GENE MUTATION AND V(D)J RECOMBINATION IN THE RADIOSENSITIVE IRS LINES

The lines irs1, irs2 and irs3, derived from V79-4 hamster cells, are s ensitive to DNA-damaging agents including ionizing radiations. However , unlike some other radiosensitive lines, the irs lines show no appare nt defect in the repair of DNA strand breaks. We have now assessed the mis-repair of DNA damage in the irs lines by measuring spontaneous an d X-ray induced frequencies of mutation in the HPRT gene. irs1 was fou nd to be hypermutable, showing instability in spontaneous mutant frequ ency and an elevation of the radiation-induced frequency relative to t he parental line. In contrast, irs2 and irs3 showed similar mutational responses to the parental line. The results support other lines of ev idence suggesting that irs1 has a mis-repair phenotype. The irs2 line has previously been shown to have a phenotype similar to cells from th e human disorder ataxia-telangiectasia and this similarity is maintain ed in their mutational response to X-rays. The irs lines were also tes ted for ability to undergo V(D)J recombination, since this process has recently been found to be defective in some radiosensitive lines with impaired double-strand break repair. Using an extrachromosomal vector containing a V(D)J rearrangement cassette, correct recombination was shown to occur at similar frequencies to parental V79-4 cells in each of the three irs lines. Thus the irs lines indicate that processes oth er than DNA double-strand break repair also control radiosensitivity, in particular those processes which may affect the regulation of DNA r epair.