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

<p>Abstract</p> <p>Background</p> <p>To investigate the etiology of <it>MLH1 </it>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.</p> <p>Methods</p> <p>We studied a set of 46 MSI-H colon tumors cases with <it>MLH1 </it>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: <it>BRAF, KRAS</it>, <it>GADD45A </it>and the <it>MLH1 </it>-93G>A polymorphism. When available, normal colon and leukocyte DNA was tested for <it>GADD45A </it>mutations and germline <it>MLH1 </it>methylation. SNP array analysis was performed on a subset of tumors.</p> <p>Results</p> <p>We identified two cases (33 and 60 years) with <it>MLH1 </it>germline promoter methylation. <it>BRAF </it>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 <it>BRAF </it>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. <it>GADD45A </it>sequencing yielded an unclassified variant. Tumors from both age groups showed infrequent copy number changes and loss-of-heterozygosity.</p> <p>Conclusion</p> <p>Somatic or germline <it>GADD45A </it>mutations did not explain sporadic MSI-H colon cancer. Although germline <it>MLH1 </it>methylation was found in two individuals, locus-specific somatic <it>MLH1 </it>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 <it>BRAF</it>.</p