Early onset MSI-H colon cancer with MLH1 promoter methylation, is there a genetic predisposition?

Abstract

BackgroundTo investigate the etiology of MLH1 promoter methylation in mismatch repair (MMR) mutation-negative early onset MSI-H colon cancer. As this type of colon cancer is associated with high ages, young patients bearing this type of malignancy are rare and could provide additional insight into the etiology of sporadic MSI-H colon cancer.MethodsWe studied a set of 46 MSI-H colon tumors cases with MLH1 promoter methylation which was enriched for patients with an age of onset below 50 years (n = 13). Tumors were tested for CIMP marker methylation and mutations linked to methylation: BRAF, KRAS, GADD45A and the MLH1 -93G>A polymorphism. When available, normal colon and leukocyte DNA was tested for GADD45A mutations and germline MLH1 methylation. SNP array analysis was performed on a subset of tumors.ResultsWe identified two cases (33 and 60 years) with MLH1 germline promoter methylation. BRAF mutations were less frequent in colon cancer patients below 50 years relative to patients above 50 years (p-value: 0.044). CIMP-high was infrequent and related to BRAF mutations in patients below 50 years. In comparison with published controls the G>A polymorphism was associated with our cohort. Although similar distribution of the pathogenic A allele was observed in the patients with an age of onset above and below 50 years, the significance for the association was lost for the group under 50 years. GADD45A sequencing yielded an unclassified variant. Tumors from both age groups showed infrequent copy number changes and loss-of-heterozygosity.ConclusionSomatic or germline GADD45A mutations did not explain sporadic MSI-H colon cancer. Although germline MLH1 methylation was found in two individuals, locus-specific somatic MLH1 hypermethylation explained the majority of sporadic early onset MSI-H colon cancer cases. Our data do not suggest an intrinsic tendency for CpG island hypermethylation in these early onset MSI-H tumors other than through somatic mutation of BRAF.

Methylation of the 3p22 region encompassing MLH1 is representative of the CpG island methylator phenotype in colorectal cancer

“…5,33,38-42 We found no evidence for an association between the MLH1 promoter germline SNP c.-93G4A genotype and an increased risk of MLH1 methylation or microsatellite instability in a case-control study of this cohort, arguing against a cis-acting role for this SNP in conferring susceptibility to methylation. In previous studies in which a positive association was found between the G4A genotype and microsatellite instability, either familial cases were retained in the cohort or the study group comprised early-onset cases, [8][9][10][11] suggesting that the genotype may be linked to founder mutations that give rise to familial or early-onset cancer phenotypes in these populations. Although in one study of a large colorectal cancer cohort, the c.-93G4A genotype was shown to be associated with MLH1 methylation, the BRAF V600E mutation as well as CIMP þ among sporadic microsatellite unstable cancers, nevertheless a stronger association between the G4A genotype was demonstrated among the microsatellite unstable cases with a positive family history.…”

“…6 The c.-93G4A SNP (rs1800734) within the MLH1 promoter has been associated with an increased risk of microsatellite instability or MLH1 methylation in some colorectal and endometrial cancer populations, but not in others. [7][8][9][10] However, these associations have been disputed on the basis that linkage disequilibrium with pathogenic mutations could not be ruled out in populations with a high incidence of familial cancer. 11 Recently, promoter reporter assays showed that the A allele of this SNP conferred reduced transcriptional activity compared with the G allele, consistent with the notion that this allele might predispose to promoter methylation.…”

Refining prognosis and identifying targetable pathways for high-risk endometrial cancer; a TransPORTEC initiative

“…27 Contamination of the carcinoma tissue by stromal or inflammatory cells was unavoidable and tumors with a partially methylated phenotype were scored as methylated.…”

MLH1 methylation screening is effective in identifying epimutation carriers

“…8 Approximately 40 index cases of constitutional MLH1 methylation have been reported. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] However, the prevalence of MLH1 constitutional epimutations is still unknown. Most studies addressing this issue have enriched their sampling with patients affected with CRC showing loss of MLH1 protein expression.…”

“…13,17,20,22 In other cases, series were enriched for patients with CRC at an age of onset below 50 years. 9,14,17,23 In a very few cases genetic alterations in cis (gross rearrangements and variants in the promoter region) have been identified as responsible for the methylation. 13,16,19 In these cases, an autosomal dominant pattern is readily observed.…”