J. M. D. Wheeler

BACKGROUND: Ulcerative colitis associated colorectal cancer (UCACRC) has several distinctive clinicopathological and genetic features which differ from sporadic colorectal cancer (SCRC). Hypermethylation of the E-cadherin gene (CDH1) has not been described previously in colorectal cancer. AIMS: A panel of SCRC and UCACRC were investigated for mutations inCDH1, and for hypermethylation of the promoter region of CDH1. SUBJECTS AND METHODS: DNA was available from 14 patients with UCACRC and from 14 with SCRC. All exons ofCDH1 were amplified with the polymerase chain reaction (PCR) and screened using single strand conformational polymorphism and direct sequencing. Hypermethylation of theCDH1 promoter region was determined by methylation specific PCR following bisulphite modification, and compared with E-cadherin protein expression from a previous immunohistochemistry study. RESULTS: Thirteen of 28 cancers (46%) were hypermethylated in theCDH1 promoter region—eight cancers (57%) in the UCACRC group and five cancers (36%) in the SCRC group (NS)—and this correlated with reduced E-cadherin expression (pCDH1 in either group although there were several polymorphisms. CONCLUSION: We have demonstrated hypermethylation of the promoter region inCDH1 in 46% of colorectal cancers studied. There was no difference between the UCACRC and SCRC groups. Just as there are specific differences in the genetic changes between UCACRC and SCRC, there is also likely to be a large degree of overlap among the genetic pathways of these cancers.

INTRODUCTION: Hypermethylation of the promoter region of the hMLH1 gene is associated with absent expression of MLH1 protein in sporadic colorectal cancers with microsatellite instability (MSI+), and it has been proposed that methylation may be a mechanism of inactivation in Knudson's hypothesis. The incidence of hypermethylation of thehMLH1 promoter in hereditary non-polyposis colorectal cancer (HNPCC) versus MSI+ sporadic colorectal cancer was investigated and compared. METHODS: DNA was available from 10 HNPCC colorectal cancers (median age 58 years, range 39-67) with germline mutations in hMLH1 and 10 MSI+ sporadic colorectal cancers (mean age 79 years, range 41-85). MSI was determined by amplification of BAT26 and TGF-β RII. The methylation status of the hMLH1 promoter was studied by the polymerase chain reaction (PCR) basedHpaII restriction enzyme assay technique. Evidence of allelic loss at hMLH1 was searched for in the HNPCC colorectal cancers. RESULTS: All cases were confirmed to be MSI+. The promoter region of hMLH1 was hypermethylated in seven of 10 MSI+ sporadic cancers versus 0 of 10 HNPCC cancers (phMLH1 was observed in eight of the 10 HNPCC colorectal cancers. CONCLUSION: While mutations and allelic loss are responsible for the MSI+ phenotype in HNPCC cancers, the majority of MSI+ sporadic cancers are hypermethylated in the promoter region of hMLH1. These data further support our argument that tumours from HNPCC patients, which almost always acquire a raised mutation rate, mostly follow a different pathway from MSI+ sporadic tumours.

Fifteen to twenty-five percent of sporadic colorectal carcinomas are replication error (RER) positive. Because the frequency of mutations in the mismatch repair genes (hMLH1 and hMSH2) is low in these tumors, we have investigated the role of mutational inactivation, methylation of the promoter region, and loss of heterozygosity (LOH) as a possible explanation for the mutator phenotype of RER+ colorectal cancer cell lines. Genomic DNA was extracted from a panel of 49 human colorectal cancer cell lines. The RER status was determined by amplification of BAT-26. All exons of hMLH1 and hMSH2 were amplified with the PCR and screened by using single-strand conformational polymorphism and direct sequencing. The methylation status was ascertained by methylation-specific PCR after bisulfite modification of DNA. Western blotting for hMLH1 was performed on methylated cell lines before and after the addition of the demethylating agent 5-azacytidine. LOH was sought by genescan analysis of amplified CA repeat markers and indirectly by determining the number of homozygotes in the cell lines and human random controls. Twelve cell lines from ten tumors (24%) were RER+. Hypermethylation of the hMLH1 promoter occurred in five of ten (50%) RER+ tumors, whereas three of thirty-two (6%) RER tumors showed partial methylation. None of the fully methylated cell lines expressed hMLH1, although all reexpressed hMLH1 after treatment with 5-azacytidine. There was no LOH in the RER+ tumors in either hMLH1 or hMSH2. Our results suggest that mutations of hMLH1 together with hypermethylation of the promoter region, but not LOH, are the cause of the mutator phenotype in the majority (70%) of RER+ tumors.