The relationship between 1,2-dimethylhydrazine dose and the induction of colon tumours: tumour development in female SWR mice does not require a K-ras mutational event

In this study we have investigated the relationship between the dose of 1,2 -dimethylhydrazine (DMH) and the yield land location) of tumours in a mouse strain susceptible to colon tumour induction. Female SWR mice were injecte d with 6.8 mg/kg DMH i.p. once a week for 1, 5, 10 and 20 weeks and the ani mals were followed for almost 2 years, Administration of increasing doses o f DMH resulted in a dose-dependent decrease in survival time. Colon tumours developed in 26, 76 and 87% of mice given a total dose of 34, 68 and 136 m g/kg DMH, respectively: no tumours were detected in animals treated with a total dose of 6.8 mg/kg, Most colon tumours (79%) were located in the dista l colon with the remainder being found in the mid colon and none were detec ted in either the proximal colon or small intestine. As mutations in the K- ms gene are thought to be key events in the pathogenesis of human and roden t colon tumours, we determined the frequency of codon 12 and 13 K-ras mutat ions in these tumours by restriction site mutation analysis and/or DNA sequ encing. A total of 50 colon tumour samples were analysed for codon 12 mutat ions and of these 29 were also screened for codon 13 mutations. No mutation s were detected in either of these codons, The mutational activation of the K-ras gene is not an essential step in the development of DMH-induced colo n tumours in female SWR mice and if similar considerations apply to humans, then the aetiological role of alkylating agents may be underestimated from the prevalence of K-ras GC-->AT transitions in human tumours.