Lynch syndrome (LS) has been known for a century. The syndrome was originally recognised as a dominant predisposition to cancers of multiple organs, including those of the gastrointestinal tract and female reproductive organs. In 1993–1995, LS was linked to germline mutations in DNA (deoxyribonucleic acid) mismatch repair (MMR) genes
MLH1
,
MSH2
,
MSH6
and
PMS2
. Soon thereafter, promoter methylation as an alternative mechanism of DNA MMR gene inactivation was discovered. Such ‘epigenetic mutations’ (epimutations) may be somatic (e.g. biallelic methylation of
MLH1
in sporadic microsatellite‐unstable cancers) or constitutional (monoallelic methylation of
MLH1
or
MSH2
in normal tissue, involving cells derived from all three germ layers and causing predisposition to LS cancers). To date, epigenetic mechanisms are known to operate at all stages of LS tumourigenesis, from constitutional predisposition to cancer initiation and progression, and involve DNA MMR genes as well as multiple other growth‐regulatory genes. This review discusses the role of epigenetic mechanisms in the pathogenesis of LS and more broadly, LS as a model of epigenetic mechanisms underlying common human cancers.
Key Concepts
Lynch syndrome (LS) is caused by mutations in one of the DNA mismatch repair genes;
MLH1
,
MSH2
,
MSH6
or
PMS2
.
DNA methylation, miRNAs and histone modifications are involved in epigenetic regulation of MMR genes.
Epigenetic regulation has been linked to constitutional predisposition, cancer initiation and progression in LS.
MLH1
gene may be constitutionally affected by primary epimutation (has no known genetic basis and is usually not inherited) or secondary epimutation (associated with genetic
cis
‐ or
trans
‐acting alteration, which can be transmitted to offspring).
EPCAM
deletions may induce secondary epimutations of the
MSH2
gene.
MMR genes typically, but not always, require two hit inactivation for tumour initiation.
LS spectrum tumours have epigenetic profiles characteristic of tumour type.
Environmental factors interact with genetic and epigenetic factors, influencing LS phenotype.