Different Mutation Frequencies and Spectra among Organs by N‐Methyl‐N‐nitrosourea in rpsL (strA) Transgenic Mice

The frequencies and spectra of N‐methyl‐N‐nitrosourea (MNU)‐induced in vivo somatic mutations were determined in rpsL (strA) transgenic mice. The wild‐type rpsL gene, which exhibits a streptomycin‐sensitive (Sm(s)) phenotype, was used as the rescue marker gene. Studies of mutation spectra among different organs and tissues were simplified using this system because of the short coding sequence (375 bp) of the rpsL gene. MNU administration to transgenic mice significantly elevated the mutation frequencies in various adult organs. Two distinctive patterns of mutation spectrum were observed, depending on the organs tested. Mutations derived from labile organs (spleen and thymus) were predominantly G:C to A:T transitions, as expected for MNU mutagenesis. Stable organs like the liver and brain, however, carried many fewer G:C to A:T transitions but significantly more single base deletions, of which the spectrum was very similar to that of background mutations in the rpsL transgenic mice. This spectrum difference among more and less proliferating organs was confirmed by the predominant occurrence of G:C to A:T transitions in fetal liver cells exposed to transplacental MNU treatment. In addition, most (approximately 90%) of the G:C to A:T transitions induced by MNU were detected in the first nucleotide of some 5′‐G‐(C or G)‐3’ sequences, many of which corresponded to the middle guanine residue of 5′‐purine‐G‐(C or G)‐3’ sequences. It is thus suggested that at particular sites, the neighboring bases in both the 5’ side and 3’ side seem to influence either the susceptibility to DNA damage or the ability to repair MNUinduced lesions.