Genetic and epigenetic modification of MLH1 accounts for a major share of microsatellite-unstable colorectal cancers.
Microsatellite instability (MSI) is a hallmark of hereditary nonpolyposis colorectal cancer, and in these patients, results from inherited defects in DNA mismatch repair genes, mostly MSH2 and MLH1. MSI also occurs in 15% of sporadic colorectal cancers, but in these tumors, its basis is less well characterized. We investigated 46 sporadic MSI+ colorectal cancers for changes in MSH2 and MLH1 protein expression, followed by the analysis of somatic mutation, loss of heterozygosity (LOH), and promoter hypermethylation as possible underlying defects. Most cases (36/46, 78%) showed lost or reduced MLH1 expression. Among these, a majority (83%) was associated with MLH1 promoter hypermethylation, whereas the rates of LOH and somatic mutation of MLH1 were 24% and 13%, respectively. Hypermethylation and LOH were inversely correlated, suggesting that they had alternative functions in the inactivation of MLH1. MSH2 expression was lost in 7/46 (15%), and of these, 2 (29%) showed LOH and/or somatic mutation of MSH2. We conclude that most sporadic MSI+ colorectal cancers have an MLH1-associated etiology and that epigenetic modification is a major mechanism of MLH1 inactivation. Moreover, we found a significantly lower prevalence for MLH1 promoter hypermethylation in hereditary nonpolyposis colorectal cancer tumors with MLH1 germline mutations (12/26, 46%), which might explain some differences that are known to occur in the clinicopathological characteristics and tumorigenic pathways between sporadic and hereditary MSI+ colorectal cancers.
Kuismanen
S A
SA
Departments of Medical Genetics and Pathology, Haartman Institute, University of Helsinki, Helsinki, Finland.
Holmberg
M T
MT
Salovaara
R
R
de la Chapelle
A
A
Peltomäki
P
P
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D015179:Colorectal Neoplasms / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D019175:DNA Methylation; D004273:DNA, Neoplasm / Q000235:genetics / Q000378:metabolism; D004268:DNA-Binding Proteins; D006801:Humans; D007150:Immunohistochemistry; D019656:Loss of Heterozygosity; D018895:Microsatellite Repeats / Q000235:genetics*; D000070957:MutL Protein Homolog 1; D051718:MutS Homolog 2 Protein; D009154:Mutation; D009363:Neoplasm Proteins / Q000032:analysis / Q000235:genetics*; D009687:Nuclear Proteins; D011401:Promoter Regions, Genetic; D011518:Proto-Oncogene Proteins / Q000032:analysis / Q000235:genetics
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D004273:DNA, Neoplasm; D004268:DNA-Binding Proteins; C501486:MLH1 protein, human; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; C497172:MSH2 protein, human; D000070957:MutL Protein Homolog 1; D051718:MutS Homolog 2 Protein