Common and Distinctive Pathogenetic Features of Arteriovenous Malformations in Hereditary Hemorrhagic Telangiectasia 1 and Hereditary Hemorrhagic Telangiectasia 2 Animal Models—Brief Report

Abstract

Objective-Hereditary hemorrhagic telangiectasia is a genetic disorder characterized by visceral and mucocutaneous arteriovenous malformations (AVMs). Clinically indistinguishable hereditary hemorrhagic telangiectasia 1 and hereditary hemorrhagic telangiectasia 2 are caused by mutations in ENG and ALK1, respectively. In this study, we have compared the development of visceral and mucocutaneous AVMs in adult stages between Eng-and Alk1-inducible knockout (iKO) models. Approach and Results-Eng or Alk1 were deleted from either vascular endothelial cells (ECs) or smooth muscle cells in adult stages using Scl-CreER and Myh11-CreER lines, respectively. Latex perfusion and intravital spectral imaging in a dorsal skinfold window chamber system were used to visualize remodeling vasculature during AVM formation. Global Eng deletion resulted in lethality with visceral AVMs and wound-induced skin AVMs. Deletion of Alk1 or Eng in ECs, but not in smooth muscle cells, resulted in wound-induced skin AVMs. Visceral AVMs were observed in EC-specific Alk1-iKO but not in Eng-iKO. Intravital spectral imaging revealed that Eng-iKO model exhibited more dynamic processes for AVM development when compared with Alk1-iKO model. In this study, we examined the effect of wounding in the development of mucocutaneous telangiectases and investigated the cellular origin of de novo AVMs in both HHT1 and HHT2 mouse models. Conclusions-Both Materials and MethodsMaterials and Methods are available in the online-only Supplement. ResultsTo investigate the role of ENG in adult mice, Eng was globally deleted using the tamoxifen-inducible ROSA26 CreER mouse strain (R26 CreER ). 10 We found that 3 consecutive day injection of tamoxifen at 2.5 mg/25 g body weight was the most effective regimen for R26CreER/+ ;Eng 2f/2f mice (Figure I in the online-only Data Supplement). In 3 to 4 days after the first tamoxifen injection, the R26 CreER/+ ;Eng 2f/2f mutant mice displayed signs of illness, such as slow movement, diarrhea, and dehydration, and died around day 4 to 10 (n>30). To analyze the subdermal vessels in the back skin, vascular casting with blue latex was performed at days 5 to 8 by infusing it into the left ventricle. Subdermal vessels were unaffected in the intact back skins of tamoxifen-injected adult R26CreER/+ ;Eng 2f/2f mice (data not shown) and also wounded skin of tamoxifen-injected adult R26CreER/+ ;Eng 2f/+ mice ( Figure IIA in the online-only Data Supplement; n=6). However, areas of wounds in mid-dorsum and ear of tamoxifen-injected R26 CreER/+ ;Eng 2f/2f mice showed dilated and tortuous vessels, and the latex dye was found in both arteries and veins, indicating the presence of AV shunts ( Figure IIB in the online-only Data Supplement; n=13). However, blood vessels away from the wound in Eng-iKOs had normal morphology and latex only in arterial branches.To determine the vascular cell type where ENG plays a critical role for the development of the vascular network at adult stages, we used 2 cell-type-specific inducible Cre lines: Scl-CreER 11 fo...

Genetic and Molecular Basis for Hereditary Hemorrhagic Telangiectasia

“…De novo arteriovenous connections are visible prior to venous and arterial dilation, and one or more of these connections enlarges into a high-flow, looping shunt as smaller connections regress [48]. Similar results were observed in endothelial cellspecific Acvrl1 and Eng deletion models, although more dynamic generation and resolution of shunts was observed in the latter [51].…”

“…To understand the cellular basis of vascular lesions in HHT, telangiectasia development has been followed in Acvrl1-and Eng-deleted adult mice during subdermal wound healing; microscopic imaging was performed via a window chamber once per day for 8-9 days [48,51]. In the adult global Acvrl1 deletion model, angiogenic sprouting into the wound is initially decreased, and vessels at the perimeter show increased remodeling.…”

“…The mouse wound healing and zebrafish embryonic vascular development models present quite different explanations for the origin of HHT-associated AVMs, with de novo versus pre-existing arteriovenous connections forming the basis of the shunt [48,51,54]. As in zebrafish, Acvrl1 expression in mouse is nearly exclusively arterial, and expression is increased in feeding arteries adjacent to wounds and ligated vessels, suggesting that it is likely regulated by mechanical force [98].…”

Alk1 controls arterial endothelial cell migration in lumenized vessels

“…However, these conclusions were reached from static observations of independent lesions. In Alk1-and Eng-deleted adult mice, wound-induced subdermal AVMs develop via angiogenic elongation of both arteries and veins, with de novo arterial-venous connections developing prior to vessel dilation (Garrido-Martin et al, 2014;Park et al, 2009). Although these latter findings were derived from longitudinal analysis, imaging of vascular growth was performed at daily intervals and was not at cellular resolution.…”

Conflicting Forces of Warfarin and Matrix Gla Protein in the Artery Wall

“…These knockout mice also have excessive angiogenic sprouting (11), which may result in a more friable vasculature. AVMs are also found in hereditary hemorrhagic telangiectasia (HHT) 1 and 2, which results from mutations in the endoglin ( Eng ) (HHT1) or activin receptor-like kinase 1 ( Alk1 ) (HHT2) genes(12). Endoglin is a co-receptor required for ALK1 signaling.…”