Expression of genes coding for histone variants and histone-associated proteins in pluripotent stem cells and mouse preimplantation embryos.

The histone code is an epigenetic regulatory system thought to play a crucial role in cellular events such as development, differentiation and in the maintenance of pluripotency. In order to gain an insight into the role variant histones may play during mammalian development; we studied gene expression of histone variants and remodelling enzymes in mouse embryonic stem (ES) cells and during mouse preimplantation development. Using quantitative reverse-transcription PCR (qRT-PCR) we document the gene expression pattern of 12 histone variants and 2 of their associated remodelling enzymes in undifferentiated ES cells and during preimplantation embryo development. All histone variants were detected in undifferentiated ES cells, with H2AZ showing the highest expression levels of all the histone variants tested. The results also show that H2A variant levels tend to increase later in embryo development whilst H3 variant levels are elevated in early preimplantation stages. In addition, the expression of SWI/SNF, a remodeler protein involved in specifically remodelling H2A-H2B dimers, mirrors the expression of H2B and H2A variants, and the H3-H4 specific chaperone CAF-1 expression mirrors H3 variant expression. These results provide a foundation for further studies on the functions of histone variants during development, differentiation and in pluripotency.