Mapping post - translational histone modifications and the TH2B variant in spermatogenesis

Abstract

Epigenetics has been defined by Goldberg et al. [1] as the study of heritable changes in gene expression or cellular phenotype which arises without changes in the DNA sequence. Post-translational histone modification and exchange of histone variants are two important epigenetic mechanisms. Histones and histone variants have been shown to be involved in many cellular events, for example they largely contribute to regulation of gene expression.

Chromatin immunoprecipitation (ChIP) is a versatile tool to study protein-DNA interactions in general and the location of post-translational histone modifications in particular. We used this approach to investigate three post-translational histone modifications and a histone variant, during spermatogenesis where global changes in the epigenome are known to occur.

We assessed occupancy of the histone modifications H3K4me3, H3K9me2 and H3K27me3 and the testis specific histone variant TH2B on regulatory sequences of genes encoding histone modifying enzymes, chromatin remodelers and genes important to early embryo development in mice pachytene spermatocytes (PCS) and round spermatids (RS).

The objective of this thesis was to initiate the investigation of the role of TH2B and the bivalent marks H3K27me3 and H3K4me3 in spermatogenesis. This study revealed that the facultative repressive mark H3K9me2 and TH2B showed strong correlation at gene regulatory sequences in PCS and RS cells. Moreover, TH2B correlated inversely with the activating mark H3K4me3. This may indicate that TH2B plays a role as a testis specific repressive histone variant. Additionally, we observed significant levels of the bivalent marks H3K4me3 and H3K27me3 on the Hoxd4 and Oct4 promoter in PCS and RS. On a speculative note, this could perhaps be suggestive of preprogramming for expression in early embryo development.