Bevacizumab in the treatment of hereditary hemorrhagic telangiectasia

Hepatic manifestations of hereditary haemorrhagic telangiectasia

Hereditary haemorrhagic telangiectasia is a genetic condition of abnormal blood vessel formation resulting from an imbalance of pro- and anti-angiogenic products of the transforming growth factor β/bone morphogenetic protein signalling pathway which contributes to vascular remodelling and maintenance. Hepatic vascular malformations are common although less frequently symptomatic, but may result in high-output cardiac failure, portal hypertension and biliary ischaemia. Whilst the understanding of the genetic and cell signalling pathways that are the hallmark of hereditary haemorrhagic telangiectasia have been clarified, there remain challenges in therapy for these patients. Only patients with symptomatic hepatic vascular malformations require treatment, with most (63%) responding to first-line medical therapy. For non-responders, bevacizumab is effective in reducing cardiac output in those with heart failure secondary to hepatic vascular malformations as well as other manifestations of the disease. Although liver transplantation is the only curative option, optimal timing is critical. Novel anti-angiogenetic drugs and those that target aberrant cell signalling pathway are being explored.

[Iron deficiency anemia in a patient with hereditary hemorrhagic telangiectasia. Case report]

Rendu-Osler-Weber disease or hereditary hemorrhagic telangiectasia (HHT) is a rare autosomal dominant disease. It is characterized by vascular dysplasia with the formation of telangiectasias on the skin, mucous membranes of the respiratory and digestive tracts, arteriovenous malformations (AVMs) in the internal organs, which is manifested by bleeding. Diagnosis is based on Curacao criteria: recurrent and spontaneous nosebleeds, multiple telangiectases on the characteristic localizations, AVMs in one or more of the internal organs, a family history of HHT (i.e. first-degree relative who meets these same criteria for definite HHT). Therapy is aimed at preventing and stopping gastrointestinal, nosebleeds, correction of iron deficiency anemia. A promising method of therapy is the use of angiogenesis inhibitors, in particular bevacizumab. The article presents a description of a clinical case of HHT in a 49-year-old woman with telangiectisia on the mucous membrane of the tongue, gastrointestinal tract and liver AVMs.

Generation of a Syngeneic Heterozygous ACVRL1(wt/mut) Knockout iPS Cell Line for the In Vitro Study of HHT2-Associated Angiogenesis

Hereditary hemorrhagic telangiectasia (HHT) type 2 is an autosomal dominant disease in which one allele of the ACVRL1 gene is mutated. Patients exhibit disturbances in TGF-beta/BMP-dependent angiogenesis and, clinically, often present with severe nosebleeds as well as a reduced quality of life. The aim of our study was to use CRISPR/Cas9 to knockout ACVRL1 in normal induced pluripotent stem cells (iPSCs) and evaluate the effects on TGF-beta- and BMP-related gene expression as well as angiogenesis. The CRISPR/Cas9 knockout of the ACVRL1 gene was carried out in previously characterized wild-type (ACVRL1^wt/wt) iPSCs. An HHT type 2 iPS cell line was generated via a single-allele knockout (ACVRL1^wt/mut) in wild-type (ACVRL1^wt/wt) iPSCs, resulting in a heterozygous 17 bp frameshift deletion in the ACVRL1 gene [NG_009549.1:g.13707_13723del; NM_000020.3:c.1137_1153del]. After the generation of embryoid bodies (EBs), endothelial differentiation was induced via adding 4 ng/mL BMP4, 2% B27, and 10 ng/mL VEGF. Endothelial differentiation was monitored via immunocytochemistry. An analysis of 151 TGF-beta/BMP-related genes was performed via RT-qPCR through the use of mRNA derived from single iPS cell cultures as well as endothelial cells derived from EBs after endothelial differentiation. Differential TGF-beta/BMP gene expression was observed between ACVRL1^wt/wt and ACVRL1^wt/mut iPSCs as well as endothelial cells. EBs derived from CRISPR/Cas9-designed ACVRL1 mutant HHT type 2 iPSCs, together with their isogenic wild-type iPSC counterparts, can serve as valuable resources for HHT type 2 in vitro studies.

Hereditary Hemorrhagic Telangiectasia Associating Neuropsychiatric Manifestations with a Significant Impact on Disease Management-Case Report and Literature Review

(1) Background: Genetic hereditary hemorrhagic telangiectasia (HHT) is clinically diagnosed. The clinical manifestations and lack of curative therapeutic interventions may lead to mental illnesses, mainly from the depression-anxiety spectrum. (2) Methods: We report the case of a 69-year-old patient diagnosed with HHT and associated psychiatric disorders; a comprehensive literature review was performed based on relevant keywords. (3) Results: Curaçao diagnostic criteria based the HHT diagnosis in our patient case at 63 years old around the surgical interventions for a basal cell carcinoma, after multiple episodes of epistaxis beginning in childhood, but with a long symptom-free period between 20 and 45 years of age. The anxiety-depressive disorder associated with nosocomephobia resulted in a delayed diagnosis and low adherence to medical monitoring. A comprehensive literature review revealed the scarcity of publications analyzing the impact of psychiatric disorders linked to this rare condition, frequently associating behavioral disengagement as a coping strategy, psychological distress, anxiety, depression, and hopelessness. (4) Conclusions: As patients with HHT face traumatic experiences from disease-related causes as well as recurring emergency hospital visits, active monitoring for mental illnesses and psychological support should be considered as part of the initial medical approach and throughout the continuum of care.

Endovascular treatment in the multimodality management of brain arteriovenous malformations: report of the Society of NeuroInterventional Surgery Standards and Guidelines Committee

BACKGROUND

The purpose of this review is to summarize the data available for the role of angiography and embolization in the comprehensive multidisciplinary management of brain arteriovenous malformations (AVMs METHODS: We performed a structured literature review for studies examining the indications, efficacy, and outcomes for patients undergoing endovascular therapy in the context of brain AVM management. We graded the quality of the evidence. Recommendations were arrived at through a consensus conference of the authors, then with additional input from the full Society of NeuroInterventional Surgery (SNIS) Standards and Guidelines Committee and the SNIS Board of Directors.

RESULTS

The multidisciplinary evaluation and treatment of brain AVMs continues to evolve. Recommendations include: (1) Digital subtraction catheter cerebral angiography (DSA)-including 2D, 3D, and reformatted cross-sectional views when appropriate-is recommended in the pre-treatment assessment of cerebral AVMs. (I, B-NR) . (2) It is recommended that endovascular embolization of cerebral arteriovenous malformations be performed in the context of a complete multidisciplinary treatment plan aiming for obliteration of the AVM and cure. (I, B-NR) . (3) Embolization of brain AVMs before surgical resection can be useful to reduce intraoperative blood loss, morbidity, and surgical complexity. (IIa, B-NR) . (4) The role of primary curative embolization of cerebral arteriovenous malformations is uncertain, particularly as compared with microsurgery and radiosurgery with or without adjunctive embolization. Further research is needed, particularly with regard to risk for AVM recurrence. (III equivocal, C-LD) . (5) Targeted embolization of high-risk features of ruptured brain AVMs may be considered to reduce the risk for recurrent hemorrhage. (IIb, C-LD) . (6) Palliative embolization may be useful to treat symptomatic AVMs in which curative therapy is otherwise not possible. (IIb, B-NR) . (7) The role of AVM embolization as an adjunct to radiosurgery is not well-established. Further research is needed. (III equivocal, C-LD) . (8) Imaging follow-up after apparent cure of brain AVMs is recommended to assess for recurrence. Although non-invasive imaging may be used for longitudinal follow-up, DSA remains the gold standard for residual or recurrent AVM detection in patients with concerning imaging and/or clinical findings. (I, C-LD) . (9) Improved national and international reporting of patients of all ages with brain AVMs, their treatments, side effects from treatment, and their long-term outcomes would enhance the ability to perform clinical trials and improve the rigor of research into this rare condition. (I, C-EO) .

CONCLUSIONS

Although the quality of evidence is lower than for more common conditions subjected to multiple randomized controlled trials, endovascular therapy has an important role in the management of brain AVMs. Prospective studies are needed to strengthen the data supporting these recommendations.

Pulmonary arterial hypertension in hereditary hemorrhagic telangiectasia associated with ACVRL1 mutation: a case report

INTRODUCTION

Hereditary hemorrhagic telangiectasia is an autosomal dominant condition with an estimated prevalence of 1 in 5000. It is characterized by the presence of abnormalities of vascular structures, and may affect many organ systems, including the lungs, brain, spinal cord, gastrointestinal tract, and liver. A causative mutation is identified in approximately 97% of patients with definite hereditary hemorrhagic telangiectasia in one of three genes including a mutation in endoglin, a mutation in a locus mapped to chromosome 5, and an activin receptor-like kinase-1 (ACVRL1) mutation that is associated with an increased incidence of primary pulmonary hypertension. Pulmonary arterial hypertension is a rare (15-25 cases per million people) but severe vascular disorder. Heritable pulmonary arterial hypertension is associated with several gene mutations, with 75% having a mutation in the bone morphogenetic protein receptor 2 (BMPR2). However, the remaining 25% of patients have other associated genetic mutations including ACVLR1, which is also associated with hereditary hemorrhagic telangiectasia. Pulmonary arterial hypertension is a rare complication in patients with hereditary hemorrhagic telangiectasia (< 1% of the hereditary hemorrhagic telangiectasia population). We describe a case report with this rare occurrence.

CASE PRESENTATION

A 70-year-old white/caucasian Irish male presented for screening for hereditary hemorrhagic telangiectasia due to a history of recurrent epistaxis (once/week) and a family history suggestive of pulmonary hypertension. Genetic testing confirmed an ACVRL1 mutation, while an echocardiogram and right heart catheterization confirmed pulmonary arterial hypertension. On examination, he had several mucocutaneous telangiectasia across his face. He was commenced on tadalafil and macitentan. However, this led to increased iron deficiency anemia and pedal edema. Selexipag was also added to his drug regime. He continues to require intermittent admissions for diuresis and blood transfusions.

CONCLUSION

The association of hereditary hemorrhagic telangiectasia and pulmonary arterial hypertension is rare (< 1%). Here we describe a case of hereditary hemorrhagic telangiectasia complicated with pulmonary arterial hypertension as a result of an ACVRL1 mutation. We also describe the clinical challenges of treating these two conditions together, as treatment options for pulmonary arterial hypertension tend to worsen hereditary hemorrhagic telangiectasia symptoms.

Molecular Biomarkers and Drug Targets in Brain Arteriovenous and Cavernous Malformations: Where Are We?

Vascular malformations of the brain (VMB) comprise abnormal development of blood vessels. A small fraction of VMBs causes hemorrhages with neurological morbidity and risk of mortality in patients. Most often, they are symptomatically silent and are detected at advanced stages of disease progression. The most common forms of VMBs are arteriovenous and cavernous malformations in the brain. Radiopathological features of these diseases are complex with high phenotypic variability. Early detection of these malformations followed by preclusion of severe neurological deficits such as hemorrhage and stroke is crucial in the clinical management of patients with VMBs. The technological advances in high-throughput omics platforms have currently infused a zest in translational research in VMBs. Besides finding novel biomarkers and therapeutic targets, these studies have withal contributed significantly to the understanding of the etiopathogenesis of VMBs. Here we discuss the recent advances in predictive and prognostic biomarker research in sporadic and familial arteriovenous malformations as well as cerebral cavernous malformations. Furthermore, we analyze the clinical applicability of protein and noncoding RNA-based molecular-targeted therapies which may have a potentially key role in disease management.

Diagnostic yield of capsule endoscopy for small bowel arteriovenous malformations in patients with hereditary hemorrhagic telangiectasia: a systematic review and meta-analysis

Background and study aims  Small bowel arteriovenous malformations (AVMs) pose a bleeding risk and have traditionally been diagnosed by invasive enteroscopic procedures in patients with hereditary hemorrhagic telangiectasia (HHT). Capsule endoscopy (CE) is emerging as a safe and non-invasive alternative for small intestinal evaluation, but its diagnostic yield and utility in diagnosing small bowel AVMs in HHT patients are understudied. The aim of this study was to meta-analyze the utility of CE for diagnosing AVMs in HHT patients. Methods  A meta-analysis and systematic review of the literature on CE in HHT patients identified in the PubMed, EMBASE, Scopus, and Cochrane databases from inception to March 2018 were conducted. Summary effects were estimated using a random effects model. Results  After applying exclusion criteria, five studies (n = 124 patients) were eligible for meta-analysis. The pooled diagnostic yield for visualization of small bowel AVMs by CE was 77.0 % (95 % CI 65.8 - 85.4 %, P  < 0.001). Conclusions  CE has a good diagnostic yield for small bowel AVMs in HHT. It can be regarded as a sufficient, noninvasive diagnostic modality for identifying small bowel AVMs in HHT patients.

Vascular deficiency of Smad4 causes arteriovenous malformations: a mouse model of Hereditary Hemorrhagic Telangiectasia

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant vascular disorder that leads to abnormal connections between arteries and veins termed arteriovenous malformations (AVM). Mutations in TGFβ pathway members ALK1, ENG and SMAD4 lead to HHT. However, a Smad4 mouse model of HHT does not currently exist. We aimed to create and characterize a Smad4 endothelial cell (EC)-specific, inducible knockout mouse (Smad4^f/f;Cdh5-Cre^ERT2) that could be used to study AVM development in HHT. We found that postnatal ablation of Smad4 caused various vascular defects, including the formation of distinct AVMs in the neonate retina. Our analyses demonstrated that increased EC proliferation and size, altered mural cell coverage and distorted artery-vein gene expression are associated with Smad4 deficiency in the vasculature. Furthermore, we show that depletion of Smad4 leads to decreased Vegfr2 expression, and concurrent loss of endothelial Smad4 and Vegfr2 in vivo leads to AVM enlargement. Our work provides a new model in which to study HHT-associated phenotypes and links the TGFβ and VEGF signaling pathways in AVM pathogenesis.

Bevacizumab for Refractory Gastrointestinal Bleeding in Rendu-Osler-Weber Disease

Rendu-Osler-Weber disease, also known as hereditary hemorrhagic telangiectasia, is a rare autosomal dominant disorder which is often characterized by recurrent epistaxis, mucocutaneous and gastrointestinal telangiectasias, and visceral arteriovenous malformations. Patients with gastrointestinal involvement can present with a wide spectrum of severity, which may vary from uncomplicated iron deficiency anemia to continuous and refractory bleeding. We present the case of a 62-year-old female, who was admitted with anemia following several episodes of melena, and whose endoscopic examination revealed multiple angiodysplasias in the stomach and small bowel. Despite endoscopic and medical treatment attempts with hormonal agents and octreotide, she developed persistent hemorrhage and severe anemia, requiring frequent red blood cell transfusions. Immediately after initiating bevacizumab (7.5 mg/kg, every 3 weeks), complete cessation of bleeding episodes was observed. Currently, after 1 year of follow-up, she maintained sustained remission without the occurrence of adverse events.

Endoglin and alk1 as therapeutic targets for hereditary hemorrhagic telangiectasia

INTRODUCTION

Hereditary Haemorrhagic Telangiectasia (HHT) is as an autosomal dominant trait characterized by frequent nose bleeds, mucocutaneous telangiectases, arteriovenous malformations (AVMs) of the lung, liver and brain, and gastrointestinal bleedings due to telangiectases. HHT is originated by mutations in genes whose encoded proteins are involved in the transforming growth factor β (TGF-β) family signalling of vascular endothelial cells. In spite of the great advances in the diagnosis as well as in the molecular, cellular and animal models of HHT, the current treatments remain just at the palliative level. Areas covered: Pathogenic mutations in genes coding for the TGF-β receptors endoglin (ENG) (HHT1) or the activin receptor-like kinase-1 (ACVRL1 or ALK1) (HHT2), are responsible for more than 80% of patients with HHT. Therefore, ENG and ALK1 are the main potential therapeutic targets for HHT and the focus of this review. The current status of the preclinical and clinical studies, including the anti-angiogenic strategy, have been addressed. Expert opinion: Endoglin and ALK1 are attractive therapeutic targets in HHT. Because haploinsufficiency is the pathogenic mechanism in HHT, several therapeutic approaches able to enhance protein expression and/or function of endoglin and ALK1 are keys to find novel and efficient treatments for the disease.

Thalidomide in angiodysplasia-related bleeding

Gastrointestinal haemorrhage from angiodysplastic lesions is not only difficult to identify, but often refractory to endoscopic intervention. Patients often require substantial transfusion support. Thalidomide has emerged as a promising medical strategy in angiodysplasia-related bleeding. We present our experience and report the findings from a review of the literature. Despite its side-effect profile, thalidomide remains the therapeutic modality with the best evidence in this difficult clinical scenario.

Screening for arteriovenous malformations in hereditary haemorrhagic telangiectasia

OBJECTIVE

To determine whether patients with hereditary haemorrhagic telangiectasia were being screened according to international guidelines, and to review recent evidence in order to provide up-to-date guidelines for the initial systemic management of hereditary haemorrhagic telangiectasia.

METHODS

A retrospective case note analysis was conducted, assessing patients in terms of screening for: genetics, cerebral arteriovenous malformations, pulmonary and hepatic arteriovenous malformations, and gastrointestinal telangiectasia. Databases searched included Medline, the Cumulative Index to Nursing and Allied Health Literature, and Embase.

RESULTS

Screening investigations were most frequently performed for hepatic arteriovenous malformations and least frequently for genetics. Recent data suggest avoiding routine genetic and cerebral arteriovenous malformation screening because of treatment morbidities; performing high-resolution chest computed tomography for pulmonary arteriovenous malformation screening; using capsule endoscopy (if possible) to reduce complications from upper gastrointestinal endoscopy; and omitting routine liver enzyme testing in favour of Doppler ultrasound.

CONCLUSION

Opportunities for systemic arteriovenous malformation screening are frequently overlooked. This review highlights the need for screening and considers the form in which it should be undertaken.

Pulmonary Hypertension in Hereditary Hemorrhagic Telangiectasia

BACKGROUND

A subset of patients with hereditary hemorrhagic telangiectasia (HHT) develops pulmonary hypertension (PH) by mechanisms including pulmonary arterial hypertension, high flow, and elevated pulmonary arterial wedge pressure (PAWP). We aimed to describe echocardiographic and hemodynamic characteristics of patients with coexisting HHT and PH.

METHODS

We conducted a single-center cohort study of patients with confirmed HHT who underwent right-sided heart catheterization (RHC) and transthoracic two-dimensional echocardiography for suspected PH between June 1, 2003 and September 1, 2013 at Mayo Clinic Rochester, Minnesota.

RESULTS

Of 38 patients with confirmed HHT who underwent RHC and echocardiography, 28 (74%) had a mean pulmonary artery pressure (MPAP) ≥ 25 mm Hg. Of those 28, 12 (43%) had pulmonary arterial hypertension. Two patients had normal PAWP and pulmonary vascular resistance (PVR), with PH secondary to either an atrial septal defect or high cardiac flow. Fourteen patients (50%) had elevated PAWP (≥ 15 mm Hg), nine with evidence of high flow. RHC in all 28 patients demonstrated a MPAP of 41 ± 11 mm Hg, PAWP of 17 ± 10 mm Hg, and PVR of 4.5 ± 4.2 Wood units. Echocardiography demonstrated moderate/severe right ventricular dysfunction in nine patients (32%). The presence of PH trended toward worse survival (P = .06).

CONCLUSIONS

PH in patients with HHT occurs by different mechanisms, and there is a trend toward worse survival in patients who develop PH despite the mechanism. The equal predilection toward all subtypes of PH illustrates the necessity of RHC to clarify the hemodynamics.

Successful therapy with bevacizumab in a case of hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT), genetic disorder manifested by uncontrolled multisystem angiogenesis with epistaxis, gastrointestinal bleeding, iron-deficiency anaemia, and arteriovenous malformations (AVM) is often related with increased levels of vascular endothelial growth factor (VEGF). Bevacizumab, a VEGF inhibitor, reduces epistaxis, telangiectasias, and iron-deficiency anaemia. A case of a female patient with HHT and chronic gastrointestinal bleeding is presented. The patient required iron supplementation and multiple blood transfusions. Bevacizumab brought marked symptomatic improvement and allowed for transfusion-independence. It is intended to apply for approval of the indications for bevacizumab administration in HHT as the 'orphan drug'.

BMP9 Crosstalk with the Hippo Pathway Regulates Endothelial Cell Matricellular and Chemokine Responses

Endoglin is a type III TGFβ auxiliary receptor that is upregulated in endothelial cells during angiogenesis and, when mutated in humans, results in the vascular disease hereditary hemorrhagic telangiectasia (HHT). Though endoglin has been implicated in cell adhesion, the underlying molecular mechanisms are still poorly understood. Here we show endoglin expression in endothelial cells regulates subcellular localization of zyxin in focal adhesions in response to BMP9. RNA knockdown of endoglin resulted in mislocalization of zyxin and altered formation of focal adhesions. The mechanotransduction role of focal adhesions and their ability to transmit regulatory signals through binding of the extracellular matrix are altered by endoglin deficiency. BMP/TGFβ transcription factors, SMADs, and zyxin have recently been implicated in a newly emerging signaling cascade, the Hippo pathway. The Hippo transcription coactivator, YAP1 (yes-associated protein 1), has been suggested to play a crucial role in mechanotransduction and cell-cell contact. Identification of BMP9-dependent nuclear localization of YAP1 in response to endoglin expression suggests a mechanism of crosstalk between the two pathways. Suppression of endoglin and YAP1 alters BMP9-dependent expression of YAP1 target genes CCN1 (cysteine-rich 61, CYR61) and CCN2 (connective tissue growth factor, CTGF) as well as the chemokine CCL2 (monocyte chemotactic protein 1, MCP-1). These results suggest a coordinate effect of endoglin deficiency on cell matrix remodeling and local inflammatory responses. Identification of a direct link between the Hippo pathway and endoglin may reveal novel mechanisms in the etiology of HHT.

Research on potential biomarkers in hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT) is a genetically heterogeneous disorder, involving mutations in two predominant genes known as Endoglin (ENG; HHT1) and activin receptor-like kinase 1 (ACVRL1/ALK1; HHT2), as well as in some less frequent genes, such as MADH4/SMAD4 (JP-HHT) or BMP9/GDF2 (HHT5). The diagnosis of HHT patients currently remains at the clinical level, according to the “Curaçao criteria,” whereas the molecular diagnosis is used to confirm or rule out suspected HHT cases, especially when a well characterized index case is present in the family or in an isolated population. Unfortunately, many suspected patients do not present a clear HHT diagnosis or do not show pathogenic mutations in HHT genes, prompting the need to investigate additional biomarkers of the disease. Here, several HHT biomarkers and novel methodological approaches developed during the last years will be reviewed. On one hand, products detected in plasma or serum samples: soluble proteins (vascular endothelial growth factor, transforming growth factor β1, soluble endoglin, angiopoietin-2) and microRNA variants (miR-27a, miR-205, miR-210). On the other hand, differential HHT gene expression fingerprinting, next generation sequencing of a panel of genes involved in HHT, and infrared spectroscopy combined with artificial neural network patterns will also be reviewed. All these biomarkers might help to improve and refine HHT diagnosis by distinguishing from the non-HHT population.

Mouse models of hereditary hemorrhagic telangiectasia: recent advances and future challenges

Hereditary hemorrhagic telangiectasia (HHT) is a genetic disorder characterized by a multi-systemic vascular dysplasia and hemorrhage. The precise factors leading to these vascular malformations are not yet understood and robust animal models of HHT are essential to gain a detailed understanding of the molecular and cellular events that lead to clinical symptoms, as well as to test new therapeutic modalities. Most cases of HHT are caused by mutations in either endoglin (ENG) or activin receptor-like kinase 1 (ACVRL1, also known as ALK1). Both genes are associated with TGFβ/BMP signaling, and loss of function mutations in the co-receptor ENG are causal in HHT1, while HHT2 is associated with mutations in the signaling receptor ACVRL1. Significant advances in mouse genetics have provided powerful ways to study the function of Eng and Acvrl1 in vivo, and to generate mouse models of HHT disease. Mice that are null for either Acvrl1 or Eng genes show embryonic lethality due to major defects in angiogenesis and heart development. However mice that are heterozygous for mutations in either of these genes develop to adulthood with no effect on survival. Although these heterozygous mice exhibit selected vascular phenotypes relevant to the clinical pathology of HHT, the phenotypes are variable and generally quite mild. An alternative approach using conditional knockout mice allows us to study the effects of specific inactivation of either Eng or Acvrl1 at different times in development and in different cell types. These conditional knockout mice provide robust and reproducible models of arteriovenous malformations, and they are currently being used to unravel the causal factors in HHT pathologies. In this review, we will summarize the strengths and limitations of current mouse models of HHT, discuss how knowledge obtained from these studies has already informed clinical care and explore the potential of these models for developing improved treatments for HHT patients in the future.

Molecular pathways: can activin-like kinase pathway inhibition enhance the limited efficacy of VEGF inhibitors?

The vascular endothelial growth factor (VEGF) pathway is critical for tumor angiogenesis. However, VEGF pathway inhibition has been limited by intrinsic and acquired resistance. Simultaneously targeting multiple steps involved in tumor angiogenesis is a potential means of overcoming this resistance. Activin like kinase 1 (ALK1) and endoglin (ENG) have effects on angiogenesis that are distinct from those of VEGF. Whereas VEGF is important for vessel initiation, ALK1 and endoglin are involved in vessel network formation. Thus, ALK1 and endoglin pathway inhibitors are attractive partners for VEGF-based combination antiangiogenic therapy. Genetic evidence supports a role for this receptor family and its ligands, bone morphogenetic proteins (BMP) 9 and 10, in vascular development. Patients with genetic alterations in ALK1 or endoglin develop hereditary hemorrhagic telangiectasia, a disorder characterized by abnormal vessel development. There are several inhibitors of the ALK1 pathway advancing in clinical development for treatment of various tumor types, including renal cell and ovarian carcinomas. Targeting of alternate angiogenic pathways, particularly in combination with VEGF pathway blockade, holds the promise of optimally inhibiting angiogenically driven tumor progression. Clin Cancer Res; 20(11); 2838-45. ©2014 AACR.

Endothelial signaling and the molecular basis of arteriovenous malformation

Arteriovenous malformations occur when abnormalities of vascular patterning result in the flow of blood from arteries to veins without an intervening capillary bed. Recent work has revealed the importance of the Notch and TGF-β signaling pathways in vascular patterning. Specifically, Notch signaling has an increasingly apparent role in arterial specification and suppression of branching, whereas TGF-β is implicated in vascular smooth muscle development and remodeling under angiogenic stimuli. These physiologic roles, consequently, have implicated both pathways in the pathogenesis of arteriovenous malformation. In this review, we summarize the studies of endothelial signaling that contribute to arteriovenous malformation and the roles of genes implicated in their pathogenesis. We further discuss how endothelial signaling may contribute to vascular smooth muscle development and how knowledge of signaling pathways may provide us targets for medical therapy in these vascular lesions.