IDENTIFICATION OF PATHOGENIC DEFECTS IN HEREDITARY NONPOLYPOSIS COLORECTAL CANCER: THE CONTRIBUTION OF GERMLINE ALLELE-SPECIFIC EXPRESSION

The identification of germline mutations in hereditary nonpolyposis colorectal cancer (HNPCC) families is crucial for efficient clinical management of mutation carriers. In a relevant fraction of cases mutations are not identified or have an unclear pathogenic potential. We discuss the results of extensive analyses with previously developed or novel assays in 140 HNPCC patients providing insights useful for clinical translation of genetic testing with traditional or next generation methods. Analyses included germline mismatch repair (MMR) gene screening for allele-specific expression (ASE), point mutations,rearrangements and promoter methylation. Germline EPCAM rearrangements were screened. Tumors were tested for microsatellite instability and immunohistochemical MMR expression. Mutation negative probands were screened for germline APC and MYH point mutations. As expected, the majority of germline defects detected were MMR point mutations (80%) and rearrangements (16.5%). Remarkably, ASE analysis detected germline MMR defects in 8/22 (36.5%) informative probands analyzed and in 3 cases imbalanced ASE was the only germline MMR defect identified. In one additional case ASE provided evidence for the pathogenic role of a putative splicing defect. Only one EPCAM rearrangement was found in our study. No germline MMR promoter methylation was detected. Notably, APC or biallelic MYH mutations were detected in 2 MMR mutation negative probands. Our results show that ASE complements other analyses in the identification of germline defects predisposing to CRC. They indicate that EPCAM rearrangements are not frequent in HNPCC, they underline the importance of screening APC and MYH in MMR mutation negative probands and caution about the confounding potential of genetic heterogeneity within families.