by Monya Baker
G9a silences gene expression two ways
As embryonic stem cells differentiate, the pluripotency gene known as Oct4 goes on lockdown. In fact, the gates to gene expression are doublelocked: the gene-encoding DNA strands are wound up into a structure called heterochromatin, in which the DNA is complexed with histones and other proteins in such a way that it is inaccessible to the transcriptional machinery. Furthermore, gene-expression machinery is kept at bay by chemical modifications to the DNA that signals the start of a gene. New work published in Nature Structural and Molecular Biology1 shows not only that both of these modifications are regulated by a single master protein, the histone methyltransferase G9a, but that this enzyme apparently brings about the inactivation of many early embryonic genes.
Led by Howard Cedar and Yehudit Bergman of Hadassah Hebrew University in Jerusalem, Israel, a team of researchers examined mouse embryonic stem (ES) cells as they began to differentiate. They used microarrays to compare cells at different stages of differentiation as well as cells that could and could not express G9a. This identified a number of genes besides Oct4 that were newly methylated during differentiation; in other words, as cells lost pluripotency, a set of genes was silenced by way of chemical modification to certain regions of DNA, and this methylation also seems to be under the control of G9a.
G9a helps convert chromatin into heterochromatin, in which gene expression is blocked. Interestingly, DNA methylation and heterochromatinization seem to be independent of each other. The researchers blocked the heterochromatinization activity of G9a by introducing a mutation into the protein that prevents it from methylating a lysine 9 residue on histone protein H3. This mutation did not, however, change patterns of gene methylation.
G9a seems to regulate DNA methylation by recruiting two well known DNA methyltransferases (Dnmt3a and Dnmt3b) to relevant sites in the genome, sites where G9a is helping to create heterochromatin. The authors speculate that other histone methyltransferases may also promote DNA methylation as well as heterochromatinization.
G9a directs both processes as mouse embryos transition between pre- and post-implantation stages. For a cell to be reprogrammed back to a pluripotent state, these states must be reversed, and while heterochromatin can be remodelled during cell division, DNA methylation patterns are often faithfully reproduced, and this, the authors believe, constitutes the main barrier to reprogramming. "G9a seems to serve as a master regulator involved in turning off pluripotency," says Cedar. His future work will be aimed at understanding how these processes reverse themselves when somatic cell genomes are reprogrammed to pluripotency.
Editor's note: Outside comment for this highlight came in too late to be included. You can read it here.
Source: Nature