Familial primary ovarian insufficiency associated with a SYCE1 point mutation: Defective meiosis elucidated in humanized mice

Objective: To investigate if nonsense mutation SYCE1 c.613C>T -found in women with familial primary ovarian insufficiency (POI)- is actually responsible for infertility, and to elucidate the involved molecular mechanisms. Design: As most fundamental mammalian oogenesis events occur during the embryonic phase, thus hindering the study of POI9s etiology/pathogeny in infertile women, we have used CRISPR/Cas9 technology to generate a mouse model line with an equivalent genome alteration (humanized mice). Setting: Academic research laboratories. Interventions: We present the characterization of the biallelic mutant mice phenotype, compared to wild type and monoallelic littermates. Animals: Studies were conducted employing the generated humanized mice. All studies were performed for both genders, except otherwise stated. Main outcome measures: reproductive capability by fertility tests; gonadal histological analysis; evaluation of chromosome synapsis and synaptonemal complex (SC) assembly by immunolocalizations; protein studies by Western blotting; transcript quantification by RT-qPCR. Results: The studied mutation proved to be the actual cause of the infertile phenotype, both in female and male mice homozygous for the change, confirming infertility of genetic origin with a recessive mode of inheritance. The mechanisms that lead to infertility are related to chromosome synapsis defects; no putative truncated SYCE1 protein was observed, and Syce1 transcript was hardly detected in biallelic mutants. Conclusions: We present for the first time the generation of humanized mice to study the actual consequences of a SC component mutation found in women with familial POI. By this approach we could confirm the suspected etiology, and shed light on the underlying molecular mechanism.