Repairability during G1 of lesions eliciting sister chromatid exchanges induced by methylmethanesulfonate or ethylmethanesulfonate in bromodeoxyuridine-substituted and unsubstituted DNA strands

The repairability during G(1) of DNA lesions eliciting sister chromatid exchanges (SCE) induced by methylmethanesulfonate (MMS) and ethylmethanesulfonate (EMS) in BrdU-substituted and unsubstituted DNA strands was determined in murine salivary gland cells in vivo. The SCE frequency was determined after exposure to MMS or EMS during early and late G(1) in the first or second cell division. The inducibility and repairability of SCE-eliciting lesions during G(1) in BrdU-substituted and unsubstituted strands were estimated considering that in the first division induction occurs on the unsubstituted strand and during the second division in one unsubstituted and one BrdU- substituted DNA strand. The results indicate that DNA lesions induced by MMS are 50% repaired in both the BrdU-substituted and unsubstituted strands and those induced by EMS are 60% repaired in the unsubstituted strand but only 20% in the BrdU-substituted strand. The increase in sensitivity of the BrdU-substituted strand to SCE induction with respect to the unsubstituted strand was 155 and 45% for MMS and EMS, respectively. These results imply that SCE-inducing lesions produced by MMS and EMS are only partially repaired and that BrdU incorporation could sensitize DNA not only to the induction of lesions eliciting SCE, but also to the induction of non-repairable lesions.