Gene amplification in fibroblasts from ataxia telangiectasia (AT) patientsand in X-ray hypersensitive AT-like Chinese hamster mutants

In search of functions involved in the regulation of gene amplification, an d given the relevance of chromosome breakage in initiating the process, we analyzed the gene amplification ability of cells hypersensitive to inducers of DNA double-strand breaks and defective in cell cycle control: two human fibroblast strains derived from patients affected by ataxia telangiectasia (AT) and two hamster mutant cell lines belonging to complementation group XRCC8 of the rodent X-ray-sensitive mutants. These mutants are considered h amster models of AT cells. To measure gene amplification, the frequency and the rate of occurrence of N-(phosphonacetyl)-L-aspartate resistant cells w ere determined. In both hamster mutants, these two parameters were increase d by about an order of magnitude compared with parental cells, suggesting t hat amplification ability was increased by the genetic defect. In primary A T fibroblasts, as in normal human fibroblasts, gene amplification was undet ectable and a block in the G(1) phase of the cell cycle was induced upon PA LA treatment. These results suggest that in AT fibroblasts, where only the ATM gene is mutated, ATM-independent mechanisms prevent gene amplification, while, in the immortalized hamster cell lines, which are already permissiv e for gene amplification, the AT-like defect increases the probability of g ene amplification.