HIGH YIELDS OF K-RAS MUTATIONS IN INTRADUCTAL PAPILLARY MUCINOUS TUMORS AND INVASIVE ADENOCARCINOMAS INDUCED BY N-NITROSO(2-HYDROXYPROPYL) (2-OXOPROPYL)AMINE IN THE PANCREAS OF FEMALE SYRIAN-HAMSTERS

Ductal adenocarcinoma, the most common form of pancreatic cancer in hu mans, is associated with activation of the K-ras oncogene in similar t o 90% of cases. In contrast, K-ras mutations are found in <50% of the relatively rare intraductal papillary mucinous tumor (IPRMT), which ar ises in the main pancreatic ducts. Since both adenocarcinomas and IPMT s are believed to arise from ductal cells and progress through similar sequences of morphological changes (i.e. flat hyperplasia, papillary hyperplasia, atypia and carcinoma in situ), it is clear that such prog ression may not always necessitate activation of the ras oncogene. Exp erimentally ductal adenocarcinomas of the pancreas can be induced in t he hamster model by a brief treatment with N-nitroso(2-hydroxypropyl) (2-oxopropyl)amine (HPOP), while IPMTs can be induced by a combined tr eatment with HPOP and erotic acid (OA) in an initiation/promotion sche dule. Since animals are exposed to the carcinogen only once, initiated normal epithelium is expected to give rise to a wide spectrum of neop lastic and preneoplastic lesions, progression of which will depend on the extent of mutagenesis induced at initiation in the targeted cells. In order to investigate the role of K-rns in progression of IPMTs as compared with adenocarcinomas we have examined the presence of K-ras m utations in the above two types of experimentally induced pancreatic c ancers, as well as in associated and preneoplastic lesions. K-ras muta tions at codons 12, 13 and 61 were determined by a designed restrictio n fragment length polymorphism method using mismatched nested primers in 77 neoplastic and preneoplastic foci microdissected from 20 pancrea ses. Mutations were found in all foci of atypical hyperplasia, in carc inomas in situ and invasive cancer, whether such lesions originated in lobular tissue or in the main pancreatic duct. Mutations were also fo und in papillary hyperplasia and flat hyperplasia in small ducts and a lso in the main duct at high frequency. With one exception, all ras mu tations were G-->A transitions at the second base of codon 12. Mutatio ns were occasionally accompanied by excessive presence of the mutant r as allele or loss of the wild-type ras allele, events that were more f requent in atypical hyperplasia (5/17), carcinomas in situ (5/14), IPM Ts (2/5) and invasive adenocarcinomas (2/5) than in flat hyperplasia ( 0/6) or papillary hyperplasia (2/18). Our results demonstrate that: (i ) K-ras mutations, predominantly G-->A transitions, are present in all experimentally induced hamster tumors; (ii) the incidence of K-ras mu tations in IPMTs is lower in humans than in the hamster model; (iii) a dvanced lesions in both adenocarcinomas and IPMTs were frequently asso ciated with an excess of the mutant over the wild-type K-ras allele. I t is likely that both adenocarcinomas and IPMTs induced chemically in the hamster model arise by mechanisms which involve early activation o f K-ras. Such a mechanism seems to be applicable only in a fraction of human IPMTs.