[RAS, BRAF, and Mismatch Repair Deficiency in Young-Onset Colorectal Cancer]

Between January 2020 and November 2024, we retrospectively analyzed the results of mismatch repair protein immunohistochemistry(MMR-IHC)/microsatellite instability(MSI)test, RAS, and BRAF genetic tests in 48 consecutive patients under 50 years of age who underwent primary tumor resection of colorectal cancer. Of these, 35 patients underwent MMR-IHC/ MSI testing, revealing 32 proficient MMR(pMMR)/non-MSI-high and 3 deficient MMR(dMMR)/MSI-high, 2 of whom were diagnosed as having Lynch syndrome. RAS/BRAF testing was performed in 28 patients, identifying 6 with KRAS variants, while no BRAFV600E or KRASG12C were found. The frequencies of dMMR and Lynch syndrome in patients under 50 years old was comparable to our previous report in which testing was conducted as part of research. Our results suggest that in patients under 50 years old(, 1)the utility of BRAF testing as an adjunctive diagnostic tool for Lynch syndrome is limited, and (2)BRAFV600E or KRASG12C was not detected;however, a larger accumulation of cases is necessary.