Title: Disorders of Sex Development
Funder: Medical Research Council

Abstract: My programme is investigating the mechanisms by which signaling events impact on gene expression using cell fate determination in the embryonic mouse gonad as a model system. The “decision” as to whether an embryo develops as a male or a female is known as sex determination and in mammals this process requires expression of the Y-linked gene SRY in XY embryos to establish commitment of the gonadal primordium, in particular somatic cells, to a testicular fate. In recent years we have established a role for mitogen-activated protein kinase (MAPK) signaling in testis determination. Using a forward genetic screen in mice we identified the boygirl mutant, which causes XY gonadal sex reversal due to loss of the kinase MAP3K4. Analysis of XY gonad development in mutant embryos revealed a reduction in Sry expression at a key stage, leading to ovary development. Our studies in mice became directly disease-relevant when we collaborated with clinical geneticists and identified mutations in MAP3K1 as the cause of XY sex reversal in humans. This suggests that MAPK signaling plays an important role in mouse and human testis determination and disruption to the signal transduction pathway may underlie additional disorders of sex development (DSD) in humans. MAP3K1 is now a gene routinely screened during diagnosis of novel cases of DSD. More recently we have been focusing on how an essentially ubiquitous signaling pathway such as MAPK is employed in a time- and tissue-dependent fashion so as to cause specific phenotypes when disrupted. We demonstrated that mice lacking GADD45g also exhibit XY gonadal sex reversal, again due to a disruption to Sry expression. Analysis of MAPK signaling in gonads from Gadd45g and Map3k4 mutants revealed a reduction in the activation of p38 MAPK. We also examined the phosphorylation status of GATA4, a protein known to regulate Sry expression, and showed that in Gadd45g and Map3k4 mutants, pGATA4 levels were significantly reduced. Thus, a GADD45g/MAP3K4/p38 MAPK/GATA4 pathway is implicated in the control of Sry expression during testis determination. The study of the regulation of gene expression requires an understanding of how chromatin modifications are controlled: in this programme our focus is on how signals to the epigenome regulate the expression of testis determining genes. It is notable that in other contexts all the molecules we have established as testis-determining have been shown to directly or indirectly affect chromatin. Examining chromatin structure and occupancy in vivo is challenging in a system where tissue availability is limiting. However, technological progress in the area of epigenomics is rapid and it is becoming feasible to examine genome-wide studies of chromatin using just a few thousand cells. Utilising reporter lines that will allow purification of gonadal somatic cells, and our unique collection of mutants disrupting testis determination, it is our aim to examine the transcriptomics and associated chromatin modifications that underlie normal testis development and those disrupted in the mutants we have characterized. The ultimate aim is to determine the epigenomics of testis determination in vivo and establish how signaling molecules, in particular, regulate this process. We are also collaborating with clinical geneticists attempting to identify novel genes causing DSDs in humans using exome and whole genome sequencing. We are generating and studying sexual development in mice with mutations in candidate genes identified in this way: this will aid in confirming pathogenicity and provides mechanistic insight into genetic pathways underlying DSDs in the human population.


Research activity codes: 1.1 NORMAL BIOLOGICAL DEVELOPMENT AND FUNCTIONING
Health categories: REPRODUCTIVE HEALTH AND CHILDBIRTH