The role and molecular mechanism of HMGN proteins in global genome repair after UV irradiation

High Mobility Group Nucleosomal Binding proteins (HMGNs) are a family of non-histone proteins that unfold chromatin by interacting with histone tails, competing with histone H1, and modulating post-translational modification of histones. By virtue of these architectural chromatin changes, HMGNs regulate transcription, replication and DNA repair. Nucleotide excision repair is a major DNA repair pathway that removes bulky DNA adducts caused by ultraviolet (UV) irradiation. The chromatin structure restricts the access of repair complexes to DNA lesions, and, therefore, remodeling and unfolding the chromatin is essential. We found that HMGN null cells (derived from DT40 chicken cells) were sensitive to UV irradiation. These cells also had a high apoptotic rate and a prolonged arrest in the G2/M and mitotic phases following UV irradiation. Moreover, UV-hypersensitive HMGN null cells showed low cyclobutane pyrimidine dimers (CPD) removal rates as compared to the wild-type cells. These results demonstrate the functional significance of HMGN proteins in nucleotide excision repair-global genome repair (NER-GGR). A global wave of core histone deacetylation peaking 4 hours after UV irradiation was observed in chicken and human cell lines. Following the nadir of acetylation, the acetylation levels gradually returned to the pre-UV steady state levels. We demonstrated that the wild-type cells but not HMGN1a/N2 null cells had a global increase in acetylation of histones H4K5 and H3K9 48 hours after UV irradiation. At 72 hours after UV irradiation H3K14 acetylation increased, a phenomenon that happened also in HeLa cells 24 hours after UV irradiation. Immunoprecipitations of HMGN1 from HeLa cells followed by histone acetyltransferase (HAT) assays demonstrated an associated HAT activity before UV irradiation and at 10 hours and 24 hours after UV irradiation. By co-immunoprecipitation assays, we showed that before UV irradiation and 2 minutes after UV irradiation, HMGN1 associated with p300, and 10 minutes and 24 hours following UV irradiation, HMGN1 associated with CBP. These results showed that HMGNs are involved in NER-GGR, but the association of HMGNs to p300/CBP and to the HAT-activity did not correlate with the peak of the CPD removal, indicating that HMGNs are involved in NER-GGR by HAT-dependent and independent pathways.