A case of early onset rectal cancer of Lynch syndrome with a novel deleterious PMS2 mutation

Heterozygous deleterious mutation of the PMS2 gene is a cause of Lynch syndrome, an autosomal dominant cancer disease. However, the frequency of PMS2 mutation is rare compared with that of the other causative genes; MSH2, MLH1 and MSH6. PMS2 mutation has so far only been reported once from a Japanese facility. Detection of PMS2 mutation is relatively complicated due to the existence of 15 highly homologous pseudogenes, and its gene conversion event with the pseudogene PMS2CL. Therefore, for PMS2 mutation analysis, it is crucial to clearly distinguish PMS2 from its pseudogenes. We report here a novel deleterious 11 bp deletion mutation of exon 11 of PMS2 distinguished from PMS2CL in a 34-year-old Japanese female with rectal cancer. PMS2 mutated at c.1492del11 results in a truncated 500 amino acid protein rather than the wild-type protein of 862 amino acids. This is supported by the fact that, although there is usually concordance between MLH1 and PMS2 expression, cells were immunohistochemically positive for MLH1, whereas PMS2 could not be immunohistochemically stained using an anti-C-terminal PMS2 antibody, or effective PMS2 mRNA degradation with NMD caused by the frameshift mutation.