Variant mutational activation of the K-ras oncogene in renal mesenchymal tumors induced in newborn F344 rats by methyl(methoxymethyl)nitrosamine.

Renal mesenchymal tumors were induced at high incidence in F344 rats by a single intraperitoneal injection of methyl(methoxymethyl)nitrosamine (DMN-OMe) within 48 h after birth. DNAs from 18 of 35 mesenchymal tumors contained transforming ras sequences in NIH3T3 transfection assays: K-ras (17/18) or N-ras (1/18). Single-stranded conformational polymorphism analysis or dideoxy sequencing of polymerase chain reaction-amplified K-ras gene fragments revealed that these neoplasms contained a variety of activating mutations in the K-ras oncogene. Alterations in codon 12 predominated and included GGT --> GAT transitions, GGT --> GTT or TGT transversions, and previously reported insertion mutations, although some tumors expressed more than one mutation and the pattern of mutations even varied within tumors. Mutations were also found in exons 2 and 3. In addition, tumor transplantability into syngeneic hosts correlated positively and significantly with K-ras activation. Renal mesenchymal tumors with transforming mutations in exon 1 were often successfully passaged (10/12) while tumors which lacked mutations in exon 1 were infrequently transplantable (2/14). While the observed base substitutions in K-ras are consistent with adduct formation, the presence of insertion mutations and intratumor heterogeneity of alterations suggest that ras activation in DMN-OMe-induced tumors is not necessarily an early event in tumorigenesis.