Mutation analysis in hereditary haemorrhagic telangiectasia in Germany reveals 11 novel ENG and 12 novel ACVRL1/ALK1 mutations

Pathogenic Variant Frequencies in Hereditary Haemorrhagic Telangiectasia Support Clinical Evidence of Protection from Myocardial Infarction

Hereditary haemorrhagic telangiectasia (HHT) is a vascular dysplasia inherited as an autosomal dominant trait, due to a single heterozygous loss-of-function variant, usually in ACVRL1 (encoding activin receptor-like kinase 1 [ALK1]), ENG (encoding endoglin [CD105]), or SMAD4. In a consecutive single-centre series of 37 positive clinical genetic tests performed in 2021-2023, a skewed distribution pattern was noted, with 30 of 32 variants reported only once, but ACVRL1 c.1231C>T (p.Arg411Trp) identified as the disease-causal gene in five different HHT families. In the same centre's non-overlapping 1992-2020 series where 110/134 (82.1%) HHT-causal variants were reported only once, ACVRL1 c.1231C>T (p.Arg411Trp) was identified in nine further families. In a 14-country, four-continent HHT Mutation Database where 181/250 (72.4%) HHT-causal variants were reported only once, ACVRL1 c.1231C>T (p.Arg411Trp) was reported by 12 different laboratories, the adjacent ACVRL1 c.1232G>A (p.Arg411Gln) by 14, and ACVRL1 c.1120C>T (p.Arg374Trp) by 18. Unlike the majority of HHT-causal ACVRL1 variants, these encode ALK1 protein that reaches the endothelial cell surface but fails to signal. Six variants of this type were present in the three series and were reported 6.8-25.5 (mean 8.9) times more frequently than the other ACVRL1 missense variants (all p-values < 0.0039). Noting lower rates of myocardial infarction reported in HHT, we explore potential mechanisms, including a selective paradigm relevant to ALK1's role in the initiating event of atherosclerosis, where a plausible dominant negative effect of these specific variants can be proposed. In conclusion, there is an ~9-fold excess of kinase-inactive, cell surface-expressed ACVRL1/ALK1 pathogenic missense variants in HHT. The findings support further examination of differential clinical and cellular phenotypes by HHT causal gene molecular subtypes.

BMP Signaling Pathway in Dentin Development and Diseases

BMP signaling plays an important role in dentin development. BMPs and antagonists regulate odontoblast differentiation and downstream gene expression via canonical Smad and non-canonical Smad signaling pathways. The interaction of BMPs with their receptors leads to the formation of complexes and the transduction of signals to the canonical Smad signaling pathway (for example, BMP ligands, receptors, and Smads) and the non-canonical Smad signaling pathway (for example, MAPKs, p38, Erk, JNK, and PI3K/Akt) to regulate dental mesenchymal stem cell/progenitor proliferation and differentiation during dentin development and homeostasis. Both the canonical Smad and non-canonical Smad signaling pathways converge at transcription factors, such as Dlx3, Osx, Runx2, and others, to promote the differentiation of dental pulp mesenchymal cells into odontoblasts and downregulated gene expressions, such as those of DSPP and DMP1. Dysregulated BMP signaling causes a number of tooth disorders in humans. Mutation or knockout of BMP signaling-associated genes in mice results in dentin defects which enable a better understanding of the BMP signaling networks underlying odontoblast differentiation and dentin formation. This review summarizes the recent advances in our understanding of BMP signaling in odontoblast differentiation and dentin formation. It includes discussion of the expression of BMPs, their receptors, and the implicated downstream genes during dentinogenesis. In addition, the structures of BMPs, BMP receptors, antagonists, and dysregulation of BMP signaling pathways associated with dentin defects are described.

Founder Effects in Hereditary Hemorrhagic Telangiectasia

A founder effect can result from the establishment of a new population by individuals from a larger population or bottleneck events. Certain alleles may be found at much higher frequencies because of genetic drift immediately after the founder event. We provide a systematic literature review of the sporadically reported founder effects in hereditary hemorrhagic telangiectasia (HHT). All publications from the ACVRL1, ENG and SMAD4 Mutation Databases and publications searched for terms “hereditary hemorrhagic telangiectasia” and “founder” in PubMed and Scopus, respectively, were extracted. Following duplicate removal, 141 publications were searched for the terms “founder” and “founding” and the etymon “ancest”. Finally, 67 publications between 1992 and 2020 were reviewed. Founder effects were graded upon shared area of ancestry/residence, shared core haplotypes, genealogy and prevalence. Twenty-six ACVRL1 and 12 ENG variants with a potential founder effect were identified. The bigger the cluster of families with a founder mutation, the more remarkable is its influence to the populational ACVRL1/ENG ratio, affecting HHT phenotype. Being aware of founder effects might simplify the diagnosis of HHT by establishing local genetic algorithms. Families sharing a common core haplotype might serve as a basis to study potential second-hits in the etiology of HHT.

Clinical presentation and treatment paradigms in patients with hereditary hemorrhagic telangiectasia and spinal vascular malformations

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant disorder that causes angiodysplasia and results in mucocutaneous telangiectasias and arteriovenous malformations of organs. Although central nervous system vascular malformations can occur anywhere along the neuraxis, spinal vascular malformations are rare. We present our experience with the presentation and management of spinal vascular malformations in patients with HHT. Of the more than 800 patients with the diagnosis of HHT screened at our institution from 1995 through 2017, four patients with spinal vascular malformations (age range 1 month-77 years; 2 male, 2 female) were identified, three of whom came to clinical attention after significant neurological deterioration from previously unknown malformations. A review of the literature including our patients demonstrated 29 total spinal arteriovenous fistulas (AVFs) in 28 HHT patients (69% male). The lesions were located predominantly in the thoracic spine (65.5%). Three lesions were not treated, 17 were treated with embolization, 6 were surgically resected, and 3 were treated with embolization and surgery. In 14 cases, the patients presented with hemorrhage of the AVF. Overall, 79% of patients achieved complete or near-complete occlusion, with 75% reporting improvement in neurological function. Discovery of spinal lesions often occurs after neurological decline because current screening protocols do not include evaluation of the patient for spinal lesions. Most patients benefit from intervention, which is tailored to the characteristics of the patient and their malformation. Given the often-severe neurological deficit encountered at presentation, we favor a protocol that screens HHT patients for spinal vascular malformations.

Endoglin in liver fibrogenesis: Bridging basic science and clinical practice

Endoglin, also known as cluster of differentiation CD105, was originally identified 25 years ago as a novel marker of endothelial cells. Later it was shown that endoglin is also expressed in pro-fibrogenic cells including mesangial cells, cardiac and scleroderma fibroblasts, and hepatic stellate cells. It is an integral membrane-bound disulfide-linked 180 kDa homodimeric receptor that acts as a transforming growth factor-β (TGF-β) auxiliary co-receptor. In humans, several hundreds of mutations of the endoglin gene are known that give rise to an autosomal dominant bleeding disorder that is characterized by localized angiodysplasia and arteriovenous malformation. This disease is termed hereditary hemorrhagic telangiectasia type I and induces various vascular lesions, mainly on the face, lips, hands and gastrointestinal mucosa. Two variants of endoglin (i.e., S- and L-endoglin) are formed by alternative splicing that distinguishes from each other in the length of their cytoplasmic tails. Moreover, a soluble form of endoglin, i.e., sol-Eng, is shedded by the matrix metalloprotease-14 that cleaves within the extracellular juxtamembrane region. Endoglin interacts with the TGF-β signaling receptors and influences Smad-dependent and -independent effects. Recent work has demonstrated that endoglin is a crucial mediator during liver fibrogenesis that critically controls the activity of the different Smad branches. In the present review, we summarize the present knowledge of endoglin expression and function, its involvement in fibrogenic Smad signaling, current models to investigate endoglin function, and the diagnostic value of endoglin in liver disease.

Pulmonary arterio-venous malformations in a patient with a novel mutation in exon 10 of the ACVRL1 gene

Mutations of the ACVRL1 gene are a cause of hereditary haemorrhagic telangiectasia (HHT) type 2. In this case report, we present a patient with isolated pulmonary arterio-venous malformations (PAVMs) without other diagnostic criteria for HHT and a novel mutation in exon 10 of the ACVRL1 gene. Other mutations in exon 10 of ACVRL1 have been linked to the development of pulmonary artery hypertension, but PAVMs are a rare manifestation of HHT associated with ACVRL1 mutations. A disrupted endothelial TGF-beta/BMP signaling cascade underlies the pathogenesis of HHT, but the exact mechanism of the disease remains unelucidated. In particular, the factors that influence the variable clinical presentation are not fully understood.

HHT diagnosis by Mid-infrared spectroscopy and artificial neural network analysis

BACKGROUND

The vascular disorder Hereditary Hemorrhagic Telangiectasia (HHT) is in general an inherited disease caused by mutations in the TGF-β/BMP receptors endoglin or ALK1 or in rare cases by mutations of the TGF-β signal transducer protein Smad4 leading to the combined syndrome of juvenile polyposis and HHT. HHT is characterized by several clinical symptoms like spontaneous and recurrent epistaxis, multiple telangiectases at sites like lips, oral cavity, fingers, nose, and visceral lesions like gastrointestinal telangiectasia, pulmonary, hepatic, cerebral or spinal arteriovenous malformations. The disease shows an inter- and intra-family variability in penetrance as well as symptoms from mild to life threatening. Penetrance is also depending on age. Diagnosis of the disease is based on the presence of some of the listed symptoms or by genetic testing. HHT diagnosis is laborious, time consuming, costly and sometimes uncertain. Not all typical symptoms may be present, especially at a younger age, and genetic testing does not always identify the disease causing mutation.

METHODS

Infrared (IR) spectroscopy was investigated as a potential alternative to the current diagnostic methods. IR-spectra were obtained by Fourier-transform Mid-IR spectroscopy from blood plasma from HHT patients and a healthy control group. Spectral data were mathematically processed and subsequently classified and analysed by artificial neural network (ANN) analyses and by visual analysis of scatter plots of the dominant principal components.

RESULTS

The analyses showed that for HHT a disease specific IR-spectrum exists that is significantly different from the control group. Furthermore, at the current stage with the here used methods, HHT can be diagnosed by Mid-IR-spectroscopy in combination with ANN analysis with a sensitivity and specificity of at least 95%. Visual analysis of PCA scatter plots revealed an inter class variation of the HHT group.

CONCLUSION

IR-spectroscopy in combination with ANN analysis can be considered to be a serious alternative diagnostic method compared to clinical and genetically based methods. Blood plasma is an ideal candidate for diagnostic purposes, it is inexpensive, easy to isolate and only minimal amounts are required. In addition, IR-spectroscopy measurement times are fast, less than one minute, and diagnosis is not based on interpretation of may be uncertain clinical data. And last but not least, the method is inexpensive.

Hyperactive BMP signaling induced by ALK2(R206H) requires type II receptor function in a Drosophila model for classic fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia Ossificans Progressiva (FOP) is an autosomal dominant disorder characterized by episodic deposition of heterotopic bone in place of soft connective tissue. All FOP-associated mutations map to the BMP type I receptor, ALK2, with the ALK2(R206H) mutant form found in the vast majority of patients. The mechanism(s) regulating the expressivity of hyperactive ALK2(R206H) signaling throughout a patient's life is not well understood.

RESULTS

In Drosophila, human ALK2(R206H) receptor induces hyperactive BMP signaling. As in vertebrates, elevated signaling associated with ALK2(R206H) in Drosophila is ligand-independent. We found that a key determinant for ALK2(R206H) hyperactivity is a functional type II receptor. Furthermore, our results indicate that like its Drosophila ortholog, Saxophone (Sax), wild-type ALK2 can antagonize, as well as promote, BMP signaling.

CONCLUSIONS

The dual function of ALK2 is of particular interest given the heterozygous nature of FOP, as the normal interplay between such disparate behaviors could be shifted by the presence of ALK2(R206H) receptors. Our studies provide a compelling example for Drosophila as a model organism to study the molecular underpinnings of complex human syndromes such as FOP.

Lack of significant estrogen and progesterone receptor expression in nasal telangiectasias in hereditary hemorrhagic telangiectasia: an immunohistochemical analysis

CONCLUSION

This immunohistochemical study of estrogen and progesterone receptors could not confirm a significant expression in nasal telangiectasias. Thus, a specific effect of these hormones or anti-hormone therapy on malformed nasal vessels has to be questioned and only offered under strict clinical control.

OBJECTIVE

The efforts to control recurrent epistaxis in hereditary hemorrhagic telangiectasia (HHT) using alternative methods are very intense. Hormone or anti-hormone therapy has frequently been postulated and the reported results are controversial. Therefore it was important to find an explanation regarding a possible impact of hormonal therapies by immunohistochemical evaluation of progesterone and estrogen receptor expression on nasal telangiectasias of affected patients.

METHODS

Tissue samples of nasal mucosa with evidence of telangiectasias from 14 patients with HHT were analyzed for the expression of progesterone and estrogen receptors on the nuclei of endothelial cells of the malformed vessels using immunohistochemistry.

RESULTS

Progesterone receptors were not detected in any of the cases and only two cases showed a weak expression of estrogen receptors with an immunoreactive score of 2/12.

Soluble endoglin specifically binds bone morphogenetic proteins 9 and 10 via its orphan domain, inhibits blood vessel formation, and suppresses tumor growth

Endoglin (CD105), a transmembrane protein of the transforming growth factor β superfamily, plays a crucial role in angiogenesis. Mutations in endoglin result in the vascular defect known as hereditary hemorrhagic telangiectasia (HHT1). The soluble form of endoglin was suggested to contribute to the pathogenesis of preeclampsia. To obtain further insight into its function, we cloned, expressed, purified, and characterized the extracellular domain (ECD) of mouse and human endoglin fused to an immunoglobulin Fc domain. We found that mouse and human endoglin ECD-Fc bound directly, specifically, and with high affinity to bone morphogenetic proteins 9 and 10 (BMP9 and BMP10) in surface plasmon resonance (Biacore) and cell-based assays. We performed a function mapping analysis of the different domains of endoglin by examining their contributions to the selectivity and biological activity of the protein. The BMP9/BMP10 binding site was localized to the orphan domain of human endoglin composed of the amino acid sequence 26-359. We established that endoglin and type II receptors bind to overlapping sites on BMP9. In the in vivo chick chorioallantoic membrane assay, the mouse and the truncated human endoglin ECD-Fc both significantly reduced VEGF-induced vessel formation. Finally, murine endoglin ECD-Fc acted as an anti-angiogenic factor that decreased blood vessel sprouting in VEGF/FGF-induced angiogenesis in in vivo angioreactors and reduced the tumor burden in the colon-26 mouse tumor model. Together our findings indicate an important role of soluble endoglin ECD in the regulation of angiogenesis and highlight efficacy of endoglin-Fc as a potential anti-angiogenesis therapeutic agent.

Functional analysis of saxophone, the Drosophila gene encoding the BMP type I receptor ortholog of human ALK1/ACVRL1 and ACVR1/ALK2

In metazoans, bone morphogenetic proteins (BMPs) direct a myriad of developmental and adult homeostatic events through their heterotetrameric type I and type II receptor complexes. We examined 3 existing and 12 newly generated mutations in the Drosophila type I receptor gene, saxophone (sax), the ortholog of the human Activin Receptor-Like Kinase1 and -2 (ALK1/ACVRL1 and ALK2/ACVR1) genes. Our genetic analyses identified two distinct classes of sax alleles. The first class consists of homozygous viable gain-of-function (GOF) alleles that exhibit (1) synthetic lethality in combination with mutations in BMP pathway components, and (2) significant maternal effect lethality that can be rescued by an increased dosage of the BMP encoding gene, dpp+. In contrast, the second class consists of alleles that are recessive lethal and do not exhibit lethality in combination with mutations in other BMP pathway components. The alleles in this second class are clearly loss-of-function (LOF) with both complete and partial loss-of-function mutations represented. We find that one allele in the second class of recessive lethals exhibits dominant-negative behavior, albeit distinct from the GOF activity of the first class of viable alleles. On the basis of the fact that the first class of viable alleles can be reverted to lethality and on our ability to independently generate recessive lethal sax mutations, our analysis demonstrates that sax is an essential gene. Consistent with this conclusion, we find that a normal sax transcript is produced by saxP, a viable allele previously reported to be null, and that this allele can be reverted to lethality. Interestingly, we determine that two mutations in the first class of sax alleles show the same amino acid substitutions as mutations in the human receptors ALK1/ACVRl-1 and ACVR1/ALK2, responsible for cases of hereditary hemorrhagic telangiectasia type 2 (HHT2) and fibrodysplasia ossificans progressiva (FOP), respectively. Finally, the data presented here identify different functional requirements for the Sax receptor, support the proposal that Sax participates in a heteromeric receptor complex, and provide a mechanistic framework for future investigations into disease states that arise from defects in BMP/TGF-beta signaling.

Mutation analysis of "Endoglin" and "Activin receptor-like kinase" genes in German patients with hereditary hemorrhagic telangiectasia and the value of rapid genotyping using an allele-specific PCR-technique

BACKGROUND

Hereditary hemorrhagic telangiectasia (HHT), also known as Rendu-Osler-Weber syndrome, is an autosomal dominant disorder which is clinically characterised by recurrent epistaxis, mucocutaneous telangiectasia and visceral arteriovenous malformations. Genetic linkage studies identified two genes primarily related to HHT: endoglin (ENG) on chromosome 9q33-34 and activin receptor-like kinase1 (ACVRL1) on chromosome 12q13. We have screened a total of 41 unselected German patients with the suspected diagnosis of HHT. Mutation analysis for the ENG and ACVRL1 genes in all patients was performed by PCR amplification. Sequences were then compared to the HHT database http://www.hhtmutation.org sequences of the ENG mRNA (accession no. BC014271.2) and the ACVRL1 mRNA (accession no. NM000020.1).

RESULTS

We identified 15 different mutations in 18 cases by direct sequencing. Among these mutations, one novel ENG mutation could be detected which has not yet been described in the literature before. The genotype-phenotype correlation was consistent with a higher frequency of pulmonary arteriovenous malformations in patients with ENG mutations than in patients with ACVRL1 mutations in our collective.

CONCLUSION

For rapid genotyping of mutations and SNPs (single nucleotide polymorphisms) in ENG and ACVRL1, allele-specific PCR methods with sequence-specific primers (PCR-SSP) were established and their value analysed.

Hereditary hemorrhagic telangiectasia type 1 and 2 mutations in Finland

CONCLUSION

The finding of several new unique mutations suggests that the genes causing hereditary hemorrhagic telangiectasia (HHT), i.e. endoglin (ENG) and activin receptor-like kinase 1 (ACVRL1), have a relatively high mutation rate. As no single founder mutation was found, analysis of the whole coding sequences of ENG and ACVRL1 genes remains the first choice in genetic testing of new index patients with HHT.

OBJECTIVES

Our aim was to characterize specific mutations causing HHT in our hospital in Helsinki serving a population of 1 million inhabitants.

PATIENTS AND METHODS

HHT patients were searched from our hospital discharge records and their diagnoses were verified by review of patient records and interviews. Eight index patients who fulfilled HHT phenotypic criteria were tested. ENG and ACVRL1 mutations were identified by DNA sequencing of ENG and ACVRL1 coding regions.

RESULTS

Of the eight index patients, four had a mutation in the ENG gene, three in the ACVRL1 gene, and one had no mutations. All the mutations were different and all the four ENG mutations and one of the ACVRL1 mutations were new and had not been described previously in other populations. All the affected first-degree relatives had the same mutation as the index case.

Mutation study of Spanish patients with hereditary hemorrhagic telangiectasia

Background

Hereditary Hemorrhagic Telangiectasia (HHT) is an autosomal dominant and age-dependent vascular disorder characterised mainly by mutations in the Endoglin (ENG) or activin receptor-like kinase-1 (ALK1, ACVRL1) genes.

Methods

Here, we have identified 22 ALK1 mutations and 15 ENG mutations, many of which had not previously been reported, in independent Spanish families afflicted with HHT.

Results

We identified mutations in thirty-seven unrelated families. A detailed analysis of clinical symptoms was recorded for each patient analyzed, with a higher significant presence of pulmonary arteriovenous malformations (PAVM) in HHT1 patients over HHT2. Twenty-two mutations in ALK1 and fifteen in ENG genes were identified. Many of them, almost half, represented new mutations in ALK1 and in ENG. Missense mutations in ENG and ALK1 were localized in a tridimensional protein structure model.

Conclusion

Overall, ALK1 mutations (HHT2) were predominant over ENG mutations (HHT1) in our Spanish population, in agreement with previous data from our country and other Mediterranean countries (France, Italy), but different to Northern Europe or North America. There was a significant increase of PAVM associated with HHT1 over HHT2 in these families.

Detection of a significant association between mutations in the ACVRL1 gene and hepatic involvement in German patients with hereditary haemorrhagic telangiectasia

Hereditary haemorrhagic telangiectasia (HHT) is a heterogeneous multisystemic dysplasia of the vascular tissue. This autosomal dominant inherited disorder shows a wide variation in its phenotypic expression. Between 8 and 78% of the HHT patients show arteriovenous malformations of the liver. The molecular basis for hepatic manifestation is still unknown. Two genes are known to play a major role in the development of HHT: activin A receptor type II-like 1 gene (ACVRL1) and ENG. Previously, we and others showed that hepatic involvement is associated with mutations in the ACVRL1 gene, but rarely caused by ENG mutations. Here, we report about the sequencing analysis of a new cohort of 18 adult HHT patients. In these patients, we identified eight novel (four in ACVRL1 and four in ENG) and eight already known mutations. Statistical analysis of our entire data revealed significant differences in the distribution of ACVRL1 and ENG mutations among HHT patients with and without liver involvement (p = 0.0016). The positive predictive value for type 2 HHT (ACVRL1 positive) patients to develop liver disease until the age of 52 years is 68.4%. We conclude that molecular genetic testing of HHT patients is important for prognosis with respect to liver disease.

Analysis of ENG and ACVRL1 genes in 137 HHT Italian families identifies 76 different mutations (24 novel). Comparison with other European studies

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant disorder causing vascular dysplasias. About 70-80% of HHT patients carries mutations in ENG or ACVRL1 genes, which code for a TGFbeta receptor type III and I respectively. Molecular data on a large cohort of Italian HHT patients are presented, discussing the significance of missense and splice site mutations. Mutation analysis in ENG and ACVRL1 genes was performed using single strand conformation polymorphisms (SSCP), denaturing high performance liquid chromatography (DHPLC) and subsequent direct sequencing. Overall, 101 mutations were found, with ACVRL1 involved in 71% of cases. The highest number of mutations (28/101 subjects, 14/76 different mutations referring to both genes) was in ACVRL1, exon 3. Mutation analysis was then extended to a total of 356 family members, and 162 proven to carry the mutation. New polymorphisms were identified in both genes, and evidence that ENG P131L change is not a disease-causing mutation was also provided. An in silico analysis was performed in order to characterize splice-site mutations. These results were compared to other European national studies and data from Italy, France and Spain were consistent for an higher incidence of ACVRL1 mutations.

Long-term follow-up in 12 children with pulmonary arteriovenous malformations: confirmation of hereditary hemorrhagic telangiectasia in all cases

OBJECTIVE

To assess whether pulmonary arteriovenous malformation (PAVM) is associated with hereditary hemorrhagic telangiectasia (HHT).

STUDY DESIGN

This study was a review of 12 children (sex ratio = 1) including family history, mutation analysis, and long-term follow-up.

RESULTS

Five children were under age 3 years when PAVM was diagnosed. Presentations included pulmonary symptoms (n = 8), cerebral abscess (n = 2), and transient ischemic attack (TIA) (n = 1); 1 patient was asymptomatic. Nine of the 12 children (75%) had a family history of PAVM. The diagnosis of HHT was confirmed in all cases. A mutation in ENG was found in 9 of the 10 children available for testing. No mutation in ACVRL1 was found. During long-term follow-up (mean, 16 years), the following complications occurred: TIA (n = 2), hemoptysis (n = 2), and cerebral abscess (n = 2). Nine children experienced recurrence of PAVM. The children with no recurrence were those without a family history of PAVM.

CONCLUSIONS

The diagnosis of HHT should be considered in a child with an apparently isolated PAVM. Because serious complications may occur at any age, we recommend screening for PAVM and long-term follow-up in children from families with HHT, especially those with an ENG mutation.

Hereditary hemorrhagic telangiectasia: clinical features in ENG and ALK1 mutation carriers

BACKGROUND

Hereditary hemorrhagic telangiectasia (HHT) is a genetic disorder characterized by epistaxis, mucocutaneous telangiectases and visceral arteriovenous malformations (AVMs), particularly in the brain (CAVMs), lungs (PAVMs), liver (HAVMs) and gastrointestinal tract (GI). The identification of a mutated ENG (HHT1) or ALK-1 (HHT2) gene now enables a genotype-phenotype correlation.

OBJECTIVE

To determine the incidence of visceral localizations and evaluate phenotypic differences between ENG and ALK1 mutation carriers.

METHODS

A total of 135 consecutive adult patients were subjected to mutational screening in ENG and ALK1 genes and instrumental tests to detect AVMs, such as chest-abdomen multislice computed tomography (MDCT), brain magnetic resonance imaging and magnetic resonance angiography (MRI/MRA), upper endoscopy, were offered to all patients, independent of presence of clinical symptoms. The 122 patients with identified mutations were enrolled in the study and genotype-phenotype correlations were established.

RESULTS

PAVMs and CAVMs were significantly more frequent in HHT1 (75% vs. 44%, P < 0.0005; 20% vs. 0%, P < 0.002, respectively) and HAVMs in HHT2 (60% vs. 84%, P < 0.01). No age difference was found for PAVMs whereas HAVMs were significantly higher in older patients in both HHT1 and HHT2. Neurological manifestations secondary to CAVMs/PAVMs were found only in HHT1 patients, whereas severe liver involvement was detected only in HHT2. Respiratory symptoms were mainly detected in HHT1.

CONCLUSIONS

Our study evidences a higher visceral involvement in HHT1 and HHT2 compared with previous reports. HHT1 is more frequently associated with congenital AVM malformations, such as CAVMs and PAVMs whereas HHT2 predominantly involves the liver. The ENG gene should be first targeted for mutational screening in the presence of large PAVM in patients < 45 years.

Structural model of human endoglin, a transmembrane receptor responsible for hereditary hemorrhagic telangiectasia

Endoglin is a type I membrane protein expressed as a disulphide-linked homodimer on human vascular endothelial cells whose haploinsufficiency is responsible for the dominant vascular dysplasia known as hereditary hemorrhagic telangiectasia (HHT). Structurally, endoglin belongs to the zona pellucida (ZP) family of proteins that share a ZP domain of approximately 260 amino acid residues at their extracellular region. Endoglin is a component of the TGF-beta receptor complex, interacts with the TGF-beta signalling receptors types I and II, and modulates cellular responses to TGF-beta. Here, we have determined for the first time the three-dimensional structure of the approximately 140 kDa extracellular domain of endoglin at 25 A resolution, using single-particle electron microscopy (EM). This reconstruction provides the general architecture of endoglin, which arranges as a dome made of antiparallel oriented monomers enclosing a cavity at one end. A high-resolution structure of endoglin has also been modelled de novo and found to be consistent with the experimental reconstruction. Each subunit comprises three well-defined domains, two of them corresponding to ZP regions, organised into an open U-shaped monomer. This domain arrangement was found to closely resemble the overall structure derived experimentally and the three modelled de novo domains were tentatively assigned to the domains observed in the EM reconstruction. This molecular model was further tested by tagging endoglin's C terminus with an IgG Fc fragment visible after 3D reconstruction of the labelled protein. Combined, these data provide the structural framework to interpret endoglin's functional domains and mutations found in HHT patients.