Two PMS2 mutations in a Turcot syndrome family with small bowel cancers

We report the clinicopathological, genetic, and immunohistochemical characterization of an atypical Turcot syndrome (TS) family with small bowel cancer. The tumor family history of a patient with cafe-au-lait spots (CALS) and early onset adenomas, duodenal cancer, and glioblastoma was positive for colonic adenoma (mother), jejunal (maternal grandfather), lung (father), and colorectal (paternal uncle) cancers. PMS2 genetic testing identified the nonsense 1951C>T (Q643X) and the missense 161C>T (S46I) mutations. PMS2 expression was absent in the proband's duodenal cancer with high microsatellite instability. The normal cells also displayed no PMS2 expression and some degree of instability. Our findings point out the association between PMS2 and TS, and support the hypothesis that patients with a few polyps, small bowel tumors with a very early onset, glioblastoma, and CALS should be considered as a variant of hereditary nonpolyposis colorectal cancer. A recessive model of inheritance caused by compound heterozygous mutations was consistent with the observed severe clinical phenotype and has important implications for predicting cancer risk in both the proband and his relatives.