Andreas Lux

Assignment of Transforming Growth Factor β1 and β3 and a Third New Ligand to the Type I Receptor ALK-1

Germ line mutations in one of two distinct genes, endoglin or ALK-1, cause hereditary hemorrhagic telangiectasia (HHT), an autosomal dominant disorder of localized angiodysplasia. Both genes encode endothelial cell receptors for the transforming growth factor β (TGF-β) ligand superfamily. Endoglin has homology to the type III receptor, betaglycan, although its exact role in TGF-β signaling is unclear. Activin receptor-like kinase 1 (ALK-1) has homology to the type I receptor family, but its ligand and corresponding type II receptor are unknown. In order to identify the ligand and type II receptor for ALK-1 and to investigate the role of endoglin in ALK-1 signaling, we devised a chimeric receptor signaling assay by exchanging the kinase domain of ALK-1 with either the TGF-β type I receptor or the activin type IB receptor, both of which can activate an inducible PAI-1 promoter. We show that TGF-β1 and TGF-β3, as well as a third unknown ligand present in serum, can activate chimeric ALK-1. HHT-associated missense mutations in the ALK-1 extracellular domain abrogate signaling. The ALK-1/ligand interaction is mediated by the type II TGF-β receptor for TGF-β and most likely through the activin type II or type IIB receptors for the serum ligand. Endoglin is a bifunctional receptor partner since it can bind to ALK-1 as well as to type I TGF-β receptor. These data suggest that HHT pathogenesis involves disruption of a complex network of positive and negative angiogenic factors, involving TGF-β, a new unknown ligand, and their corresponding receptors.

Hereditary haemorrhagic telangiectasia: a questionnaire based study to delineate the different phenotypes caused by endoglin and ALK1 mutations

Background: Hereditary haemorrhagic telangiectasia (HHT) is an autosomal dominant vascular dysplasia characterised by mucocutaneous telangiectasis, epistaxis, gastrointestinal haemorrhage, and arteriovenous malformations in the lung and brain. Causative mutations for HHT have been identified in two genes, endoglin and ALK1, which encode proteins involved in serine-threonine kinase signalling in the endothelial cell. Methods: A number of people affected with HHT had completed a postal questionnaire as part of an international study to delineate the HHT phenotype. We identified questionnaires completed by subjects in whom we had identified a mutation in endoglin or ALK1. Further questionnaires were sent to families with known mutations. Data were only included from questionnaires returned by people known to carry disease causing mutations. Results: Questionnaires were completed by 83 subjects with known mutations. Of these, 49 had endoglin mutations (HHT1) and 34 had ALK1 mutations (HHT2). Subjects with HHT1 reported an earlier onset of epistaxis (p=0.01) and telangiectasis (p=0.0001) than those with HHT2. Pulmonary arteriovenous malformations were only reported in the endoglin mutation group in our study (p<0.001). Conclusions: Our questionnaire based study provides evidence that the HHT phenotype caused by mutations in endoglin (HHT1) is distinct from, and more severe than, HHT caused by mutations in ALK1 (HHT2). This has significant implications for diagnosis, screening, and treatment in the two different forms of HHT, as well as for understanding the pathogenesis of the disease.

Endoglin structure and function - Determinants of endoglin phosphorylation by transforming growth factor-beta receptors

Determination of the functional relationship between the transforming growth factor-β (TGFβ) receptor proteins endoglin and ALK1 is essential to the understanding of the human vascular disease, hereditary hemorrhagic telangiectasia. TGFβ1 caused recruitment of ALK1 into a complex with endoglin in human umbilical vein endothelial cells (HUVECs). Therefore, we examined TGFβ receptor-dependent phosphorylation of endoglin by the constitutively active forms of the TGFβ type I receptors ALK1, ALK5, and the TGFβ type II receptor, TβRII. Of these receptors, TβRII preferentially phosphorylated endoglin on cytosolic domain serine residues Ser634 and Ser635. Removal of the carboxyl-terminal tripeptide of endoglin, which comprises a putative PDZ-liganding motif, dramatically increased endoglin serine phosphorylation by all three receptors, suggesting that the PDZ-liganding motif is important for the regulation of endoglin phosphorylation. Constitutively active (ca)ALK1, but not caALK5, phosphorylated endoglin on cytosolic domain threonine residues. caALK1-mediated threonine phosphorylation required prior serine phosphorylation, suggesting a sequential mechanism of endoglin phosphorylation. Wild-type, but not a threonine phosphorylation-defective endoglin mutant blocked cell detachment and the antiproliferative effects of caALK1 expressed in HUVECs. These results suggest that ALK1 is a preferred TGFβ receptor kinase for endoglin threonine phosphorylation in HUVECs and indicate a role for endoglin phosphorylation in the regulation of endothelial cell adhesion and growth by ALK1.

Mutation and expression analysis of the endoglin gene in hereditary hemorrhagic telangiectasia reveals null alleles

Hereditary Hemorrhagic Telangiectasia (HHT) is an autosomal dominant disorder characterized by multisystemic vascular dysplasia and recurrent hemorrhage from the sites of vascular lesions. Two genes have been identified for HHT. Endoglin, a TGF‐β binding protein which maps to chromosome 9q3, is the gene for HHT1. The type and location of most of the previously described mutations in the endoglin (ENG) gene suggested a dominant‐negative model of receptor–complex dysfunction for the molecular basis of this disorder. In this article we describe 11 novel ENG mutations in HHT kindreds, which include missense and splice‐site mutations. Two identical missense mutations in unrelated families disrupt the start codon of the gene. In addition, some frameshift and nonsense mutations lead to very low or undetectable levels of transcript from the mutant allele. These combined data suggest that the nature of most ENG mutations is to create a null (nonfunctional) allele, and that there is no requirement for the synthesis of a truncated endoglin protein in the pathogenesis of HHT. Hum Mutat 11:286–294, 1998.

Overlapping spectra of SMAD4 mutations in juvenile polyposis (JP) and JP–HHT syndrome†

Juvenile polyposis (JP) and hereditary hemorrhagic telangiectasia (HHT) are clinically distinct diseases caused by mutations in SMAD4 and BMPR1A (for JP) and endoglin and ALK1 (for HHT). Recently, a combined syndrome of JP–HHT was described that is also caused by mutations in SMAD4. Although both JP and JP–HHT are caused by SMAD4 mutations, a possible genotype:phenotype correlation was noted as all of the SMAD4 mutations in the JP–HHT patients were clustered in the COOH‐terminal MH2 domain of the protein. If valid, this correlation would provide a molecular explanation for the phenotypic differences, as well as a pre‐symptomatic diagnostic test to distinguish patients at risk for the overlapping but different clinical features of the disorders. In this study, we collected 19 new JP–HHT patients from which we identified 15 additional SMAD4 mutations. We also reviewed the literature for other reports of JP patients with HHT symptoms with confirmed SMAD4 mutations. Our combined results show that although the SMAD4 mutations in JP–HHT patients do show a tendency to cluster in the MH2 domain, mutations in other parts of the gene also cause the combined syndrome. Thus, any mutation in SMAD4 can cause JP–HHT. Any JP patient with a SMAD4 mutation is, therefore, at risk for the visceral manifestations of HHT and any HHT patient with SMAD4 mutation is at risk for early onset gastrointestinal cancer. In conclusion, a patient who tests positive for any SMAD4 mutation must be considered at risk for the combined syndrome of JP–HHT and monitored accordingly.

Endoglin structure and function: Determinants of endoglin phosphorylation by TGFβ receptors

Determination of the functional relationship between the transforming growth factor-β (TGFβ) receptor proteins endoglin and ALK1 is essential to the understanding of the human vascular disease, hereditary hemorrhagic telangiectasia. TGFβ1 caused recruitment of ALK1 into a complex with endoglin in human umbilical vein endothelial cells (HUVECs). Therefore, we examined TGFβ receptor-dependent phosphorylation of endoglin by the constitutively active forms of the TGFβ type I receptors ALK1, ALK5, and the TGFβ type II receptor, TβRII. Of these receptors, TβRII preferentially phosphorylated endoglin on cytosolic domain serine residues Ser634 and Ser635. Removal of the carboxyl-terminal tripeptide of endoglin, which comprises a putative PDZ-liganding motif, dramatically increased endoglin serine phosphorylation by all three receptors, suggesting that the PDZ-liganding motif is important for the regulation of endoglin phosphorylation. Constitutively active (ca)ALK1, but not caALK5, phosphorylated endoglin on cytosolic domain threonine residues. caALK1-mediated threonine phosphorylation required prior serine phosphorylation, suggesting a sequential mechanism of endoglin phosphorylation. Wild-type, but not a threonine phosphorylation-defective endoglin mutant blocked cell detachment and the antiproliferative effects of caALK1 expressed in HUVECs. These results suggest that ALK1 is a preferred TGFβ receptor kinase for endoglin threonine phosphorylation in HUVECs and indicate a role for endoglin phosphorylation in the regulation of endothelial cell adhesion and growth by ALK1.

Identification of Tctex2β, a Novel Dynein Light Chain Family Member That Interacts with Different Transforming Growth Factor-β Receptors

Endoglin is a membrane-inserted protein that is preferentially synthesized in angiogenic vascular endothelial and smooth muscle cells. Endoglin associates with members of the transforming growth factor-β (TGF-β) receptor family and has been identified as the gene involved in hereditary hemorrhagic telangiectasia. Although endoglin is known to affect cell responses to TGF-β, its mode of action is largely unknown. We performed yeast two-hybrid screening of a human placental cDNA library and isolated a new endoglin-binding partner, a novel 221-amino acid member of the Tctex1/2 family of cytoplasmic dynein light chains named Tctex2β, as the founder of a new Tctex1/2 subfamily. The interaction was localized exclusively to the cytoplasmic domain of endoglin. Reverse transcription-PCR showed expression of Tctex2β in a wide range of tissues, including vascular endothelial and smooth muscle cells, placenta, and testis, as well as in several tumor cell lines. High expression levels were found in human umbilical vein endothelial cells and the large cell lung cancer cell line. Forced expression of Tctex2β had a profound inhibitory effect on TGF-β signaling. Additional Tctex2β-interacting receptors were identified to be the TGF-β type II receptor and most likely beta-glycan, but not ALK5, ALK1, or the bone morphogenetic protein type II receptor. Upon fluorescence tagging, co-localization of Tctex2β and endoglin, as well as Tctex2β, endoglin, and the TGF-β type II receptor, was observed by different microscopy techniques. Our findings link endoglin for the first time to microtubule-based minus end-directed transport machinery, suggesting that some endoglin functions might be regulated and directed by its interaction with the cytoplasmic dynein light chain Tctex2β.

NOVEL MISSENSE AND FRAMESHIFT MUTATIONS IN THE ACTIVIN RECEPTOR-LIKE KINASE-1 GENE IN HEREDITARY HEMORRHAGIC TELANGIECTASIA

Hereditary Hemorrhagic Telangiectasia (HHT) is an autosomal dominant disorder characterized by multisystemic vascular dysplasia and recurrent hemorrhage. One of the causative genes is the activin receptor‐like kinase‐1 (ALK‐1) gene located on chromosome 12q13. ALK‐1 is an endothelial cell type I receptor for the TGF‐β superfamily of ligands. As a number of mutations have been identified in the kinase domain of ALK‐1, we initiated a mutation analysis specifically targeting the first four coding exons of ALK‐1 in order to determine if mutations in the extracellular and transmembrane domains are also present in HHT. Six new mutations have been identified. Three frameshift mutations were identified in exons encoding the extracellular and transmembrane domains. These mutations would grossly truncate the ALK‐1 protein and are thus classic null alleles. Three new missense mutations within the exons encoding the extracellular domain, in addition to two previously described missense mutations, are located at or near highly conserved cysteines. These mutations may disrupt intra‐ or inter‐molecular disulfide bridges required for ligand binding. The combined data suggest that both severe and subtle changes in the ALK‐1 amino acid sequence can lead to receptor dysfunction and result in the HHT disease phenotype. Hum Mutat 12:137, 1998.

Genetic and molecular analyses of PEG10 reveal new aspects of genomic organization, transcription and translation.

The paternally expressed gene PEG10 is a retrotransposon derived gene adapted through mammalian evolution located on human chromosome 7q21. PEG10 codes for at least two proteins, PEG10-RF1 and PEG10-RF1/2, by -1 frameshift translation. Overexpression or reinduced PEG10 expression was seen in malignancies, like hepatocellular carcinoma or B-cell acute and chronic lymphocytic leukemia. PEG10 was also shown to promote adipocyte differentiation. Experimental evidence suggests that the PEG10-RF1 protein is an inhibitor of apoptosis and mediates cell proliferation. Here we present new data on the genomic organization of PEG10 by identifying the major transcription start site, a new splice variant and report the cloning and analysis of 1.9 kb of the PEG10 promoter. Furthermore, we show for the first time that PEG10 translation is initiated at a non-AUG start codon upstream of the previously predicted AUG codon as well as at the AUG codon. The finding that PEG10 translation is initiated at different sides adds a new aspect to the already interesting feature of PEG10s −1 frameshift translation mechanism. It is now important to unravel the cellular functions of the PEG10 protein variants and how they are related to normal or pathological conditions. The generated promoter-reporter constructs can be used for future studies to investigate how PEG10 expression is regulated. In summary, our study provides new data on the genomic organization as well as expression and translation of PEG10, a prerequisite in order to study and understand the role of PEG10 in cancer, embryonic development and normal cell homeostasis.

Novel missense and frameshift mutations in the activin receptor-like kinase-1 gene in hereditary hemorrhagic telangiectasia. Mutations in brief no. 164. Online.

Hereditary hemmorrhagic telangiectasia (HHT) is an autosomal dominant disorder characterized by multisystemic vascular dyplasia and recurrent hemorrhage. One of the causative genes is the activin receptor-like kinase-1 (ALK-1) gene located on chromosome 12q13. ALK-1 is an endothelial cell type I receptor for the TGF-beta superfamily of ligands. As a number of mutations have been identified in the kinase domain of ALK-1, we initiated a mutation analysis specifically targeting the first four coding exons of ALK-1 in order to determine if mutations in the extracellular and transmembrane domains are also present in HHT. Six new mutations have been identified. Three frameshift mutations were identified in exons encoding the extracellular and transmembrane domains. These mutations would grossly truncate the ALK-1 protein and are thus classic null alleles. Three new missense mutations within the exons encoding the extracellular domain, in addition to two previously described missense mutations, are located at or near highly conserved cysteines. These mutations may disrupt intra- or inter-molecular disulfide bridges required for ligand binding. The combined data suggest that both severe and subtle changes in the ALK-1 amino acid sequence can lead to receptor dysfunction and result in the HHT disease phenotype.Marfan Syndrome (MfS) is an autosomal dominant inherited connective tissue disorder with variable phenotypic expression of cardiovascular, skeletal and ocular manifestations. Cardiovascular complications, such as aortic aneurysm and dissection drastically reduce life expectancy of individuals with MfS, whereas preventive surgery substantially improves the prognosis of these patients. A number of mutations in the fibrillin 1 (FBN1) gene associated with MfS have been identified to date, demonstrating considerable molecular heterogeneity. One region, however, located around exon 24, exhibits a striking clustering of mutations, which are associated with a severe, socalled neonatal form of MfS. Here we report the first mutation (G2950A) in exon 24 of the neonatal region of the FBN1 gene, associated with a classic MfS phenotype. The mutation leads to the subsitution of valin by isoleucin (V984I), both uncharged amino acids, which only differ in a single methyl group. This defect was identified in a proband with cardiovascular manifestations of MfS by SSCP analysis of PCR-amplified genomic DNA, direct PCR sequencing and RFLP analysis. The substitution was neither detected in the unaffected 4-year old daughter of the proband, nor in 3 of his healthy family members nor in 108 allels from control individuals, suggesting that this mutation is causative for MfS in the patient. Since no other family member of the proband is affected by MfS, the defect described is sporadic. In summary, we identified a novel defect in exon 24 of the neonatal region of the FBN1 gene in a patient with a classic phenotype of MfS, suggesting that conservative substitutions in this region may lead to a less severe phenotype of the disease. This finding further demonstrates the remarkable phenotypic heterogeneity associated with FBN1 mutations and stresses the significance of modifying genes and individual alterations in protein function for the pheontypic expression of the disease.Nephrogenic diabetes insipidus (NDI) is a rare, mostly X‐linked recessive disorder characterized by renal tubular resistance to the antidiuretic effect of arginine vasopressin. The gene responsible for the X‐linked NDI, the G‐protein‐coupled vasopressin V2 receptor, has been localized on the Xq28 region. In this study we present three NDI families from Hungary with three different missense mutations in the vasopressin V2 receptor gene. After the mutations in the affected probands in each family had been characterized, other family members were screened by restriction enzyme analysis. The N317K and the W323S mutations have not been detected previously. The C112R is an already known mutation. The N317K was a de novo mutation in the patient. The C112R and the W323S were found in the mothers of the patients as carriers and in all other patients, but not in the unaffected members of the families. Segregation of the mutations was consistent with the clinically observed symptoms as well as their severity. As conclusion, these findings provide further evidence that X‐linked NDI results from defects in the V2 receptor gene. Hum Mutat 12:137–138, 1998.