p16(INK4a) expression begins early in human colon neoplasia and correlatesinversely with markers of cell proliferation

Background & Aims: p16(INK4a) is a cell cycle inhibitor and a major tumor-s uppressor protein, but the regulation of p16(INK4a) is poorly understood an d the physiologic settings in which it exerts its antiproliferative effects are unknown, A role for p16(INK4a) i, intestinal neoplasia is suggested by the observation that the promoter region is methylated in a subset of huma n colon tumors, We examined the expression of the protein in specimens repr esenting the full spectrum of neoplastic progression in the human colon and determined whether expressing cells showed evidence of cell cycle inhibiti on. Methods: We studied p16(INK4a) expression by immunoprecipitation, immun oblotting, reverse-transcription polymerase chain reaction (RT-PCR), immuno histochemistry, and immunofluorescence in matched normal and neoplastic col onic tissue from 70 patients. Results: p16(INK4a) expression was very low i n normal mucosa, with staining observed in rare epithelial cells at the bas e of crypts, A distinctly higher expression was found in 4 of 7 aberrant cr ypt foci, 32 of 36 adenomas, 18 of 28 primary carcinomas, and 5 of 5 metast atic carcinomas, Within each neoplasm p16(INK4a) staining was heterogeneous , with higher expression commonly seen in areas bordering normal tissue. p1 6(INK4a) Staining correlated inversely with that of Ki67, cyclin A, and the retinoblastoma protein, suggesting that cell cycle progression was inhibit ed. Conclusions: These results suggest that p16(INK4a) expression begins in the earliest detectable stages of neoplastic progression in the human colo n and exerts a continuous, piecemeal constraint on tumor growth.