Abstract TMP5: Bone Marrow-derived And Clonally Expanded Alk1 ^- Endothelial Cells Contribute To Brain Arteriovenous Malformation In Mice

Background and Purpose: Homozygous mutation in arteriovenous malformation (AVM) causative genes in a fraction of endothelial cells (ECs) causes brain AVMs (bAVMs). Heterozygous mutation of endoglin, an AVM causative gene, in bone marrow (BM) caused capillary dysplasia in mouse brain. We tested if (1) homozygous mutation of activin receptor-like kinase 1 ( Alk1, another AVM causative gene) in BM derived ECs (BMDECs) can cause bAVM in brain angiogenic region, (2) Alk1 - ECs clonally expand in bAVM, and (3) the burden of Alk1 - ECs correlates with bAVM severity. Methods: The BMs of Pdgfb iCreER; Alk1 2 f/2f ;Ai14 mice with EC-specific tamoxifen-inducible Cre and Alk1 floxed alleles were transplanted into wild-type mice to analyze the role of BMDECs in bAVM development. Pdgfb iCreER; Alk1 2 f/2f ;confetti +/- mice with Cre-regulated confetti transgene were used to study EC clonal expansion. Brain AVMs were induced by intra-brain injection of an adeno-associated viral vector expressing vascular endothelial growth factor followed by intra-peritoneal injection of tamoxifen. Results: Wild-type mice transplanted with Pdgfb iCreER; Alk1 2 f/2f ;Ai14 BM developed bAVMs after bAVM induction. Recombined BMDECs were detected in bAVMs. The presence of clusters of ECs expressing same confetti color in bAVMs suggests that Alk1 - ECs had been clonally expanded. Increasing tamoxifen dose increased the number of Alk1 - ECs and abnormal vessels in bAVMs of Pdgfb iCreER; Alk1 2 f/2f mice. Conclusion: Homozygous mutation of Alk1 in BMDECs is sufficient to cause bAVM development in brain angiogenic region; clonal expansion of Alk1 - ECs provides a likely mechanism for how a fraction of mutant ECs causes bAVM; and the burden of Alk1 - ECs correlates with AVM severity.