e12546 Background: The clinical diagnosis of Lynch syndrome is usually established when a family fulfills the Amsterdam Criteria, and confirmed by identification of MMR deficiency and/or germline mutations in one of the MMR genes. In a subset of families with Amsterdam criteria, no evidence of DNA mismatch repair deficiency can be identified and this clinical entity has been designated “Familial Colorectal Cancer Type X (FCCTX)”. Methods: The prevalence of MMR deficiency was assessed in a group of 25 individuals with colorectal or endometrial cancer from 20 families fulfilling Amsterdam criteria in Southern Brazil. MMR deficiency was assessed by IHC testing using a panel of antibodies against MSH2, MLH1, MSH6, and PMS2. In cases showing loss of nuclear expression of MLH1, presence of the BRAF p.V600E mutation and microsatellite instability were assessed in the tumor. Results: Fourteen of the 20 families studied fulfilled Amsterdam II criteria. MMR deficiency was identified in the tumor of 11 index cases and in the remaining 9, nuclear expression of all four MMR proteins was normal, suggesting the diagnosis of FCCTX. This FCCTX phenotype was observed in both Amsterdam I and Amsterdam II families. CRC with MMR-deficiency was diagnosed at an earlier age (41.2 years) than CRC showing MMR proficiency (47.2 years), although the difference did not reach significance. Conclusions: FCCTX is a frequent differential diagnosis in hereditary colorectal cancer in patients presenting to high risk cancer genetics clinics in Southern Brazil and further molecular characterization of hereditary colorectal cancer families with these features is warranted.
Diagnosis and management of hereditary colorectal cancer syndromes: Lynch syndrome as a model