Molecular subtypes and the evolution of treatment decisions in metastatic colorectal cancer

Colorectal cancer (CRC) has clinically-relevant molecular heterogeneity at multiple levels: genomics, epigenomics, transcriptomics and microenvironment features. Genomic events acquired during carcinogenesis remain drivers of cancer progression in the metastatic setting. For example, KRAS and NRAS mutations defi ne a population refractory to EGFR monoclonal antibodies, BRAFV600E mutations associate with poor outcome under standard therapies and response to targeted inhibitors in combinations, while HER2 amplifi cations confer unique sensitivity to double HER2 blockade. Multiple rare gene alterations driving resistance to EGFR monoclonal antibodies have been described with significant overlap in primary and acquired mechanisms, in line with a clonal selection process. In this context, sequential analysis of circulating tumor DNA has the potential to guide drug development in a treatment refractory setting. Rare kinase fusion events and complex alterations in genes involved in DNA damage repair have been described, with emerging evidence for targetability. On the other hand, transcriptomic subtypes and pathway activation signatures have also shown prognostic and potential predictive value in metastatic CRC. These markers refl ect stromal and immune microenvironment interactions with cancer cells. For example, the microsatellite instable (MSI) or POLE ultramutant CRC population is particularly sensitive to immune checkpoint inhibitors, while tumors with a mesenchymal phenotype are characterized by activation of immunosuppressive molecules that mandate stratifi ed development of novel immunotherapy combinations. In this manuscript we review the expanding landscape of targetable oncogenic alterations and signatures in metastatic CRC and discuss the clinical implementation of novel molecular diagnostic tests.