Background: Mismatch repair deficient/microsatellite instability high (dMMR/MSI-H) colorectal cancer (CRC) defines a molecular subtype with distinct clinicopathologic characteristics including an excellent response to immunotherapy. Although BRAF mutations are established as a negative prognostic marker in CRC, whether they retain their negative prognostic impact in or alter the response to immunotherapy in dMMR/MSI-H CRC remains unknown. Herein, we present a systematic review and meta-analysis of the impact of BRAF mutations on the overall survival (OS) and immune checkpoint inhibitor (ICI) response in dMMR/MSI-H CRC. Methods: Studies published from inception to 26 January 2021 were searched in PubMed, Embase, and major conference proceedings (AACR, ASCO, and ESMO). Eligible studies included the following: 1) observational studies reporting outcomes based on BRAF mutation status in dMMR/MSI-H CRC patients and 2) experimental studies of ICI reporting outcomes based on BRAF mutation status in dMMR/MSI-H CRC patients. A summary hazard ratio (HR) was calculated for OS in BRAF mutated (BRAFmut) vs. BRAF wild type (BRAFwt) patients (pts) with the random effects meta-analysis (REM). A summary odds ratio (OR) was calculated for objective response rate (ORR) in BRAFmut vs. BRAFwt pts treated with ICI with the REM. Results: Database search conducted according to PRISMA guidelines found 4221 studies in total. Initial screening identified 30 studies and after full-text review, 9 studies (N = 4158 pts) were included for the meta-analysis of prognosis (analysis A) and 3 studies (N = 178 pts) were included for the meta-analysis of ICI response (analysis B). The outcome measures are summarized in the table below. Analysis A showed that in stage I-IV dMMR/MSI-H CRC pts, BRAFmut was associated with worse OS than BRAFwt (HR 1.57, 1.23-1.99). The heterogeneity was low (I² = 21%). Subgroup analysis showed no significant difference in the prognostic impact of BRAF mutation status between stage IV only and stage I-IV CRC pts. Analysis B showed no difference in ORR (OR 1.04, 0.48-2.25) between BRAFmut vs. BRAFwt dMMR/MSI-H pts who received ICI. The heterogeneity was low (I² = 0%). Conclusions:BRAF mutations retain their negative prognostic impact in dMMR/MSI-H stage I-IV and stage IV CRC but are not associated with differential ICI response. Limitations include the following: analysis A was based on retrospective studies; also, the impact of BRAF status on the survival outcome of ICI could not be assessed due to limited number of studies.

*Lower HR favors BRAFmut**Higher OR favors BRAFmut.