Atypical (non-V600E) BRAF mutations in metastatic colorectal cancer in population and real-world cohorts

BRAF-V600E mutation (mt) is a strong negative prognostic and predictive biomarker in metastatic colorectal cancer (mCRC). Non-V600Emt, designated atypical BRAFmt (aBRAFmt) are rare, and little is known about their frequency, co-mutations and prognostic and predictive role. These were compared between mutational groups of mCRC patients collected from three Nordic population-based or real-world cohorts. Pathology of aBRAFmt was studied. The study included 1449 mCRC patients with 51 (3%) aBRAFmt, 182 (13%) BRAF-V600Emt, 456 (31%) RAS&BRAF wild-type (wt) and 760 (52%) RASmt tumours. aBRAFmt were seen in 2% of real-world and 4% of population-based cohorts. Twenty-six different aBRAFmt were detected, 11 (22%) class 2 (serrated adenocarcinoma in 2/9 tested), 32 (64%) class 3 (serrated in 15/25) and 4 (8%) unclassified. aBRAFmt patients were predominantly male, had more rectal primaries, less peritoneal metastases, deficient mismatch repair in one (2%), and better survival after metastasectomy (89% 5-year overall survival [OS]-rate) compared with BRAF-V600Emt. aBRAFmt and BRAF-V600Emt had poorer performance status and received fewer treatment lines than RAS&BRAFwt and RASmt. OS among aBRAFmt (median 14.4 months) was longer than for BRAF-V600Emt (11.2 months), but shorter than for RAS&BRAFwt (30.5 months) and RASmt (23.4 months). Addition of bevacizumab trended for better OS for the aBRAFmt. Nine patients with aBRAFmt received cetuximab/panitumumab without response. aBRAFmt represents a distinct subgroup differing from other RAS/BRAF groups, with serrated adenocarcinoma in only half. OS for patients with aBRAFmt tumours was slightly better than for BRAF-V600Emt, but worse than for RASmt and RAS&BRAFwt. aBRAFmt should not be a contraindication for metastasectomy.