ESPE Yearbook of Paediatric Endocrinology

Brief Summary: This molecular study explored the role of microRNAs (miRNAs) in mammalian sex determination. It used bulk and single-cell RNA-seq (scRNA-seq) alongside time-course expression analyses in a knockout mouse model to assess the role of the miR-17^~92 cluster in this process.

miRNAs are small, single-stranded, non-coding RNA molecules, 21 to 23 nucleotides in length, that mediate post-transcriptional gene regulation. Hurtado A et al. demonstrated that deleting the miR-17^~92 cluster induces complete gonadal dysgenesis in XY mice. The miR-17^~92 cluster, known as OncomiR-1, is a polycistronic cluster containing six miRNA genes (miR-17, miR-18a, miR-19a, miR-20a, miR-19b-1, and miR-92a-1) involved in various developmental and pathogenic processes. Members of this cluster are expressed in mouse gonads of both sexes during and after the sex determination stage. Additionally, these miRNAs are evolutionarily conserved and linked to sex development and sex change in non-mammalian vertebrates.

These authors used the Cre/LoxP system to generate XY knockout (KO) mice lacking the miR-17^~92 cluster (Tg(CAG- cre)^1Nagy;miR-17^~92^del/del). XY KO mice developed as phenotypic females with two uterine horns and ovaries. Histological analysis of XY KO gonads revealed absence of Sertoli cells at the sex determination stage and a clear ovarian morphology. Sry expression was delayed in XY KO gonads. Double immunofluorescence analysis showed that XY KO gonads did not express the testicular marker Sox9 but instead expressed the ovarian marker Foxl2.

Bulk RNA-seq on gonads revealed that both XY and XX mutants acquired a female-like expression profile, with most genes displaying an ovarian-specific expression pattern. Furthermore, molecular pathways involved in cell proliferation and growth control (e.g., Ras protein signal transduction, MAP kinase activity, ERK1/ERK2 cascade) were altered. Consistently, miR-17^~92 KO gonads were ^~50% smaller than wild-type controls. Analysis of proliferation rates in gonads showed that the genital ridge of mutant gonads contained fewer proliferating progenitor cells than controls.

This study highlights, for the first time, the role of miR-17^~92 in controlling early gonadal growth and Sry expression dynamics, underlines the importance of non-coding genome elements such as miRNAs in sex determination and adds complexity to our understanding of this process.