Cerebral cavernous malformations (CCMs) are capillary-venous malformations located in the central nervous system that often lead to neurological deficits and cerebral hemorrhage. Pharmacological treatment limiting disease progression is dearly needed, as available therapy is limited to surgical lesion eradication or stereotactic radiosurgery. CCM affects up to 0.5% of the human population and can occur either in a sporadic or familial form. Loss-of-function mutations in any of three genes CCM1, CCM2 and CCM3 have been associated to familial CCM. Postnatal endothelial-specific deletion of any of the three Ccm genes in mice results in the development of multiple brain vascular malformations that faithfully resemble human CCM lesions. Here we describe that CCM malformations are formed by endothelial cells (ECs) undergoing endothelial-to-mesenchymal transition (EndMT), a phenotype that recapitulates most of the previously observed functional changes that are responsible for dysplasia and fragility of CCM lesions. Ccm1 deletion leads to activation of the MEKK3-MEK5-ERK5-MEF2 signaling cascade resulting in a marked upregulation of the transcription factor Kru?ppel-like factor 4 (KLF4) in ECs in vivo. KLF4 promotes an endogenous production of bone morphogenetic protein 6 (BMP6) in ECs that, in turn, activates the transforming growth factor-? (TGF-?) and bone morphogenetic protein (BMP) signalling pathway. KLF4 transcriptional activity and KLF4-dependent TGF-?/BMP pathway activation are responsible for the EndMT switch observed in the absence of Ccm1. Interestingly, using both a pharmacological treatment to inhibit TGF-?/BMP pathway or a genetic approach based on endothelial-specific Ccm1 and Klf4 double knockout mice, we strongly reduce the development and progression of CCM lesions. Importantly loss of Klf4 almost abolishes mouse mortality due to brain hemorrhage in endothelial Ccm1-ablated mice. These data indicate that KLF4, TGF- ?/BMP pathway and EndMT are crucial events for CCM pathogenesis and unveil KLF4 as a key therapeutic target for CCM.