Purpose: Double-strand break repair via homologous recombination is essential in maintaining genetic integrity. RAD51 and XRCC3 are involved in the repair of DNA by this pathway, and polymorphisms have been identified in both the RAD51 (RAD51-G135C) and XRCC3 (XRCC3-Thr241Met) genes. The object of this study was to examine whether these polymorphisms may modulate susceptibility to the development of acute myeloid leukemia (AML), a disease that is characterized by genetic instability. Experimental Design: We studied the distribution of polymorphisms in RAD51 and XRCC3 in 216 cases of de novo AML, 51 cases of therapy-related AML (t-AML), and 186 control subjects using PCR followed by restriction enzyme digestion. The polymorphic deletion of the detoxification gene glutathione S-transferase M1 (GSTM1) was also examined by PCR. Results: The risk of the development of AML was found to be significantly increased when both variant RAD51-135C and XRCC3-241Met alleles are present [odds ratio (OR), 3.77; 95% confidence interval (CI), 1.39–10.24], whereas the risk of t-AML development is even higher (OR, 8.11; 95% CI, 2.22–29.68), presumably because of the large genotoxic insult these patients receive after their exposure to radiotherapy or chemotherapy. If we further divide the AML group into patients in which the burden of DNA damage is increased, because of the deletion of the GSTM1 gene, the risk of development of AML is further increased (OR, 15.26; 95% CI, 1.83–127.27). Conclusions: These results strongly suggest that DNA double-strand breaks and their repair are important in the pathogenesis of both de novo and t-AML.

Polymorphisms in several DNA repair genes have been described. These polymorphisms may affect DNA repair capacity and modulate cancer susceptibility by means of gene-environment interactions. We investigated DNA repair capacity and its association with acute myeloblastic leukemia (AML). We studied polymorphisms in 3 DNA repair genes: XRCC1, XRCC3, and XPD. We also assessed the incidence of a functional polymorphism in theNQO1 gene, which is involved in protection of cells from oxidative damage. We genotyped the polymorphisms by using polymerase chain reaction–restriction fragment-length polymorphism analysis in 134 patients with de novo AML, 34 with therapy-related AML (t-AML), and 178 controls. The distributions of theXRCC3 Thr241Met and NQO1 Pro187Ser genotypes were not significantly different in patients and controls. However, the distribution of the XRCC1 Arg399Gln genotypes was significantly different when comparing the t-AML and control groups (χ², P = .03). The presence of at least oneXRCC1 399Gln allele indicated a protective effect for the allele in controls compared with patients with t-AML (odds ratio 0.44; 95% confidence interval, 0.20-0.93). We found no interactions between the XRCC1 or XRCC3 and NQO1genotypes. We also found no differences in the distribution of the XPD Lys751Gln or XRCC1 Arg194Trp genotypes. Our data provide evidence of a protective effect against AML in individuals with at least one copy of the variant XRCC1 399Gln allele compared with those homozygous for the common allele.

XRCC1 is a key component of DNA base excision repair, single strand break repair, and backup nonhomologous end-joining pathway. XRCC1 (X-ray repair cross-complementing gene 1) deficiency promotes genomic instability, increases cancer risk, and may have clinical application in breast cancer. We investigated XRCC1 expression in early breast cancers (n = 1,297) and validated in an independent cohort of estrogen receptor (ER)-α–negative breast cancers (n = 281). Preclinically, we evaluated XRCC1-deficient and -proficient Chinese hamster and human cancer cells for synthetic lethality application using double-strand break (DSB) repair inhibitors [KU55933 (ataxia telangectasia–mutated; ATM inhibitor) and NU7441 (DNA-PKcs inhibitor)]. In breast cancer, loss of XRCC1 (16%) was associated with high grade (P < 0.0001), loss of hormone receptors (P < 0.0001), triple-negative (P < 0.0001), and basal-like phenotypes (P = 0.001). Loss of XRCC1 was associated with a two-fold increase in risk of death (P < 0.0001) and independently with poor outcome (P < 0.0001). Preclinically, KU55933 [2-(4-Morpholinyl)-6-(1-thianthrenyl)-4H-pyran-4-one] and NU7441 [8-(4-Dibenzothienyl)-2-(4-morpholinyl)-4H-1-benzopyran-4-one] were synthetically lethal in XRCC1-deficient compared with proficient cells as evidenced by hypersensitivity to DSB repair inhibitors, accumulation of DNA DSBs, G₂–M cell-cycle arrest, and induction of apoptosis. This is the first study to show that XRCC1 deficiency in breast cancer results in an aggressive phenotype and that XRCC1 deficiency could also be exploited for a novel synthetic lethality application using DSB repair inhibitors. Cancer Res; 73(5); 1621–34. ©2012 AACR.

Prostaglandin E₂ (PGE₂) can increase thelial vascular endogrowth factor A (VEGF-A) production but the mechanisms involved are unclear. Here we characterized the transcriptional mechanisms involved in human airway smooth muscle cells (HASMC). PGE₂ increased VEGF-A mRNA and protein but not mRNA stability. PGE₂ stimulated the activity of a transiently transfected 2068-bp (–2018 to +50) VEGF-A promoter-driven luciferase construct. Functional 5′ deletional analysis mapped the PGE₂ response element to the 135-bp sequence (–85/ +50) of the human VEGF-A promoter. PGE₂-induced luciferase activity was reduced in cells transfected with a 135-bp VEGF promoter fragment containing mutated Sp-1 binding sites but not in cells transfected with a construct containing mutated EGR-1 binding sites. Electrophoretic mobility shift assay and chromatin immunoprecipitation assay confirmed binding of Sp-1 to the VEGF promoter. PGE₂ increased phosphorylation of Sp-1 and luciferase activity of a transfected Sp-1 reporter construct. PGE receptor agonists EP₂ (ONO-AE1 259) and EP₄ (ONO-AE1 329) mimicked the effect of PGE₂, and reverse transcription-PCR, Western blotting, and flow cytometry confirmed the presence of EP₂ and EP₄ receptors. VEGF protein release and Sp-1 reporter activity were increased by forskolin and isoproterenol, which increase cytosolic cAMP, and the cAMP analogue, 8-bromoadenosine-3′,5′-cyclophosphoric acid. These studies suggest that PGE₂ increases VEGF transcriptionally and involves the Sp-1 binding site via a cAMP-dependent mechanism involving EP₂ and EP₄ receptors.

An apurinic/apyrimidinic (AP) site is an obligatory cytotoxic intermediate in DNA Base Excision Repair (BER) that is processed by human AP endonuclease 1 (APE1). APE1 is essential for BER and an emerging drug target in cancer. We have isolated novel small molecule inhibitors of APE1. In this study, we have investigated the ability of APE1 inhibitors to induce synthetic lethality (SL) in a panel of DNA double‐strand break (DSB) repair deficient and proficient cells; i) Chinese hamster (CH) cells: BRCA2 deficient (V‐C8), ATM deficient (V‐E5), wild type (V79) and BRCA2 revertant [V‐C8(Rev1)]. ii) Human cancer cells: BRCA1 deficient (MDA‐MB‐436), BRCA1 proficient (MCF‐7), BRCA2 deficient (CAPAN‐1 and HeLa SilenciX cells), BRCA2 proficient (PANC1 and control SilenciX cells). We also tested SL in CH ovary cells expressing a dominant‐negative form of APE1 (E8 cells) using ATM inhibitors and DNA‐PKcs inhibitors (DSB inhibitors). APE1 inhibitors are synthetically lethal in BRCA and ATM deficient cells. APE1 inhibition resulted in accumulation of DNA DSBs and G2/M cell cycle arrest. SL was also demonstrated in CH cells expressing a dominant‐negative form of APE1 treated with ATM or DNA‐PKcs inhibitors. We conclude that APE1 is a promising SL target in cancer.