Maintenance of genomic stability is paramount for survival of an organism; failure to repair DNA damage ultimately leads to the accumulation of genetically unstable cells and the onset of different human diseases including cancer. DNA single strand breaks and base oxidation/alkylation are among the most frequent types of DNA damage occurring spontaneously in cells. Base excision repair (BER), which copes with the majority of these lesions, is therefore a fundamental DNA repair system. Accordingly, it is important to understand how BER is regulated, and particularly, how and if BER is affected by the cellular load of DNA damage. Although functions of key BER proteins are well-defined, regulation of their expression is poorly understood. During BER, the protein XRCC1 is particularly important. It functions as a scaffold, stabilising repair complexes at sites of DNA damage thereby promoting efficient DNA repair. As a central coordinator in BER, it is therefore of great interest to understand how expression of XRCC1 is controlled. In this thesis I demonstrate that modulation of XRCC1 expression is mediated by transcription factor Sp1. Importantly, Sp1 is also affected during the DNA damage response, suggesting an indirect mechanism promoting BER modulation in response to the cellular DNA damage load. In fact, I show that, in response to persistent DNA strand breaks, the key DNA damage signalling factor ATM phosphorylates Sp1. This initiates Sp1 degradation, negatively affecting BER. Therefore, this thesis identifies a mechanism involving signalling from ATM that regulates BER in response to persistent DNA damage, which I link to susceptibility to apoptosis and cell elimination. I hypothesise that regulation of DNA repair in response to persistent DNA damage constitutes a mechanism to promote the elimination of potentially pre-cancerous cells that accumulate unrepairable levels of DNA lesions.

X-ray repair cross complementing Group 1 (XRCC1) protein plays an important role in base excision repair. Single nucleotide polymorphisms (SNPs) in XRCC1 gene may affect DNA repairing ability, genetic susceptibility, and prognosis to oral and pharyngeal squamous cell cancer (OPSCC). This study was carried out to evaluate the association of three XRCC1 SNPs with the risk and prognosis of OPSCC. A total of 509 OPSCC cases and 678 cancer-free controls were recruited to detect the genotypes of XRCC1 by PCR-RFLP. Then, 447 case patients with surgical treatment and safety margins were included in the survival analysis. No association was observed for XRCC1 194 and the risk of OPSCC. As compared with the wild Arg/Arg genotype, the combined genotypes of 280 Arg/His and His/His were with decreased risk (AOR=0.73, 95% CI, 0.52-1.03, p = 0.069) of OPSCC and with a significantly decreased risk (AOR=0.67, 95% CI, 0.47-0.97, p = 0.035) of oral cavity. As compared with the Arg/Arg genotype of XRCC1 399, the Gln/Gln genotype was with the increased risk of OPSCC (AOR=2.06, 95%CI: 1.21-3.51, p = 0.008) and oral cavity cancer (AOR=1.89, 95%CI: 1.08-3.33, p = 0.026). We defined the ¡§putative high risk haplotypes¡¨ as ¡§Arg-Arg-Gln and Trp-Arg-Gln¡¨. The AOR were 1.29 (95% CI, 1.04-1.60, p = 0.020) for the ¡§putative high risk haplotypes¡¨ as compared with other haplotypes for OPSCC. Then, two putative high risk haplotypes were combined into ¡§putative high risk diplotypes¡¨. The AOR for the ¡§high risk diplotypes¡¨ were 1.98 (95% CI, 1.18-3.33, p = 0.010) as compared with other diplotypes for OPSCC. No association between XRCC1 polymorphisms and clinicopathological outcomes, except XRCC1 280. Those carriers of XRCC1 280His allele (combined Arg/His and His/His genotypes) were associated with late onset (≥50 yrs) of oral cavity cancers. No association between genetic variants in XRCC1 and disease-specific survival except XRCC1 399. Patients with 399 Arg/Gln and Gln/Gln genotypes showed a significant better survival as compared to Arg/Arg genotype carriers (AHR 0.41 95% CI: 0.18-0.93), especially for those patients younger than 50 years (p = 0.012), in pathological stage III or IV (p = 0.044), and without postoperative radiotherapy (p = 0.012). In summary, XRCC1 280 Arg/His and His/His genotypes were associated with decreased risk of oral cavity cancer. 399 Gln/Gln genotype was associated with increased risk of OPSCC and oral cavity cancer. The putative ¡§high risk haplotypes and diplotypes¡¨ were with increased risk of OPSCC. However, 399 Arg/Gln and Gln/Gln genotypes were prognostic factors, especially for those with young age, aggressive tumor stage, and without postoperative radiotherapy for oro and hypopharynx patients. These findings suggest that XRCC1 polymorphisms may play a role in the development and prognosis of OPSCC.

Effector caspase activation is a critical regulatory step in apoptosis, as well as an essential inductive cut in numerous non-death related processes that occur within complex cell systems. Here we report two novel studies detailing mechanisms by which effector caspase activation advances muscle cell differentiation. In the first study, we demonstrate that caspase 3 triggered DNA damage leads to rapid formation of XRCC1 repair foci within differentiating myonuclei, which dissipates as the maturation program proceeds. Skeletal myoblast deletion of XRCC1 does not impact cell growth, yet leads to perinatal lethality, with sustained DNA damage and impaired myofiber development. These observations demonstrate that the temporal deployment of the XRCC1-related DNA repair mechanisms are effector caspase mediated, and essential for muscle cell differentiation. In the second study, we sought to investigate whether effector caspase enzymes altered chromatin structure to promote the early differentiation of muscle progenitor cells. Past research has shown that Matrix Attachment region proteins known as Special AT-rich binding proteins are expressed abundantly in stem and progenitor cells, showing rapid decrease in expression as the cell advances into its mature phenotype. Here we demonstrate that effector caspase-7 is responsible for cleavage of Satb2, rather than caspase 3. Satb2 degradation alters the expressed genetic profile leading to acceleration of the muscle differentiation program. Our cumulative work adds novel roles in which effector caspases are vital in the development of cells.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / A radiaÃÃo ionizante à considerada um fator de risco para o desenvolvimento de diversos tipos de neoplasias pelos danos causados à molÃcula de DNA, sendo de extrema importÃncia, portanto, o monitoramento de populaÃÃes humanas expostas a ela. O municÃpio de Monte Alegre no estado do Parà apresenta uma das maiores Ãreas de mineraÃÃo do urÃnio do mundo, que se estende aos municÃpios vizinhos de Prainha e Alenquer. Este trabalho teve como objetivos a avaliaÃÃo do potencial genotÃxico do urÃnio presente em rochas nas residÃncias de indivÃduos dos municÃpios de Monte Alegre, Prainha e Alenquer atravÃs do ensaio do cometa alcalino em linfÃcitos perifÃricos e a determinaÃÃo das freqÃÃncias de polimorfismos nos genes de reparo do DNA XRCC1 e XRCC3 e no gene de metabolizaÃÃo GSTM1 atravÃs de seqÃenciamento direto de DNA. Na anÃlise do cometa alcalino, nÃo houve diferenÃa estatisticamente significante entre os Ãndices de Dano (IDs) das populaÃÃes de Monte Alegre (ID = 32,01  1,57), Prainha (ID = 45,80  1,12) e Alenquer (ID = 44,30  0,62) e o do controle negativo (ID = 42,00  5,75) (p > 0,05). AtravÃs do seqÃenciamento direto de DNA, observou-se a presenÃa de polimorfismos nos genes XRCC1 e XRCC3 em regiÃes de Ãntrons e de Ãxons. No gene XRCC1, as freqÃÃncias alÃlicas variantes para o polimorfismo Arg194Trp nas populaÃÃes de Monte Alegre, Prainha e Alenquer foram 12%, 13% e 7% e, para Arg399Gln, 28%, 30% e 32%, respectivamente. No gene XRCC3, as freqÃÃncias do alelo variante do polimorfismo Thr241Met encontradas nas populaÃÃes de Monte Alegre, Prainha e Alenquer foram, respectivamente, 28%, 13% e 33%. Em relaÃÃo ao gene GSTM1, as freqÃÃncias obtidas de ausÃncia do gene apresentaram os valores de 36%, 31% e 40%, respectivamente Ãs populaÃÃes de Monte Alegre, Prainha e Alenquer. No geral, todas as freqÃÃncias alÃlicas nos Ãxons dos genes XRCC1 e XRCC3 e de ausÃncia do gene GSTM1 foram estatisticamente semelhantes entre os trÃs municÃpios e se mostraram em concordÃncia com freqÃÃncias obtidas em outros estudos com populaÃÃes brasileiras. Conclui-se que, dentre as populaÃÃes estudadas, nÃo houve aumento da incidÃncia de dano ao DNA pela exposiÃÃo ao urÃnio, o que pode se explicar pela baixa radiaÃÃo nessas localidades. AlÃm disso, as freqÃÃncias alÃlicas dos polimorfismos encontrados nos genes de reparo XRCC1 e XRCC3, assim como as de ausÃncia do gene de metabolizaÃÃo GSTM1 nÃo diferiram das encontradas em populaÃÃes de outras regiÃes do Brasil. Desse modo, nas regiÃes estudadas, possivelmente nÃo existe tendÃncia ao desenvolvimento de cÃncer induzido pela exposiÃÃo ao urÃnio. / Radiation is considered a risk factor for the development of several types of cancers caused by damage into the DNA molecule and is of extreme importance, therefore, the monitoring of human populations exposed to it. The municipality of Monte Alegre in the state of Parà in Brazil has one of the largest uranium mining areas of the world, which extends to the neighboring municipalities of Prainha and Alenquer. This work assessed the genotoxic potential of exposure to uranium in rocks found in dwellings in individuals from the municipalities of Monte Alegre, Prainha and Alenquer through the alkaline comet assay in peripheral blood lymphocytes and the determination of the frequencies of polymorphisms in DNA repair genes XRCC1 and XRCC3 and in carcinogen-metabolism gene GSTM1 through direct DNA sequencing. The analysis of the alkaline comet assay indicated that there was no statistically significant difference between the Damage Indexes (DIs) of Monte Alegre (DI = 32.01  1.57), Prainha (DI = 45.80  1.12) and Alenquer (DI = 44.30  0.62) and of the negative control (DI = 42.00  5.75) (p > 0.05). Through direct DNA sequencing, there were polymorphisms in XRCC1 and XRCC3 genes in regions of introns and exons. In XRCC1 gene, the variant allele frequencies for Arg194Trp polymorphism in Monte Alegre, Prainha and Alenquer populations were 12%, 13% and 7%, and for Arg399Gln, 28%, 30% and 32% respectively. In XRCC3 gene, the frequencies of the variant allele of Thr241Met polymorphism found in Monte Alegre, Prainha and Alenquer populations were, respectively, 28%, 13% and 33%. For GSTM1 gene, the frequencies obtained for the absence of this gene were 36%, 31% and 40%, respectively to Monte Alegre, Prainha and Alenquer populations. In general, the absence frequencies of GSTM1 gene and of allelic frequencies in XRCC1 and XRCC3 exons were statistically similar among the three municipalities and were in agreement with frequencies obtained in other studies with Brazilian populations. As a conclusion, among the populations studied, there was no increased incidence of DNA damage by exposure to uranium, which can be explained by the low radiation in these locations and that the allelic frequencies of polymorphisms found in the DNA repair genes XRCC1 and XRCC3, and the absence frequencies of GSTM1 gene did not differ from those in populations from other regions of Brazil. Thus, in the studied area, possibly there is no tendency to the development of cancer induced by exposure to uranium.

Betel quid¡]BQ¡^chewing is recognized as a major risk factor for oral precancerous lesions¡]OPLs¡^in Taiwan. The compositions of Betel quid could cause DNA damage. X-ray repair cross complementing Group 1¡]XRCC1¡^plays a crucial role in the process of DNA repair. Polymorphisms in XRCC1 gene may affect DNA repairing ability and modulate the susceptibility of OPLs. The aim of this study was to investigate the association of XRCC1 genetic variants with the risk of BQ-related oral precancerous lesions, including oral leukoplakia¡]OL¡^ and oral submucous fibrosis ( OSF ). A total of 449 males¡]169 OL cases, 82 OSF cases, and 208 healthy controls¡^who habitually chewed BQ were recruited. The genotypes were determined by PCR-RFLP and TaqMan real-time assays. The C allele and T/C+C/C genotypes at XRCC1 -77 were associated with the reduced risk of OL ( AOR=0.54, 95%CI:0.34-0.85 and AOR=0.47, 95%CI:0.28-0.78, respectively ). The 399Gln allele and 399 Arg/Gln+Gln/Gln genotypes were associated with the increased risk of OL¡]AOR=1.94; 95%CI: 1.41-2.67 and AOR=2.64; 95%CI: 1.73-4.03, respectively¡^and OSF ( AOR=1.67; 95%CI: 1.11-2.49 and AOR=2.30; 95%CI: 1.35-3.91, respectively ). The haplotypes or diplotypes contain fewer risk alleles¡]-77T or 399Gln¡^ were with lower risk of OL¡]both Ptrend<0.001¡^and OSF (Ptrend=0.056 and Ptrend=0.040, respectively). In conclusion, our results suggest that polymorphisms of XRCC1 at -77 and 399 may be associated with the risk of OPLs.

Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro / O câncer de mama é o tipo de câncer mais comum em mulheres brasileiras e o principal responsável pelos óbitos neste grupo. Segundo dados do Instituto Nacional do Câncer, em 2008, na região Sudeste do Brasil, o câncer de mama foi o mais incidente, com um risco estimado de 68,12 casos novos por 100 mil. No Estado do Rio de Janeiro a estimativa foi de 7.680 casos, sendo 4.160 na capital. O presente estudo teve como objetivo determinar, através da metodologia de PCRRFLP, as freqüências alélicas e genotípicas dos polimorfismos PIN3 Ins 16pb e Arg72Pro, do gene TP53, e Arg194Trp e Arg399Gln, do gene XRCC1, em 105 casos de câncer de mama e analisar a relação entre esses polimorfismos e os dados sócio-demográficos das pacientes, a história familiar de câncer de mama e as características clínico-patológicas do tumor. Por esta razão, neste estudo realizamos um estudo do tipo caso-caso, utilizando-se pacientes diagnosticadas com carcinoma ductal infiltrante. Em relação ao papel desses polimorfismos genéticos no prognóstico da doença, foi observada uma associação da variante Arg194Trp de XRCC1 com a agressividade do tumor através de uma correlação positiva com o grau de Elston III (OR = 3,99; IC 95% 1,08 14,66; p = 0,044). Análises subseqüentes contando com um número maior de indivíduos podem auxiliar na confirmação deste achado e na elucidação do papel da variante Trp194 no prognóstico do câncer de mama. / Breast cancer is the most common cancer in Brazilian women and the main responsible for their cancer death. According to data of the National Institute of Cancer, in 2008, in the Southeast region of Brazil, breast cancer had the highest incidence with an estimated risk of 68.12 new cases in one hundred thousand. In the State of Rio de Janeiro, the estimative was of 7.680 cases, 4.160 of them in the capital. The present study aimed to determine, through PCR-RFLP methodology, the alleles and genotypes frequencies of the polymorphisms PIN3 Ins 16pb and Arg72Pro, in the TP53 gene; Arg194Trp and Arg399Gln, in the XRCC1 gene, in 105 cases of breast cancer, and analyze the relationship between these polymorphisms and patients socio-demographic data, family history of breast cancer and tumor clinicopathological characteristics. Hence, in this study we performed a case-case study, using patients diagnosed with infiltrating ductal carcinoma. In respect with the role of these genetic polymorphisms in breast cancer prognosis, it was observed an association with the variant Arg194Trp of XRCC1 and tumor aggressiveness by a positive correlation with Elston grade III (OR = 3.99; CI 95% 1.08 14.66; p = 0.044). Further analyses including a larger group of study can help to confirm this finding and elucidate the role of Trp194 variant in breast cancer prognosis.

Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro / O câncer de mama é o tipo de câncer mais comum em mulheres brasileiras e o principal responsável pelos óbitos neste grupo. Segundo dados do Instituto Nacional do Câncer, em 2008, na região Sudeste do Brasil, o câncer de mama foi o mais incidente, com um risco estimado de 68,12 casos novos por 100 mil. No Estado do Rio de Janeiro a estimativa foi de 7.680 casos, sendo 4.160 na capital. O presente estudo teve como objetivo determinar, através da metodologia de PCRRFLP, as freqüências alélicas e genotípicas dos polimorfismos PIN3 Ins 16pb e Arg72Pro, do gene TP53, e Arg194Trp e Arg399Gln, do gene XRCC1, em 105 casos de câncer de mama e analisar a relação entre esses polimorfismos e os dados sócio-demográficos das pacientes, a história familiar de câncer de mama e as características clínico-patológicas do tumor. Por esta razão, neste estudo realizamos um estudo do tipo caso-caso, utilizando-se pacientes diagnosticadas com carcinoma ductal infiltrante. Em relação ao papel desses polimorfismos genéticos no prognóstico da doença, foi observada uma associação da variante Arg194Trp de XRCC1 com a agressividade do tumor através de uma correlação positiva com o grau de Elston III (OR = 3,99; IC 95% 1,08 14,66; p = 0,044). Análises subseqüentes contando com um número maior de indivíduos podem auxiliar na confirmação deste achado e na elucidação do papel da variante Trp194 no prognóstico do câncer de mama. / Breast cancer is the most common cancer in Brazilian women and the main responsible for their cancer death. According to data of the National Institute of Cancer, in 2008, in the Southeast region of Brazil, breast cancer had the highest incidence with an estimated risk of 68.12 new cases in one hundred thousand. In the State of Rio de Janeiro, the estimative was of 7.680 cases, 4.160 of them in the capital. The present study aimed to determine, through PCR-RFLP methodology, the alleles and genotypes frequencies of the polymorphisms PIN3 Ins 16pb and Arg72Pro, in the TP53 gene; Arg194Trp and Arg399Gln, in the XRCC1 gene, in 105 cases of breast cancer, and analyze the relationship between these polymorphisms and patients socio-demographic data, family history of breast cancer and tumor clinicopathological characteristics. Hence, in this study we performed a case-case study, using patients diagnosed with infiltrating ductal carcinoma. In respect with the role of these genetic polymorphisms in breast cancer prognosis, it was observed an association with the variant Arg194Trp of XRCC1 and tumor aggressiveness by a positive correlation with Elston grade III (OR = 3.99; CI 95% 1.08 14.66; p = 0.044). Further analyses including a larger group of study can help to confirm this finding and elucidate the role of Trp194 variant in breast cancer prognosis.

Les modifications post-traductionnelles des protéines par des polymères d’ADP-ribose (PAR) ou par phosphorylation permet l’assemblage des complexes de la réparation de l’ADN à la chromatine endommagée dont les fonctions sont essentielles pour assurer le maintien de la stabilité du génome. En réponse aux lésions de l’ADN, l’activité de synthèse de PAR des protéines PARP1 et PARP2 est fortement stimulée. Les PAR servent de signalisation pour le recrutement de multiples protéines, dont la protéine plateforme XRCC1.Les études menées au cours de cette thèse ont porté sur l’étude de la régulation des fonctions des protéines PARP1, PARP2 dans la réparation des cassures double brins (CDB) et l’étude des modifications de XRCC1 par phosphorylation en réponse à des dommages de l’ADN. En utilisant des substrats permettant de mesurer l’efficacité des différentes voies de réparation des CDB, nous avons démontré que PARP2, et non PARP1, est impliqué dans la régulation du choix des voies de la réparation des CDB. Plus spécifiquement, nous avons montré que PARP2 stimule l’initiation de la résection des extrémités des CDB dépendante de CtIP, indépendamment de son activité catalytique. Par des approches de vidéo-microscopie, nous avons pu déterminer que PARP2 limite l’accumulation de 53BP1 aux sites de dommages induits par micro-irradiation laser. Nous proposons que la protéine PARP2, en limitant le recrutement de la protéine 53BP1 aux sites de dommages, favorise la réparation des CDB dépendante de la résection des extrémités d’ADN, au détriment de la voie canonique de jonction des extrémités. Ces résultats sont les premiers démontrant un rôle de PARP2 dans le choix des voies de réparation des CDB.En parallèle, nous avons analysé comment la phosphorylation régule les fonctions de la protéine XRCC1. Par des approches in vitro et in vivo, nous avons pu déterminer que l’interdomaine 1 de XRCC1 est phosphorylé par la kinase CDK5. En réponse aux dommages induits par un agent alkylant, XRCC1 est activement déphosphorylé in vivo. De plus, nous avons observé que lorsque l’interdomaine 1 ne peut pas être phosphorylé in vitro, l’interaction de XRCC1 avec les PAR synthétisés par PARP1 et PARP2 augmente, et le recrutement de XRCC1 aux sites de dommages de l’ADN est accru. Ces résultats indiquent pour la première fois que la déphosphorylation de XRCC1 en réponse à un stress génotoxique participe activement à son recrutement aux sites de dommages.Dans leur ensemble, ces travaux ont contribué à améliorer nos connaissances fondamentales des réseaux de protéines impliquées dans la prise en charge des dommages de l’ADN. La compréhension de ces mécanismes est essentielle non seulement car ils participent au maintien de la stabilité du génome mais aussi du fait du développement exponentiel de nouvelles stratégies anti-tumorales qui visent à inhiber les voies de la réparation dans la but de cibler spécifiquement les cellules cancéreuses. / Post-translational modifications of proteins by polymers of ADP-ribose (PAR) or by phosphorylation allow the assembly of DNA repair protein complexes at damaged chromatin and are crucial to ensure genome stability. In response to DNA insults, the synthesis of PAR by the PARP1 and PARP2 proteins is strongly induced. PAR act as a signaling platform for the recruitment of multiples proteins at the sites of DNA damages, including the scaffold protein XRCC1. Research conducted during this PhD have been focused on studying the regulation of PARP1 and PARP2 functions in double-strands break repair (DSBR), and in investigating the role of XRCC1 modifications by phosphorylation in response to DNA damage.Using DNA repair assay allowing us to assess the accuracy of the different DSBR pathways, we demonstrated that PARP2, and not PARP1, is involved in the regulation of DNA double-strands break repair pathway choice. More precisely, we showed that PARP2 stimulates CtIP dependent initiation of end-resection at DSB, independently of its catalytic activity. By live cell imaging, we were able to determine that PARP2 limit 53BP1 accumulation at DNA damage sites induced by laser-microirradiation. We propose that by limiting 53BP1 accumulation at DNA damage sites, PARP2 stimulate DSB repair pathway that depend on DNA end-resection, thus counteracting the canonical end-joining pathway. These results are the first demonstrating a role for PARP2 in DNA DBSR pathway choice.In addition, we analyzed how the functions of XRCC1 are regulated by phosphorylation. Using in vitro and in vivo approaches, we were able to demonstrate that the linker 1 region of XRCC1 is phosphorylated by the CDK5 kinase. XRCC1 is actively dephosphorylated in response to DNA damage induced by an alkylating agent in vivo. We also observed that when the linker 1 cannot be phosphorylated, the XRCC1 interaction between the PAR synthetized by PARP1 and PARP2 is stimulated, and XRCC1 recruitement at the sites of DNA damage is far more efficient. These evidences indicate for the first time that the dephosphorylation of XRCC1 actively participate in its recruitment at the site of DNA damage. Put together, this work contributed to strengthen our fundamental knowledge of the protein network involved in the DNA damage response. Knowledge of those mechanisms is crucial since they participate in maintaining genome stability, and because new antitumoral drugs targeting DNA repair pathways in the attempt to specifically killed tumor cells are exponentially released.

Caspase-3 has demonstrated a non-apoptotic function in several developmental programs including skeletal muscle differentiation, yet the mechanism of action has not been fully elucidated. Under apoptotic conditions Caspase-3 induces DNA fragmentation through activation of CAD. Recent observations have demonstrated CAD activity and the resulting DNA strand breaks are also vital for skeletal muscle differentiation. These breaks are transient in nature, suggesting an active DNA repair program to maintain genomic integrity. The aim of this study was to delineate the DNA repair mechanism coordinated with caspase/CAD mediated DNA damage. It was found that XRCC1 formed punctate nuclear foci early in myoblast differentiation concurrent to the induction of DNA damage. Caspase-3 inhibition caused attenuation of the formation of DNA lesions and XRCC1 foci in differentiating myoblasts. Targeted reduction in XRCC1 expression impaired myoblast differentiation. These results suggest that XRCC1 may play a role in repairing the DNA damage associated with myoblast differentiation.

Caspase-3 has demonstrated a non-apoptotic function in several developmental programs including skeletal muscle differentiation, yet the mechanism of action has not been fully elucidated. Under apoptotic conditions Caspase-3 induces DNA fragmentation through activation of CAD. Recent observations have demonstrated CAD activity and the resulting DNA strand breaks are also vital for skeletal muscle differentiation. These breaks are transient in nature, suggesting an active DNA repair program to maintain genomic integrity. The aim of this study was to delineate the DNA repair mechanism coordinated with caspase/CAD mediated DNA damage. It was found that XRCC1 formed punctate nuclear foci early in myoblast differentiation concurrent to the induction of DNA damage. Caspase-3 inhibition caused attenuation of the formation of DNA lesions and XRCC1 foci in differentiating myoblasts. Targeted reduction in XRCC1 expression impaired myoblast differentiation. These results suggest that XRCC1 may play a role in repairing the DNA damage associated with myoblast differentiation.