Induction of invasive squamous cell carcinomas in the forestomach of (C3H x MSM)F1, MSM, and C3H mice by N-methyl-N-nitrosourea and mutational analysis of the H-ras and p53 genes.

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Genetic analysis of tumors developing in F1 hybrids between genetically separate strains of mice makes it possible to search for loss of heterozygosity (LOH), information on which provides clues to finding tumor-suppressor genes. For this purpose, however, reproducible carcinogenic conditions for the organ of interest need to be first determined. In the present study, a forestomach model of squamous cell carcinomas (SCCs), induced in (C3H x MSM)F1 mice by N-methyl-N-nitrosourea (MNU), was established and mutational changes in the H-ras and p53 genes were examined in tumors. Male (C3H x MSM)F1, MSM and C3H mice were given MNU by i.g. intubation once a week at a dose of 0.03 mg/g body weight for 10 weeks, then kept without further treatment. At experimental weeks 38-46, markedly invasive SCCs were observed in the forestomach at incidences of 9/14 (64.3%), 9/16 (56.3%), and 2/10 (20.0%), respectively. In the three strains of mice, DNA analysis of SCCs by PCR-SSCP analysis followed by direct DNA sequencing revealed low incidences of point mutations in the H-ras (4/20, 20%) and p53 (3/20, 15%) genes. The results demonstrate the usefulness of the present animal experimental protocol for induction of high grade SCC in the forestomach of (C3H x MSM)F1 mice, and suggest the possibility that point mutations in the H-ras or p53 genes may play some role in pathways leading to the development of such lesions.