We investigated the prevalence and characteristics of defective mismatch repair (dMMR) in colorectal cancer (CRC) patients who would potentially benefit from anti-programmed cell death protein 1 (PD-1) immunotherapy. Medical records were obtained and reviewed for 1147 patients who underwent surgical resection of stage I-IV CRC, in whom universal screening for Lynch syndrome using immunohistochemistry for MMR proteins had been undertaken. The molecular characteristics of dMMR CRCs were also investigated. Defective MMR accounted for 5.2% of stage I-IV CRC patients, including 12 (1.0% of all CRC patients) who had stage IV disease or recurrence after curative resection (n = 6 each). These 12 patients included patients with LS (n = 3) and Lynch-like syndrome (n = 1). Defective MMR tumors were predominantly located in the right-sided colon (P < 0.01). Approximately 1% of stage I-IV CRC patients could potentially benefit from anti-PD-1 immunotherapy, while one-third would require genetic counseling and/or MMR gene testing.
Keywords: Anti-PD-1 blockade; Colorectal cancer; Defective mismatch repair.