Synergism between DNA methylation and macroH2A1 occupancy in epigenetic silencing of the tumor suppressor gene p16(CDKN2A).

Promoter hypermethylation and heterochromatinization is a frequent event leading to gene inactivation and tumorigenesis. At the molecular level, inactivation of tumor suppressor genes in cancer has many similarities to the inactive X chromosome in female cells and is defined and maintained by DNA methylation and characteristic histone modifications. In addition, the inactive-X is marked by the histone macroH2A, a variant of H2A with a large non-histone region of unknown function. Studying tumor suppressor genes (TSGs) silenced in cancer cell lines, we find that when active, these promoters are associated with H2A.Z but become enriched for macroH2A1 once silenced. Knockdown of macroH2A1 was not sufficient for reactivation of silenced genes. However, when combined with DNA demethylation, macroH2A1 deficiency significantly enhanced reactivation of the tumor suppressor genes p16, MLH1 and Timp3 and inhibited cell proliferation. Our findings link macroH2A1 to heterochromatin of epigenetically silenced cancer genes and indicate synergism between macroH2A1 and DNA methylation in maintenance of the silenced state.

Aberrant epigenetic silencing of tumor suppressor genes is reversed by direct reprogramming.

Direct reprogramming procedures reset the epigenetic memory of cells and convert differentiated somatic cells into pluripotent stem cells. In addition to epigenetic memory of cell identity, which is established during development, somatic cells can accumulate abnormal epigenetic changes that can contribute to pathological conditions. Aberrant promoter hypermethylation and epigenetic silencing of tumor suppressor genes (TSGs) are now recognized as an important mechanism in tumor initiation and progression. Here, we have studied the fate of the silenced TSGs p16(CDKN2A) during direct reprogramming. We find that following reprogramming, p16 expression is restored and is stably maintained even when cells are induced to differentiate. Large-scale methylation profiling of donor cells identified aberrant methylation at hundreds of additional sites. Methylation at many, but not all these sites was reversed following reprogramming. Our results suggest that reprogramming approaches may be applied to repair the epigenetic lesions associated with cancer.