, male Inhbb M364T/M364T mutant mice retained normal fertility. Serum hormone analyses however, indicated that the Inhbb M364T variant resulted in reduced circulating levels of activin B, but did not affect FSH production. We also examined the effect of this p.Met360Thr, and an additional INHBB variant (c.314C>T: p.Thr105Met) found in another infertile man, on inhibin B and activin B in vitro biosynthesis. It was found that both INHBB variants resulted in a significant disruption to activin B in vitro biosynthesis. Together, this analysis supports that INHBB variants that limit activin B production have consequences for testis composition in males.

AXDND1 is required to balance spermatogonial commitment and for sperm tail formation in mice and humans

“…For age series data, a t-test was used to compare genotypes at each age point. Significance of apoptosis data was assessed using a script in RStudio as previously described (47).…”

Targeting activins and inhibins to treat reproductive disorders and cancer cachexia

“…Evidence in the mouse suggests that activin A plays a dominant role in testis function relative to activin B and inhibin B, as β B -subunit null mice have normal testes and fertility (Vassalli et al 1994). However, in men, maintenance of β B -subunit production is important for normal testis function (Meachem et al 2001, Houston et al 2022.…”

“…Human genetic and diagnostic studies corroborate the findings in mice highlighting the importance of inhibins and activins in the maintenance of reproductive health. It has been demonstrated that variations in the INHA, INHBA and INHBB genes are associated with premature ovarian failure (Harris et al 2005, Chand et al 2007), ovarian cancer (Tournier et al 2014) and male infertility (Li et al 2015, Houston et al 2022. Only recently, two discrete INHBB gene variations were identified in infertile men (Houston et al 2022).…”

“…It has been demonstrated that variations in the INHA, INHBA and INHBB genes are associated with premature ovarian failure (Harris et al 2005, Chand et al 2007), ovarian cancer (Tournier et al 2014) and male infertility (Li et al 2015, Houston et al 2022. Only recently, two discrete INHBB gene variations were identified in infertile men (Houston et al 2022). It was found that an INHBB gene variant c.1079T>C:p.Met360Thr, located in the mature region of the inhibin βB-subunit, resulted in significantly reduced serum activin B levels and altered testis germ cell content in corresponding Inhbb M364T/ M364T mice.…”

“…It was found that an INHBB gene variant c.1079T>C:p.Met360Thr, located in the mature region of the inhibin βB-subunit, resulted in significantly reduced serum activin B levels and altered testis germ cell content in corresponding Inhbb M364T/ M364T mice. The second INHBB gene variant c.314C>T:p. Thr105Met, which lies within the prodomain of the inhibin βB-subunit, diminished the in vitro production of mature activin B (Houston et al 2022). Together, this analysis supports that INHBB gene variants that limit activin B production have consequences for testis composition in males.…”