BRAFV600E-mutated colorectal cancer: primary and acquired resistance to targeted treatment (MASCARA study)

BRAF-mutant metastatic colorectal cancer (mCRC) is a clinically aggressive subtype with limited treatment options and poor prognosis. The BRAFV600E mutation, found in approximately 10% of CRC cases, is associated with rapid disease progression and resistance to standard therapies. Although combination treatments such as Encorafenib and Cetuximab have shown clinical benefit, predictive biomarkers for response remain largely undefined. The MASCARA study investigates the clinical and molecular characteristics of BRAF-mutant mCRC, aiming to identify biomarkers predictive of treatment response. A cohort of 50 patients, with 30 evaluated for clinical and genomic data, was analyzed using whole-exome sequencing (WES), circulating tumor DNA (ctDNA) profiling, and spatial transcriptomics. Preliminary results show a median age of 69 years, a predominance of female patients (70%), and tumors primarily located in the right colon (80%). Liver (40%) and lung (30%) were the most common metastatic sites. RECIST-based assessment revealed that 20% of patients achieved complete or partial response, while 80% were non-responders. Genomic profiling confirmed BRAFV600E in all cases and identified frequent alterations in TP53, BRINP3, NOTCH3, LRP2, and FBXW7. Most tumors were microsatellite stable (80%) with high tumor mutational burden (TMB-H, 87%). Resistance-associated mutations (e.g., RAS, PIK3CA, SMAD4) were detected via ctDNA, suggesting clonal evolution during treatment. Spatial transcriptomics highlighted immune-related differences between responders and non-responders. Responders exhibited upregulation of antigen presentation genes (CD74, HLA-DRA, HLA-DRB), indicating a more immune-permissive tumor microenvironment (TME). In contrast, non-responders showed enrichment of immune evasion markers (SSX1, CCL15) and downregulation of tumor suppressors (ACTB, EPCAM). Pathway analysis revealed enrichment of immune activation in responders and survival/immune evasion pathways (e.g., FGFR2, GPCR signaling) in non-responders. These findings underscore the combined impact of genetic alterations and TME composition in modulating therapy response and support the development of personalized therapeutic strategies.