Immunohistochemical "null" pattern of all four mismatch repair proteins in Lynch syndrome-associated small bowel adenocarcinoma: a case report and comprehensive genomic profiling.

Immunohistochemistry for mismatch repair (MMR) proteins is important for evaluating the molecular background of MMR-deficient tumors, including those with Lynch syndrome (LS). The four major MMR proteins function as heterodimers (MLH1/PMS2 and MSH2/MSH6), and usually only one of the MMR subsystems is impaired. However, rare cases of concurrent immunohistochemical loss of all four MMR proteins, termed "null" phenotype, have been reported. We present the first reported case of small bowel adenocarcinoma (SBA) showing a null-phenotype. A 62-year-old man developed jejunal cancer. Immunohistochemical analysis showed complete loss of all four MMR proteins in the entire tumor. Genetic testing revealed LS with a germline pathogenic variant in MSH2, and somatic mutations in MSH2 and MLH1 were also detected. No BRAF V600E mutation was found. Combined with comprehensive cancer genomic profiling, this report provides insight into an exceptional MMR-deficient tumor and SBA, a relatively rare disease given its surface area.