Abstract Body: Background and purpose: Brain arteriovenous malformations (bAVMs) can cause life threatening intra-cranial hemorrhage. There is no effective drug therapy available to treat bAVM patients. Mutation of AVM causative genes in endothelial cells (ECs) induce bAVM development. Mutation of activin receptor-like kinase 1 (Alk1) or endoglin (ENG) causes a familiar form of bAVM. Transgene mediated Alk1 expression in ECs cured AVM phenotypes in mice with Alk1 or Eng mutation. Adeno-associated viral vectors (AAVs) can be engineered to target different cells. We hypothesize that by screening engineered AAV vectors, an AAV variant can be identified that can transduce brain ECs specifically. Methods: A single-stranded CBA promoter driven tdTomato transgene was packaged into three engineered AAV capsids (AAV.cc47, AAV.cc84 and AAV1RX) and AAV9 (a vector that can penetrate the blood brain barrier). The vectors were delivered intravenously (i.v.) or intranasally (i.n.) to wild-type mice. A CMV promoter driven Alk1 transgene was packaged into AAV.cc84 and delivered to PdgfbiCreER;Alk1^f/f mice through i.v. followed by induction of bAVM. Transduced cells in the brain and other organs, vessel density, pericytes, abnormal vessels in the bAVMs, and liver inflammation were analyzed histologically. Liver and kidney function were measured enzymatically. We also tested if AAV.cc84 can transduce human ECs by applying it to cultured HUVECs. Results: After i.v. delivery, AAV9 and AAV1RX transduced some brain cells. AAV9 also transduced some hepatocytes. AAV.cc47 did not transduce any brain cells but many hepatocytes. AAV.cc84 transduced many brain ECs and some hepatocytes. After i.n. delivery, AAV9 and AAV1RX transduced some brain cells, while AAV.cc84 predominantly transduced brain ECs. None of these vectors transduced hepatocytes. AAV.cc47 showed robust transduction in brain cells and hepatocytes. AAV.cc84-Alk1 restored Alk1 expression in about 50% of brain ECs of TM treated PdgfbiCreER; Alk1^f/f mice after i.v. injection, reduced vessel density and abnormal vessels, and increased vascular pericyte coverage. AAV.cc84 also transduced cultured HUVECs. Conclusion: The engineered AAV vector, AAV.cc84 can transduce brain ECs after i.v. and i.n. delivery effectively. AAV.cc84-Alk1 restored Alk1 expression in brain ECs and reduce bAVM severity in Alk1 deficient mice. Thus, this vector can be used for developing gene therapies to treat patients with HHT or sporadic bAVMs.