Immunohistochemical detection of the p53 oncoprotein in tumours of melanocytic origin.

We employed the polyclonal anti-p53 antibody NCL-CM1 to cultured cells and pathological tissues in order to investigate the expression of p53 oncoprotein in human malignant melanomas. The results in the cultured cells showed that the antigenic determinant was sensitive to formalin fixation, resulting in a lower reactivity than with fixation by alcohol. In pathological tissues, the expression of p53 oncoprotein increased with progression of the tumour. Among 79 melanomas 37 (47%) showed distinct nuclear labelling and the highest proportion of reactive cells was observed in metastatic melanomas (mean 4.8%). An immunocytochemical study also revealed the presence of mutant-type oncoprotein in human melanoma cell lines, which was recognized by monoclonal antibody P240, and we confirmed that the molecular weight of the antigens recognized by both antibodies was 53 kDa by Western blot analysis. Therefore, although the presence of point mutations in human melanomas is yet to be confirmed our data suggest that the antigen detected by NCL-CM1 is a mutant-type or a complex of mutant and wild-type p53 oncoproteins. This antibody may be useful in retrospective studies of tumours of melanocytic origin.