Investigating the contribution of paternal nucleosomes using the Gcn5 knock-out mouse

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ABSTRACT Infertility affects approximately 15% of couples in the United States, with males contributing in nearly 50% of these cases. One potential mechanism underlying infertility are alterations to the paternal epigenome. Mammalian spermatozoa exhibit a unique, highly compacted and condensed DNA structure that is strongly dependent on epigenetic mechanisms, including histone hyperacetylation followed by nucleosome eviction. Specifically, 90-99% of sperm nucleosomes are evicted and replaced with protamines, allowing for this remarkable degree of compaction. Human sperm exhibiting altered ratios of protamines or excess histone retention are associated with infertility and altered embryogenesis following IVF/ICSI. However, the exact cause of altered embryogenesis as a result of abnormal nucleosome retention, and ultimately, potential regulatory functions and mechanisms by which paternally contributed histones affect early development, remain largely unknown. We have previously developed a conditional mouse mutant where the histone acetyltransferase Gcn5 is ablated in pre-meiotic germ cells (Gcn5cKO). One relevant feature of this model is that mature sperm have increased histone retention and decreased fertility. This model provides the ability to study the effect of paternal nucleosome contributions to the embryo, and determine the consequences of abnormal paternal nucleosome contribution on embryonic development. We propose that the paternal epigenome, specifically nucleosomes, play a role in regulating early embryonic chromatin and transcriptional dynamics, thus leading to proper embryonic development. We will utilize our Gcn5cKO mouse model to introduce an abnormal complement of sperm nucleosomes into embryos and investigate the following Specific Aims: (1) To determine if excess paternal nucleosomes alter the kinetics and successful development of pre-implantation embryos and (2) determine if abnormally retained paternal nucleosomes alter chromatin dynamics and transcription in pre-implantation embryos. Together, the proposed research will provide important insight into the mechanisms governing early embryonic development, including the effects of the paternal epigenome on chromatin dynamics and zygotic genome activation. It will additionally provide evidence as to why abnormal paternal chromatin results in infertility and altered embryogenesis in humans. The results of these studies have the potential to ultimately impact clinical management of patients diagnosed with infertility.