The cancer susceptibility in hereditary nonpolyposis colorectal cancer (HNPCC) syndrome is associated with defective DNA mismatch repair (MMR). An inherited defect in one MMR allele and an acquired defect in the other allele lead to loss of MMR function in the respective polypeptide that accelerates tumour progression. Accordingly, lack of the mutated protein and instability at short tandem repeat sequences such as microsatellites in the genome are the main molecular characteristics of the HNPCC tumours.
More than 450 different MMR gene mutations and 100 intragenic polymorphisms, affecting mostly the MMR genes, MLH1, MSH2, and MSH6, are listed in the international HNPCC mutation database (http://www.nfdht.nl). The clinical phenotype varies between families. Contrary to most mutations affecting the MLH1 and MSH2 genes, a significant proportion of MSH6 mutations occur in HNPCC families with less typical clinical features. Carriers of MSH6 mutations often display late age at onset, carcinomas of the endometrium, and low or no microsatellite instability (MSI) in tumours (Miyaki et al, 1997; Kolodner et al, 1999; Wijnen et al, 1999; Wu et al, 1999; Parc et al, 2000; Wagner et al, 2001; Berends et al, 2002; Peterlongo et al, 2003; Cederquist et al, 2004). Moreover, one-third of MSH6 mutations are nontruncating, and so do not necessarily destroy the encoded protein. These mutations may be associated with a positive immunohistochemical (IHC) staining for the MSH6 protein, which makes them even more difficult to interpret. In the mutation database, 12 MSH6 amino-acid substitutions are listed as pathogenic mutations, while 16 nucleotide changes in the MSH6 coding sequence are listed as polymorphisms. In both groups some changes are found to segregate in putative HNPCC families, while others occur in single endometrial cancer (EC) or colorectal cancer (CRC) patients, which impedes segregation analysis and makes their interpretation impossible.
To address the question, which, if any, MSH6 missense mutations cause susceptibility to HNPCC, we previously studied the functionality of six MSH6 variants (S144I, G566R, P1087T, P1087R, R1095H, and L1354Q) (Kariola et al, 2002, 2003). Here, we expanded the study by analysing five novel MSH6 missense mutations (R128L, P623L, K728T, G881K+S, and E1193K). Distinct from the previous cases, these mutations were derived from a population-based study of EC and CRC patients whose tumours showed MSI, the main hallmark of HNPCC. Our experimental results in conjunction with the clinical characteristics of the patients provide evidence that most MSH6 missense mutations found in a population-based case series of patients with MSI positive tumours are unlikely to be deleterious.