Stewart

Project Title: Specialized roles of H3K4 methyltransferases during mouse development

Nucleosomes at Pol II promoters are characterized by several distinct features including trimethylation on histone 3 lysine 4 (H3K4me3). In yeast, only one enzyme, Set1, is responsible for H3K4 methylation. The Set1 complex (Set1C) also includes a PHD finger protein, Spp1, that binds to H3K4me3. Hence Set1C contains both the enzyme and a binding module for H3K4me3 and is an example of the epigenetic feed-forward principle of positive reinforcement. Mammals possess at least six enzymes capable of methylating H3K4, including two Set1 orthologues and four Mlls. All six are found in Set1C-like complexes that also probably include H3K4me3 binding PHD fingers. Hence the maintenance of promoter nucleosomes in mammals may also have an epigenetic component. Although bulk somatic H3K4me3 appears to be redundantly maintained by the six enzymes, each of the six appears to be uniquely required during mouse development. To elucidate the essential roles of these factors, we are using conditional mutagenesis and have made most progress with Mll2. Mll2 is required in at least two phases, which are unexpectedly separated by a phase when it is not required.

Mll2 is required for bulk H3K4me3 during oogenesis and early development, however not H3K4me1, indicating that mono- and tri-methylations are conveyed by different enzymes. During this period the epigenome is reprogrammed to establish pluripotency in the epiblast. Whereas Mll2 is required during reprogramming, it is no longer required once pluripotency has been established and other methyltransferases are responsible for bulk

H3K4me3. After the pluripotent phase, Mll2 is again required around gastrulation, apparently for neurogenesis (whereas its sister gene, Mll1 appears to be dedicated to mesenchymal differentiation). These data reveal unexpected specializations and switching of epigenetic roles amongst H3K4 methyltransferases during mouse development.