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DNA double-strand breaks (DSBs) are one of the most dangerous forms of DNA damage, as a single DSB, if unrepaired, can induce cell cycle arrest and apoptosis. DSBs are caused by environmental sources, includings ionizing radiation (IR), exposure to genotoxic compounds and from endogenous sources, plus replication fork collapse during DNA replication and repair events, such as DNA-interstrand crosslinks repair intermediates. In dividing mammalian cells, DSBs repair is mediated by the error-free-Rad51 related homologous recombinational repair pathway (HRR). The HRR process requires the assembly of multienzymatic complexes visualized immunocytochemically as Rad51 nuclear foci. In vertebrates, five Rad51 paralogs (Rad51B, Rad51C, Rad51D, XRCC2, and XRCC3) are expressed in mitotically growing cells and are thought to play mediating roles in HRR, although their precise functions remain unclear. Rad51 paralog defective cell lines present similar phenotypes: spontaneous chromosomal aberrations, high sensitivity to killing by cross-linking agents and mild sensitivity to IR. Among the five paralogs, Rad51C was found to be a central component present in two complexes, Rad51C-XRCC3 and Rad51B-Rad51C-Rad51D-XRCC2. Rad51C protein exhibits three biochemical activities, including DNA binding, ATPase, and DNA duplex separation. It has been reported recently that XRCC3 protein levels were reduced in Rad51C-depleted HeLa cells. Moreover we report here that Xrcc3 protein levels in XRCC3 overexpressing MCF-7 cells correlates with specific increases in Rad51C protein levels (PL) (Rad51C/PL==1.4XRCC3/PL-73, r=0.90, p=0.04). Taken together these results suggest that XRCC3 and Rad51C are dependent for their stability upon heterodimerization with each other. To test hypothesis we determined by western blot using specific antibodies, Xrcc3, Xrcc2 and Rad51C protein levels in the NCI-60 cell line panel. Our results demonstrates that Xrcc3 protein levels correlates specifically with Rad51C protein levels in the NCI-60 cell line panel(r=0.00018). These results confirm previous results in specific cancer cell lines (Masson JY, et al, Genes Dev.15:3296-307,2001; Wiese C et al Nucleic Acids Res30:1001-8,2002.) and suggest that the Rad51C-XRCC3 association is widely present in human cancer cell lines of diverse origin.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]