Transfection of SV40-transformed ataxia-telangiectasia fibroblasts with mouse DNA corrects hypersensitivity to neocarzinostatin and activates fibronectin gene expression.

: SV40-transformed ataxia-telangiectasia (SV40-AT) fibroblasts were cotransfected with a plasmid carrying the neomycin-resistance gene as well as DNA from primary mouse embryo fibroblasts. The transfected fibroblasts were seeded under selective conditions and neomycin-resistant (neor) colonies were obtained and tested for the effect of the carcinogen neocarzinostatin (NCS) on DNA synthesis. Whereas the primary A-T and SV40-transformed A-T fibroblasts did not respond to carcinogen treatment and continued to synthesize DNA on damaged templates, normal fibroblasts stopped DNA synthesis after NCS treatment. Among the neomycin-resistant colonies, cells of two colonies responded to NCS treatment by the cessation of DNA synthesis like normal fibroblasts. When DNA from such a colony was transfected into SV40-AT cells, four neor colonies were isolated of which one had regained the normal phenotype. This study provides the first clue that mouse DNA can partly correct the A-T genetic defect expressed in SV40-transformed fibroblasts. Two of the neor colonies with the corrected phenotype expressed a 3.5 kb fibronectin RNA that was detectable by a rat fibronectin DNA probe but not by the human fibronectin DNA probe containing the cell attachment sequence. The latter probe did not detect fibronectin mRNA in the SV40-AT cells but detected expression of the 8.6 kb fibronectin RNA in the two neor colonies of transfected SV40-AT fibroblasts in which the response to NCS was repressed. The results suggest that "correction" of the A-T gene defect in SV40-AT fibroblasts might be associated with regulation of human fibronectin gene(s) expression.