CpG island hypermethylation in human colorectal tumors is not associated with DNA methyltransferase overexpression

The molecular basis of aberrant hypermethylation of CpG islands observed in a subset of human colorectal tumors is unknown. One potential mechanism is the up-regulation of DNA (cytosine-5)-methyltransferases, Recently, two ne w mammalian DNA methyltransferase genes hare been identified, which are ref erred to as DNMT3A and DNMT3B. The encoded proteins differ from the predomi nant mammalian DNA methyltransferase DNMT1 in that they have a substantiall y higher ratio of de novo to maintenance methyltransferase activity. We hav e used a highly quantitative 5' nuclease fluorogenic reverse transcription- PCR method (TaqMan) to analyze the expression of all three DNA methyltransf erase genes in 25 individual colorectal adenocarcinoma specimens and matche d normal mucosa samples. In addition, we examined the methylation patterns of four CpG islands [APC, ESR1 (estrogen receptor), CDKN2A (p16), and MLH1] to determine whether individual tumors show a positive correlation between the level of DNA methyltransferase expression and the frequency of CpG isl and hypermethylation. All three methyltransferases appear to be up-regulate d in tumors when RNA levels are normalized using either ACTB (beta-actin) o r POLR2A (RNA pol II large subunit), but not when RNA levels are normalized with proliferation-associated genes, such as H4F2 (histone H4) or PCNA, Th e frequency or extent of CpG island hypermethylation in individual tumors d id not correlate with the expression of any of the three DNA methyltransfer ases. Our results suggest that deregulation of DNA methyltransferase gene e xpression does not play a role in establishing tumor-specific abnormal DNA methylation patterns in human colorectal cancer.