Irreparable DNA cross-links and mammalian cell lethality with bifunctional alkylating agents.

The appearance and removal of DNA cross-links in C3H10T1/2 cells has been investigated with closely related bifunctional alkylating agents having widely different kinetics of alkylating activity. Concentrations of nitrogen mustard, phosphoramide mustard and melphalan demonstrating equivalent levels of cell lethality indicate a correlation between the appearance of residual DNA cross-links, transformation and the loss of colony formation ability. Residual DNA cross-links appear to remain after repair is nearly complete by 24 h and consist of both the interstrand DNA and DNA-protein varieties. These adducts occur in the general population of cells despite the fact that these cells retain their DNA cross-link repair capabilities. Less than 100 interstrand DNA cross-links and a 10-fold greater number of DNA-protein cross-links are estimated to remain in the DNA after a 66% lethal dose of these compounds. Due to the obvious complications involving DNA replication in the presence of such lesions, residual DNA cross-links may possibly be associated with lethality in these cells. Current evidence demonstrating the formation of additional sites of alkylation such as irreparable phosphotriester adducts by several alkylating agents may suggest a role for a specific type of DNA damage in this process.