Relationship between specific alkylated bases and mutations at two gene loci induced by ethylnitrosourea and diethyl sulfate in CHO cells

DNA adduct formation and induction of mutations at gene loci, hypoxanthine-guanine-phosphoribosyltranferase (HPRT) and Na,K-ATPase, were determined simultaneously in Chinese hamster ovary (CHO) cells after treatment with 2 ethylating agents, ethylnitrosourea (ENU) or diethyl sulfate (DES). Doses of DES and ENU, which resulted in equal levels of O⁶-ethylguanine (O⁶-EtGua) and O⁶-ethylthymine (O⁴-EtThy) in the DNA, were found to induce very similar frequencies of 6-thioguanine-resistant (6-TG^r) mutants. Formation of these DNA adducts might therefore be correlated with mutations induced at the HPRT locus.

When, however, the same analysis was applied to ouabain-resistant (our^r) mutants, it was found that, at similar levels of O⁶-EtGua and O⁴-EtThy, DES induced many more oua^r mutants than ENU. This result supports the notion that primary DNA lesions other than O⁶-EtGua and O⁴-EtThy are involved in the fixation of ENU- and DES-induced mutations at the Na,K-ATPase gene locus.