Human apurinic/apyrimidinic endonuclease (APE1) is an essential enzyme in DNA base excision repair that cuts the DNA backbone immediately adjacent to the 5' side of abasic sites to facilitate repair synthesis by DNA polymerase beta (ref. 1). Mice lacking the murine homologue of APE1 die at an early embryonic stage. Here we report that APE1 has a DNA exonuclease activity on mismatched deoxyribonucleotides at the 3' termini of nicked or gapped DNA molecules. The efficiency of this activity is inversely proportional to the gap size in DNA. In a base excision repair system reconstituted in vitro, the rejoining of nicked mismatched DNA depended on the presence of APE1, indicating that APE1 may increase the fidelity of base excision repair and may represent a new 3' mispaired DNA repair mechanism. The exonuclease activity of APE1 can remove the anti-HIV nucleoside analogues 3'-azido-3'-deoxythymidine and 2',3'-didehydro-2', 3'-dideoxythymidine from DNA, suggesting that APE1 might have an impact on the therapeutic index of antiviral compounds in this category.

Obesity and the metabolic syndrome have evolved to be major health issues throughout the world. Whether loss of genome integrity contributes to this epidemic is an open question. DNA polymerase η (pol η), encoded by the xeroderma pigmentosum (XP-V) gene, plays an essential role in preventing cutaneous cancer caused by UV radiation-induced DNA damage. Herein, we demonstrate that pol η deficiency in mice (pol η −/− ) causes obesity with visceral fat accumulation, hepatic steatosis, hyperleptinemia, hyperinsulinemia, and glucose intolerance. In comparison to WT mice, adipose tissue from pol η −/− mice exhibits increased DNA damage and a greater DNA damage response, indicated by upregulation and/or phosphorylation of ataxia telangiectasia mutated (ATM), phosphorylated H 2 AX (γH 2 AX), and poly[ADP-ribose] polymerase 1 (PARP-1). Concomitantly, increased cellular senescence in the adipose tissue from pol η −/− mice was observed and measured by up-regulation of senescence markers, including p53, p16 Ink4a , p21, senescence-associated (SA) β-gal activity, and SA secretion of proinflammatory cytokines interleukin 6 (IL-6) and tumor necrosis factor α (TNF-α) as early as 4 wk of age. Treatment of pol η −/− mice with a p53 inhibitor, pifithrin-α, reduced adipocyte senescence and attenuated the metabolic abnormalities. Furthermore, elevation of adipocyte DNA damage with a high-fat diet or sodium arsenite exacerbated adipocyte senescence and metabolic abnormalities in pol η −/− mice. In contrast, reduction of adipose DNA damage with N-acetylcysteine or metformin ameliorated cellular senescence and metabolic abnormalities. These studies indicate that elevated DNA damage is a root cause of adipocyte senescence, which plays a determining role in the development of obesity and insulin resistance.T he human genome is constantly challenged by exogenous and endogenous DNA damaging agents. To ensure genome integrity, human cells have faithful DNA replication machinery and DNA repair systems that are coordinated by DNA damage response networks. In response to different extents or types of DNA lesions, the DNA damage response activates appropriate cellular responses, including transient or permanent (senescence) cell cycle arrest, or apoptosis, to minimize the detrimental effects of DNA lesions (1). Reduction or deficiency in DNA repair/replication enzyme activity is well documented to increase vulnerability for the development of cancer, neurodegenerative diseases, and aging (2). In addition, defective DNA repair enzymes are associated with the metabolic symptom; for example, DNA glycosylase (Neil1)-and OGG1-deficient mice are obese (3-5), and nucleotide excision repair protein ERCC1-XPF deficiency causes lipodystrophy (6). Furthermore, DNA damage response protein ataxia telangiectasia mutated (ATM) suppresses JNK activity through p53 signaling and mediates an antioxidant action that has been suggested to be relevant to the metabolic syndrome (7). Nucleotide excision repair XP-A protein may affect metabolism by altering mitochondrial...

Human DNA polymerase (pol ) can replicate across UV-induced pyrimidine dimers, and defects in the gene encoding pol result in a syndrome called xeroderma pigmentosum variant (XP-V). XP-V patients are prone to the development of cancer in sun-exposed areas, and cells derived from XP-V patients demonstrate increased sensitivity to UV radiation and a higher mutation rate compared with wild-type cells. pol has been shown to replicate across a wide spectrum of DNA lesions introduced by environmental or chemotherapeutic agents, or during nucleotide starvation, suggesting that the biological roles for pol are not limited to repair of UV-damaged DNA. The high error rate of pol requires that its intracellular activity be tightly regulated. Here, we show that the phosphorylation of pol increased after UV irradiation, and that treatment with caffeine, siRNA against ATR, or an inhibitor of PKC (calphostin C), reduced the accumulation of pol at stalled replication forks after UV irradiation or treatment with cisplatin and gemcitabine. Site-specific mutagenesis (S587A and T617A) of pol at two putative PKC phosphorylation sites located in the proteinprotein interaction domain prevented nuclear foci formation induced by UV irradiation or treatment with gemcitabine/cisplatin. In addition, XP-V cell lines stably expressing either the S587A or T617A mutant form of pol were more sensitive to UV radiation and gemcitabine/cisplatin than control cells expressing wild-type pol . These results suggest that phosphorylation is one mechanism by which the cellular activity of pol is regulated. xeroderma pigmentosum variant ͉ lesion bypass X eroderma pigmentosum (XP) is an autosomal recessive condition characterized by premature skin aging, pigmentary changes, photosensitivity, and malignant tumor development. The manifestations associated with XP are because of a cellular hypersensitivity to UV radiation resulting from defects in any one of a number of genes encoding nucleotide excision repair (XP-A to -G) proteins (1, 2). Human DNA polymerase (pol ) is an important enzyme that replicates across pyrimidine dimers introduced by UV radiation (3), and defects in the gene encoding pol result in xeroderma pigmentosum variant (XP-V) syndrome (3, 4). Similar to patients with other forms of XP, patients with XP-V are highly sensitive to UV radiation and prone to the development of skin cancer (5). Furthermore, cells derived from XP-V patients exhibit a higher mutation rate (6) than the wild-type cells. In addition to pyrimidine dimers, pol has been shown to replicate across 8-hydroxyurea-induced lesions, O 6 -methylguanine, and cisplatin cross-linked intrastrand GG sites (7). Recently, we have shown that pol incorporates, extends, and bypasses chemotherapeutic nucleoside analogs AraC and gemcitabine (8). pol is also involved in Ig hypermutation (9), strand invasion during homologous recombination (10), and replication during nucleotide starvation (11). These studies suggest that in addition to its roles in the protection of cells from DNA damage, p...

PurposeThe development of resistance against anticancer drugs has been a persistent clinical problem for the treatment of locally advanced malignancies in the head and neck mucosal derived squamous cell carcinoma (HNSCC). Recent evidence indicates that the DNA translesion synthesis (TLS) polymerase η (Pol η; hRad30a gene) reduces the effectiveness of gemcitabine/cisplatin. The goal of this study is to examine the relationship between the expression level of Pol η and the observed resistance against these chemotherapeutic agents in HNSCC, which is currently unknown. MethodsSixty-four mucosal derived squamous cell carcinomas of head and neck (HNSCC) from 1989 and 2007 at the City of Hope National Medical Center (Duarte, CA) were retrospectively analyzed. Pretreatment samples were immunostained with anti-Pol η antibody and the correlation between the expression level of Pol η and clinical outcomes were evaluated. Forty-nine cases treated with platinum (n=40) or gemcitabine (n=9) based chemotherapy were further examined for Pol η expression level for comparison with patient response to chemotherapy. ResultsThe expression of Pol η was elevated in 67% of the head and neck tumor samples. Pol η expression level was significantly higher in grade 1 to grade 2 tumors (well to moderately differentiated). The overall benefit rate (complete response+ partial response) in patients treated with platinum and gemcitabine based chemotherapy was 79.5%, where low Pol η level was significantly associated with high complete response rate (p=0.03), although not associated with overall survival. Furthermore, no significant correlation was observed between Pol η expression level with gender, age, tobacco/alcohol history, tumor stage and metastatic status.ConclusionsOur data suggest that Pol η expression may be a useful prediction marker for the effectiveness of platinum or gemcitabine based therapy for HNSCC.

Significance Mitochondria are often considered a source of reactive oxygen species (ROS) that cause damage to cellular components including nuclear DNA. This endogenous damage is thought to underlie neurodegeneration, especially in diseases such as Cockayne syndrome (CS) that lack transcription-coupled DNA repair (TCR). We find no evidence, however, for any nuclear DNA damage from increased mitochondrial ROS. Our results indicate that the neurodegenerative symptoms in CS may be mitochondrial in origin, independent of nuclear DNA damage and repair.