Guidelines on Genetic Evaluation and Management of Lynch Syndrome

A Consensus Statement by the US Multi-Society Task Force on Colorectal Cancer

Francis M Giardiello MD; John I Allen; Jennifer E Axilbund; C Richard Boland; Carol A Burke; Randall W Burt; James M Church; Jason A Dominitz; David A Johnson; Tonya Kaltenbach; Theodore R Levin; David A Lieberman; Douglas J Robertson; Sapna Syngal; Douglas K Rex

Disclosures

Am J Gastroenterol. 2014;109(8):1159-1179. 

In This Article

Genetic Alterations

Germline Mutations

LS is caused by inactivation of one of several DNA MMR genes. These genes function to maintain fidelity of the DNA during replication by correction of nucleotide base mis-pairs and small insertions or deletions generated by mis-incorporations or slippage of DNA polymerase during DNA replication. Germline mutation in the MMR genes MLH1, MSH2, MSH6, and PMS2 cause LS.[10,76–79] Also, deletions of the terminal codon of the EPCAM gene (previously called the TACSTD1 gene), located just upstream from the MSH2 gene, result in silencing of the MSH2 gene in tissues that express EPCAM and, consequently, produce a phenotype very similar to LS.[80] In an investigation of 2 families, when the deletion is isolated to the stop codon of EPCAM, a colon-only phenotype occurs.[81] In another study, if the deletion also includes critical portions of the MSH2 promoter, a full LS phenotype results.[82] Mutations in MLH1 and MSH2 account for up to 90% and MSH6 about 10% of mutations found in LS families. In the past, PMS2 mutations have been identified rarely because of the presence of multiple PMS2 pseudogenes, which confuse genetic diagnostics.[83,84] A recent study found PMS2 mutations in 6% of all LS families.[85]

Germline Epimutations

Rare patients have been reported with germline MLH1 hypermethylation. These patients do not have MLH1 sequence variations or rearrangements. This epimutation appears to be mosaic, involving different tissues to varying extents and is typically reversible so that offspring are usually unaffected, but inheritance has been demonstrated in a few families. Patients with this epimutation have early-onset LS and/or multiple LS cancers.[86]

Tumor Alterations

LS is caused by a single dominant mutation inherited in the germline, which increases risk for cancer. The LS cancers form only after a second hit (by one of several genetic damage mechanisms) occurs within somatic tissue, which causes loss of function to the normal (wild-type) allele inherited from the unaffected parent; this results in total loss of DNA MMR activity in that cell and subsequent MSI. Therefore, the disease is inherited as a Mendelian dominant. However, the tumors occur after somatic biallelic gene inactivation, with one mutation inherited and the other acquired.

Microsatellite Instability. MSI is a phenomenon manifested by ubiquitous mutations at simple repetitive sequences (microsatellites) found in the tumor DNA (but not in the DNA of the adjacent normal colorectal mucosa) of individuals with MMR gene mutations.[87] MSI is characterized by abnormal expansion or contraction of these microsatellite repeats. Microsatellite repeats are normally found through the genome primarily in intronic sequences. MSI in CRC indicates a defect in one of the MMR genes caused by either somatic changes of the gene (hypermethylation of the MLH1 promoter) or a germline defect (LS). MSI is found in most (>90%) colon malignancies in patients with LS (due to germline MMR gene mutation) and in 12% of patients with sporadic CRC (due to somatic hypermethylation of the MLH1 gene).[87] MSI is graded as MSI-high (≥30% of markers are unstable), MSI-low (<30% of markers are unstable), and MS-stable (no markers are unstable).[88] Most CRCs in LS are MSI-high. The significance of MSI-low tumors is controversial. Some evidence suggests that MSI-low is due to MSH6 germline mutation in certain cases,[89] but this phenomenon is most often caused by somatic inactivation of the MSH3 gene, which is common and not inherited.[90,91] Somatic down-regulation of MSH3 is accompanied by MSI-low, as well as mutations at trinucleotide and tetranucleotide repeats, but not mutations at mononucleotide and dinucleotide repeats, which are used for standard ascertainment of MSI.[90] In addition, germline mutations in MLH3 have not been associated with an LS phenotype.[92,93]

Loss of Expression of DNA Mismatch Repair Proteins. IHC of CRCs utilizing antibodies to the MMR gene proteins MLH1, MSH2, MSH6, and PMS2 evaluates for the loss of MMR protein expression and assists in the identification of patients with LS.[94] Deleterious alterations (either germline or somatic) in specific DNA MMR are indicated by loss or partial production of the MMR protein produced by that gene. MSH2 and MSH6 proteins are often lost concurrently and indicate MSH2 mutation. Isolated loss of MSH2 or MSH6 on IHC testing has high specificity for a germline mutation of the MSH2 or MSH6 gene, respectively, hence the diagnosis of LS. Also, loss of the MSH2 protein can be caused by germline mutation in the EPCAM gene rather than MSH2 gene. Similarly, MLH1 and PMS2 proteins are also often lost together; this generally indicates loss of MLH1 function either due to germline mutation or somatic (not germline) silencing of the MLH1 gene (see Somatic methylation of MLH1). Isolated loss of PMS2 protein generally indicates an underlying germline PMS2 mutation.

Somatic Methylation of MLH1. Aberrant MLH1 gene promoter methylation is a somatic event that is confined to the CRC and is rarely inherited. Aberrant methylation of MLH1 is responsible for causing loss of MLH1 protein expression and results in MSI found in approximately 12% of sporadic cancers.[95] The methylation of MLH1 must be biallelic to abrogate MMR activity.

BRAF Mutations. The BRAF gene, a member of the RAF-RAS gene family, encodes a cytoplasmic serine/threonine kinase, an important component of the mitogen-activated protein kinase signaling pathway. Somatic mutations in the BRAF gene, largely at codon 600, are noted in 15% of sporadic CRCs. These are CRCs that develop through a methylation pathway called CpG island methylator phenotype. These cancers can also demonstrate MSI-high through somatic promoter methylation of MLH1.

Somatic BRAF V600 mutations have been detected predominantly in sporadic CRC[96,97] of the type discussed here. Consequently, the presence of a BRAF mutation in an MSI-high CRC is usually, but not always, evidence against the presence of LS.[98]

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