Distinct Genomic Landscape In Colorectal Mucinous Carcinoma Via Comprehensive Genomic Profiling

222 Background: Mucinous carcinoma (MC), accounting for 10-15% of colorectal cancer (CRC), has long been associated with an inferior response to treatment. MC as an unfavorable prognostic factor in early stage CRC remains questionable. In this study, we used a large CRC cohort to define the genomic landscape of MC and identify prognostic and clinically actionable information. Methods: The 1226 CRC patient cohort included 997 adenocarcinomas (AC), 129 MC, and 110 adenocarcinomas with mucous composition (AMC). All of the pathological sections were verified by an experienced pathologist. FFPE tumor samples and matched peripheral blood were sequenced using a 450-cancer-related gene panel. Results: MC was significantly associated with right colon (p<0.001) and regional lymph node metastasis (p=0.004) compared to AC. We found a higher mutation rate of BRAF V600E (10.9% vs. 3.3%), PIK3CA (28.7% vs. 19.2%), SMAD4 (34.1% vs. 19.1%), BRCA1/2 (16.3% vs. 6.8%) and homology recombination pathway (40.3% vs. 22.7%), and a lower rate of TP53 (53.5% vs. 79.5%), APC (46.5% vs. 75.1%,), and HER2 amplification (0% vs. 2.1%,) in MC than in AC. MC had a significantly higher proportion of microsatellite instability-high (MSI-H) tumors (22.5% vs. 6.8%, p<0.001). Furthermore, in tumors with MSI stable, POLE mutation was more frequent in MC than in AC (7.0% vs. 2.6%, p=0.094) and resulted in dramatic elevated tumor mutational burden (TMB, range 79.5-591.5 muts/Mb), indicating up to 30% of MC patients may benefit from immunotherapy. MSI-H was associated with better prognosis (5-y DFS, MSI-H 86.7% vs. MSS 56.7%) in stage II/III CRC patients with MC. Importantly, AMC mimicked the genomic features of MC rather than AC. Conclusions: For the first time, comprehensive genomic analysis revealed that MC had distinct molecular features, indicating promising clinically application for both immunotherapy and targeted therapy. Early stage MC had a diverse prognosis due to MSI status and should be interpreted differently. The similarity of molecular profile between AMC and MC suggested the possible usage of MC, but not AC, clinical strategy for AMC.