Genomics of sexual cell fate transdifferentiation in the mouse gonad

Sex determination in mammals hinges on a cell fate decision in the fetal bipotential gonad between male Sertoli cells and female granulosa cells. While this decision normally is permanent, loss of key cell fate regulators such as the transcription factors Dmrt1 and Foxl2 can cause postnatal transdifferentiation from Sertoli to granulosa-like or vice versa. Here we examine the mechanism of transdifferentiation in mice carrying either a null mutation of Dmrt1 or a point mutation, R111G, that alters the DNA binding motif and causes human XY gonadal dysgenesis and sex reversal. We first define genes misexpressed during transdifferentiation and then show that female transcriptional regulators driving transdifferentiation in the mutant XY gonad (ESR2, LRH1, FOXL2) bind chromatin sites related to those normally bound in the XX ovary. We then define gene expression changes at the onset of transdifferentiation and abnormal chromatin compartments that may help destabilize cell fate and initiate transdifferentiation. We model the R111G mutation in mice and show that it causes dominant gonadal dysgenesis, analogous to its human phenotype but less severe. We show that R111G partially feminizes the testicular transcriptome and causes dominant disruption of DMRT1 binding specificity in vivo. These data help illuminate how transdifferentiation occurs when sexual cell fate maintenance is disrupted and identify chromatin sites and transcripts that may play key roles in the transdifferentiation process. Overall design: RNA-seq (30 samples), ChIP-seq (17 samples, and Hi-C (2 samples) on gonads and purified Sertoli cells.