BRAF variants and oncogenic fusions to define sporadic MMR-deficient colorectal cancer subtypes.

Abstract

211 Background: Mismatch-repair deficient (MMRd) colorectal cancers (CRC) are rare tumors exquisitely sensitive to immunotherapy. The molecular events that initiate and sustain MMRd tumors remain controversial. The majority of sporadic MMRd CRCs exhibit BRAF-V600E variants with MLH1 hypermethylation, however BRAF-wild-type(wt) tumors exhibit high rates of rare gene fusions. Methods: We performed an integrated molecular and clinical analysis for 253 patients with MMRd sporadic CRC who received MSK-IMPACT genomic profiling at Memorial Sloan Kettering Cancer Center. Tumors were classified based on BRAF-V600E variants and gene fusion events. We evaluated the tumor burden of molecular alterations and disproportionate co-occurrence of specific driver variants of significance between each subtype. We used allele-specific FACETS analysis and MLH1 promoter hypermethylation assessments to infer the MMR-gene inactivation mechanism per tumor. We also examined differences in tumor phenotype and patient clinical outcomes. Patient progression-free-survival after treatment and overall survival was assessed with the Kaplan-Meier curves, and multivariable analyses were performed with Cox proportional hazards regression modeling. Results: Patients exhibited BRAF-V600E-mutated (mut: n=178), BRAF-V600E-wt/gene-fusion positive (n=42), and BRAF-V600E-wt/fusion-negative (n=33) disease. Gene fusion events were mutually exclusive with BRAF-V600E variants and occurred in NTRK1 (n=13), NTRK3 (n=8; one with co-occurring NTRK1 fusion), RET (n=8), BRAF (n=8), ALK (n=5), and FGFR2 (n=1). BRAF-V600E-mut and fusion-positive tumors display similar predominant MLH1 silencing, WNT-signaling activators, and co-alterations. In contrast, BRAF-V600E-wt/fusion-negative tumors exhibit MLH1 inactivation associated with increased copy number alterations, and exhibit distinct co-occurring APC and TCFF7L2 variants. Both fusion-positive and BRAF-V600E-wt/fusion-negative tumors disproportionately arise in the transverse and left colon. In a multivariable analysis of 73 patients with stage IV MMRd CRC, fusion-positive status was an independently-significant, positive prognostic marker. For patients treated with immune checkpoint inhibitors (ICIs), tumor regression rates were high in BRAF-V600E-mut (63%; n=22/35) and fusion-positive (62.5%, n=5/8) subtypes, with slightly poorer outcomes in BRAF-V600E-wt/fusion-negative (40%, n=2/5) tumors. Patients with fusion-positive tumors exhibited clinical benefit to NTRK inhibition. Conclusions: In sporadic MMRd CRC, gene fusion-positive tumors exhibit an improved prognosis with similar molecular and treatment outcomes to BRAF-V600E-mut tumors. BRAF-V600E-wt/fusion-negative tumors are a unique sporadic MMRd subtype driven by alternative WNT-activation and potentially lower immunotherapy responses in small numbers of patients.