ENHANCED CHROMOSOMAL RESPONSE OF ATAXIA-TELANGIECTASIA CELLS TO SPECIFIC TYPES OF DNA DOUBLE-STRAND BREAKS

The chromosomal response of two ataxia-telangiectasia (A-T) lymphoblas toid cell lines (A-T-PA and A-T-KM) to restriction endonucleases (RE) is compared with that of a normal (N-SW) lymphoblastoid cell fine. The RE used were PvuII (generating DNA double-strand breaks with blunt te rmini), BamHI (cohesive termini with 4 base, 5' over-hangs) and PstI ( cohesive termini with 4 base 3' overhangs). Chromatid aberrations were analysed in cells 5 h after treatment. Cells were porated using strep tolysin O to allow entry of RE. Both A-T lines showed an enhanced freq uency of chromatid breaks in G(2) phase compared with normal cells in response to RE. The enhanced response of A-T cells was most marked in the case of PvuII treatment when the enhancement ratios were 2.5 and 4 .2 for A-T-PA and A-T-KM respectively. However, the frequency of DNA d ouble-strand breaks (dsb), measured by neutral filter elution, were co nsiderably lower in A-T-PA cells than N-SW, due to a lower efficiency of poration. When A-T-PA cells were treated with streptolysin O at a h igher concentration (0.3 Units/ml), a condition that apparently led to a similar level of poration in A-T-PA as in N-SW cells treated with 0 .06 Units/ml as judged by the similar number of dsb induced in the two lines for a given PvuII concentration, the enhancement ratio for A-T- PA cells treated with PvuII increased from 2.5 to 5.8. BamHI and PstI were found to be less clastogenic in all three cell lines as found pre viously for Chinese hamster cells, although part of this effect may be due to a lower activity, particularly in the case of PstI. However, e ven at a 4-6-fold higher concentration, BamHI was still less clastogen ic than PvuII. It is concluded that dsb with blunt termini are more cl astogenic than those with cohesive termini. The results suggest that t he chromosomal sensitivity of A-T cells may result from a defect causi ng a higher rate of conversion of dsb into chromatid aberrations.