Epigenetic reprogramming in mammalian reproduction: contribution from histone variants.

Development of the mammalian embryo is, by definition, epigenetic. At the level of the nucleosome, the building block of the chromatin, changes in chromatin structure can be regulated through histone content. Apart from the canonical histones whose synthesis is restricted to S-phase, different histone variants have been identified. Histone variants can help to establish specialised chromatin regions and to regulate developmental and cell differentiation processes. While the role of histone variants has been extensively explored in differentiated cells, less is known in germ cells and embryos. Increasing lines of evidence suggest that the functions and/or properties of histone variants in embryos might be different to those in somatic cells. During reprogramming, histone variants such as H3.3 or H2A.Z are candidates to play potential important roles. We suggest that H3.3 has an important role in setting up a 'transition' signature, and provides the possibility to infer changes in chromatin architecture independent of DNA replication. This should confer flexibility during important developmental processes. The specific pathways through which H3.3 could regulate different chromatin conformations at different loci and the identification of specific proteins responsible for this deposition are an important challenge for future investigation. Lastly, the set of variants incorporated within the nucleosome can have important consequences in the regulation of epigenetic mechanisms during development.