Research Project 4: Epigenetic Regulation of Retrotransposon Silencing and Male Fertility in the Mouse

Project Summary Mammalian germ cells undergo genome-wide epigenetic reprogramming during development. During this process, retrotransposons (repetitive DNA elements) that account for ~40% of the mammalian genome are transiently activated but subsequently become permanently silenced through multiple epigenetic mechanisms. Although retrotransposons play important roles in genome evolution, their mobilization can be detrimental to genome integrity and a failure to silence retrotransposons in germ cells can cause sterility. We previously identified TEX15 as a testis-specific protein. In the previous funding period, we found that TEX15 is essential for epigenetic silencing of retrotransposons in male germ cells. Furthermore, TEX15 is associated with a Piwi protein in testis and functions as a nuclear effector for the piRNA pathway in retrotransposon silencing. TEX15 is a large protein with a motif of unknown function. Our preliminary results have shown that TEX15 is associated with chromatin-binding epigenetic factors. Importantly, we also found that loss of TEX15 or its interacting partner in postnatal germ cells results in production of sperm with abnormal “mushroom” shaped head morphology. Thus, we hypothesize that TEX15 regulates the epigenetic landscape of retrotransposons in male germ cells and plays an important role in spermiogenesis through its associated chromatin modifiers. In this project, we will 1) determine the epigenetic state associated with retrotransposon activation in Tex15-deficient male germ cells and define the functional domains and genomic targets of TEX15 and 2) characterize the function of TEX15 and its associated chromatin-binding proteins in regulating gene expression critical for spermiogenesis. Finally, human exome sequencing studies have shown that nonsense mutations in human TEX15 causes non- obstructive azoospermia (NOA), a severe form of infertility in men. Strikingly, NOA patients frequently exhibit heterogeneity in testicular histopathology, the causes of which are enigmatic. We for the first time propose revertant mosaicism as a mechanism for heterogeneity in testicular histopathology in NOA men and plan to model this human male infertility condition using our unique Tex15 revertant mosaicism mouse mutant. Together, our genetic, genomic, and proteomic studies will elucidate the TEX15-mediated epigenetic program in silencing of retrotransposons in germ cells and its role in spermiogenesis. These results will also have strong implications in understanding human male infertility and provide guidance to treatment outcome consultation in IVF clinics.