The molecular basis for the DNA repair dysfunction observed in mutant Chinese hamster ovary cell lines of X-ray repair cross complementing group 1 (XRCC1) is unknown and the exact role of the XRCC1 protein remains unclear. To help clarify the role of the XRCC1 gene we analyzed four mutant cell lines of this complementation group and a revertant cell line for XRCC1 protein content and for sequence alterations in the XRCC1 coding region. Immunoblot analysis of cellular extracts indicated that each of four mutant lines was lacking XRCC1 protein, whereas the repair-proficient revertant line derived from one of these mutants contained a normal level of XRCC1. Although each of these cell lines expressed XRCC1 mRNA, we found in all cases a distinct point mutation resulting in crucial alterations in the encoded XRCC1 protein sequence of 633 amino acids. Two of the mutations cause non-conservative amino acid changes, Glu102→Lys and Cys390→Tyr, at positions that are invariant among hamster, mouse and human XRCC1 sequences and are located in putative functional domains. A third debilitating mutation disrupts RNA splicing, generating multiple transcripts of different length that contain deletions spanning a region of > 100 amino acids in the midsection of the XRCC1 coding sequence. A fourth mutation results in a termination codon that shortens the open reading frame to 220 amino acids, however, in the revertant cell line a further mutation in the same codon, Stop221→Leu, permits translation of a full-length functional variant protein. These mutational data indicate the importance of the putative functional regions in XRCC1, such as the BRCA1 C-terminal (BRCT) domain found in common with BRCA1 and other DNA repair and cell cycle checkpoint proteins, and also regions necessary for interaction with DNA polymerase β and DNA ligase III.
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