No abstract

To counteract the deleterious effects of genotoxic injury, cells have set up a sophisticated network of DNA repair pathways. We show that Gal4-VP16 and RAR transcriptional activators stimulate nucleotide excision repair (NER). This DNA repair activation is not coupled to transcription since it occurs in Cockayne syndrome cells (which are transcription-coupled repair deficient) and is observed in vitro in the presence of alpha-amanitin and in the absence of the basal transcription factors. Using a reconstituted dual incision assay, we also show that binding of activators to their cognate sequences induces a local chromatin remodeling mediated by ATP-driven chromatin remodeling and acetyltransferase activities to facilitate DNA repair.

Evidence for the epigenetic regulation of hematopoietic stem cells (HSCs) is growing, but the genome-wide epigenetic signature of HSCs and its functional significance remain unclear. In this study, from a genome-wide comparison of CpG methylation in human CD34+ and CD34− cells, we identified a characteristic undermethylation dip around the transcription start site of promoters and an overmethylation of flanking regions in undifferentiated CD34+ cells. This “bivalent-like” CpG methylation pattern around the transcription start site was more prominent in genes not associated with CpG islands (CGI−) than CGI+ genes. Undifferentiated hematopoietic cells also exhibited dynamic chromatin associated with active transcription and a higher turnover of histone acetylation than terminally differentiated cells. Interestingly, inhibition of chromatin condensation by chemical treatment (5-azacytidine, trichostatin A) enhanced the self-renewal of “stimulated” HSCs in reconstituting bone marrows but not “steady-state” HSCs in stationary phase bone marrows. In contrast, similar treatments on more mature cells caused partial phenotypic dedifferentiation and apoptosis at levels correlated with their hematopoietic differentiation. Taken together, our study reveals that the undifferentiated state of hematopoietic cells is characterized by a unique epigenetic signature, which includes dynamic chromatin structures and an epigenetic plasticity that correlates to level of undifferentiation.