Mutational landscape and evolutionary pattern of peritoneal metastases in colorectal cancer.

e15533 Background: Peritoneal metastases occur in approximately 10% of all colorectal cancer (CRC) patients, which are associated with significant morbidity and worse clinical outcome. To date, the evolutionary pattern and molecular mechanisms of peritoneal metastases remains largely unknown, which provided little insights into the prevention and treatment of this subgroup of CRC patients. Methods: We performed whole-exome sequencing (WES) on 49 samples of matched primary tumors, peritoneal metastases, and lymph node metastases from 11 CRC patients. Phylogenetic reconstruction and evolutionary analyses were performed to investigate the mode and route of metastatic dissemination. The timing of metastatic dissemination was inferred using SCIMET. Metastases-associated driver events were analyzed and validated in two independent cohorts. Finally, therapeutic actionability of somatic alterations was assessed. Results: A characterization of metastatic dissemination found that 3 out of 11 patients displayed monoclonal dissemination. Whereas the other 8 patients exhibited polyclonal dissemination, of which 6 were monophyletic and 2 were polyphyletic. In five patients, we observed evidence for the metastasis-seeding-metastasis mode among peritoneal metastases. Additionally, the peritoneal metastases were mainly seeded by the primary tumor rather than the lymph node metastases. Metastatic timing inference found that most (9 out of 11) of the peritoneal metastases occurred early before the primary tumor became clinically detectable. Genetic alterations associated with peritoneal metastases were identified, including somatic mutations in TP53, SMAD4, KRAS, RNF43, and SMARCB1, as well as focal amplifications (8p24) and losses (17p12). Analysis of therapeutic actionability identified potentially targetable oncogenic alterations in all patients. Conclusions: Our results provided important insights into the evolutionary features of peritoneal metastases, which had profound implications for the prediction, prevention, and treatment of peritoneal metastasis in CRC.