TGFβ signaling directs serrated adenomas to the mesenchymal colorectal cancer subtype

The heterogeneous nature of colorectal cancer (CRC) complicates prognosis and is suggested to be a determining factor in the efficacy of adjuvant therapy for individual patients. Based on gene expression profiling, CRC is currently classified into four consensus molecular subtypes (CMSs), characterized by specific biological programs, thus suggesting the existence of unifying developmental drivers for each CMS. Using human organoid cultures, we investigated the role of such developmental drivers at the premalignant stage of distinct CRC subtypes and found that TGF beta plays an important role in the development of the mesenchymal CMS4, which is of special interest due to its association with dismal prognosis. We show that in tubular adenomas (TAs), which progress to classical CRCs, the dominating response to TGF beta is death by apoptosis. By contrast, induction of a mesenchymal phenotype upon TGF beta treatment prevails in a genetically engineered organoid culture carrying a BRAFV600E mutation, constituting a model system for sessile serrated adenomas (SSAs). Our data indicate that TGF beta signaling is already active in SSA precursor lesions and that TGF beta is a critical cue for directing SSAs to the mesenchymal, poor-prognosis CMS4 of CRC.