The expression of the retinoblastoma gene (Rb-1) in colorectal carcinomas was studied by Western blotting and immunohistochemistry. Western blot analysis using monoclonal antibodies raised against synthetic peptides of the Rb-1 gene product (pRB) revealed that colorectal carcinomas overexpressed pRB with a molecular weight of around 110 kD when compared to normal mucosa. Immunohistochemical staining of formalin-fixed, paraffin-embedded tissue sections from 48 colorectal carcinomas demonstrated that pRB expression was exclusively localized to the nucleus. More than 50% of the carcinoma cells expressed nuclear pRB in 14 tumors (29.2%), while 10-50% of the carcinoma cells did so in 21 tumors (43.8%), and less than 10% of cells did so in 13 tumors (27.1%). Although there was no clear correlation between pRB expression and clinico-pathologic parameters such as tumor stage, tumor size, depth of invasion, and lymph node metastasis, a higher incidence of pRB expression was observed in well to moderately differentiated adenocarcinoma than in the signet ring cell carcinoma. Thus the present study demonstrated for the first time that the oncosuppressor gene, Rb-1, is overexpressed at the protein level in most colorectal carcinomas.

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