Role of DNA methylation and histone modifications in structural maintenance of heterochromatin domains (chromocenters)

Effects of DNA methylation inhibitor; 5-azacytidine (5-aza-C); and histone acetylation inhibitor, trichostatine A (TSA), on the structure of pericentric heterochromatin of L929 mouse cells have been studied. 5-aza-C treatment for 48 h resulted in the transformation of ovoid chromocenters into elongated structures in 85% of cells. Hypotonic treatment of these cells reveals tandemly arranged DAPI-positive globules that are well distinguishable by light microscopy. Similar globular units can be observed in hypotonic-treated control cells. TSA treatment for 48 h causes dramatic decrease in HP1α content in cells. In 25% of treated cells chromocenters became highly decondensed and can not be reliably detected by light and electron microscopy. 85% cells demonstrate globular chromocenters with low HP1α content. Hypotonic treatment induces transformation of compact chromocenters into ring-like structures that can be either single or clustered. Rings are formed by uniform fiber in which no globular subunits are detected. The data obtained are discussed concerning several mechanisms of heterochromatin structure maintenance and the role of epigenetic factors.