A Case of Resistance to Systemic Therapy in Hypermutation of Colorectal Cancer

A 78-year-old man was admitted with diarrhea. Colonoscopy and computed tomography(CT)revealed rectal cancer with multiple liver metastases. Low anterior resection was performed for local control. After the operation, 5 courses of mFOLFOX6 plus bevacizumab chemotherapy were administered as first-line systemic therapy, but CT showed progressive disease with liver metastases. After the first-line systemic therapy, 2 courses of FOLFIRI plus bevacizumab chemotherapy were performed as second-line systemic therapy, but CT also revealed progressive disease with liver metastases. We retrospectively performed comprehensive genomic sequencing with a 415-gene panel and found that the patient had a hypermutation subtype. Interestingly, the panel also revealed that he had mismatch-repair(MMR)deficiency with MSH2 mutation, which is reported as a possible cause of resistance to 5-fluorouracil in colorectal cancer.