How heterochromatin spreads

Heterochromatin spreading plays a key role in the genome partitioning process that shapes cell fate. Heterochromatin exists in small regions (nucleation sites) in stem cells (for example, embryonic or hematopoietic stem cells) that expand differentially depending on the lineage track. How heterochromatin spreading from small nucleation sites is developmentally regulated to drive lineage fates, or how lineages decisions direct differential spreading is completely unknown. We focus on a key enzyme that is involved in assembling histone 3 lysine 9 (H3K9) dimethylated (me2) heterochromatin in differentiation. This enzyme is a heterodimer of the G9a and GLP histone methyltransferase.

In our in vitro studies, we try to answer the following questions: 1. How does G9a/GLP engage with its nucleosomal substrate? We have shown that the heterodimer is specifically enhanced in its catalytic activity on nucleosomes. 2. How does G9a/GLP, or other H3K9 methylases, together with heterochromatin structural proteins propagate methylation along the chromatin chain? We use biochemistry and biophysics with recombinant chromatin reagents and a variety of single-molecule platforms.

In our in vivo studies we try to address the following questions: 1. How do heterochromatin domains behave within a cell cycle and 2. How does a domain of H3K9 methylation reach a particular size? We address these problems using single cell imaging of fission yeast together with the Finkelstein lab and epigenomic and imaging methods in mouse stem cells during differentiation.