Fosl1 is a transcriptional effector of BRAF ^V600E-driven intestinal tumorigenesis

The serrated neoplasia pathway is an alternate route to colorectal cancer (CRC) development where BRAF ^V600E is the most common initiating genetic alteration. BRAF^V600E-driven tumorigenesis requires gene expression changes mediated by activation of the ERK MAPK signaling pathway. However, the key effectors of this process are elusive. Here, we identify the ERK-regulated transcription factor Fosl1, one such effector. We show that Fosl1 is dispensable for the initiation of BRAF^V600E-driven serrated neoplasia in mice but promotes progression of the disease by regulating the expression of genes involved in inflammation, immunity, cell cycle control, fetal-like programming, and gastric metaplasia. Notably, transgenic Fosl1 expression alone was sufficient to induce tumors with a BRAF ^V600E-like serrated morphology and transcriptional profile. These findings reveal a mechanism through which oncogenic BRAF-driven ERK signaling reprograms transcription to drive serrated neoplasia.