THE DEFECT IN THE AT-LIKE HAMSTER-CELL MUTANTS IS COMPLEMENTED BY MOUSE CHROMOSOME-9 BUT NOT BY ANY OF THE HUMAN-CHROMOSOMES

X-ray-sensitive Chinese hamster V79 cells mutants, V-C4, V-E5 and V-G8 , show an abnormal response to X-ray-induced DNA damage. Like ataxia t elangiectasia (AT) cells, they display increased cell killing, chromos omal instability and a diminished inhibition of DNA synthesis followin g ionizing radiation. To localize the defective hamster gene (XRCC8) o n the human genome, human chromosomes were introduced into the AT-like hamster mutants, by microcell mediated chromosome transfer. Although, none of the human chromosomes corrected the defect in these mutants, the defect was corrected by a single mouse chromosome, derived from th e A9 microcell donor cell line. In four independent X-ray-resistant mi crocell hybrid clones of V-E5, the presence of the mouse chromosome wa s determined by fluorescent in situ hybridization, using a mouse cot-l probe. By PCR analysis with primers specific for different mouse chro mosomes and Southern blot analysis with the mouse Ldlr probe, the mous e chromosome 9, was identified in all four X-ray-resistant hybrid clon es. Segregation of the mouse chromosome 9 from these hamster-mouse mic rocell hybrids led to the loss of the regained X-ray-resistance, confi rming that mouse chromosome 9 is responsible for complementation of th e defect in V-E5 cells. The assignment of the mouse homolog of the ATM gene to mouse chromosome 9, and the presence of this mouse chromosome only in the radioresistant hamster cell hybrids suggest that the hams ter AT-like mutants are homologous to AT, although they are not comple mented by human chromosome 11.