Staining patterns of p53 immunohistochemistry and their biological significance in colorectal cancer.

Immunohistochemistry (IHC) is a cheap and rapid method to detect p53 inactivation but the results are often discordant with gene mutation analysis. This study aimed to investigate whether there is a difference in the immunohistochemical staining patterns of p53-positive cells on comparing tumours with inactivating gene mutations with those without. Tissues of 142 colorectal cancers were investigated for p53 inactivation simultaneously by IHC and gene analysis using SSCP of exons 4-9 and sequencing. In addition, tumours were investigated immunohistochemically for the expression of mdm-2 protein, known to be transcriptionally transactivated by the wild-type (wt) p53 gene. p53-positive cells of tumours without detectable p53 gene mutations were microdissected using a PALM laser microscope system and subjected to p53 sequence analysis. Among the 142 cases of colorectal cancer (male/female=88/54; mean age 66a+/-11 years, range 24-90 years), 74% (n=105) of tumours were positive by p53 IHC and mutations in the p53 gene were found in 51% (73 patients). In 16% (12 patients) with mutations in the p53 gene, IHC for p53 was negative. In tumours with mutations in the p53 gene and positive p53 IHC, staining of all nuclei of the tumour was more frequently (57/61, 93%) found than in tumours without p53 gene mutations, where staining of scattered single cells was predominantly seen (29/44, 66%; p<0.0001). mdm-2 positivity (n=33) showed only staining of scattered single cells, predominantly (24/33, 82%; p<0.0001) in tumours without gene mutations. Single cell microdissection followed by mutation analysis of scattered p53-positive cells revealed no gene mutations. A scattered positive immunohistochemical reactivity of p53 in colorectal cancer cells might therefore represent a functionally active non-mutated p53 gene and should not be considered as a marker of gene mutation and inactivation.