To investigate the role of the Fhit gene in carcinogen induction of neoplasia, we have inactivated one Fhit allele in mouse embryonic stem cells and produced (129͞SvJ ؋ C57BL͞6J) F1 mice with a Fhit allele inactivated (؉͞؊). Fhit ؉͞؉ and ؉͞؊ mice were treated intragastrically with nitrosomethylbenzylamine and observed for 10 wk posttreatment. A total of 25% of the ؉͞؉ mice developed adenoma or papilloma of the forestomach, whereas 100% of the ؉͞؊ mice developed multiple tumors that were a mixture of adenomas, squamous papillomas, invasive carcinomas of the forestomach, as well as tumors of sebaceous glands. The visceral and sebaceous tumors, which lacked Fhit protein, were similar to those characteristic of Muir-Torre familial cancer syndrome.esophageal͞gastric cancer ͉ N-nitrosomethylbenzylamine ͉ carcinogeninduced tumorigenesis ͉ Fhit knockout mice ͉ tumor suppressor gene

Mice carrying one inactivated Fhit allele (Fhit ؉͞؊ mice) are highly susceptible to tumor induction by N-nitrosomethylbenzylamine, with 100% of Fhit ؉͞؊ mice exhibiting tumors of the forestomach͞ squamocolumnar junction vs. 25% of Fhit ؉͞؉ controls. In the current study a single N-nitrosomethylbenzylamine dose was administered to Fhit ؉͞؉, ؉͞؊, and ؊͞؊ mice to compare carcinogen susceptibility in ؉͞-and ؊͞؊ Fhit-deficient mice. At 29 weeks after treatment, 7.7% of wild-type mice had tumors. Of the Fhit ؊͞؊ mice 89.5% exhibited tumors (average 3.3 tumors͞mouse) of the forestomach and squamocolumnar junction; half of the ؊͞؊ mice had medium (2 mm diameter) to large (>2 mm) tumors. Of the Fhit ؉͞؊ mice 78% exhibited tumors (average 2.4 tumors͞mouse) and 22% showed medium to large tumors. Untreated Fhit-deficient mice have been observed for up to 2 years for spontaneous tumors. Fhit ؉͞؊ mice (average age 21 mo) exhibit an average of 0.94 tumors of different types; Fhit ؊͞؊ mice (average age 16 mo) also showed an array of tumors (average 0.76 tumor͞mouse). The similar spontaneous and induced tumor spectra observed in mice with one or both Fhit alleles inactivated suggests that Fhit may be a one-hit tumor suppressor gene in some tissues. D eletion at chromosome region 3p14.2 is among the earliest events observed in many cancers and is observed in very early preneoplastic lesions of lung, cervix, and breast (1-5). The Ϸ2-Mb fragile FHIT gene is the likely target of these diseaseassociated deletions, which result in loss of portions of one or both FHIT alleles, with concomitant loss of at least half of the normal complement of Fhit protein. Whether loss of one FHIT allele predisposes to further genetic changes is not known, but clonal expansion of the 3p14.2-deleted cells occurs. In the majority of frankly malignant lesions of most cancer types, one or both FHIT alleles are damaged with resultant reduction or loss of the proapoptatic Fhit protein (refs. 6 and 7 for review), loss that has been correlated with worse outcome in some tumor types (8-11).The FHIT gene, in both mouse and human (12-15), encompasses a constitutive chromosomal fragile site, which is susceptible to DNA gaps and breaks on exposure to carcinogens, a susceptibility that explains the frequent alterations to the gene in preneoplastic and neoplastic lesions. Because the FHIT locus is so susceptible to damage and is inactivated early in many cancers and its loss may have prognostic significance in a large fraction of cancers, Fhit interacting proteins, substrates, and effectors are attractive targets for inclusion in cancer prevention and therapy strategies (16,17). To establish models for exploring the role of Fhit in cancer induction, prevention, and therapy, we have developed mouse strains carrying one or two inactivated Fhit alleles (18) and have studied the development of spontaneous and N-nitrosomethylbenzylamine (NMBA)-induced tumors in adult mice with intact or inactivated Fhit alleles. In a previous study, we had observed that 100% of Fhit ϩ͞Ϫ m...