Cockayne Syndrome (CS) is an autosomal recessive disorder, characterized by growth failure, neurological abnormalities, premature aging symptoms, and cutaneous photosensitivity, but no increased cancer incidence. CS is divided into two complementation groups: CSA (mutation in CKN1) and CSB (mutation in ERCC6). Of the patients suffering from CS, 80% have mutations in the CSB gene. Emerging evidence indicates a role for CSB in facilitating the BER response. For instance, we recently identified a novel physical and functional interaction between CSB and the major human abasic endonuclease, APE1, which operates centrally in the BER pathway. However, the precise molecular contributions of CSB to this repair process remain poorly defined. Furthermore, although CSB harbors the seven conserved ATPase motifs found in the SWI2/SNF2 helicase-like superfamily of chromatin remodeling proteins, its biochemical properties remain largely uncharacterized. We are determining the molecular functions of CSB in BER and how these newly-defined activities potentially contribute to DNA damage responses in disease manifestation.? ? XRCC1 is a non-enzymatic scaffold protein that operates to coordinate single-strand break repair (SSBR), which is a sub-pathway of BER critical for repairing oxidative DNA strand breaks. Functional interactions with Aprataxin (APTX) and tyrosyl DNA phosphodiesterase 1 (TDP1) proteins that are defective in hereditary spinocerebellar ataxias and that process complex SSB ends imply a connection between XRCC1 (and SSBR in general) and neurodegenerative disease. Recently, we have found that shRNA lentiviral-mediated XRCC1 knockdown in human SH-SY5Y neuroblastoma cells results in a largely selective increase in sensitivity of the nondividing (i.e. terminally differentiated) cell population to the redox-cycling agents, menadione and paraquat; this reduced survival was accompanied by an accumulation of DNA strand breaks. Moreover, knockdown of XRCC1 in primary human fetal brain neurons lead to enhanced sensitivity to menadione, as indicated by increased levels of DNA strand breaks relative to control cells. The cumulative results implicate XRCC1, and more broadly SSBR, in the protection of non-dividing neuronal cells from the genotoxic consequences of oxidative stress. We are currently assessing the role of this protein in age-related pathologies using heterozygous mice, and will concomitantly evaluate the effect of XRCC1 haploinsufficiency on neurodegeneration and cancer proneness following defined insults.
