CCMs arise from endothelial cell loss of KRIT1, CCM2 or PDCD10, non-homologous proteins that form an adaptor complex. How disruption of the CCM complex results in disease remains controversial, with numerous signalling pathways (including Rho, SMAD and Wnt/β-catenin) and processes such as endothelial mesenchymal transition (EndMT) proposed to have causal roles. CCM2 binds to MEKK3 1).
Although a role for these three genes in the formation of these intracranial vascular lesions has been established since the 1990s, additional works have further elucidated the molecular mechanisms by which mutations in these genes and the resultant aberrant proteins interact, leading to the formation of CCMs.
The three CCM proteins coded by KRIT1, CCM2, and PDCD10 form a trimeric protein complex. Germline loss-of-function mutations in any of these genes may lead to the formation of CCMs. Therefore, it is reasonable to assume that a molecular pathway exists that requires all three proteins to function together correctly for proper cellular function. Moreover, research is demonstrating how each component protein is capable of interacting with numerous other signaling and cytoskeletal molecules allowing for a diverse range of functions in molecular signaling pathways via unique protein–protein interactions.
Significant research findings from 2000 to 2015 have further enhanced our understanding of the pathogenesis of CCM formation. The use of advanced sequencing technologies to characterize genomic mutations and the identification of new signaling pathways and protein–protein interactions have led to great strides in understanding the molecular genetics involved in the development of CCMs. However, many unanswered questions remain, and future studies are clearly needed to improve our understanding of CCM pathogenesis. “Gene to protein to disease” mechanisms involved in the pathogenesis of CCMs should shed further light on potential therapeutic targets. 2).
The Phosphoinositide 3 kinase (PI3K)/Akt pathway is known to play a major role in angiogenesis. Studies have shown that the phosphatase and tensin homologue deleted on chromosome ten (PTEN), a tumor suppressor, is an antagonist regulator of the PI3K/Akt pathway and mediates angiogenesis by activating vascular endothelial growth factor (VEGF) expression.
Understanding the biology of these proteins with respect to their signaling counterpart will help to guide future research towards new therapeutic targets applicable for CCM treatment 3).
Tang et al. identify endothelial Toll-like receptor 4 (TLR4) and the gut microbiome as critical stimulants of cerebral cavernous malformation formation. Activation of TLR4 by Gram negative bacteria or lipopolysaccharide accelerates CCM formation, and genetic or pharmacologic blockade of TLR4 signalling prevents CCM formation in mice. Polymorphisms that increase expression of the TLR4 gene or the gene encoding its co-receptor CD14 are associated with higher CCM lesion burden in humans. Germ-free mice are protected from CCM formation, and a single course of antibiotics permanently alters CCM susceptibility in mice. These studies identify unexpected roles for the microbiome and innate immune signalling in the pathogenesis of a cerebrovascular disease, as well as strategies for its treatment 5).