The sperm epigenome: implications for the embryo.

Recent advances, including the human genome project and numerous studies of cancer and other diseases, have shown that the genetic code is not simply limited to the sequence of the four bases of DNA but also includes epigenetic programming, heritable changes that affect gene expression [Riggs A, Martinssen R, Russo V (2007) Introduction. In: Riggs A, Martinssen R, Russo V (eds) Epigenetics mechanisms of gene regulation. Cold Spring Harbor Press, New York]. The science of epigenetics is important in understanding many diseases and biological processes, including in identifying the causes of disease and better understanding the mechanisms by which the environment can affect gene expression [Carrell Fertil Steril 97 (2):267-274, 2012]. This chapter will focus on the epigenome of sperm and particularly highlight the potential role of the sperm epigenome in embryogenesis.The sperm epigenome is unique and highly specialized because of the unique nature and function of sperm and because of the diverse requirements for successful fertilization. Due to the need for motility, sperm chromatin must be compacted and highly organized. During spermiogenesis the chromatin is packaged tightly into the sperm head by the replacement of most histones with protamines. This allows for protection of the DNA from the hostile environment in the female reproductive tract. Remaining histones can have chemical modifications to the tails of the protein that either facilitate or repress gene transcription. Sperm, like embryonic stem cells, have a unique pattern of histone modifications that includes both activating and silencing marks in the promoters of genes associated with development. These bivalent marks, along with DNA hypomethylation, comprise a unique state in which the key genes are "poised" for possible activation in embryogenesis. Sperm epigenetic abnormalities have been linked with multiple diseases including male factor infertility and poor embryogenesis.