Correlations Between P53 Gene Alterations And Protein Expression In Colorectal Carcinomas.
O.P.F. Clausen3, G.I. Meling1, R.A. Lothe2, P. De Angelis3, Y. Chen3, T.O. Rognum1, 1Inst. of Forensic Medicine, 3Inst. of Pathology, The National Hospital, 2Dept. of Genetics, Inst. of Cancer Research, The Norwegian Radium Hospital, Oslo, Norway.
Alterations of the P53 tumor suppressor gene are associated with colorectal carcinogenesis. We therefore examined interrelations between P53 gene alterations and protein expression in 200 colorectal carcinomas, in order to study their prognostic significance. 162 tumors were heterozygous at two RFLP loci, both located to chromosome band 17p13, and one within the P53 gene. LOH was found in 68% of the tumors, P53 gene mutations were examined within conserved domains (exons 5-8) with CDGE and sequencing, and 45% of the tumors had point mutations. Protein expression was measured by immunohistochemistry in histological sections, and 46% of the tumors showed more than 5% positive tumor cells. P53 expression was measured in a subset of the same tumors with immunoblotting as well as with immunohistochemistry. The results from both methods were comparable. Nearly all the tumors that had P53 mutations showed LOH (92%), and a majority of the tumors with mutations also showed P53 expression (72%). Among the tumors without P53 mutations, 48% showed LOH, whereas a minority of these tumors expressed P53 protein (24%). A major conclusion from this study is that very few colorectal carcinomas show expression of the P53 protein without having P53 gene alterations (mutations and/or LOH) (5/49), and that the majority of those expressing P53 protein had mutations as well as LOH (33/49). Five percent P53 positive cells as a cut-off level for positivity was based on the fact that there were no significant differences in genetic alterations between the groups without and with less than 5% positive cells.