p16(INK4a) promoter methylation and 9p21 allelic loss in colorectal carcinomas: relation with immunohistochemical p16(INK4a) expression and with tumor budding

In colorectal carcinomas, p16(INK4a) inactivation is known to occur by allelic loss and by promoter methylation, but mutations are rare. p16(INK4a) is up-regulated in tumor buds, and the consequent shutdown of proliferation may be a prerequisite for tumor budding. Fifty-seven colorectal carcinomas from a consecutive series were investigated. Using DNA from tissue homogenates, p16(INK4a) promoter methylation was seen in 17 of 57 tumors by methylation-specific polymerase chain reaction, and this could be confirmed using DNA from laser-capture microdissected material in 16 of these cases. A total loss of immunohistochemical p16(INK4a) expression was seen in 6 of 17 tumors with promoter methylation. Quantification of immunohistochemical p16(INK4a) expression for the remaining 11 cases revealed statistically lower frequencies of expression as compared with cases without p16(INK4a) promoter methylation. 9p21 allelic loss was observed in 9 cases, but p16(INK4a) expression in these carcinomas was not reduced. Attempted linear regression of p16(INK4a) expression in tumor buds on the degree of tumor budding, as counted on pan-cytokeratin immunostains, did not show a correlation. p16(INK4a) promoter methylation can completely abrogate p16(INK4a) expression in colorectal carcinomas. In many cases, however, it has an appreciable but only modulatory influence on p16(INK4a) expression. Possibly, methylations are heterozygous, and/or mosaic in colorectal carcinomas and/or methylations are not totally stable but can be lost between carcinoma cell replication cycles. Up-regulation of p16(INK4a) does not seem to be a strict requirement for tumor budding, hence, the absence of a correlation.