INCREASED REMOVAL OF 3-ALKYLADENINE REDUCES THE FREQUENCIES OF HPRT MUTATIONS INDUCED BY METHYLMETHANESULFONATE AND ETHYLMETHANESULFONATE IN CHINESE-HAMSTER FIBROBLAST CELLS

We have previously reported the isolation of mammalian cell lines expr essing the 3-methyladenine DNA glycosylase I (tag) gene from E.coli. T hese cells are 2-5 fold more resistant to the toxic effects of methyla ting agents than normal cells (15). Kinetic measurements of 3-methylad enine removal from the genome in situ show a moderate (3-fold) increas e in Tag expressing cells relative to normal as compared to a high (50 -fold) increase in exogenous alkylated DNA in vitro by cell extracts. Excision of 7-methylguanine is as expected, unaffected by the tag(+) g ene expression. The frequency of mutations formed in the hypoxanthine phosphoribosyl transferase (hprt) locus was investigated after methylm ethanesulfonate (MMS), ethylmethanesulfonate (EMS), N-nitroso-N-methyl urea (NMU) and N-nitroso-N-ethylurea (NEU) exposure. Tag expression re duced the frequency of MMS and EMS induced mutations to about half the normal rate, whereas the mutation frequency in cells exposed to NMU o r NEU is not affected by the tag(+) gene expression. These results ind icate that after exposure to compounds which produce predominantly N-a lkylations in DNA, a substantial proportion of the mutations induced i s formed at 3-alkyladenine residues in DNA.