Aberrant methylation of the Cyclooxygenase 2 CpG island in colorectal tumors

Cyclooxygenases (COXs) are key enzymes that convert arachidonic acid to pro staglandins. Overexpression of one of the COX isozymes, COX2, has been show n to play an important role in colorectal cancer progression. Recently, how ever, low expression of COX2 has been reported in a subset of colorectal an d gastric cancers. Aberrant CpG island methylation and associated transcrip tional silencing are common in colorectal cancer, and we therefore investig ated the potential role of methylation in the transcriptional silencing of COX2. We examined the methylation status of the COX2 5' CpG island in a ser ies of tumor cell lines. Among the 33 cell lines examined, dense methylatio n (>70%) of COX2 was detected in 5 cell lines, and partial methylation was detected in 10 cell lines. Detailed methylation mapping using bisulfite gen omic sequencing revealed that loss of expression of COX2 mRNA was closely c orrelated with methylation of a region upstream of exon 1, and expression c ould be restored by demethylation using the DNA methyltransferase inhibitor 5-aza-deoxycytidine. Aberrant methylation of COX2 was also detected in 12 of 92 (13%) unselected sporadic primary colorectal cancers and 7 of 50 (14% ) colorectal adenomas. COX2 methylation was strongly associated with the pr esence of the CpG island methylator phenotype (P < 0.01), inversely related to p53 gene mutation (P < 0.01), and unrelated to microsatellite instabili ty status. We propose that COX2 expression in colorectal tumors is modulate d by functional factors that favor high expression and by the CpG island me thylator phenotype that favors silencing in a subset of cases. These result s raise the possibility that tumors with COX2 methylation may be less sensi tive to treatment using specific COX2 inhibitors.