Hereditary haemorrhagic telangiectasia: pathophysiology, diagnosis and treatment

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.

Dietary supplement use and nosebleeds in hereditary haemorrhagic telangiectasia - an observational study

Understanding potential provocations of haemorrhage is important in a range of clinical settings, and particularly for people with abnormal vasculature. Patients with hereditary haemorrhagic telangiectasia (HHT) can report haemorrhage from nasal telangiectasia in real time, and suggested dietary factors may precipitate nosebleeds. To examine further, nosebleed severity, dietary supplement use, and blood indices were evaluated in an unselected group of 50 HHT patients recruited from a specialist UK service. Using the validated Epistaxis Severity Score, nosebleed severity ranged from 0 to 9.1 out of 10 (median 3.9). Using a Food Frequency Questionnaire, 24/50 (48%) participants reported use of dietary supplements in the previous year. A third (18/50; 36%) had used self prescribed, non-iron containing dietary supplements, ingesting between 1 and 3 different supplements each day. Eight (16%) used fish oils. Despite having more severe epistaxis (p = 0.012), the 12 iron supplement users had higher serum iron concentrations, and were able to maintain their red blood cell indices. In contrast, there was no evident benefit for the participants using non iron supplements. Furthermore, platelet counts and serum fibrinogen tended to be lower in fish oil/supplement users, and one fish oil user demonstrated reduced in vitro platelet aggregation. In conclusion, in this small study, a third of HHT patients used non-iron dietary supplements, and one in six ingested fish oils, unaware of their known anti-platelet activity. The scale of use, and potential of these "natural health supplements" to exacerbate nosebleeds has not been appreciated previously in HHT.

Pathogenesis of Brain Arteriovenous Malformations

Brain arteriovenous malformations (bAVMs) represent a high risk of intracranial hemorrhages, which are substantial causes of morbidity and mortality of bAVMs, especially in children and young adults. Although a variety of factors leading to hemorrhages of bAVMs are investigated extensively, their pathogenesis is still not well elucidated. The author has reviewed the updated data of genetic aspects of bAVMs, especially focusing on clinical and experimental knowledge from hereditary hemorrhagic telangiectasia, which is the representative genetic disease presenting with bAVMs caused by loss-of-function in one of the two genes: endoglin and activin receptor-like kinase 1. This knowledge may allow us to infer the pathogensis of sporadic bAVMs and in the development of new medical therapies for them.

Neurological involvement in hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant disorder characterized by epistaxis, telangiectases, and multi-organ vascular dysplasia. Head and neck localizations of HHT are recurrent, frequent associated with serious complications. The aim of this study was to describe the clinical and imaging patterns of neurological involvement in HHT and to discuss the role of interventional radiology in the management of HHT patients. Based on a multidisciplinary experience of twenty years at our center, we report here the different aspects of neurological involvement of HHT. Depending on the genetic type of the disease, vascular abnormalities may affect different organs. The knowledge of neurological involvement according to specific localization of HHT makes detection easier. As cerebral or spinal arteriovenous fistula may be present in patients with epistaxis or pulmonary arteriovenous malformations (PAVMs), radiologists should be able to detect high-risk lesions and prevent related complications. Finally, we review indications and techniques of embolization for hemorrhagic lesions and emphasize that endovascular therapies are very effective and safe in experienced hands. Head and neck imaging is commonly used for the diagnosis of HHT. Imaging plays also a key role for patient evaluation before treatment as pluridisciplinary management is needed.

Interaction Between ALK1 Signaling and Connexin40 in the Development of Arteriovenous Malformations

OBJECTIVE

To determine the role of Gja5 that encodes for the gap junction protein connexin40 in the generation of arteriovenous malformations in the hereditary hemorrhagic telangiectasia type 2 (HHT2) mouse model.

APPROACH AND RESULTS

We identified GJA5 as a target gene of the bone morphogenetic protein-9/activin receptor-like kinase 1 signaling pathway in human aortic endothelial cells and importantly found that connexin40 levels were particularly low in a small group of patients with HHT2. We next took advantage of the Acvrl1(+/-) mutant mice that develop lesions similar to those in patients with HHT2 and generated Acvrl1(+/-); Gja5(EGFP/+) mice. Gja5 haploinsufficiency led to vasodilation of the arteries and rarefaction of the capillary bed in Acvrl1(+/-) mice. At the molecular level, we found that reduced Gja5 in Acvrl1(+/-) mice stimulated the production of reactive oxygen species, an important mediator of vessel remodeling. To normalize the altered hemodynamic forces in Acvrl1(+/-); Gja5(EGFP/+) mice, capillaries formed transient arteriovenous shunts that could develop into large malformations when exposed to environmental insults.

CONCLUSIONS

We identified GJA5 as a potential modifier gene for HHT2. Our findings demonstrate that Acvrl1 haploinsufficiency combined with the effects of modifier genes that regulate vessel caliber is responsible for the heterogeneity and severity of the disease. The mouse models of HHT have led to the proposal that 3 events-heterozygosity, loss of heterozygosity, and angiogenic stimulation-are necessary for arteriovenous malformation formation. Here, we present a novel 3-step model in which pathological vessel caliber and consequent altered blood flow are necessary events for arteriovenous malformation development.

Endoglin regulates mural cell adhesion in the circulatory system

The circulatory system is walled off by different cell types, including vascular mural cells and podocytes. The interaction and interplay between endothelial cells (ECs) and mural cells, such as vascular smooth muscle cells or pericytes, play a pivotal role in vascular biology. Endoglin is an RGD-containing counter-receptor for β1 integrins and is highly expressed by ECs during angiogenesis. We find that the adhesion between vascular ECs and mural cells is enhanced by integrin activators and inhibited upon suppression of membrane endoglin or β1-integrin, as well as by addition of soluble endoglin (SolEng), anti-integrin α5β1 antibody or an RGD peptide. Analysis of different endoglin mutants, allowed the mapping of the endoglin RGD motif as involved in the adhesion process. In Eng (+/-) mice, a model for hereditary hemorrhagic telangectasia type 1, endoglin haploinsufficiency induces a pericyte-dependent increase in vascular permeability. Also, transgenic mice overexpressing SolEng, an animal model for preeclampsia, show podocyturia, suggesting that SolEng is responsible for podocytes detachment from glomerular capillaries. These results suggest a critical role for endoglin in integrin-mediated adhesion of mural cells and provide a better understanding on the mechanisms of vessel maturation in normal physiology as well as in pathologies such as preeclampsia or hereditary hemorrhagic telangiectasia.

Mice Lacking Endoglin in Macrophages Show an Impaired Immune Response

Endoglin is an auxiliary receptor for members of the TGF-β superfamily and plays an important role in the homeostasis of the vessel wall. Mutations in endoglin gene (ENG) or in the closely related TGF-β receptor type I ACVRL1/ALK1 are responsible for a rare dominant vascular dysplasia, the Hereditary Hemorrhagic Telangiectasia (HHT), or Rendu-Osler-Weber syndrome. Endoglin is also expressed in human macrophages, but its role in macrophage function remains unknown. In this work, we show that endoglin expression is triggered during the monocyte-macrophage differentiation process, both in vitro and during the in vivo differentiation of blood monocytes recruited to foci of inflammation in wild-type C57BL/6 mice. To analyze the role of endoglin in macrophages in vivo, an endoglin myeloid lineage specific knock-out mouse line (Eng^fl/flLysMCre) was generated. These mice show a predisposition to develop spontaneous infections by opportunistic bacteria. Eng^fl/flLysMCre mice also display increased survival following LPS-induced peritonitis, suggesting a delayed immune response. Phagocytic activity is impaired in peritoneal macrophages, altering one of the main functions of macrophages which contributes to the initiation of the immune response. We also observed altered expression of TGF-β1 target genes in endoglin deficient peritoneal macrophages. Overall, the altered immune activity of endoglin deficient macrophages could help to explain the higher rate of infectious diseases seen in HHT1 patients.

Endoglin is a transmembrane protein and an auxiliary receptor for TGF-β with an important role in the homeostasis of the vessel wall. However, endoglin was originally identified as a human cell surface antigen expressed in a pre-B leukemic cell line. Mutations in ENG are responsible for the Hereditary Hemorrhagic Telangiectasia type 1 (HHT1) or Rendu-Osler-Weber syndrome. HHT is a rare disease, with a prevalence of 1/5,000 to 1/8,000. It is an autosomal dominant disorder characterized by a multisystemic vascular dysplasia, recurrent hemorrhages and arteriovenous malformations in internal organs. Interestingly, endoglin expression is also triggered during the monocyte-macrophage differentiation process. In our laboratory, we described that up-regulation of endoglin during in vitro differentiation of blood monocytes is age-dependent and impaired in monocytes from HHT patients, suggesting a role of endoglin in macrophages. In the present work, we first analyzed endoglin expression during differentiation of peripheral blood monocytes to macrophages under in vitro and in vivo conditions. Next, to investigate endoglin’s role in macrophage function in vivo, a myeloid-lineage specific endoglin knock-out mouse line was generated (Eng^fl/flLysMCre). Endoglin deficiency in macrophages predisposed animals to spontaneous infections and led to delayed endotoxin-induced mortality. Phagocytic activity by peritoneal macrophages was reduced in the absence of endoglin and altered expression of TGF-β target genes was consistent with an altered balance of TGF-β signaling. The results show a novel role for endoglin in mouse macrophages, which if analogous in human macrophages, may explain, at least in part, the increased infection rates seen in HHT patients.

Low Dose Iron Treatments Induce a DNA Damage Response in Human Endothelial Cells within Minutes

BACKGROUND

Spontaneous reports from patients able to report vascular sequelae in real time, and recognition that serum non transferrin bound iron may reach or exceed 10μmol/L in the blood stream after iron tablets or infusions, led us to hypothesize that conventional iron treatments may provoke acute vascular injury. This prompted us to examine whether a phenotype could be observed in normal human endothelial cells treated with low dose iron.

METHODOLOGY

Confluent primary human endothelial cells (EC) were treated with filter-sterilized iron (II) citrate or fresh media for RNA sequencing and validation studies. RNA transcript profiles were evaluated using directional RNA sequencing with no pre-specification of target sequences. Alignments were counted for exons and junctions of the gene strand only, blinded to treatment types.

PRINCIPAL FINDINGS

Rapid changes in RNA transcript profiles were observed in endothelial cells treated with 10μmol/L iron (II) citrate, compared to media-treated cells. Clustering for Gene Ontology (GO) performed on all differentially expressed genes revealed significant differences in biological process terms between iron and media-treated EC, whereas 10 sets of an equivalent number of randomly selected genes from the respective EC gene datasets showed no significant differences in any GO terms. After 1 hour, differentially expressed genes clustered to vesicle mediated transport, protein catabolism, and cell cycle (Benjamini p = 0.0016, 0.0024 and 0.0032 respectively), and by 6 hours, to cellular response to DNA damage stimulus most significantly through DNA repair genes FANCG, BLM, and H2AFX. Comet assays demonstrated that 10μM iron treatment elicited DNA damage within 1 hour. This was accompanied by a brisk DNA damage response pulse, as ascertained by the development of DNA damage response (DDR) foci, and p53 stabilization.

SIGNIFICANCE

These data suggest that low dose iron treatments are sufficient to modify the vascular endothelium, and induce a DNA damage response.

Transcription factor KLF6 upregulates expression of metalloprotease MMP14 and subsequent release of soluble endoglin during vascular injury

After endothelial injury, the transcription factor Krüppel-like factor 6 (KLF6) translocates into the cell nucleus to regulate a variety of target genes involved in angiogenesis, vascular repair and remodeling, including components of the membrane transforming growth factor beta (TGF-β) receptor complex such as endoglin and activin receptor-like kinase 1. The membrane metalloproteinase 14 (MMP14 or MT1-MMP) targets endoglin to release soluble endoglin and is involved in vascular inflammation and endothelial tubulogenesis. However, little is known about the regulation of MMP14 expression during vascular wounding. In vitro denudation of monolayers of human endothelial cell monolayers leads to an increase in the KLF6 gene transcriptional rate, followed by an upregulation of MMP14 and release of soluble endoglin. Concomitant with this process, MMP14 co-localizes with endoglin in the sprouting endothelial cells surrounding the wound border. MMP14 expression at mRNA and protein levels is increased by ectopic KLF6 and downregulated by KLF6 suppression in cultured endothelial cells. Moreover, after wire-induced endothelial denudation, Klf6^+/− mice show lower levels of MMP14 in their vasculature compared with their wild-type siblings. Ectopic cellular expression of KLF6 results in an increased transcription rate of MMP14, and chromatin immunoprecipitation assays show that KLF6 interacts with MMP14 promoter in ECs, this interaction being enhanced during wound healing. Furthermore, KLF6 markedly increases the transcriptional activity of different reporter constructs of MMP14 gene promoter. These results suggest that KLF6 regulates MMP14 transcription and is a critical player of the gene expression network triggered during endothelial repair.

Bazedoxifene, a new orphan drug for the treatment of bleeding in hereditary haemorrhagic telangiectasia

Hereditary haemorrhagic telangiectasia (HHT), or Rendu-Osler-Weber syndrome, is a dominant genetic vascular disorder. In HHT, blood vessels are weak and prone to bleeding, leading to epistaxis and anaemia, severely affecting patients' quality of life. Development of vascular malformations in HHT patients is originated mainly by mutations in ACVRL1/ALK1 (activin receptor-like kinase type I) or Endoglin (ENG) genes. These genes encode proteins of the TGF-β signalling pathway in endothelial cells, controlling angiogenesis. Haploinsufficiency of these proteins is the basis of HHT pathogenicity. It was our objective to study the efficiency of Bazedoxifene, a selective estrogen receptor modulator (SERM) in HHT, looking for a decrease in epistaxis, and understanding the underlying molecular mechanism. Plasma samples of five HHT patients were collected before, and after 1 and 3 months of Bazedoxifene treatment. ENG and ALK1 expression in activated mononuclear cells derived from blood, as well as VEGF plasma levels, were measured. Quantification of Endoglin and ALK1 mRNA was done in endothelial cells derived from HHT and healthy donors, after in vitro treatment with Bazedoxifene. Angiogenesis was also measured by tubulogenesis and wound healing assays. Upon Bazedoxifene treatment, haemoglobin levels of HHT patients increased and the quantity and frequency of epistaxis decreased. Bazedoxifene increased Endoglin and ALK1 mRNA levels, in cells derived from blood samples and in cultured endothelial cells, promoting tube formation. In conclusion, Bazedoxifene seems to decrease bleeding in HHT by partial compensation of haploinsufficiency. The results shown here are the basis of a new orphan drug designation for HHT by the European Medicine Agency (EMA).

A Dormant Microbial Component in the Development of Preeclampsia

Preeclampsia (PE) is a complex, multisystem disorder that remains a leading cause of morbidity and mortality in pregnancy. Four main classes of dysregulation accompany PE and are widely considered to contribute to its severity. These are abnormal trophoblast invasion of the placenta, anti-angiogenic responses, oxidative stress, and inflammation. What is lacking, however, is an explanation of how these themselves are caused. We here develop the unifying idea, and the considerable evidence for it, that the originating cause of PE (and of the four classes of dysregulation) is, in fact, microbial infection, that most such microbes are dormant and hence resist detection by conventional (replication-dependent) microbiology, and that by occasional resuscitation and growth it is they that are responsible for all the observable sequelae, including the continuing, chronic inflammation. In particular, bacterial products such as lipopolysaccharide (LPS), also known as endotoxin, are well known as highly inflammagenic and stimulate an innate (and possibly trained) immune response that exacerbates the inflammation further. The known need of microbes for free iron can explain the iron dysregulation that accompanies PE. We describe the main routes of infection (gut, oral, and urinary tract infection) and the regularly observed presence of microbes in placental and other tissues in PE. Every known proteomic biomarker of "preeclampsia" that we assessed has, in fact, also been shown to be raised in response to infection. An infectious component to PE fulfills the Bradford Hill criteria for ascribing a disease to an environmental cause and suggests a number of treatments, some of which have, in fact, been shown to be successful. PE was classically referred to as endotoxemia or toxemia of pregnancy, and it is ironic that it seems that LPS and other microbial endotoxins really are involved. Overall, the recognition of an infectious component in the etiology of PE mirrors that for ulcers and other diseases that were previously considered to lack one.

Vascular Integrity in the Pathogenesis of Brain Arteriovenous Malformation

Brain arteriovenous malformation (bAVM) is an important cause of intracranial hemorrhage (ICH), particularly in the young population. ICH is the first clinical symptom in about 50 % of bAVM patients. The vessels in bAVM are fragile and prone to rupture, causing bleeding into the brain. About 30 % of unruptured and non-hemorrhagic bAVMs demonstrate microscopic evidence of hemosiderin in the vascular wall. In bAVM mouse models, vascular mural cell coverage is reduced in the AVM lesion, accompanied by vascular leakage and microhemorrhage. In this review, we discuss possible signaling pathways involved in abnormal vascular development in bAVM.

Efficacy and safety of thalidomide for the treatment of severe recurrent epistaxis in hereditary haemorrhagic telangiectasia: results of a non-randomised, single-centre, phase 2 study

BACKGROUND

Hereditary haemorrhagic telangiectasia is a genetic disease that leads to multiregional angiodysplasia. Severe recurrent epistaxis is the most common presentation, frequently leading to severe anaemia. Several therapeutic approaches have been investigated, but they are mostly palliative and have had variable results. We aimed to assess the efficacy of thalidomide for the reduction of epistaxis in patients with hereditary haemorrhagic telangiectasia that is refractory to standard therapy.

METHODS

We recruited patients aged 17 years or older with hereditary haemorrhagic telangiectasia who had severe recurrent epistaxis refractory to minimally invasive surgical procedures into an open-label, phase 2, non-randomised, single-centre study at IRCCS Policlinico San Matteo Foundation (Pavia, Italy). We gave patients thalidomide at a starting dose of 50 mg/day orally. If they had no response, we increased the thalidomide dose by 50 mg/day increments every 4 weeks, until a response was seen, up to a maximum dose of 200 mg/day. After patients had achieved a response, they continued treatment for 8-16 additional weeks. The primary endpoint was the efficacy of thalidomide measured as the percentage of patients who had reductions of at least one grade in the frequency, intensity, or duration of epistaxis. We followed up patients each month to assess epistaxis severity score and transfusion need, and any adverse events were reported. We included all patients who received any study drug and who participated in at least one post-baseline assessment in the primary efficacy population. The safety population consisted of all patients who received any dose of study treatment. This trial is registered with ClinicalTrials.gov, number NCT01485224.

FINDINGS

Between Dec 1, 2011, and May 12, 2014, we enrolled 31 patients. Median follow-up was 15·9 months (IQR 10·1-22·3). Three (10%, 95% CI 2-26) patients had a complete response, with bleeding stopped, 28 (90%, 95% CI 74-98) patients had partial responses. Overall, all 31 (100%, 89-100) patients responded to therapy with a significant decrease in all epistaxis parameters (p<0·0001 for frequency, intensity, and duration). A response was achieved by 25 (81%) patients at 50 mg/day of thalidomide, five (16%) patients at 100 mg/day, and one (3%) patient at 150 mg/day. Patients had only non-serious, grade 1 adverse effects, the most common of which were constipation (21 patients), drowsiness (six patients), and peripheral oedema (eight patients). One patient died a month after the end of treatment, but this was not deemed to be related to treatment.

INTERPRETATION

Low-dose thalidomide seems to be safe and effective for the reduction of epistaxis in patients with hereditary haemorrhagic telangiectasia. Our findings should be validated by further studies with larger patient populations, longer follow-up, and that also assess the benefit for quality of life.

FUNDING

Telethon Foundation.

Molecular controls of arterial morphogenesis

Formation of arterial vasculature, here termed arteriogenesis, is a central process in embryonic vascular development as well as in adult tissues. Although the process of capillary formation, angiogenesis, is relatively well understood, much remains to be learned about arteriogenesis. Recent discoveries point to the key role played by vascular endothelial growth factor receptor 2 in control of this process and to newly identified control circuits that dramatically influence its activity. The latter can present particularly attractive targets for a new class of therapeutic agents capable of activation of this signaling cascade in a ligand-independent manner, thereby promoting arteriogenesis in diseased tissues.

Functional and splicing defect analysis of 23 ACVRL1 mutations in a cohort of patients affected by Hereditary Hemorrhagic Telangiectasia

Hereditary Hemorrhagic Telangiectasia syndrome (HHT) or Rendu-Osler-Weber (ROW) syndrome is an autosomal dominant vascular disorder. Two most common forms of HHT, HHT1 and HHT2, have been linked to mutations in the endoglin (ENG) and activin receptor-like kinase 1 (ACVRL1or ALK1) genes respectively. This work was designed to examine the pathogenicity of 23 nucleotide variations in ACVRL1 gene detected in more than 400 patients. Among them, 14 missense mutations and one intronic variant were novels, and 8 missense mutations were previously identified with questionable implication in HHT2. The functionality of missense mutations was analyzed in response to BMP9 (specific ligand of ALK1), the maturation of the protein products and their localization were analyzed by western blot and fluorescence microscopy. The splicing impairment of the intronic and of two missense mutations was examined by minigene assay. Functional analysis showed that 18 out of 22 missense mutations were defective. Splicing analysis revealed that one missense mutation (c.733A>G, p.Ile245Val) affects the splicing of the harboring exon 6. Similarly, the intronic mutation outside the consensus splicing sites (c.1048+5G>A in intron 7) was seen pathogenic by splicing study. Both mutations induce a frame shift creating a premature stop codon likely resulting in mRNA degradation by NMD surveillance mechanism. Our results confirm the haploinsufficiency model proposed for HHT2. The affected allele of ACVRL1 induces mRNA degradation or the synthesis of a protein lacking the receptor activity. Furthermore, our data demonstrate that functional and splicing analyses together, represent two robust diagnostic tools to be used by geneticists confronted with novel or conflicted ACVRL1 mutations.

Increased endothelial cell permeability in endoglin-deficient cells

Endoglin (ENG) is a TGF-β superfamily coreceptor essential for vascular endothelium integrity. ENG mutations lead to a vascular dysplasia associated with frequent hemorrhages in multiple organs, whereas ENG null mouse embryos die at midgestation with impaired heart development and leaky vasculature. ENG interacts with several proteins involved in cell adhesion, and we postulated that it regulates vascular permeability. The current study assessed the permeability of ENG homozygous null (Eng(-/-)), heterozygous (Eng(+/-)), and normal (Eng(+/+)) mouse embryonic endothelial cell (EC) lines. Permeability, measured by passage of fluorescent dextran through EC monolayers, was increased 2.9- and 1.7-fold for Eng(-/-) and Eng(+/-) ECs, respectively, compared to control ECs and was not increased by TGF-β1 or VEGF. Prolonged starvation increased Eng(-/-) EC permeability by 3.7-fold with no effect on control ECs; neutrophils transmigrated faster through Eng(-/-) than Eng(+/+) monolayers. Using a pull-down assay, we demonstrate that Ras homolog gene family (Rho) A is constitutively active in Eng(-/-) and Eng(+/-) ECs. We show that the endothelial barrier destabilizing factor thrombospondin-1 and its receptor-like protein tyrosine phosphatase are increased, whereas stabilizing factors VEGF receptor 2, vascular endothelial-cadherin, p21-activated kinase, and Ras-related C3 botulinum toxin substrate 2 are decreased in Eng(-/-) cells. Our findings indicate that ENG deficiency leads to EC hyperpermeability through constitutive activation of RhoA and destabilization of endothelial barrier function.

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.

Genome-wide transcriptional and functional analysis of endoglin isoforms in the human promonocytic cell line U937

Endoglin is an auxiliary cell surface receptor for TGF-β family members. Two different alternatively spliced isoforms, long (L)-endoglin and short (S)-endoglin, have been reported. S-endoglin and L-endoglin proteins vary from each other in their cytoplasmic tails that contain 14 and 47 amino acids, respectively. A critical role for endoglin in vascular development has primarily been studied in endothelial cells. In addition, endoglin expression is upregulated during monocyte-to-macrophage differentiation; however, little is known about its role in this myeloid context. To investigate the function of endoglin in monocytes, stable transfectants expressing the two endoglin isoforms in the promonocytic human cell line U937 were generated. The differential gene expression fingerprinting of these endoglin transfectants using DNA microarrays and further bioinformatics analysis showed a clear alteration in essential biological functions, mainly those related to "Cellular Movement", including cell adhesion and transmigration. Interestingly, these cellular functions are highly dependent on adhesion molecules, including integrins α1 (CD49a, ITGA1 gene), αL (CD11a, ITGAL gene), αM (CD11b, ITGAM gene) and β2 (CD18, ITGB2 gene) and the chemokine receptor CCR2 (CD192, CCR2 gene), which are downregulated in endoglin transfectants. Moreover, activin A (INHBA gene), a TGF-β superfamily member involved in macrophage polarization, was distinctly affected in each endoglin transfectant, and may contribute to the regulated expression of integrins. These data were confirmed by quantitative PCR, flow cytometry and functional tests. Taken together, these results provide new insight into endoglin function in monocytes.

Bleeding and clotting in hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT) is a relatively common inherited vascular disorder that was first described in 1864, and is notable for epistaxis, telangiectasia, and arterial venous malformations. While genetic tests are available, the diagnosis remains clinical, and is based on the Curacao criteria. Patients with HHT are at increased risk for both bleeding and clotting events. Because of these competing complications, hematologists are often faced with difficult clinical decisions. While the majority of management decisions revolve around bleeding complications, it is not infrequent for these patients to require anticoagulation for thrombosis. Any anticoagulation recommendations must take into account the bleeding risks associated with HHT. Recent reviews have found that HHT patients can be safely anticoagulated, with the most frequent complication being worsened epistaxis. Large clinical trials have shown that factor IIa and Xa inhibitors have less intracranial bleeding than warfarin, and basic coagulation research has provided a possible mechanism. This article describes the anticoagulation dilemma posed when a 62-year-old female patient with a history of bleeding events associated with HHT was diagnosed with a pulmonary embolism. The subsequent discussion focuses on the approach to anticoagulation in the HHT patient, and addresses the role of the new oral anticoagulants.

Circulatory contributors to the phenotype in hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT) is mechanistically and therapeutically challenging, not only because of the molecular and cellular perturbations that generate vascular abnormalities, but also the modifications to circulatory physiology that result, and are likely to exacerbate vascular injury. First, most HHT patients have visceral arteriovenous malformations (AVMs). Significant visceral AVMs reduce the systemic vascular resistance: supra-normal cardiac outputs are required to maintain arterial blood pressure, and may result in significant pulmonary venous hypertension. Secondly, bleeding from nasal and gastrointestinal telangiectasia leads to iron losses of such magnitude that in most cases, diet is insufficient to meet the ‘hemorrhage adjusted iron requirement.’ Resultant iron deficiency restricts erythropoiesis, leading to anemia and further increases in cardiac output. Low iron levels are also associated with venous and arterial thromboses, elevated Factor VIII, and increased platelet aggregation to circulating 5HT (serotonin). Third, recent data highlight that reduced oxygenation of blood due to pulmonary AVMs results in a graded erythrocytotic response to maintain arterial oxygen content, and higher stroke volumes and/or heart rates to maintain oxygen delivery. Finally, HHT-independent factors such as diet, pregnancy, sepsis, and other intercurrent illnesses also influence vascular structures, hemorrhage, and iron handling in HHT patients. These considerations emphasize the complexity of mechanisms that impact on vascular structures in HHT, and also offer opportunities for targeted therapeutic approaches.

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.

Genetic variation in the functional ENG allele inherited from the non-affected parent associates with presence of pulmonary arteriovenous malformation in hereditary hemorrhagic telangiectasia 1 (HHT1) and may influence expression of PTPN14

HHT shows clinical variability within and between families. Organ site and prevalence of arteriovenous malformations (AVMs) depend on the HHT causative gene and on environmental and genetic modifiers. We tested whether variation in the functional ENG allele, inherited from the unaffected parent, alters risk for pulmonary AVM in HHT1 mutation carriers who are ENG haploinsufficient. Genetic association was found between rs10987746 of the wild type ENG allele and presence of pulmonary AVM [relative risk = 1.3 (1.0018-1.7424)]. The rs10987746-C at-risk allele associated with lower expression of ENG RNA in a panel of human lymphoblastoid cell lines (P = 0.004). Moreover, in angiogenically active human lung adenocarcinoma tissue, but not in uninvolved quiescent lung, rs10987746-C was correlated with expression of PTPN14 (P = 0.004), another modifier of HHT. Quantitative TAQMAN expression analysis in a panel of normal lung tissues from 69 genetically heterogeneous inter-specific backcross mice, demonstrated strong correlation between expression levels of Eng, Acvrl1, and Ptpn14 (r2 = 0.75-0.9, P < 1 × 10(-12)), further suggesting a direct or indirect interaction between these three genes in lung in vivo. Our data indicate that genetic variation within the single functional ENG gene influences quantitative and/or qualitative differences in ENG expression that contribute to risk of pulmonary AVM in HHT1, and provide correlative support for PTPN14 involvement in endoglin/ALK1 lung biology in vivo. PTPN14 has been shown to be a negative regulator of Yap/Taz signaling, which is implicated in mechanotransduction, providing a possible molecular link between endoglin/ALK1 signaling and mechanical stress. EMILIN2, which showed suggestive genetic association with pulmonary AVM, is also reported to interact with Taz in angiogenesis. Elucidation of the molecular mechanisms regulating these interactions in endothelial cells may ultimately provide more rational choices for HHT therapy.

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.

Pericytes as targets in hereditary hemorrhagic telangiectasia

Defective paracrine Transforming Growth Factor-β (TGF-β) signaling between endothelial cells and the neighboring mural cells have been thought to lead to the development of vascular lesions that are characteristic of Hereditary Hemorrhagic Telangiectasia (HHT). This review highlights recent progress in our understanding of TGF-β signaling in mural cell recruitment and vessel stabilization and how perturbed TGF-β signaling might contribute to defective endothelial-mural cell interaction affecting vessel functionalities. Our recent findings have provided exciting insights into the role of thalidomide, a drug that reduces both the frequency and the duration of epistaxis in individuals with HHT by targeting mural cells. These advances provide opportunities for the development of new therapies for vascular malformations.

[Bevacizumab: a new success in hereditary hemorrhagic telangiectasia]

INTRODUCTION

Hereditary hemorrhagic telangiectasia (HHT), is a rare, hereditary vascular dysplasia, characterized by recurrent epistaxis, mucocutaneous telangiectasias and visceral arteriovenous malformations. The vascular endothelial growth factor VEGF seems to play a crucial role in the pathogenesis of this disease. Recently bevacizumab, a humanized monoclonal VEGF inhibitor, has shown promise in treating patients with HHT.

CASE REPORT

A 66-year-old man, having HHT since the age of 30 years with recurrent epistaxis related to telangiectasia at the nasal septum and chronic iron deficiency anemia requiring frequent blood transfusions with iron infusions. The assessment of his disease showed septal perforation, telangiectasis in the proximal jejunum and terminal ileum, and pulmonary arteriovenous malformations. There was no improvement, despite iron infusions, repeated blood transfusions and cauterization. The patient was treated with bevacizumab at a dose of 5mg/kg/infusion every 2 weeks and was given 6 cycles. Bevacizumab, was effective without side effects.

DISCUSSION

It has been hypothesized that HHT is related to an imbalanced state between antiangiogenic factors and proangiogenic factors. Mutations of 3 genes are actually identified in HHT: ENG, ACVRL1, MADH4. The management of patients with HHT currently based on screening for visceral arteriovenous malformations and symptomatic measures are often disappointing. However, the angiogenic nature of this disease suggests an interesting therapy by using angiogenesis inhibitor. Therefore, bevacizumab was introduced as a potential therapy for HHT. Some clinical cases or small series report the efficacy of bevacizumab, in HHT with recurrent epistaxis, refractory iron deficiency anemia, gastrointestinal bleeding and also in liver vascular malformations with high cardiac output failure.

CONCLUSION

The use of modulators of angiogenesis such as bevacizumab is a possible therapeutic target in HHT.

Cardiopulmonary exercise testing demonstrates maintenance of exercise capacity in patients with hypoxemia and pulmonary arteriovenous malformations

BACKGROUND

Patients with pulmonary arteriovenous malformations (PAVMs) are unusual because hypoxemia results from right-to-left shunting and not airway or alveolar disease. Their surprisingly well-preserved exercise capacity is not generally appreciated.

METHODS

To examine why exercise tolerance is preserved, cardiopulmonary exercise tests were performed while breathing room air in 21 patients with radiologically proven PAVMs, including five restudied 3 to 12 months after embolization when their PAVMs had regressed. Where physiologic matching was demonstrable, comparisons were made with 12 healthy control subjects.

RESULTS

The majority of patients achieved their predicted work rate despite a resting arterial oxygen saturation (SaO₂) of 80% to 96%. Peak work rate and oxygen consumption (VO₂) were no lower in patients with more hypoxemia. Despite higher SaO₂ following embolization (median, 96% and 90%; P = .009), patients achieved similar work rates and similar peak VO₂. Strikingly, treated patients reset to virtually identical peak oxygen pulses (ie, VO₂ per heart beat) and in many cases to the same point on the peak oxygen pulse/work rate plot. The 21 patients had increased minute ventilation (VE) for given increases in CO₂ production (VE/VCO₂ slope), but perceived dyspnea was no greater than in the 12 control subjects or in the same patients before compared to after embolization comparison. Overall, work rate and peak VO₂ were associated not with oxygenation parameters but with VE/VCO₂ slope, BMI, and anaerobic threshold.

CONCLUSIONS

Patients with hypoxemia and PAVMs can maintain normal oxygen delivery/VO₂ during peak exercise. Following improvement of SaO₂ by embolization, patients appeared to reset compensatory mechanisms and, as a result, achieved similar peak VO₂ per heart beat and peak work rates.

Endoglin involvement in integrin-mediated cell adhesion as a putative pathogenic mechanism in hereditary hemorrhagic telangiectasia type 1 (HHT1)

Mutations in the endoglin gene (ENG) are responsible for ∼50% of all cases with hereditary hemorrhagic telangiectasia (HHT). Because of the absence of effective treatments for HHT symptoms, studies aimed at identifying novel biological functions of endoglin which could serve as therapeutic targets of the disease are needed. Endoglin is an endothelial membrane protein, whose most studied function has been its role as an auxiliary receptor in the TGF-β receptor complex. However, several lines of evidence suggest the involvement of endoglin in TGF-β-independent functions. Endoglin displays, within its zona pellucida domain, an RGD motif, which is a prototypic sequence involved in integrin-based interactions with other proteins. Indeed, we have recently described a novel role for endothelial endoglin in leukocyte trafficking and extravasation via its interaction with leukocyte integrins. In addition, functional, as well as protein and gene expression analysis have shown that ectopic endoglin represses the synthesis of several members of the integrin family and modulates integrin-mediated cell adhesions. This review focuses on the tight link between endoglin and integrins and how the role of endothelial endoglin in integrin-dependent cell adhesion processes can provide a better understanding of the pathogenic mechanisms leading to vascular lesions in endoglin haploinsufficient HHT1 patients.

Essential role for TMEM100 in vascular integrity but limited contributions to the pathogenesis of hereditary haemorrhagic telangiectasia

AIMS

TMEM100 was previously identified as a downstream target of activin receptor-like kinase 1 (ALK1; ACVRL1) signalling. Mutations on ALK1 cause hereditary haemorrhagic telangiectasia (HHT), a vascular disorder characterized by mucocutaneous telangiectases and visceral arteriovenous malformations (AVMs). The aims of this study are to investigate the in vivo role of TMEM100 at various developmental and adult stages and to determine the extent to which TMEM100 contributed to the development of AVMs as a key downstream effector of ALK1.

METHODS AND RESULTS

Blood vasculature in Tmem100-null embryos and inducible Tmem100-null neonatal and adult mice was examined. We found that TMEM100 deficiency resulted in cardiovascular defects at embryonic stage; dilated vessels, hyperbranching, and increased number of filopodia in the retinal vasculature at neonatal stage; and various vascular abnormalities, including internal haemorrhage, arteriovenous shunts, and weakening of vasculature with abnormal elastin layers at adult stage. However, arteriovenous shunts in adult mutant mice appeared to be underdeveloped without typical tortuosity of vessels associated with AVMs. We uncovered that the expression of genes encoding cell adhesion and extracellular matrix proteins was significantly affected in lungs of adult mutant mice. Especially Mfap4, which is associated with elastin fibre formation, was mostly down-regulated.

CONCLUSION

These results demonstrate that TMEM100 has essential functions for the maintenance of vascular integrity as well as the formation of blood vessels. Our results also indicate that down-regulation of Tmem100 is not the central mechanism of HHT pathogenesis, but it may contribute to the development of vascular pathology of HHT by weakening vascular integrity.

Increase of circulating endothelial cells in patients with Hereditary Hemorrhagic Telangiectasia

Hereditary Hemorrhagic Telangiectasia (HHT) is an autosomal dominant disorder characterized by vascular malformations. The genes known to be associated with HHT include ENG (HHT1), ACVRL1 (HHT2) and SMAD4 (JPHT). It has been reported that circulating CD34(+) cell subsets repair damaged vessels. To investigate whether mobilization of these cells is present in the peripheral blood (PB) of HTT patients, we analyzed CD34(+) cells, CD34(+)VEGFR-2(+) progenitor or mature endothelial cells, and CD34(+)CD133(+)VEGFR-2(-) hematopoietic progenitor cells (HPCs). Cytofluorimetric analysis was performed in 150 HTT patients and 43 healthy subjects (CTRLs). In HTT patients, PB CD34(+) cells were significantly increased; the frequency of endothelial cells was higher (P = 0.002), while the frequency of CD34(+)CD133(+)VEGFR-2(-) HPCs was lower (P = 0.00007) than in CTRLs. Results were comparable in patients with ENG or ACVRL1 gene mutation; in patients with ENG mutation, the frequency of the cell subsets inversely correlated with the age of the patients at time of sampling (CD34(+)), disease duration (CD34(+)VEGFR-2(+)), and age at disease onset (CD34(+)CD133(+) VEGFR-2(-)). In conclusion, HHT patients show an increase of circulating endothelial cells and a decrease of HPCs. In patients with ENG mutation, the frequency of CD34(+) endothelial cells correlates with specific clinical characteristics suggesting that their active turnover characterizes the initial phase of the disease.

Pulmonary arteriovenous malformations

Within the past decade, pulmonary arteriovenous malformations (PAVMs) have evolved from rare curiosities to not uncommon clinical states, with the latest estimates suggesting a prevalence of ~1 in 2,600. PAVMs provide anatomic right-to-left shunts, allowing systemic venous blood to bypass gas exchange and pulmonary capillary bed processing. Hypoxemia and enhanced ventilatory demands result, although both are usually asymptomatic. Paradoxical emboli lead to strokes and cerebral abscesses, and these commonly occur in individuals with previously undiagnosed PAVMs. PAVM hemorrhage is rare but is the main cause of maternal death in pregnancy. PAVM occlusion by embolization is the standard of care to reduce these risks. However, recent data demonstrate that currently recommended management protocols can result in levels of radiation exposure that would be classified as harmful. Recent publications also provide a better appreciation of the hematologic and cardiovascular demands required to maintain arterial oxygen content and oxygen consumption in hypoxemic patients, identify patient subgroups at higher risk of complications, and emphasize the proportion of radiologically visible PAVMs too small to treat by embolization. This review, therefore, outlines medical states that exacerbate the consequences of PAVMs. Chief among these is iron deficiency, which is commonly present due to concurrent hereditary hemorrhagic telangiectasia: iron deficiency impairs hypoxemia compensations by restricting erythropoiesis and increases the risk of ischemic strokes. Management of periodontal disease, dental interventions, pulmonary hypertension, and pregnancy also requires specific consideration in the setting of PAVMs. The review concludes by discussing to what extent previously recommended protocols may benefit from modification or revision.

Optimal management of hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT), also known by the eponym Osler-Weber-Rendu syndrome, is a group of related disorders inherited in an autosomal dominant fashion and characterized by the development of arteriovenous malformations (AVM) in the skin, mucous membranes, and/or internal organs such as brain, lungs, and liver. Its prevalence is currently estimated at one in 5,000 to 8,000. Most cases are due to mutations in the endoglin (HHT1) or ACVRLK1 (HHT2) genes. Telangiectasias in nasal and gastrointestinal mucosa generally present with recurrent/chronic bleeding and iron deficiency anemia. Larger AVMs occur in lungs (~40%-60% of affected individuals), liver (~40%-70%), brain (~10%), and spine (~1%). Due to the devastating and potentially fatal complications of some of these lesions (for example, strokes and brain abscesses with pulmonary AVMs), presymptomatic screening and treatment are of utmost importance. However, due to the rarity of this condition, many providers lack an appreciation for the whole gamut of its manifestations and complications, age-dependent penetrance, and marked intrafamilial variation. As a result, HHT remains frequently underdiagnosed and many families do not receive the appropriate screening and treatments. This article provides an overview of the clinical features of HHT, discusses the clinical and genetic diagnostic strategies, and presents an up-to-date review of literature and detailed considerations regarding screening for visceral AVMs, preventive modalities, and treatment options.

Functional analysis of endoglin mutations from hereditary hemorrhagic telangiectasia type 1 patients reveals different mechanisms for endoglin loss of function

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant inheritable vascular dysplasia caused by mutations in genes encoding either endoglin or activin receptor-like kinase-1 (ALK1). Functional significance of endoglin missense mutations remains largely unknown leading to a difficult discrimination between polymorphisms and pathogenic mutations. In order to study the functional significance of endoglin mutations and to help HHT1 diagnosis, we developed a cellular assay based on the ability of endoglin to enhance ALK1 response to bone morphogenetic protein 9 (BMP9). We generated and characterized 31 distinct ENG mutants reproducing human HHT1 missense mutations identified in patients of the Molecular Genetics Department in Lyon. We found that 16 mutants behaved like wild-type (WT) endoglin, and thus corresponded to benign rare variants. The 15 other variants showed defects in BMP9 response and were identified as pathogenic mutations. Interestingly, two mutants (S278P and F282V) had lost their ability to bind BMP9, identifying two crucial amino acids for BMP9 binding to endoglin. For all the others, the functional defect was correlated with a defective trafficking to the cell surface associated with retention in the endoplasmic reticulum. Further, we demonstrated that some intracellular mutants dimerized with WT endoglin and impaired its cell-surface expression thus acting as dominant-negatives. Taken together, we show that endoglin loss-of-function can result from different mechanisms in HHT1 patients. We also provide a diagnostic tool helping geneticists in screening for novel or conflicting ENG mutations.

Organ specific optical imaging of mitochondrial redox state in a rodent model of hereditary hemorrhagic telangiectasia-1

Hereditary Hemorrhagic Telangiectasia-1 (HHT-1) is a vascular disease caused by mutations in the endoglin (Eng)/CD105 gene. The objective of this study was to quantify the oxidative state of a rodent model of HHT-1 using an optical imaging technique. We used a cryofluorescence imaging instrument to quantitatively assess tissue metabolism in this model. Mitochondrial redox ratio (FAD/NADH), FAD RR, was used as a quantitative marker of the metabolic status and was examined in the kidneys, and eyes of wild-type and Eng +/- mice. Kidneys and eyes from wild-type P21, 6W, and 10M old mice showed, respectively, a 9% (±2), 24% (±0.4), 15% (±1), and 23% (±4), 33% (±0.6), and 30% (±2) change in the mean FAD RR compared to Eng +/- mice at the same age. Thus, endoglin haploinsufficiency is associated with less oxidative stress in various organs and mitigation of angiogenesis.

Hereditary hemorrhagic telangiectasia treated with low dose intravenous bevacizumab

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant disorder that leads to mucocutaneous telangiectasias, epistaxis, and gastrointestinal bleeding. Depending on the severity and manifestation of the disease, various therapeutic modalities have been used, from local bleeding control to surgery or concomitant drug therapy. Several articles under review have presented guidelines for treatment of HHT with bevacizumab as a direct anti-angiogenesis strategy. Still, neither the exact optimal dose nor the minimum effective dose of intravenous bevacizumab in patients with severe HHT has been reported. A 55-year-old man presented with long-standing epistaxis, recent melena, dizziness, and a three-generation family history of chronic epistaxis, anemia, and regular blood transfusions. Treatment with argon plasma coagulation (APC) for the gastrointestinal bleeding failed to raise hemoglobin levels, we considered using the bevacizumab. We report a patient with severe HHT, who was treated with low-dose bevacizumab (2 mg/kg) and improved substantially.

Genetics of vascular malformations

Vascular anomalies are developmental defects of the vasculature and encompass a variety of disorders. The majority of these occur sporadically, yet a few are reported to be familial. The identification of genes mutated in the different malformations provides insight into their etiopathogenic mechanisms and the specific roles the associated proteins play in vascular development and maintenance. It is becoming evident that somatic mosaicism plays a major role in the formation of vascular lesions. The importance of utilizing Next-Generating Sequencing (NGS) for high-throughput and "deep" screening of both blood and lesional DNA and RNA is thus emphasized, as the somatic changes are present in low quantities. There are several examples where NGS has already accomplished discovering these changes. The identification of all the causative genes and unraveling of a holistic overview of the pathogenic mechanisms should enable generation of in vitro and in vivo models and lead to development of more effective treatments, not only targeted on symptoms.