ENHANCEMENT OF FREQUENCIES OF RESTRICTION ENDONUCLEASE-INDUCED CHROMATID BREAKS BY ARABINOSIDE ADENINE IN NORMAL HUMAN AND ATAXIA-TELANGIECTASIA CELLS

The effect of ara A (9-beta-D-arabinofuranosyladenine) a potent inhibi tor of DNA synthesis on the frequencies of chromatid breaks induced by restriction endonucleases (RE) has been investigated in normal human and ataxia telangiectasia (AT) lymphoblastoid cells. PvuII, PstI and B amHI, which cause blunt-ended, 3'-overhang and 5'-overhang cohesive-en ded DNA double-strand breaks (dsb) respectively, were introduced into two AT cell lines (AT-KM and AT-PA) and a normal human (N-SW) cell lin e by the use of streptolysin-O poration. Controls were exposed to gamm a-irradiation and similarly treated with or without ara A. Both AT cel l lines were found to exhibit higher frequencies of chromatid breaks w hen treated with RE alone as compared with the normal cell line. The p attern of chromatid response to the three RE was shown to be similar i n all three cell lines i.e. PvuII was most clastogenic while PstI and BamHI were both less effective at inducing chromosomal aberrations. In cubation of cells with ara A resulted in an increase in frequencies of chromatid breaks in PvuII and PstI treated cells but no increase was observed in BamHI treated cells. Normal cells showed most response to ara A following treatment with PvuII and PstI (enhancement ratios 4.63 and 3.75 respectively) while AT cells were affected by ara A to a les ser extent indicating a reduced expression of damage by ara A in these lines. Since ara A is a potent inhibitor of DNA synthesis, it was con cluded from the elevated frequency of chromosomal aberrations in the p resence of ara A that rejoining of RE-induced dsb in genomic DNA of hu man cells involves nucleotide insertion at dsb termini prior to ligati on.