International Journal of Molecular Sciences Review Pulmonary Vascular Complications in Hereditary Hemorrhagic Telangiectasia and the Underlying Pathophysiology Sala Bofarid 1, Anna E. Hosman 2, Johannes J. Mager 2, Repke J. Snijder 2 and Marco C. Post 1,3,* 1 Department of Cardiology, St. Antonius Hospital, 3435 CM Nieuwegein, The Netherlands; [email protected] 2 Department of Pulmonology, St. Antonius Hospital, 3435 CM Nieuwegein, The Netherlands; [email protected] (A.E.H.); [email protected] (J.J.M.); [email protected] (R.J.S.) 3 Department of Cardiology, University Medical Center Utrecht, 3584 CM Utrecht, The Netherlands * Correspondence: [email protected]; Tel.: +31-883203000 Abstract: In this review, we discuss the role of transforming growth factor-beta (TGF-β) in the development of pulmonary vascular disease (PVD), both pulmonary arteriovenous malformations (AVM) and pulmonary hypertension (PH), in hereditary hemorrhagic telangiectasia (HHT). HHT or Rendu-Osler-Weber disease is an autosomal dominant genetic disorder with an estimated prevalence of 1 in 5000 persons and characterized by epistaxis, telangiectasia and AVMs in more than 80% of cases, HHT is caused by a mutation in the ENG gene on chromosome 9 encoding for the protein endoglin or activin receptor-like kinase 1 (ACVRL1) gene on chromosome 12 encoding for the protein ALK-1, resulting in HHT type 1 or HHT type 2, respectively. A third disease-causing mutation has been found in the SMAD-4 gene, causing a combination of HHT and juvenile polyposis coli. All Citation: Bofarid, S.; Hosman, A.E.; three genes play a role in the TGF-β signaling pathway that is essential in angiogenesis where it Mager, J.J.; Snijder, R.J.; Post, M.C. plays a pivotal role in neoangiogenesis, vessel maturation and stabilization. PH is characterized by Pulmonary Vascular Complications in elevated mean pulmonary arterial pressure caused by a variety of different underlying pathologies. Hereditary Hemorrhagic HHT carries an additional increased risk of PH because of high cardiac output as a result of anemia Telangiectasia and the Underlying Pathophysiology. Int. J. Mol. Sci. 2021, and shunting through hepatic AVMs, or development of pulmonary arterial hypertension due 22, 3471. https://doi.org/10.3390/ to interference of the TGF-β pathway. HHT in combination with PH is associated with a worse ijms22073471 prognosis due to right-sided cardiac failure. The treatment of PVD in HHT includes medical or interventional therapy. Academic Editor: Lukas J. A. C. Hawinkels Keywords: HHT; endoglin; pulmonary vascular disease Received: 2 February 2021 Accepted: 25 March 2021 Published: 27 March 2021 1. Hereditary Hemorrhagic Telangiectasia Hereditary hemorrhagic telangiectasia (HHT), additionally known as Rendu-Osler- Publisher’s Note: MDPI stays neutral Weber disease, is an autosomal-dominant inherited disease with an estimated prevalence with regard to jurisdictional claims in of 1 in 5000 individuals and higher in certain regions [1]. HHT can initially present itself published maps and institutional affil- with spontaneous recurrent epistaxis and mucocutaneous telangiectases. However, HHT is iations. additionally frequently complicated by arteriovenous malformations (AVMs) in the lung, brain, liver and digestive system [2]. Unfortunately, HHT is still underdiagnosed, and entire families remain unaware of available screening and treatment opportunities [2–4]. Diagnosing HHT can be done through genetic testing or by the use of the clinical Copyright: © 2021 by the authors. Curaçao Criteria framework. The Curaçao diagnostic criteria for HHT consist of the Licensee MDPI, Basel, Switzerland. following [5]: This article is an open access article distributed under the terms and • Frequent and recurrent epistaxis, which may be mild to severe conditions of the Creative Commons • Multiple telangiectases on characteristic sites: lips, oral cavity, fingers, and nose Attribution (CC BY) license (https:// • AVMs or telangiectases in one or more of the internal organs (lung, brain, liver, creativecommons.org/licenses/by/ intestines, stomach, and spinal cord) 4.0/). • A 1st-degree relative with HHT Int. J. Mol. Sci. 2021, 22, 3471. https://doi.org/10.3390/ijms22073471 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 3471 2 of 13 A diagnosis of HHT is considered confirmed if at least three criteria are present, and possible with two criteria, as listed above [6]. Currently, there are five different mutations known to cause HHT; this has led to the subdivision of HHT into five subtypes [7]. These mutations do not lead to abnormal proteins but cause haploinsufficiency, which leads to a reduction in concentration of functional proteins as well as an imbalance in the TGF-β signaling pathway [8]. The various types of HHT can be subdivided based on the genetic mutation in the TGF-β signaling pathway [2]: • HHT1 is caused by mutations in the ENG gene (cytogenetic location 9q34.1; OMIM187300, encoding for the protein endoglin). HHT type 1 is characterized by a higher preva- lence of pulmonary and cerebral AVMs, mucocutaneous telangiectasia, and epistaxis compared to HHT type 2 [2,9,10]. • HHT type 2 is caused by a mutation in the ACVRL1-gene (cytogenetic location 12q13.13; OMIM600376, encoding for the ALK1 protein) and has a higher prevalence of hepatic AVMs compared to HHT type 1 [2,9,11]. • HHT type 3 and HHT type 4 are linked with mutations in, respectively, chromosome 5 and 7; however, the exact genes remain unknown [12–14]. • HHT type 5 is caused by a mutation in the Growth Differentiation Factor 2 gene (GDF-2) that codes for the Bone Morphogenetic Protein 9 (BMP9) (OMIM615506) which expresses an HHT-like phenotype and is therefore classified as HHT type 5 [14,15]. • Mutations in the SMAD4 gene (cytogenetic location 18q21.2; OMIM175050) can cause a rare syndrome that is a combination of juvenile polyposis and HHT. This mutation is only found in 1–2% of HHT patients [2,14,16]. Approximately 80% of HHT patients have mutations in the ENG and ACVRL1- gene [8,14]. Excessive TGF-β activation contributes to the development of a variety of diseases, including cancer, autoimmune disease, vascular disease, and progressive multi-organ fibrosis. The TGF-β signaling pathway is involved in many cellular processes, including cell growth, cell differentiation, apoptosis, cellular homeostasis, and others [17,18] (Figure1). In endothelial cells (ECs), TGF-β can signal through two type-1 receptors: the ALK-5 pathway in which SMAD 2 and 3 are activated and the ALK-1 pathway in which SMAD 1, 5, and 8 are activated [2,7] (Figure1). All the known mutations in genes that cause HHT are found in the TGF-β signaling pathway. Several studies show biphasic effect of TGF-β in ECs [19,20]. In low concentra- tions, TGF-β is pro-angiogenetic, while in high concentrations TGF-β inhibits angiogen- esis [20,21]. ALK-1 and ALK-5 receptors play a pivotal role in the angiogenesis and quiescence of ECs [22]. Activation of the ALK-5 pathway provides a quiescent pheno- type in which proliferation and migration are inhibited while stabilization of vessels by podocytes is stimulated [22]. In contrast to ALK-5, activation of the ALK-1 receptor induces proliferation and migration and increased VEGF expression [23–25]. ALK-1 and ALK-5 not only have opposite responses but are also co-dependent as presence of ALK-5 is necessary for maximum ALK-1 activation [22]. Studies have shown that in ALK-5 KO mice both the ALK-5 and ALK-1 pathway are defective [24,26]. Furthermore ALK-1 can directly antagonize the ALK-5/SMAD-2/3 pathway on the level of the SMADs [24,26]. Endoglin is upregulated by ALK-1 and is an accessory receptor in the TGF-β signaling pathway that is specifically expressed in the proliferation of ECs and has an opposite effect on ALK-5 [2,22,27,28]. Endoglin is necessary in the TGF-β/ALK-1 signaling and indirectly inhibits the TGF-β/ALK-5 pathway [26,29]. In addition, high concentrations of Endoglin in ECs stimulate the BMP-9 [30]. Paradoxically outside the EC, Endoglin actually inhibits TGF-β [31]. TGF-β, BMP-9 and hypoxia stimulate the increase endoglin expression in ECs [30,32]. Int.Int. J. J. Mol. Mol. Sci. Sci.2021 2021, ,22 22,, 3471 x FOR PEER REVIEW 3 ofof 1315 FigureFigure 1. 1.Molecular Molecular pathophysiology pathophysiology of of hereditary hereditary hemorrhagic hemorrhagic telangiectasia telangiectasia (HHT). (HHT). Physiological Physiological signaling signaling in endothelialin endothe- lial cells occurs through ALK-1 and ALK-5. Signaling through ALK-1 results in the activation of the SMAD 1,5,8-CoS- cells occurs through ALK-1 and ALK-5. Signaling through ALK-1 results in the activation of the SMAD 1,5,8-CoSMAD4 MAD4 pathway resulting in proliferation and migration of endothelial cells (ECs). ALK-1 (and indirectly endoglin) also pathway resulting in proliferation and migration of endothelial cells (ECs). ALK-1 (and indirectly endoglin) also inhibits inhibits ALK-5 signaling. ALK-5 signaling activates the SMAD 2,3-CoSMAD4 pathway which inhibits proliferation and ALK-5migration signaling. of ECs ALK-5 and stimulate signaling the activates stabilization the SMAD of the 2,3-CoSMAD4 vessels. pathway which inhibits proliferation and migration of ECs and stimulate the stabilization of the vessels. In endothelial cells (ECs), TGF-β can signal through two type-1 receptors: the ALK-5 BMP-9 is a ligand involved in the TGF-β/ALK-1 complex, high concentrations of pathway in which SMAD 2 and 3 are activated and the ALK-1 pathway in which SMAD BMP-9 in vitro and ex vivo inhibit proliferation and migration of ECs [24,33]. In vivo, 1, 5, and 8 are activated [2,7] (Figure 1). All the known mutations in genes that cause HHT however, several studies have seen that BMP-9 in combination with TGF-β induces are found in the TGF-β signaling pathway.