Analysis of cells harboring a putative DNA repair gene reveals a lack of evidence for a second independent xeroderma pigmentosum group A correcting gene

The UV hypersensitivity of xeroderma pigmentosum (XP) complementation group A cells is restored to near-normal by transfection of the XPA gene located on human chromosome 9. However, it has been reported that a cosmid related to a cDNA on chromosome 8 is also able to partially correct the UV sensitivity of XP-A cells. We describe here an investigation of a representative cosmid transfectant, denoted 2-0-A₂. Whole cell extracts prepared from a 2-0-A₂ cells carried out DNA repair synthesis in vitro that was in the normal range, consistent with their UV-resistant phenotype. Immunoblotting indicated that 2-0-A₂ cells expressed full-length XPA protein. This was unexpected because the 2-0-A₂ cell line was thought to have been isolated by transfection of a cell line derived from patient XP2OS, and a known homozygous mutation in XP2OS prevents expression of XPA gene product. This mutation creates an AlwNI restriction endonuclease cleavage site in XPA and was not present in 2-0-A₂. These results prompted and RFLP analysis which revealed that the 2-0-A₂ cell line was not derived from XP2OS but from another line that fails to express XPA protein, XP12BE. It appears that the significant UV-resistance and DNA repair capacity of 2-0-A₂ can be ascribed to the re-expression of XPA in XP12BE, and it is unnecessary to postulate a second XP-A complementing gene to explain the results.