Background: Constitutional MLH1 epimutations are characterised by monoallelic methylation of the MLH1 promoter throughout normal tissues, accompanied by allele-specific silencing. The mechanism underlying primary MLH1 epimutations is currently unknown. The aim of this study was to perform an in-depth characterisation of constitutional MLH1 epimutations targeting the aberrantly methylated region around MLH1 and other genomic loci. Methods: Twelve MLH1 epimutation carriers, 61 Lynch syndrome patients, and 41 healthy controls, were analysed by Infinium 450 K array. Targeted molecular techniques were used to characterise the MLH1 epimutation carriers and their inheritance pattern. Results: No nucleotide or structural variants were identified in-cis on the epimutated allele in 10 carriers, in which inter-generational methylation erasure was demonstrated in two, suggesting primary type of epimutation. CNVs outside the MLH1 locus were found in two cases. EPM2AIP1-MLH1 CpG island was identified as the sole differentially methylated region in MLH1 epimutation carriers compared to controls. Conclusion: Primary constitutional MLH1 epimutations arise as a focal epigenetic event at the EPM2AIP1-MLH1 CpG island in the absence of cis-acting genetic variants. Further molecular characterisation is needed to elucidate the mechanistic basis of MLH1 epimutations and their heritability/reversibility.
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We are indebted to the patients and their families. We thank all members of the involved Genetic Counseling Cancer Units and Valencian Biobank Network. We thank Lauro Sumoy and Gabriel Rech for their support with methylation array data analysis, Miguel Àngel Peinado and Mireia Jordà for a critical revision of the manuscript. This work was funded by the Spanish Ministry of Economy and Competitiveness and cofunded by FEDER funds -a way to build Europe-(grants SAF2012-33636 and SAF2015-68016-R), Instituto de Salud Carlos III (grant PI16/00563), CIBERONC, RTICC Network (RD12/0036/0031 and RD12/0036/0008), the Spanish Association Against Cancer (080253), the Government of Catalonia (grant 2014SGR338) and Fundación Mutua Madrileña (grant AP114252013). This article is based upon work from COST Action BM1206, supported by COST (European Cooperation in Science and Technology, www.cost.eu). E.D. was supported by a grant from the Spanish Ministry of Economy and Competitiveness.
© 2018, The Author(s).