title: MGMT and MLH1 promoter methylation versus APC, KRAS and BRAF gene mutations in colorectal cancer: indications for distinct pathways and sequence of events.
abstract: BACKGROUND
  To study how caretaker gene silencing relates to gatekeeper mutations in colorectal cancer (CRC), we investigated whether O6-methylguanine DNA methyltransferase (MGMT) and Human Mut-L Homologue 1 (MLH1) promoter hypermethylation are associated with APC, KRAS and BRAF mutations among 734 CRC patients.
  
  
  METHODS
  We compared MGMT hypermethylation with G:C > A:T mutations in APC and KRAS and with the occurrence of such mutations in CpG or non-CpG dinucleotides in APC. We also compared MLH1 hypermethylation with truncating APC mutations and activating KRAS and BRAF mutations.
  
  
  RESULTS
  Only 10% of the tumors showed both MGMT and MLH1 hypermethylation. MGMT hypermethylation occurred more frequently in tumors with G:C > A:T KRAS mutations (55%) compared with those without these mutations (38%, P < 0.001). No such difference was observed for G:C > A:T mutations in APC, regardless of whether mutations occurred in CpG or non-CpG dinucleotides. MLH1 hypermethylation was less common in tumors with APC mutations (P = 0.006) or KRAS mutations (P = 0.001), but was positively associated with BRAF mutations (P < 0.001).
  
  
  CONCLUSIONS
  MGMT hypermethylation is associated with G:C > A:T mutations in KRAS, but not in APC, suggesting that MGMT hypermethylation may succeed APC mutations but precedes KRAS mutations in colorectal carcinogenesis. MLH1-hypermethylated tumors harbor fewer APC and KRAS mutations and more BRAF mutations, suggesting that they develop distinctly from an MGMT methylator pathway.
authors.affiliation: Department of Epidemiology, GROW-School for Oncology and Developmental Biology, Maastricht University, Maastricht, The Netherlands.
authors.forename: S
authors.initials: S
authors.lastname: de Vogel
authors.forename: M P
authors.initials: MP
authors.lastname: Weijenberg
authors.forename: J G
authors.initials: JG
authors.lastname: Herman
authors.forename: K A D
authors.initials: KA
authors.lastname: Wouters
authors.forename: A F P M
authors.initials: AF
authors.lastname: de Goeij
authors.forename: P A
authors.initials: PA
authors.lastname: van den Brandt
authors.forename: A P
authors.initials: AP
authors.lastname: de Bruïne
authors.forename: M
authors.initials: M
authors.lastname: van Engeland
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics* / Q000378:metabolism; D000368:Aged; D015179:Colorectal Neoplasms / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D018899:CpG Islands / Q000235:genetics; D019175:DNA Methylation / Q000235:genetics*; D015254:DNA Modification Methylases / Q000235:genetics* / Q000378:metabolism; D045643:DNA Repair Enzymes / Q000235:genetics* / Q000378:metabolism; D005260:Female; D020868:Gene Silencing; D017491:Genes, APC; D011905:Genes, ras / Q000235:genetics; D006801:Humans; D008297:Male; D008875:Middle Aged; D000070957:MutL Protein Homolog 1; D009426:Netherlands; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D017354:Point Mutation; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D011446:Prospective Studies; D011518:Proto-Oncogene Proteins / Q000235:genetics; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics; D016283:Proto-Oncogene Proteins p21(ras); D025521:Tumor Suppressor Proteins / Q000235:genetics* / Q000378:metabolism; D018631:ras Proteins / Q000235:genetics
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C117307:KRAS protein, human; C501486:MLH1 protein, human; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D025521:Tumor Suppressor Proteins; D015254:DNA Modification Methylases; C515491:MGMT protein, human; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D000070957:MutL Protein Homolog 1; D016283:Proto-Oncogene Proteins p21(ras); D018631:ras Proteins; D045643:DNA Repair Enzymes