ClinVar

Description

The p.Ala20Val variant was identified in 3 of 2290 proband chromosomes (frequency: 0.001) from individuals or families with colorectal cancer (Charames 2000, Nilbert 2009, Wasielewski 2009) and was present in 1 of 332 control chromosomes (frequency: 0.003) from healthy individuals (Wasielewski 2009). The variant was also identified in dbSNP (ID: rs63750664), with a minor allele frequency of 0.0002 (1000 Genomes Project), NHLBI Exome Sequencing Project (Exome Variant Server), HGMD, MutDB, “Mismatch Repair Genes Variant Database”, “InSiGHT Colon Cancer Database”, the ClinVar database (classified with uncertain significance by InSiGHT and Ambry Genetics; classified as likely benign by GeneDx), and UMD (1X as a likely neutral variant). The variant was identified by the Exome Variant Server project in 3 of 8566 European American alleles (frequency: 0.0003), although this low number of observations and low frequency is not substantive enough to determine the prevalence of the variant in the general population and its relationship to disease.The p.Ala20 residue is not conserved in mammals and computational analyses (PolyPhen-2, SIFT, AlignGVGD, BLOSUM, MutationTaster) do not suggest a high likelihood of impact to the protein. However, this information is not predictive enough to rule out pathogenicity. The p.Ala20Val variant occurs outside of the splicing consensus sequence and 1 of 5 in silico or computational prediction software programs (SpliceSiteFinder, MaxEntScan, NNSPLICE, GeneSplicer, HumanSpliceFinder) predict a greater than 10% difference in splicing, although this is not very predictive of pathogenicity. A study by Charames (2000) suggested this alteration result in a significant change in amino acid polarity occurring outside of the MSH6 domains known to interact with MSH2.However, three functional studies have suggested the variant has no impact on MSH6 gene and has neutral functional effect (Ali 2012, Drost 2012, Terui 2013). In addition, this variant was identified in one individual by our laboratory as co-occuring with a pathogenic variant in MSH6 (c.3932_3935dup, p.Ile1313SerfsX7), increasing the likelihood this variant does not have clinical significance. In summary, based on the above information, this variant is classified as benign.