Specific point mutations in lamin A gene have been shown to accelerate aging in humans and mice. Particularly, a de novo mutation at G608G position impairs lamin A processing to produce the mutant protein progerin, which causes the Hutchinson Gilford progeria syndrome. The premature aging phenotype of Hutchinson Gilford progeria syndrome is largely recapitulated in mice deficient for the lamin A-processing enzyme, Zmpste24. We have previously reported that Zmpste24 deficiency results in genomic instability and early cellular senescence due to the delayed recruitment of repair proteins to sites of DNA damage. Here, we further investigate the molecular mechanism underlying delayed DNA damage response and identify a histone acetylation defect in Zmpste24 −/− mice. Specifically, histone H4 was hypoacetylated at a lysine 16 residue (H4K16), and this defect was attributed to the reduced association of a histone acetyltransferase, Mof, to the nuclear matrix. Given the reversible nature of epigenetic changes, rescue experiments performed either by Mof overexpression or by histone deacetylase inhibition promoted repair protein recruitment to DNA damage sites and substantially ameliorated agingassociated phenotypes, both in vitro and in vivo. The life span of Zmpste24 −/− mice was also extended with the supplementation of a histone deacetylase inhibitor, sodium butyrate, to drinking water. Consistent with recent data showing age-dependent buildup of unprocessable lamin A in physiological aging, aged wild-type mice also showed hypoacetylation of H4K16. The above results shed light on how chromatin modifications regulate the DNA damage response and suggest that the reversal of epigenetic marks could make an attractive therapeutic target against laminopathybased progeroid pathologies. E ukaryotic cells are equipped with a surveillance machinery to orchestrate the rapid detection and repair of DNA damage. When DNA damage occurs, chromatin surrounding the doublestrand breaks (DSBs) is altered and histones are modified to facilitate access for repair proteins (1). As a rapid response to DSB induction, the histone H2A variant, H2AX, is phosphorylated at Ser139 (γ-H2AX), which in turn interacts with MDC1, a DSB repair mediator, to facilitate the further recruitment of DNA repair proteins, such as 53BP1 and BRCA1 (2-4). Interestingly, γ-H2AX accumulation has been documented both in human senescent cells and in the fibroblasts of aged mice and primates (5-8). It has been proposed that these age-associated γ-H2AX foci contain nonrepairable DSBs and may have a role in initiating aging, especially because DSBs are very toxic and are one of the most lethal forms of DNA damage. Direct evidence for nonrepairable DNA damage as an inducer of premature aging has been obtained from mouse models that lack DNA repair proteins, such as ATM, Ku70, Ku80, DNA ligase IV, and Ercc1, as well as from humans with premature aging syndromes (9, 10). Together, these studies support the idea that the inability to recruit repair proteins to sites of DNA les...