BHK cell lines with increased rates of gene amplification are hypersensitive to ultraviolet light

Four cell lines (MP1, -4, -5, -7), isolated from baby hamster kidney cells after simultaneous selection with N-(phosphonacetyl)-L-aspartate and methotrexate, have previously been shown to amplify their DNA at an increased rate. We now show that all four lines are hypersensitive to killing by UV light and mitomycin C. At high doses of UV light or mitomycin C, the MP lines survived less than 10% or less than 5% as well as parental cells, respectively. After UV irradiation, inhibition of DNA and RNA synthesis was greater in MP than in parental cells, and recovery was slower or absent. A 2- to 3.5-fold increase in the frequency of UV-induced sister chromatid exchange was also seen in the four cell lines. In MP5, unscheduled DNA replication after treatment with UV light was only approximately 70% as great as in parental cells and the other MP lines. In MP4 and MP7 cells S phase was elongated. Although their individual properties confirm that the four cell lines are independent, their common properties suggest a relationship between tolerance of DNA damage and gene amplification.