Highly sensitive MLH1 methylation analysis in blood allows the identification of low-level epigenetic mosaicism

Constitutional MLH1 epimutations are a rare cause of Lynch syndrome. Low methylation levels (≤10%) have been occasionally described. The aim of this study was the identification of patients with low levels of epigenetic mosaicism in MLH1 gene. Eighteen patients with MLH1 hypermethylated tumors and undetectable methylation in blood as assessed by Methylation-Specific Multiplex Ligation-Dependent Probe Amplification were included (MS-MLPA). Highly sensitive MS-Melting Curve Analysis (MS-MCA) at MLH1 promoter was used to screen for epigenetic mosaicism. Constitutional methylation was confirmed by other methods. Mutational analysis of hereditary cancer genes including MLH1 was performed. MS-MCA analysis identified one case (5.6%) with low levels of methylation (1-2%) in blood DNA. The patient had developed 3 gastrointestinal tumors at ages 22, 24 and 25, sharing MLH1 promoter hypermethylation and loss of heterozygosity associated with c.655A allele. The presence of low MLH1 methylation levels was confirmed by clonal bisulfite sequencing, evidencing the association with c.-93G allele (in phase with c.655G). The extension of the hypermethylated region overlaps with the reported in constitutional MLH1 epimutation carriers. No rare germline variants were identified. The use of highly sensitive techniques such as MS-MCA has demonstrated to be useful for the detection of low MLH1 methylation levels in blood. Case 29 was positive for germline MLH1 methylation. In order to confirm the presence of epimutation at the patient, her methylation values were compared against values reported for constitututional MLH1 epimutation carriers by Dámaso et al., 2019 (GEO Series=GSE107353).