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.

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 ¼ 1,297) and validated in an independent cohort of estrogen receptor (ER)-a-negative breast cancers (n ¼ 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 (DNAPKcs 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 ¼ 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, G 2 -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);

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 the NQO1 gene, which is involved in protection of cells from oxidative damage. We genotyped the polymorphisms by using polymerase chain reactionrestriction fragment-length polymorphism analysis in 134 patients with de novo AML, 34 with therapy-related AML (t-AML), and 178 controls. The distributions of the XRCC3 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 ( 2 , P ‫؍‬ .03). The presence of at least one XRCC1 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 NQO1 genotypes. 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. (Blood. 2002;100: 3761-3766)