Functional analysis of Mmd2 and related PAQR genes during sex determination in mice
Sex determination in eutherian mammals is controlled by the Y-linked gene Sry, which drives the formation of testes in male embryos. Despite extensive study, the genetic steps linking Sry action and male sex determination remain largely unknown. Here, we focused on Mmd2, a gene that encodes a member of the progestin and adipoQ receptor (PAQR) family. We show that Mmd2 is expressed during the sex-determining period in XY but not XX gonads, specifically in the Sertoli cell lineage which orchestrates early testis development. Analysis of knockout mice deficient in Sox9 and Sf1 revealed that Mmd2 operates downstream of these known sex-determining genes. However, when we used CRISPR to ablate Mmd2 in the mouse, fetal testis development appeared to progress normally. To determine if other genes might have compensated for the loss of Mmd2, we identified the closely related PAQR family members Paqr8 and Mmd as also being expressed during testis development. We used CRISPR to generate mouse strains deficient in Paqr8 and Mmd, but both knockout lines appeared phenotypically normal and fertile. Finally, we generated Mmd2;Mmd and Mmd2;Paqr8 double-null embryos and again observed normal testis development. These results may reflect functional redundancy among these factors. Our findings highlight the difficulties involved in identifying genes with a functional role in sex determination and gonadal development through expression screening and loss-of-function analyses of individual candidate genes, and may help to explain the paucity of genes in which variations have been found to cause human disorders/differences of sex development.