Abstract 66: Reduction of the Number of Endothelial Cells Harboring Mutations of AVM Causative Genes Lessens Brain AVM Severity

Background and purpose: Mutation of arteriovenous malformation (AVM) causal genes in the endothelial cells (ECs) is essential for AVM formation. However, the relationship of the mutant EC-dose and AVM severity is not clear. We tested whether reduction of mutant ECs reduces bAVM severity using three mouse models that mimic human hereditary hemorrhagic telangiectasia (HHT), a familiar form of bAVM. Majority of HHT patients have Alk1 or endoglin ( Eng ) gene mutation. Methods: Model 1 is Pdgfb iCreER; Alk1 2f/2f mouse that expresses an EC-specific estrogen inducible cre recombinase and has its Alk1 gene exon 4-6 floxed. Alk1 gene deletion was induced by treating the mice with different doses of tamoxifen (TM, 1.25, 0.75, or 0.01 mg/25g of body weight). Model 2 is RosaCreER; Eng 2f/2f mouse that expresses a estrogen inducible cre globally and has exon 5 and 6 of its Eng gene floxed. Model 3 is Pdgfb iCreER; Eng 2f/2f mice. The EC Eng deletion in Model 2 & 3 was induced by three doses of TM (TM 2.5M/25g) treatment. The icre in Pdgfb iCreER transgene has a codon mutant, which is more efficient in deletion of floxed alleles than wild-type cre. BAVMs were induced in TM treated mice through intra-brain injection of AAV-VEGF. Gene deletion efficiency was quantified by qPCR and immunostaining. BAVM phenotypes were analyzed histologically. Results: In model 1, reduction of TM dose reduces the number of Alk1 null ECs and the severity of bAVM. The number of abnormal vessels in bAVM is positively correlated with the TM doses (R=0.8) and negatively correlated with Alk1 positive ECs (R=-0.9). Reduction of TM has also reduced mouse mortality. Compared to Rosa-CreER-mediated global Eng deletion, pdgfb- icreER-induced EC Eng deletion did not increase the number of abnormal vessels, but reduced vascular mural cell-coverage and increased hemorrhage in the bAVM lesion. Conclusion: Reduction of mutant ECs reduces bAVM severity.