FUNCTIONAL COMPLEMENTATION STUDIES WITH X-RAY-SENSITIVE MUTANTS OF CHINESE-HAMSTER CELLS CLOSELY RESEMBLING ATAXIA-TELANGIECTASIA CELLS

In order to isolate a human gene complementing the defect in A-T-like hamster cell mutants, the mutants were used as recipients for genomic DNA. transfection, using either HeLa chromosomal DNA or DNA from a hum an cosmid library. Three primary transformants with an intermediate X- ray sensitivity and almost normal sensitivity to MMS, but retaining ra dioresistant DNA synthesis (RDS), were obtained. To identify the human chromosome that complements the defect in the A-T-like mutants, and t o assess the degree of complementation for survival and RDS, microcell -mediated chromosome transfer was used. At least 20 independent hybrid clones between the mutant and each one of the human chromosomes 1, 2, 4, 5, 15, 17 or 18 were isolated. All hybrid clones remained X-ray se nsitive, except one with chromosome 4, and another with chromosome 15, both showing an intermediate X-ray sensitivity. By using in situ hybr idization we found that this partial correction was due to the presenc e of a mouse chromosome. In these two hybrids containing the mouse chr omosome together with human chromosome 4 or 15, RDS was fully compleme nted only in the hybrid with chromosome 4 but not in the one containin g chromosome 15, suggesting that RDS and X-ray sensitivity may be comp lemented independently.