Histone lysine methylation in mammalian development

Histone lysine methylation has emerged as an important post-translational modification with the potential to index epigenetic information required for mammalian development. To investigate this potential, I examined the genome-wide dynamics of histone lysine methylation in mid-gestation mouse embryos. Using a panel of histone methyl-lysine antibodies, I determined the subnuclear distribution of 14 methyl-lysine derivatives of histories H3 and H4 in situ. This analysis revealed strikingly distinct genome wide distributions for some methyl-lysine derivatives, suggesting different functional properties. For instance, H3 K9 trimethylation (tMeK9-H3) exhibited dramatic cell cycle differences and appeared highly enriched in proliferative cells of the embryo. Likewise, H4 K20 monomethylation (mMeK20-H4) was also elevated in mitotic cells, but was more broadly distributed than tMeK9-H3. In contrast, the historic H4 K20 trimethyl derivative (tMeK20-H4) was progressively lost from dividing cells and became enriched in differentiating cells of the neural, and cardiac and skeletal muscle lineages, together with a concomitant loss of mMeK20-H4. These findings were corroborated in mouse primary cultures and C2C12 cells, which also revealed changes in histone H3 K36 and H3 K79 methylation during muscle differentiation. To further evaluate K20-H4 methylation changes during development, a mammalian retinogenesis model was used, enabling analysis of discrete neural cell lineages that arise from a single retinal progenitor cell population. Remarkably, progressive elevation of tMeK20-H4 was restricted to the ganglion cell layer and did not include other differentiated cell types, suggesting a lineage specific function.;To understand the mechanistic basis of histone H4-K20 methylation changes during myogenesis, published expression data and in silico gene expression analysis were used to identify histone lysine methyltransferases that were enriched or restricted to the muscle cell lineage. Two of these enzymes, Prdm12 and Smyd1, were further examined by overexpression analysis in C2C12 differentiation and mouse primary limb bud cultures. Significantly, this revealed that Prdm12 appears to promote differentiation in addition to influencing the level of dMeK20-H4. Together, these results establish an unforeseen level of spatial and temporal differences in histone lysine methylation during development, and suggest that histone H4-K20 methylation is a key regulator of myogenic differentiation.