Mural Cells Initiate Endothelial-to-Mesenchymal Transition in Adjacent Endothelial Cells in Extracranial AVMs

Extracranial arteriovenous malformations (eAVMs) are complex vascular lesions characterized by anomalous arteriovenous connections, vascular instability, and disruptions in endothelial cell (EC)-to-mural cell (MC) interactions. This study sought to determine whether eAVM-MCs could induce endothelial-to-mesenchymal transition (EndMT), a process known to disrupt vascular integrity, in the eAVM microenvironment. eAVM and paired control tissues were analyzed using RT-PCR for EC (CD31, CD34, and CDH5) and EndMT-specific markers (SNAI1, SNAI2, ACTA2/α-SMA, N-cadherin/CDH2, VIM). Immunohistochemistry (IHC) was also performed to analyze MC- (PDGFR-β and α-SMA), EC (CD31, CD34, and CDH5), and EndMT-specific markers (CDH2 and SNAI1) in sequential paraffin-embedded sections of eAVM patient biopsies and in adjacent normal tissue biopsies from the same patients. Furthermore, eAVM-MCs and MCs from normal paired tissues (NMCs) were then isolated from fresh human surgical samples using flow cytometry and co-cultured with normal human umbilical vein vascular endothelial cells (HUVECs), followed by analysis of CD31 by immunofluorescence. RT-PCR analysis did not show a significant difference in the expression of EC markers between eAVM tissues and controls, whereas expression of EndMT-specific markers was upregulated in eAVM tissues compared to controls. IHC of eAVMs and paired control tissues demonstrated regions of significant perivascular eAVM-MC expansion (PDGFR-β+, and α-SMA+) surrounding dilated, morphologically abnormal vessels. These regions contained endothelium undergoing EndMT as evidenced by loss of CD31, CD34, and CDH5 expression and upregulation of the EndMT-associated genes CDH2 and SNAI1. Isolated eAVM-MCs induced loss of CD31 in HUVECs when grown in co-culture, while NMCs did not. This study suggests that the eAVM endothelium surrounded by expanded eAVM-MCs undergoes EndMT, potentially leading to the formation of dilated and fragile vessels, and implicates the eAVM-MCs in EndMT initiation and eAVM pathology.