Pathogenic effects of TIE2-mutations causing venous malformation
(2012)
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- (en) Venous malformations (VM) are localized defects of angiogenesis, which typically involve the skin, mucosa and subcutis. They are characterized by enlarged venous channels with a relative lack of surrounding smooth muscle cells (SMCs). While mostly sporadic (>98%), 1-2% occur as an autosomal dominantly inherited trait, named cutaneomucosal venous malformation (VMCM). Both forms can be caused by gain-of-function mutations in TEK, encoding the endothelial cell (EC) -specific tyrosine kinase receptor TIE2. Eight different inherited TIE2 changes have been implicated in VMCM, all of which induce a ligand-independent receptor hyperphosphorylation in vitro, from 2- to 30-fold that of the wild-type. The most common of these (10/17 families; 58.8%) is the mildly activating R849W (Publication I). A somatic second-hit was identified in tissue-derived cDNA from a patient carrying the R849W-mutation in the germline. It was located on the wild-type allele and partially deleted the ligand-binding domain of TIE2, resulting in a loss-of-function of the receptor (Publication II). At least 50% of common sporadic VMs are also caused by somatic activating mutations in TIE2. The mutations include a frequent L914F change (85.7%), and a series of double-mutations on the same allele (in cis). Like the inherited changes, all somatic TIE2 mutations cause hyperphosphorylation in vitro. L914F is phosphorylated 12-13 fold the level of wild-type TIE2, 4 times as strongly as R849W. Double-mutants cause higher (roughly additive) levels of chronic phosphorylation as compared to their constituent single-mutant forms. Interestingly, the most common TIE2 mutants (germline R849W and somatic L914F) have distinct but overlapping effects on receptor compartmentalization and translocation in response to angiopoietin-1 (ANGPT1) ligand (Publication II). Global gene expression profiling of human umbilical vein endothelial cells (HUVECs) overexpressing wild-type TIE2, the most frequent “weak” inherited mutant R849W, and the most frequent somatic mutant L914F revealed that L914F strongly dysregulates pathways involved in vascular development and cell migration. By contrast, R849W, in the absence of ligand-stimulation, has extremely weak effects, making it indistinguishable from wild-type cells in global analyses. In addition, we inferred that the transcription factor FOXO1 is inhibited in L914F mutant cells as compared to wild-type, based on differential expression of its target genes. Amongst these is the platelet derived growth factor beta (PDGFB), a known recruiter of SMCs, which shows a significant drop at the level of secreted protein from cultured cells, as well as ex vivo, around malformed veins as compared to normal veins or arteries. AKT, which has been shown to phosphorylate and therefore inhibit FOXO1, is highly activated by the TIE2 mutant forms. Inhibition of AKT increases PDGFB secretion in mutant cells, demonstrating an AKT/FOXO1-dependent role for TIE2 in the regulation of PDGFB production. This allowed us to hypothesize that TIE2 dysregulation causes smooth muscle cell paucity in VMs at least partially due to lack of PDGFB (Publication III, in preparation). In conclusion, the insights we have gained about the effects of TIE2 mutants allow us to explain some of the key features of the disease phenotype they mediate: lack of SMCs around lesions, lack of whose support may contribute to vessel dilation. In concert with in vivo mouse models of VMs currently being generated, these data will be of great value in deciphering why VMs occur, and which pathways are potentially attractive therapeutic targets.
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Uebelhoer, M. (2012). Pathogenic effects of TIE2-mutations causing venous malformation. https://hdl.handle.net/2078.5/162394