| The incidence of infertility is rising and estimated to affect one in six couples, yet a significant number of couples have an undefined cause for their infertility. The search for causes of male infertility has been the focus of many recent investigations, however, many questions remain. We hypothesized that genetic and epigenetic states of the mature sperm are important for establishing proper germ cell identity, function, and developmental capacity. We reason that perturbations to the genome or epigenome may underlie a subset of idiopathic cases of infertility. This dissertation addresses some potential genetic causes of male infertility; focusing on the genetic causes of an abnomal protamine ratio. Our gene re-sequencing studies examined whether single nucleotide polymorphisms (SNPs) in the untranslated regions of the protamine genes (Chapter 3) or in a transcriptional regulator of the protamine genes known as YBX2 (Appendix A) may account for the altered P1/P2 ratio in infertile men. We concluded that gene mutations in these two regulatory elements were a rare cause of male infertility. As an alternative we turned to examine whether epigenetic changes in the germline may underlie some cases of male infertility. Initially we focused on characterizing the sperm chromatin of fertile men. A comprehensive analysis of the sperm epigenome revealed novel biological features that revised our understanding of the potential role of sperm chromatin in the creation of a totipotent gamete (Chapter 5). We then assessed whether changes in the chromatin landscape associate with infertility or poor reproductive outcome. Interesting, we show significant changes in DNA methylation at a number of imprinted loci (Chapter 6 and 7) and subtle changes in the overall chromatin landscape (H3K4me and H3K27me) genome-wide that may have a detrimental effect on fecundity (Chapter 7). Taken together, these studies reinforce the dogma that infertility is a complex disease and that changes in the genetic and the epigenetic states of the mature sperm may account for some, but not all, cases of idiopathic infertility. Furthermore, these data suggest that germ cell chromatin may have a significant role in germ cell function and possibly contribute to early embryonic processes and infertility. |
