Germline mutations inPMS2andMLH1in individuals with solitary loss of PMS2 expression in colorectal carcinomas from the Colon Cancer Family Registry Cohort

Abstract

ObjectivesImmunohistochemistry for DNA mismatch repair proteins is used to screen for Lynch syndrome in individuals with colorectal carcinoma (CRC). Although solitary loss of PMS2 expression is indicative of carrying a germline mutation in PMS2, previous studies reported MLH1 mutation in some cases. We determined the prevalence of MLH1 germline mutations in a large cohort of individuals with a CRC demonstrating solitary loss of PMS2 expression.DesignThis cohort study included 88 individuals affected with a PMS2-deficient CRC from the Colon Cancer Family Registry Cohort. Germline PMS2 mutation analysis (long-range PCR and multiplex ligation-dependent probe amplification) was followed by MLH1 mutation testing (Sanger sequencing and multiplex ligation-dependent probe amplification).ResultsOf the 66 individuals with complete mutation screening, we identified a pathogenic PMS2 mutation in 49 (74%), a pathogenic MLH1 mutation in 8 (12%) and a MLH1 variant of uncertain clinical significance predicted to be damaging by in silico analysis in 3 (4%); 6 (9%) carried variants likely to have no clinical significance. Missense point mutations accounted for most alterations (83%; 9/11) in MLH1. The MLH1 c.113A> G p.Asn38Ser mutation was found in 2 related individuals. One individual who carried the MLH1 intronic mutation c.677+3A>G p.Gln197Argfs*8 leading to the skipping of exon 8, developed 2 tumours, both of which retained MLH1 expression.ConclusionsA substantial proportion of CRCs with solitary loss of PMS2 expression are associated with a deleterious MLH1 germline mutation supporting the screening for MLH1 in individuals with tumours of this immunophenotype, when no PMS2 mutation has been identified.

Biochemical and structural characterization of two variants of uncertain significance in the PMS2 gene

“…Quantification of the relative PMS2 WT and variant expression levels revealed a consistent, statistically significant reduction in the expression of p.Leu42_Glu44del in both whole cell ( P < 0.001) and nuclear extracts ( P < 0.01) compared with the WT and the p.Gly207Glu variant (Figure b). While it is documented that the stability of the PMS2 protein is reliant upon the expression of MLH1 (Dudley et al., ; Mohd et al., ; Rosty et al., ), we consistently observed lower levels of MLH1 when it is coexpressed with the PMS2 p.Leu42_Glu44del variant but not with PMS2 WT or p.Gly207Glu (). This result could be a consequence of reduced stability of the MutLα complex that leads to decreased protein levels of both PMS2 p.Leu42_Glu44del and MLH1.…”

Tumor Molecular Testing Guides Anti-PD-1 Therapy and Provides Evidence for Pathogenicity of Mismatch Repair Variants

“…The MLH1 c.374C>A (p.Ala125Glu) variant is a missense variant. Prior studies have shown that in the case of the MLH1 gene, pathogenic missense variants can sometimes lead to intact expression of a nonfunctional MLH1 protein, with the PMS2 protein missing alone . The genomics of patient 1 are consistent with these reports as an isolated PMS2 loss by immunohistochemistry was seen on the gluteal biopsy (Fig.…”

Epithelioid variant of gastrointestinal stromal tumor harboring PDGFRA mutation and MLH1 gene alteration: A case report

“…In GISTs, infrequent methylation of the MLH1 gene has been reported, but to the best of our knowledge, no MLH1 mutations have been previously reported. In the present case, MLH1 c.1572 G > C transversion was found in exon 14, one of the exons serving as constitutive PMS2 dimerization domains, at an allele frequency of 50.5% . This high frequency of altered allele might indicate germline not but somatic alterations.…”