SMAD4 Deficiency Leads to Development of Arteriovenous Malformations in Neonatal and Adult Mice

Background--Hereditary hemorrhagic telangiectasia (HHT) is a rare genetic vascular disorder caused by mutations in endoglin (ENG), activin receptor-like kinase 1 (ACVRL1; ALK1), or SMAD4. Major clinical symptoms of HHT are arteriovenous malformations (AVMs) found in the brain, lungs, visceral organs, and mucosal surface. Animal models harboring mutations in Eng or Alk1 recapitulate all of these HHT clinical symptoms and have been useful resources for studying mechanisms and testing potential drugs. However, animal models representing SMAD4 mutations have been lacking. The goal of this study is to evaluate Smad4-nducible knockout (iKO) mice as an animal model of HHT and compare the phenotypes with other established HHT animal models. Methods and Results--Global Smad4 deletion was induced at neonatal and adult stages, and hemoglobin levels, gastrointestinal hemorrhage, and presence of aberrant arteriovenous connections were examined. Neonatal Smad4-iKO mice exhibited signs of gastrointestinal bleeding and AVMs in the brain, intestine, nose, and retina. The radial expansion was decreased, and AVMs were detected on both distal and proximal retinal vasculature of Smad4-iKOs. Aberrant smooth muscle actin staining was observed in the initial stage AVMs and their connecting veins, indicating abnormal arterial flow to veins. In adult mice, Smad4 deficiency caused gastrointestinal bleeding and AVMs along the gastrointestinal tract and wounded skin. HHT-related phenotypes of Smad4-iKOs appeared to be comparable with those found in Alk1-iKO and Eng-iKO mice. Conclusions--These data further confirm that SMAD signaling is crucial for normal arteriovenous network formation, and Smad4-iKO will be an alternative animal model of AVM research associated with HHT.