N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal
tumorigenesis is an important model for
squamous cell carcinoma of the human esophagus. In this 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
codon 12 Ha-ras mutation at various stages of
tumor development in the rat esophagus. 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 the potential mechanism of action by which Ha-ras contributes to promotion and progression in this model, we compared
mRNA expression of
cyclin D1 and p27 in Ha-ras mutant and Ha-ras normal
papillomas. We found no differences in
mRNA expression of either
cyclin D1 or p27 between these two
papilloma populations. Our data suggest an important paradigm shift for the role of ras mutations in this model of chemical
carcinogenesis, indicating a functional role of Ha-ras activation in promotion/progression and not in the initiation phase of NMBA-induced papillomagenesis.