Fatima S. Govani

Hereditary haemorrhagic telangiectasia: a clinical and scientific review.

The autosomal-dominant trait hereditary haemorrhagic telangiectasia (HHT) affects 1 in 5–8000 people. Genes mutated in HHT (most commonly for endoglin or activin receptor-like kinase (ALK1)) encode proteins that modulate transforming growth factor (TGF)-β superfamily signalling in vascular endothelial cells; mutations lead to the development of fragile telangiectatic vessels and arteriovenous malformations. In this article, we review the underlying molecular, cellular and circulatory pathobiology; explore HHT clinical and genetic diagnostic strategies; present detailed considerations regarding screening for asymptomatic visceral involvement; and provide overviews of management strategies.

Elevated factor VIII in hereditary haemorrhagic telangiectasia (HHT): Association with venous thromboembolism

Hereditary haemorrhagic telangiectasia (HHT) causes chronic nasal and gastrointestinal haemorrhage. Prothrombotic agents are commonly used for severe haemorrhage. Thrombotic risks have not been defined. In order to identify prothrombotic variables in HHT patients, and assess their potential functional significance, a pilot ELISA-based study comparing plasma proteins in healthy individuals with HHT to age/sex-matched non-HHT controls was validated in a full study of 309 consecutive HHT-affected individuals. In the pilot study, factor VIII (FVIII) and von Willebrand factor antigen concentrations were elevated in the HHT group compared to non-HHT controls (p<0.0013, Mann-Whitney). Service laboratory measurements confirmed high FVIII:Ag in 125 HHT-affected individuals with no recent ill-health, intervention or venous thromboemboli. FVIII:Ag levels increased with age. Logistic regression also suggested an age-independent association with HHT-associated pulmonary arteriovenous malformations (AVMs). No association was demonstrated between FVIII:Ag and acute phase response, disseminated intravascular coagulation, ABO group, pulmonary artery pressure, or markers of HHT haemorrhage. Elevated FVIII:Ag were associated with shortened activated partial thromboplastin times (APTTs), and VTE:VTE affected 20/309 (6.5%) HHT-affected individuals, at median age 61(36-71) years. Four VTE occurred in factorV Leiden heterozygotes in the months following PAVM-associated brain abscess. The strongest association with VTE was with log-transformed FVIII:Ag measured 10-132 months from VTE (odds ratio 2.41, 95% confidence intervals 1.254, 4.612, p=0.008). Age made no additional contribution to VTE risk once adjusted for FVIII:Ag. In conclusion, HHT-related elevation of FVIII:Ag levels may influence thrombotic risk in HHT. Individualised risk-benefit considerations may be helpful in HHT management.

Endothelial cell processing and alternatively spliced transcripts of factor VIII: potential implications for coagulation cascades and pulmonary hypertension.

Background Coagulation factor VIII (FVIII) deficiency leads to haemophilia A. Conversely, elevated plasma levels are a strong predictor of recurrent venous thromboemboli and pulmonary hypertension phenotypes in which in situ thromboses are implicated. Extrahepatic sources of plasma FVIII are implicated, but have remained elusive. Methodology/Principal Findings Immunohistochemistry of normal human lung tissue, and confocal microscopy, flow cytometry, and ELISA quantification of conditioned media from normal primary endothelial cells were used to examine endothelial expression of FVIII and coexpression with von Willebrand Factor (vWF), which protects secreted FVIII heavy chain from rapid proteloysis. FVIII transcripts predicted from database mining were identified by rt-PCR and sequencing. FVIII mAb-reactive material was demonstrated in CD31+ endothelial cells in normal human lung tissue, and in primary pulmonary artery, pulmonary microvascular, and dermal microvascular endothelial cells. In pulmonary endothelial cells, this protein occasionally colocalized with vWF, centered on Weibel Palade bodies. Pulmonary artery and pulmonary microvascular endothelial cells secreted low levels of FVIII and vWF to conditioned media, and demonstrated cell surface expression of FVIII and vWF Ab–reacting proteins compared to an isotype control. Four endothelial splice isoforms were identified. Two utilize transcription start sites in alternate 5′ exons within the int22h-1 repeat responsible for intron 22 inversions in 40% of severe haemophiliacs. A reciprocal relationship between the presence of short isoforms and full-length FVIII transcript suggested potential splice-switching mechanisms. Conclusions/Significance The pulmonary endothelium is confirmed as a site of FVIII secretion, with evidence of synthesis, cell surface expression, and coexpression with vWF. There is complex alternate transcription initiation from the FVIII gene. These findings provide a framework for future research on the regulation and perturbation of FVIII synthesis, and of potential relevance to haemophilia, thromboses, and pulmonary hypertensive states.

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.

Fine mapping of the hereditary haemorrhagic telangiectasia (HHT)3 locus on chromosome 5 excludes VE-Cadherin-2, Sprouty4 and other interval genes.

BackgroundThere is significant interest in new loci for the inherited condition hereditary haemorrhagic telangiectasia (HHT) because the known disease genes encode proteins involved in vascular transforming growth factor (TGF)-β signalling pathways, and the disease phenotype appears to be unmasked or provoked by angiogenesis in man and animal models. In a previous study, we mapped a new locus for HHT (HHT3) to a 5.7 Mb region of chromosome 5. Some of the polymorphic markers used had been uninformative in key recombinant individuals, leaving two potentially excludable regions, one of which contained loci for attractive candidate genes encoding VE Cadherin-2, Sprouty4 and FGF1, proteins involved in angiogenesis.MethodsExtended analyses in the interval-defining pedigree were performed using informative genomic sequence variants identified during candidate gene sequencing. These variants were amplified by polymerase chain reaction; sequenced on an ABI 3730xl, and analysed using FinchTV V1.4.0 software.ResultsInformative genomic sequence variants were used to construct haplotypes permitting more precise citing of recombination breakpoints. These reduced the uninformative centromeric region from 141.2-144 Mb to between 141.9-142.6 Mb, and the uninformative telomeric region from 145.2-146.9 Mb to between 146.1-146.4 Mb.ConclusionsThe HHT3 interval on chromosome 5 was reduced to 4.5 Mb excluding 30% of the coding genes in the original HHT3 interval. Strong candidates VE-cadherin-2 and Sprouty4 cannot be HHT3.

Directional Next-Generation RNA Sequencing and Examination of Premature Termination Codon Mutations in Endoglin/Hereditary Haemorrhagic Telangiectasia

Hereditary haemorrhagic telangiectasia (HHT) is a disease characterised by abnormal vascular structures, and most commonly caused by mutations in ENG, ACVRL1 or SMAD4 encoding endothelial cell-expressed proteins involved in TGF-β superfamily signalling. The majority of mutations reported on the HHT mutation database are predicted to lead to stop codons, either due to frameshifts or direct nonsense substitutions. The proportion is higher for ENG (67%) and SMAD4 (65%) than for ACVRL1 (42%), p < 0.0001. Here, by focussing on ENG, we report why conventional views of these mutations may need to be revised. Of the 111 stop codon-generating ENG mutations, on ExPASy translation, all except one were premature termination codons (PTCs), sited at least 50-55 bp upstream of the final exon-exon boundary of the main endoglin isoform, L-endoglin. This strongly suggests that the mutated RNA species will undergo nonsense-mediated decay. We provide new in vitro expression data to support dominant negative activity of stable truncated endoglin proteins but suggest these will not generate HHT: the single natural stop codon mutation in L-endoglin (sited within 50-55 nucleotides of the final exon-exon boundary) is unlikely to generate functional protein since it replaces the entire transmembrane domain, as would 8 further natural stop codon mutations, if the minor S-endoglin isoform were implicated in HHT pathogenesis. Finally, next-generation RNA sequencing data of 7 different RNA libraries from primary human endothelial cells demonstrate that multiple intronic regions of ENG are transcribed. The potential consequences of heterozygous deletions or duplications of such regions are discussed. These data support the haploinsufficiency model for HHT pathogenesis, explain why final exon mutations have not been detected to date in HHT, emphasise the potential need for functional examination of non-PTC-generating mutations, and lead to proposals for an alternate stratification system of mutational types for HHT genotype-phenotype correlations.

Reply to Fernández-Fernández

Dr Fernandez-Fernandez1 is right in raising the important new data regarding anti-angiogenic strategies in hereditary haemorrhagic telangiectasia (HHT), data which were published too late for inclusion in our review.2 Although the gene mutations inform us that HHT is a disorder caused by aberrant signalling by the transforming growth factor (TGF)-β superfamily, the context in which the gene mutations are deleterious, when functioning apparently perfectly well for the vast majority of vessels, has always proved tantalising. In 2009, there has been a resurgent interest in the role of angiogenesis in provoking or unmasking an HHT phenotype, with animal models particularly implicating a role for aberrant angiogenesis during wound healing.3 Pathogenesis-based treatment strategies based on case reports using anti-VEGF (bevacizumab/Avastin),4, 5, 6 interferon,7, 8 and thalidomide9, 10 are being assessed in ongoing clinical trials at HHT centres. For any efficacious agent, side-effect profiles are likely to be crucial in determining whether the use of any efficacious agents can become more widespread within the HHT patient population. In 2009, encouraging data were also reported for two better-tolerated agents, N-acetyl cysteine (trial data for 600 mg tds)11 and tamoxifen (randomised placebo-control trial data for 20 mg/day).12 These have been introduced into specialised HHT practice, reflecting the rapidly advancing field.