ClinVar

Description

The p.C333R pathogenic mutation (also known as c.997T>C), located in coding exon 6 of the MSH2 gene, results from a T to C substitution at nucleotide position 997. The cysteine at codon 333 is replaced by arginine, an amino acid with highly dissimilar properties. This mutation was detected in an individual with early-onset colorectal cancer who met Amsterdam criteria and whose tumor revealed loss of MSH2 and MSH6 protein expression on IHC (Ambry internal data). This alteration was also reported in an individual with colorectal cancer diagnosed at the age of 62; his colorectal tumor showed high microsatellite instability (MSI-H) (Adebamowo CA et al. Afr J Med Med Sci. 2000 Mar; 29(1):71-3). Based on internal structural analysis, the p.C333R alteration results in strong perturbation of the region and is significantly more destabilizing than known pathogenic variants in the region (Warren JJ et al. Mol. Cell. 2007 May; 26(4):579-92; Obmolova G et al. Nature. 2000 Oct; 407(6805):703-10). In a massively parallel cell-based functional assay testing susceptibility to a DNA damaging agent, 6-thioguanine (6-TG), this variant was reported to be functionally deleterious (Jia X et al. Am J Hum Genet, 2021 01;108:163-175). Functional studies in yeast using recombinant protein expressing the homologous residue, p.C345R, demonstrated loss of yeast MSH6 subunit interaction, reduced mismatch repair activity, and reduced protein expression compared to wild type (Gammie AE et al. Genetics. 2007 Oct;177(2):707-21; Martinez SL et al. Proc Natl Acad Sci U S A. 2010 Mar 16;107(11):5070-5). This variant is considered to be rare based on population cohorts in the Genome Aggregation Database (gnomAD). This amino acid position is highly conserved in available vertebrate species. In addition, this alteration is predicted to be deleterious by in silico analysis. Based on the supporting evidence, this alteration is interpreted as a disease-causing mutation.