Melphalan-induced mutagenesis in an SV40-based shuttle vector: predominance of A.T----T.A transversions.

The base and sequence specificity of mutagenesis by the carcinogenic antitumor agent melphalan (L-phenylalanine mustard) was examined by in vitro treatment of the plasmid pZ189 followed by replication in human 293 cells, rescue in bacteria, and sequence analysis of mutations in the supF gene. Melphalan was strongly mutagenic in this assay. The induced mutations were predominantly base substitutions, with a minor component of small deletions (3-80 base pairs). Surprisingly, A.T----T.A transversions were by far the most frequent substitutions, suggesting that modifications of adenine may play a major role in mutagenesis. More than half the substitutions were clustered in a C-T-A-A sequence in the anticodon loop. No base substitutions were detected at G-N-C sequences, which are thought to be potential sites of DNA interstrand cross-links. The results raise the possibility that the cytotoxic and mutagenic effects of melphalan may be separable.