The in vivo genetic activity profile of the monofunctional nitrogen mustard 2-chloroethylamine differs drastically from its bifunctional counterpart mechlorethamine

The property of forming crosslinks within DNA is seen as the major cause of the high carcinogenic, genotoxic and anti-neoplastic potency of bifunctional nitrogen mustards. To further investigate the importance for genotoxicity of a second reactive group in a molecule, the genetic activity profiles of the bifunctional nitrogen mustard mechlorethamine (MEC) and its monofunctional counterpart 2-chloroethylamine (CEA) were compared, using several in vivo end points in Drosophila. When post-meiotic male germ cells were alkylated by CEA and then transferred to nucleotide excision repair (NER)-proficient oocytes, no more than up to 4-fold increased forward mutation frequencies were induced. With oocytes deficient for XPG (DmXPG), frequencies were enhanced up to 50 times. For MEC mutation frequencies increased up to 40 times the background, whereas only a low hypermutability was observed when DmXPG were used instead of wild-type females, indicating that nitrogen mustard-induced monoadducts, in contrast to crosslinks, are efficiently repaired by the NER system. Specific locus mutations generated in the vermilion gene by CEA under NER(-) conditions were almost exclusively base pair substitutions (93%). The high proportion of mutations at guanine positions indicates a strong contribution of N7-alkylguanine to the mutational spectrum. MEC induced 64% deletions and other DNA rearrangements in crosses of males with DmXPG females. The small portion of point mutations (36%) was further reduced to approximately 20% with NER(+) females. Inactivation of NER had no potentiating effect on clastogenic events (chromosome loss) induced by CEA, which is in sharp contrast to the strongly enhanced forward mutation frequencies measured with DmXPG females. The weak genotoxic effectiveness of CEA under NER(+) conditions is clearly due to efficient error-free repair of monoalkyl adducts. These results further support the concept that bifunctional nitrogen mustards exert their mutagenic activity through formation of DNA crosslinks and that DNA monoadducts make only a minor contribution to their genotoxic activity.