Abstract Nucleotide excision repair (NER) is the major pathway in cells for the removal of bulky photoproducts and ensuring integrity of the genome. NER constitutes lesion recognition, excision of damaged DNA, synthesis of a DNA fragment, and ligation of the repair patch. Nonetheless, DNA within the cells is compacted into chromatin and lesion recognition requires chromatin to be remodeled to permit access of repair and checkpoint machinery to the damage site. Consequent to repair of the lesions, chromatin structure is restored to its original state so that cells resume cell cycle progression. Although several reports have demonstrated the role of chromatin remodelers, histone modifications and histone chaperones in DNA damage response (DDR), knowledge of how these epigenetic mechanisms control the DDR is largely in its infancy. We and others have recently observed that histone H3 is deacetylated on lysine 56 immediately in response to UV-induced DNA damage. Acetylation is promptly restored upon completion of repair and is necessary for recovery from DNA damage induced cell cycle arrest. However, the function of H3K56 deacetylation in the DDR remains to be determined. We found that UV-induced deacetylation of H3K56 is abrogated in the presence of TSA and sodium butyrate, both of which inhibit class I histone deacetylases (HDACs). Knockdown of either HDAC1 or HDAC2 alone did not affect UV-induced deacetylation of H3K56. However, simultaneous knockdown of both HDAC1 and 2 resulted in retention of H3K56 acetylation after UV irradiation. Consistently, both HDAC1 and HDAC2 localized to sites of UV damage. These observations are consistent with that reported by Miller et. al., indicating that HDAC1 and HDAC2 are responsible for the deacetylation of H3K56 in response to DNA damage. Since the deacetylation of H3K56 is one of the most immediate responses to UV-irradiation, we assumed it to play a crucial role in NER. Accordingly, we observed that CPD removal was affected in H1299 cells with histone H3K56Q mutation that mimics constitutive acetylation. Furthermore, we found that localization of the checkpoint kinase ATM to UV damage sites was affected in these cells. Consequently, ATM-mediated H2AX phosphorylation was also found to be defective in the mutant cells. Based on our observations, we speculate that HDAC1 and 2 mediated H3K56 deacetylation regulates NER and checkpoint signaling cascade in response to UV damage. This work was supported by PHS grants ES2388, ES12991 and CA93413 and Pelotonia postdoctoral fellowship to AB. Citation Format: Aruna Battu, Altaf Ahmad Wani. UV damage-induced histone H3 lysine 56 deacetylation regulates nucleotide excision repair and checkpoint signaling. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1786. doi:10.1158/1538-7445.AM2013-1786