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

Eddy Hj Van Roon,Jan Oosting,Carli Mj Tops,Dianhdra Erasmus,Frederik J Hes,Manon Van Engeland,Emile J De Meijer,Hans Morreau,Tom Van Wezel,Kim Ad Wouters,Anneke Middeldorp,Marjo Van Puijenbroek,Judith M Boer,Ronald Van Eijk

doi:10.1186/1471-2407-10-180

Eddy Hj Van Roon, Jan Oosting + Show 12 more

Open Access

https://doi.org/10.1186/1471-2407-10-180

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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.

Highlights

To investigate the etiology of MLH1 promoter methylation in mismatch repair (MMR) mutation-negative early onset MSI-H colon cancer
All tumors showed loss of expression of nuclear MLH1 and its heterodimer PMS2, confirming the deleterious effect of MLH1 promoter methylation. Both MSH2 and MSH6 stained positive and pathogenic germline mutations in any of the four mismatch repair genes were identified in none of the patients
CpG island DNA methylation is more frequent in older patients and is highly correlated with BRAF mutation in younger colon carcinoma patients We examined the methylation status of 31 samples (11 below 50 years, 20 above) using six CpG island methylator phenotype (CIMP) markers (MINT1, MINT2, MINT12, MINT31, RIZ1 and TIMP3) with Methylation-Specific PCR (MSP) and two CIMP markers (MINT27 and Megalin) with Combined Bisulfite Restriction Analysis (COBRA)

Summary

Introduction

To 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. High frequency of microsatellite instability (MSI-H) is the hallmark of tumors with a mismatch DNA repair (MMR) deficiency. This deficiency leads to an accumulation of somatic mutations, especially in repetitive coding or non-coding DNA sequences (microsatellites) in the genome.

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