Abstract Rationale Mutation in human arteriovenous malformation (AVM) causative genes in a fraction of endothelial cells (ECs) causes AVMs in mice. It is unclear how a small number of mutant ECs can lead to AVM formation. Objective To understand how a fraction of mutant ECs causes AVM, we tested the following hypotheses: (1) activin receptor-like kinase 1 ( Alk1 or Acvlr1 ) mutant brain ECs undergo clonal expansion upon angiogenic stimulation, (2) Alk1 mutant ECs display growth advantage, (3) the burden of Alk 1 mutant ECs correlates with AVM severity, and (4) Alk1 mutant bone marrow (BM) derived ECs alone is sufficient to cause AVM. Methods and Results We used Pdgfb iCreER; Alk1 f/f ;confetti +/− mice which express an EC-specific tamoxifen (TM)-inducible Cre recombinase, a Cre-regulated confetti transgene, and Alk1 floxed alleles. Brain AVMs were induced by direct brain injection of an adeno-associated viral vector expressing vascular endothelial growth factor (AAV-VEGF) followed with intra-peritoneal injection of TM two weeks later. Color-predominance of confetti reporter in AVMs compared to control brain ECs suggested that clonal expansion was associated with AVM development. We treated Pdgfb iCreER; Alk1 f/f with different doses of TM to create a mosaic of wild-type (WT) and mutant ECs and found that equal numbers of Alk1 + and Alk1 − ECs were proliferating. Increase of TM dose increased the number of Alk1 − ECs, the abnormal vessels in brain AVMs, the number of arteriovenous shunts in the intestines, and mouse mortality. To test if mutation of Alk1 in BM-derived ECs can cause brain AVM, we transplanted WT mice with BM of Pdgfb iCreER; Alk1 f/f mice. After AAV-VEGF and TM treatment, these mice developed AVMs in their brains and arteriovenous shunts in their intestines. Conclusion Clonal expansion of Alk1 mutant ECs could partly explain why a fraction of mutant ECs causes AVM. Mutation of AVM causal genes in BM-derived ECs is sufficient to cause AVM formation.

Alk1 mutant endothelial cells undergo clonal expansion in mouse brain arteriovenous malformations