Review of the endothelial pathogenic mechanism of TIE2-related venous malformation.

Background: Venous malformation (VM) is a type of disease involving vascular morphogenesis in humans. Clinically, VM can be sporadic or inherited. TIE2, also known as TEK or HYK, is a member of the receptor tyrosine kinase subfamily and is highly conserved among species. In 1996, an arginine-to-tryptophan substitution at position 849 (R849W) in TIE2 was found to induce hereditary VM. Additional alterations in TIE2 involved in the pathogenesis of inherited or sporadic VM have since been reported.

Methods: The relevant key literature was selectively reviewed, including case reports, reviews, research studies, and meta-analyses.

Results: TIE2 can be thought of as the basis for VM, with a potential role in determining locations, through intracorporal endothelium-specific distribution and expression from the embryonic phase. It has a sophisticated protein structure, and various point mutations destroy its function and physiologic processes by obviously different activation mechanisms, of which some inhibit dephosphorylation and others maintain phosphorylation. Extracellularly, whereas angiopoietins (ANGs) are ligands of TIE2, the chaotic balance between ANG1 and ANG2 in VM is related to their effects on switching between the cell-cell/cell-extracellular matrix contact conditions and vascular quiescence/angiogenesis state, resulting in corrupted contacts. Intracellularly, among diverse cellular pathways, phosphatidylinositol 4,5-bisphosphate 3-kinase/AKT serine-threonine kinase, mitogen-activated protein kinase, and Dok-related protein are irreplaceable keys underlying changes in endothelial morphology and behavioral biology in VM. For example, R849W and L914F (a leucine-to-phenylalanine substitution at position 914), the most important and frequent TIE2 mutations associated with VM, share similar phenotypes but differ with respect to signaling pathways, heredity, and triggering factors.

Conclusions: Based on this comprehensive analysis, we propose an avalanche theory, in which mutant TIE2 is a trigger and pathogenic core, the intercellular network is a tool, altered extracellular matrix and contacts are the final foothold, and fragile contacts are the result. Precise classification according to TIE2 mutation type in VM, especially the mutation site, is important for future targeted therapies.