A further step in how genes are silenced
A new publication from the GEN-AU teams of Thomas Jenuwein (IMP) and Denise Barlow (CeMM) changes our current understanding of how mouse and human genes are silenced. The scientists applied a powerful genome technology (so-called ChIP-Chip) to map epigenetic marks typical of expressed and silent genes directly on the chromosome itself.
The maps produced by these teams showed two unexpected features that run counter to the existing dogma of how gene silencing operates in mouse and human chromosomes. The first was the discovery that heterochromatin can exist as small focal patches that only cover the start of a silent gene and do not spread further. (Previously, heterochromatin had only been described as spreading over very large regions at the centromeres and telomeres that cap the ends of a chromosome or over an entire inactive chromosome). The second discovery is that these focal heterochromatin patches can lie inside a gene without hindering the ability of the gene to be highly expressed.
The significance of this work lies not only in these two discoveries that are essential for a basic understanding of how the expression of mouse and human genes is controlled at the level of the chromosome itself, but also, in challenging current ideas about how gene expression affects chromatin structure and how heterochromatin actually silences genes.
(Regha et al.: Active and Repressive Chromatin Is Interspersed without Spreading in an Imprinted Gene Cluster in the Mammalian Genome. Mol. Cell, August 3rd 2007)