2',3'-Dideoxycytidine (ddC), which is currently used HIV treatments, is a potent carcinogen in mice. Gavage exposures with ddC (500-1000mg/kg) for 3-6 months result in a high incidence of lymphomas of T-cell origin in B6C3F1 and NIH-Swiss mice. Molecular genetic analyses of these lymphomas have been performed in an attempt to provide information that could be useful for extrapolation between species and the assessment of human risks associated with ddC treatment. Activating mutations in ras oncogenes are common in some mouse lymphomas but single strand conformation analyses failed to reveal any alterations in a series of ddC-lymphomas. Since the p53 tumor suppressor gene is inactivated in some mouse lymphomas, ddC-lymphomas were examined for p53 mutations. Immunohistochemistry analysis revealed positive nuclear p53 staining in 23 of 107 ddC-lymphomas. However, the staining pattern was quite dispersed and weak in all but four samples, suggesting that point mutations are uncommon in most tumors. Southern analysis revealed a homozygous p53 deletion in one of 48 lymphomas. Mutations in the four strongly staining tumors are being characterized by direct sequencing. In order to map potential involvement of additional tumor suppressor genes, allelotypes of 16 B6C3F1 lymphomas were generated with microsatellites on each chromosome. The most frequent losses of heterozygosity (LOH) were observed with markers on chromosomes 12(38%), 2(31%) and 4(25%). Additional studies indicated telomeric locations for the chromosome 12 and 2 loci but candidate genes for these regions are not apparent. Chromosome 4 LOH was consistent with a syntenic region of human chromosome 9p21 that is frequently deleted in a wide variety of tumors including leukemias. These alterations appear to inactivate the MTS1 and MTS2 genes which are cyclin dependent kinase inhibitors. We have cloned and sequenced the mouse homologues of MTS1 and MTS2 in order to mutation screen the ddC-lymphomas by Southern analysis and direct sequencing.
