Further evidence for heritability of an epimutation in one of 12 cases with MLH1 promoter methylation in blood cells clinically displaying HNPCC

Abstract

Germline mutations in mismatch repair (MMR) genes, tumours with high microsatellite instability (MSI-H) and loss of MMR protein expression are the hallmarks of HNPCC (Lynch syndrome). While somatic MLH1 promoter hypermethylation is generally accepted in the tumorigenesis of sporadic tumours, abnormal MLH1 promoter methylation in normal body cells is controversially discussed as a mechanism predisposing patients to HNPCC. In all 94 patients suspected of HNPCC-syndrome with a mean age of onset of 45.5 years, MLH1-deficiency in their tumours but no germline mutation, underwent methylation-specific PCRscreening for MLH1 promoter methylation. In peripheral blood cells of 12 patients an MLH1 promoter methylation, in seven informative cases allele-specific, was found. Normal colonic tissue, buccal mucosa, and tumour tissue available from three patients also presented abnormal methylation in the MLH1 promoter. The heredity of aberrant methylation is questionable. Pro: MLH1 promoter methylation was found in a patient and his mother giving evidence for a familial predisposition for an epimutation in MLH1. Contra: a de novo set-up of methylation in one patient, a mosaic or incomplete methylation pattern in six patients, and no evidence for inheritance of MLH1 promoter methylation in the remaining families. Our findings provide strong evidence that MLH1 promoter methylation in normal body cells mimics HNPCC and constitutes a pathogenic pre-lesion in MLH1. The identification of hypermethylation as an epigenetic defect has important implications for surveillance recommendations, as these patients should be treated like Lynch syndrome patients, whereas the heritability of methylation is still under investigation.

MLH1 methylation screening is effective in identifying epimutation carriers

“…12,13,15,17 This associates with an allele-specific methylation pattern. 11,17,20,21 In these cases, methylation seems to be widespread affecting all embryonic layers being mosaicism reported. 10,12,20 The aim of our study was to investigate the prevalence of MLH1 epimutations in a series of 34 patients with MLH1-methylated CRC and no detected germline MLH1 mutations.…”

“…11,17,20,21 In these cases, methylation seems to be widespread affecting all embryonic layers being mosaicism reported. 10,12,20 The aim of our study was to investigate the prevalence of MLH1 epimutations in a series of 34 patients with MLH1-methylated CRC and no detected germline MLH1 mutations. We identified two bona fide MLH1 epimutation carriers and extensively characterized one of them.…”

“…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.…”

“…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.…”

“…In this context, the inheritance of the epimutation has only been experimentally confirmed in three cases. 10,17,20 The functional impact of these epimutations seems clear. In the few cases analyzed, methylation has been linked to allele-specific silencing of MLH1 and EPM2AIP1.…”