Incidence and effects of Ha-ras codon 12 G -> A transition mutations in preneoplastic lesions induced by N-nitrosomethylbenzylamine in the rat esophagus

N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal tumorigenesis is a n important model for squamous cell carcinoma of the human esophagus. In th is model, previous studies have shown that the GGA --> GAA Ha-ras codon 12 mutation is present in the majority of papillomas. No other Ha-ras mutation has been identified. Studies using other models of chemical carcinogenesis suggest that Ha-ras activation has a critical role during tumor initiation . We have used laser-capture microdissection and polymerase chain reaction- restriction fragment length polymorphism analysis to study the role of codo n 12 Ha-ras mutation at various stages of tumor development in the rat esop hagus. Our results indicate that Ha-ras mutation was present infrequently ( 4.3%) in premalignant lesions. The incidence of Ha-ras mutation was high in papillomas (57.1%), however, and 50% of papillomas expressed mutant Ha-ras RNA message. Additionally, there was a linear trend correlating increased incidence of Ha-ras mutation with later papilloma stage. These data suggest the role of ras activation later in neoplastic development. To evaluate th e potential mechanism of action by which Ha-ras contributes to promotion an d progression in this model, we compared mRNA expression of cyclin D1 and p 27 in Ha-ras mutant and Ha-ras normal papillomas. We found no differences i n mRNA expression of either cyclin D1 or p27 between these two papilloma po pulations. Our data suggest an important paradigm shift for the role of ras mutations in this model of chemical carcinogenesis, indicating a functiona l role of Ha-ras activation in promotion/progression and not in the initiat ion phase of NMBA-induced papillomagenesis. (C) 2001 Wiley-Liss, Inc.