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Rationale: NT-proBNP (N-terminal pro-brain natriuretic peptide), a biomarker of cardiac origin, is used to risk stratify patients with pulmonary arterial hypertension (PAH). Its limitations include poor sensitivity to early vascular pathology. Other biomarkers of vascular or systemic origin may also be useful in the management of PAH. Objectives: Identify prognostic proteins in PAH that complement NT-proBNP and clinical risk scores. Methods: An aptamer-based assay (SomaScan version 4) targeting 4,152 proteins was used to measure plasma proteins in patients with idiopathic, heritable, or drug-induced PAH from the UK National Cohort of PAH (n = 357) and the French EFORT (Evaluation of Prognostic Factors and Therapeutic Targets in PAH) study (n = 79). Prognostic proteins were identified in discovery-replication analyses of UK samples. Proteins independent of 6-minute-walk distance and NT-proBNP entered least absolute shrinkage and selection operator modeling, and the best combination in a single score was evaluated against clinical targets in EFORT. Measurements and Main Results: Thirty-one proteins robustly informed prognosis independent of NT-proBNP and 6-minute-walk distance in the UK cohort. A weighted combination score of six proteins was validated at baseline (5-yr mortality; area under the curve [AUC], 0.73; 95% confidence interval [CI], 0.63-0.85) and follow-up in EFORT (AUC, 0.84; 95% CI, 0.75-0.94; P = 9.96 × 10-6). The protein score risk stratified patients independent of established clinical targets and risk equations. The addition of the six-protein model score to NT-proBNP improved prediction of 5-year outcomes from AUC 0.762 (0.702-0.821) to 0.818 (0.767-0.869) by receiver operating characteristic analysis (P = 0.00426 for difference in AUC) in the UK replication and French samples combined. Conclusions: The plasma proteome informs prognosis beyond established factors in PAH and may provide a more sensitive measure of therapeutic response.

Rationale: Despite the increased recognition of TBX4 (T-BOX transcription factor 4)-associated pulmonary arterial hypertension (PAH), genotype-phenotype associations are lacking and may provide important insights. Objectives: To compile and functionally characterize all TBX4 variants reported to date and undertake a comprehensive genotype-phenotype analysis. Methods: We assembled a multicenter cohort of 137 patients harboring monoallelic TBX4 variants and assessed the pathogenicity of missense variation (n = 42) using a novel luciferase reporter assay containing T-BOX binding motifs. We sought genotype-phenotype correlations and undertook a comparative analysis with patients with PAH with BMPR2 (Bone Morphogenetic Protein Receptor type 2) causal variants (n = 162) or no identified variants in PAH-associated genes (n = 741) genotyped via the National Institute for Health Research BioResource-Rare Diseases. Measurements and Main Results: Functional assessment of TBX4 missense variants led to the novel finding of gain-of-function effects associated with older age at diagnosis of lung disease compared with loss-of-function effects (P = 0.038). Variants located in the T-BOX and nuclear localization domains were associated with earlier presentation (P = 0.005) and increased incidence of interstitial lung disease (P = 0.003). Event-free survival (death or transplantation) was shorter in the T-BOX group (P = 0.022), although age had a significant effect in the hazard model (P = 0.0461). Carriers of TBX4 variants were diagnosed at a younger age (P < 0.001) and had worse baseline lung function (FEV1, FVC) (P = 0.009) than the BMPR2 and no identified causal variant groups. Conclusions: We demonstrated that TBX4 syndrome is not strictly the result of haploinsufficiency but can also be caused by gain of function. The pleiotropic effects of TBX4 in lung disease may be in part explained by the differential effect of pathogenic mutations located in critical protein domains.

Most patients with rare diseases do not receive a molecular diagnosis and the aetiological variants and causative genes for more than half such disorders remain to be discovered1. Here we used whole-genome sequencing (WGS) in a national health system to streamline diagnosis and to discover unknown aetiological variants in the coding and non-coding regions of the genome. We generated WGS data for 13,037 participants, of whom 9,802 had a rare disease, and provided a genetic diagnosis to 1,138 of the 7,065 extensively phenotyped participants. We identified 95 Mendelian associations between genes and rare diseases, of which 11 have been discovered since 2015 and at least 79 are confirmed to be aetiological. By generating WGS data of UK Biobank participants2, we found that rare alleles can explain the presence of some individuals in the tails of a quantitative trait for red blood cells. Finally, we identified four novel non-coding variants that cause disease through the disruption of transcription of ARPC1B, GATA1, LRBA and MPL. Our study demonstrates a synergy by using WGS for diagnosis and aetiological discovery in routine healthcare.

Background: Pulmonary arterial hypertension (PAH) is a lethal vasculopathy characterized by pathogenic remodeling of pulmonary arterioles leading to increased pulmonary pressures, right ventricular hypertrophy, and heart failure. PAH can be associated with other diseases (APAH: connective tissue diseases, congenital heart disease, and others) but often the etiology is idiopathic (IPAH). Mutations in bone morphogenetic protein receptor 2 (BMPR2) are the cause of most heritable cases but the vast majority of other cases are genetically undefined. Methods: To identify new risk genes, we utilized an international consortium of 4241 PAH cases with exome or genome sequencing data from the National Biological Sample and Data Repository for PAH, Columbia University Irving Medical Center, and the UK NIHR BioResource – Rare Diseases Study. The strength of this combined cohort is a doubling of the number of IPAH cases compared to either national cohort alone. We identified protein-coding variants and performed rare variant association analyses in unrelated participants of European ancestry, including 1647 IPAH cases and 18,819 controls. We also analyzed de novo variants in 124 pediatric trios enriched for IPAH and APAH-CHD. Results: Seven genes with rare deleterious variants were associated with IPAH with false discovery rate smaller than 0.1: three known genes (BMPR2, GDF2, and TBX4), two recently identified candidate genes (SOX17, KDR), and two new candidate genes (fibulin 2, FBLN2; platelet-derived growth factor D, PDGFD). The new genes were identified based solely on rare deleterious missense variants, a variant type that could not be adequately assessed in either cohort alone. The candidate genes exhibit expression patterns in lung and heart similar to that of known PAH risk genes, and most variants occur in conserved protein domains. For pediatric PAH, predicted deleterious de novo variants exhibited a significant burden compared to the background mutation rate (2.45×, p = 2.5e−5). At least eight novel pediatric candidate genes carrying de novo variants have plausible roles in lung/heart development. Conclusions: Rare variant analysis of a large international consortium identified two new candidate genes—FBLN2 and PDGFD. The new genes have known functions in vasculogenesis and remodeling. Trio analysis predicted that ~ 15% of pediatric IPAH may be explained by de novo variants.

RATIONALE: Idiopathic and hereditary pulmonary arterial hypertension (PAH) are rare but comprise a genetically heterogeneous patient group. RNA-sequencing linked to the underlying genetic architecture can be used to better understand the underlying pathology by identifying key signalling pathways and stratify patients more robustly according to clinical risk. OBJECTIVES: Using a three-stage design of RNA discovery, RNA validation/model construction and model validation to define a set of PAH-associated RNAs and a single summarising RNA model score. To define genes most likely to be involved in disease development, we performed Mendelian randomisation (MR) analysis. METHODS: RNA-sequencing was performed on whole blood samples from 359 patients with idiopathic, heritable and drug-induced PAH and 72 age- and sex-matched healthy volunteers. The score was evaluated against disease severity markers including survival analysis using all-cause mortality from diagnosis. MR used known eQTL and summary statistics from a PAH GWAS. MEASUREMENTS AND MAIN RESULTS: We identified 507 genes with differential RNA expression in PAH patients compared to controls. A model of 25 RNAs was able to distinguish PAH with 87% accuracy (AUC 95% CI: 0.791-0.945) in model validation. The RNA model score was associated with disease severity and long-term survival (p=4.66x10-6) in PAH. MR detected an association between SMAD5 levels and PAH disease susceptibility (OR:0.317, 95%CI:0.129-0.776, p=0.012). CONCLUSIONS: A whole blood RNA signature of PAH, which includes RNAs relevant to disease pathogenesis, associates with disease severity and identifies patients with poor clinical outcomes. Genetic variants associated with lower SMAD5 expression may increase susceptibility to PAH.

Background: Pulmonary arterial hypertension (PAH) is a rare disease with an emerging genetic basis. Heterozygous mutations in the gene encoding the bone morphogenetic protein receptor type 2 ( BMPR2 ) are the commonest genetic cause of PAH, whereas biallelic mutations in the eukaryotic translation initiation factor 2 alpha kinase 4 gene ( EIF2AK4 ) are described in pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis. Here, we determine the frequency of these mutations and define the genotype-phenotype characteristics in a large cohort of patients diagnosed clinically with PAH. Methods: Whole-genome sequencing was performed on DNA from patients with idiopathic and heritable PAH and with pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis recruited to the National Institute of Health Research BioResource–Rare Diseases study. Heterozygous variants in BMPR2 and biallelic EIF2AK4 variants with a minor allele frequency of sorting intolerant from tolerant predictions) were identified as potentially causal. Phenotype data from the time of diagnosis were also captured. Results: Eight hundred sixty-four patients with idiopathic or heritable PAH and 16 with pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis were recruited. Mutations in BMPR2 were identified in 130 patients (14.8%). Biallelic mutations in EIF2AK4 were identified in 5 patients with a clinical diagnosis of pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis. Furthermore, 9 patients with a clinical diagnosis of PAH carried biallelic EIF2AK4 mutations. These patients had a reduced transfer coefficient for carbon monoxide (K co ; 33% [interquartile range, 30%–35%] predicted) and younger age at diagnosis (29 years; interquartile range, 23–38 years) and more interlobular septal thickening and mediastinal lymphadenopathy on computed tomography of the chest compared with patients with PAH without EIF2AK4 mutations. However, radiological assessment alone could not accurately identify biallelic EIF2AK4 mutation carriers. Patients with PAH with biallelic EIF2AK4 mutations had a shorter survival. Conclusions: Biallelic EIF2AK4 mutations are found in patients classified clinically as having idiopathic and heritable PAH. These patients cannot be identified reliably by computed tomography, but a low K co and a young age at diagnosis suggests the underlying molecular diagnosis. Genetic testing can identify these misclassified patients, allowing appropriate management and early referral for lung transplantation.

BackgroundPulmonary arterial hypertension (PAH) is a rare disorder leading to premature death. Rare genetic variants contribute to disease etiology but the contribution of common genetic variation to disease risk and outcome remains poorly characterized.MethodsWe performed two separate genome-wide association studies of PAH using data across 11,744 European-ancestry individuals (including 2,085 patients), one with genotypes from 5,895 whole genome sequences and another with genotyping array data from 5,849 further samples. Cross-validation of loci reaching genome-wide significance was sought by meta-analysis. We functionally annotated associated variants and tested associations with duration of survival.FindingsA locus atHLA-DPA1/DPB1within the class II major histocompatibility (MHC) region and a second nearSOX17were significantly associated with PAH. TheSOX17locus contained two independent signals associated with PAH. Functional and epigenomic data indicate that the risk variants nearSOX17alter gene regulation via an enhancer active in endothelial cells. PAH risk variants determined haplotype-specific enhancer activity and CRISPR-inhibition of the enhancer reducedSOX17expression. Analysis of median survival showed that PAH patients with two copies of theHLA-DPA1/DPB1risk variant had a two-fold difference (>16 years versus 8 years), compared to patients homozygous for the alternative allele.InterpretationWe have found that common genetic variation at loci inHLA-DPA1/DPB1and an enhancer nearSOX17are associated with PAH. Impairment of Sox17 function may be more common in PAH than suggested by rare mutations inSOX17. Allelic variation atHLA-DPB1stratifies PAH patients for survival following diagnosis, with implications for future therapeutic trial design.FundingUK NIHR, BHF, UK MRC, Dinosaur Trust, NIH/NHLBI, ERS, EMBO, Wellcome Trust, EU, AHA, ACClinPharm, Netherlands CVRI, Dutch Heart Foundation, Dutch Federation of UMC, Netherlands OHRD and RNAS, German DFG, German BMBF, APH Paris, Inserm, Université Paris-Sud, and French ANR.

Background - Approximately 25% of patients with pulmonary arterial hypertension (PAH) have been found to harbor rare mutations in disease-causing genes. To identify missing heritability in PAH we integrated deep phenotyping with whole-genome sequencing data using Bayesian statistics. Methods - We analyzed 13,037 participants enrolled in the NIHR BioResource - Rare Diseases (NBR) study, of which 1,148 were recruited to the PAH domain. To test for genetic associations between genes and selected phenotypes of pulmonary hypertension (PH), we used the Bayesian rare-variant association method BeviMed. Results - Heterozygous, high impact, likely loss-of-function variants in the Kinase Insert Domain Receptor (KDR) gene were strongly associated with significantly reduced transfer coefficient for carbon monoxide (KCO, posterior probability (PP)=0.989) and older age at diagnosis (PP=0.912). We also provide evidence for familial segregation of a rare nonsense KDR variant with these phenotypes. On computed tomographic imaging of the lungs, a range of parenchymal abnormalities were observed in the five patients harboring these predicted deleterious variants in KDR. Four additional PAH cases with rare likely loss-of-function variants in KDR were independently identified in the US PAH Biobank cohort with similar phenotypic characteristics. Conclusions - The Bayesian inference approach allowed us to independently validate KDR, which encodes for the Vascular Endothelial Growth Factor Receptor 2 (VEGFR2), as a novel PAH candidate gene. Furthermore, this approach specifically associated high impact likely loss-of-function variants in the genetically constrained gene with distinct phenotypes. These findings provide evidence for KDR being a clinically actionable PAH gene and further support the central role of the vascular endothelium in the pathobiology of PAH.

Rationale: Recently, rare heterozygous mutations in GDF2 were identified in patients with pulmonary arterial hypertension (PAH). GDF2 encodes the circulating BMP (bone morphogenetic protein) type 9, which is a ligand for the BMP2 receptor.Objectives: Here we determined the functional impact of GDF2 mutations and characterized plasma BMP9 and BMP10 levels in patients with idiopathic PAH.Methods: Missense BMP9 mutant proteins were expressed in vitro and the impact on BMP9 protein processing and secretion, endothelial signaling, and functional activity was assessed. Plasma BMP9 and BMP10 levels and activity were assayed in patients with PAH with GDF2 variants and in control subjects. Levels were also measured in a larger cohort of control subjects (n = 120) and patients with idiopathic PAH (n = 260).Measurements and Main Results: We identified a novel rare variation at the GDF2 and BMP10 loci, including copy number variation. In vitro, BMP9 missense proteins demonstrated impaired cellular processing and secretion. Patients with PAH who carried these mutations exhibited reduced plasma levels of BMP9 and reduced BMP activity. Unexpectedly, plasma BMP10 levels were also markedly reduced in these individuals. Although overall BMP9 and BMP10 levels did not differ between patients with PAH and control subjects, BMP10 levels were lower in PAH females. A subset of patients with PAH had markedly reduced plasma levels of BMP9 and BMP10 in the absence of GDF2 mutations.Conclusions: Our findings demonstrate that GDF2 mutations result in BMP9 loss of function and are likely causal. These mutations lead to reduced circulating levels of both BMP9 and BMP10. These findings support therapeutic strategies to enhance BMP9 or BMP10 signaling in PAH.

BACKGROUND: Rare genetic variants cause pulmonary arterial hypertension, but the contribution of common genetic variation to disease risk and natural history is poorly characterised. We tested for genome-wide association for pulmonary arterial hypertension in large international cohorts and assessed the contribution of associated regions to outcomes. METHODS: We did two separate genome-wide association studies (GWAS) and a meta-analysis of pulmonary arterial hypertension. These GWAS used data from four international case-control studies across 11 744 individuals with European ancestry (including 2085 patients). One GWAS used genotypes from 5895 whole-genome sequences and the other GWAS used genotyping array data from an additional 5849 individuals. Cross-validation of loci reaching genome-wide significance was sought by meta-analysis. Conditional analysis corrected for the most significant variants at each locus was used to resolve signals for multiple associations. We functionally annotated associated variants and tested associations with duration of survival. All-cause mortality was the primary endpoint in survival analyses. FINDINGS: A locus near SOX17 (rs10103692, odds ratio 1·80 [95% CI 1·55-2·08], p=5·13 × 10-15) and a second locus in HLA-DPA1 and HLA-DPB1 (collectively referred to as HLA-DPA1/DPB1 here; rs2856830, 1·56 [1·42-1·71], p=7·65 × 10-20) within the class II MHC region were associated with pulmonary arterial hypertension. The SOX17 locus had two independent signals associated with pulmonary arterial hypertension (rs13266183, 1·36 [1·25-1·48], p=1·69 × 10-12; and rs10103692). Functional and epigenomic data indicate that the risk variants near SOX17 alter gene regulation via an enhancer active in endothelial cells. Pulmonary arterial hypertension risk variants determined haplotype-specific enhancer activity, and CRISPR-mediated inhibition of the enhancer reduced SOX17 expression. The HLA-DPA1/DPB1 rs2856830 genotype was strongly associated with survival. Median survival from diagnosis in patients with pulmonary arterial hypertension with the C/C homozygous genotype was double (13·50 years [95% CI 12·07 to >13·50]) that of those with the T/T genotype (6·97 years [6·02-8·05]), despite similar baseline disease severity. INTERPRETATION: This is the first study to report that common genetic variation at loci in an enhancer near SOX17 and in HLA-DPA1/DPB1 is associated with pulmonary arterial hypertension. Impairment of SOX17 function might be more common in pulmonary arterial hypertension than suggested by rare mutations in SOX17. Further studies are needed to confirm the association between HLA typing or rs2856830 genotyping and survival, and to determine whether HLA typing or rs2856830 genotyping improves risk stratification in clinical practice or trials. FUNDING: UK NIHR, BHF, UK MRC, Dinosaur Trust, NIH/NHLBI, ERS, EMBO, Wellcome Trust, EU, AHA, ACClinPharm, Netherlands CVRI, Dutch Heart Foundation, Dutch Federation of UMC, Netherlands OHRD and RNAS, German DFG, German BMBF, APH Paris, INSERM, Université Paris-Sud, and French ANR.

Pulmonary hypertension (PH) commonly occurs in patients with interstitial lung disease (ILD) and is associated with worsening of symptoms and an adverse prognosis. The onset of PH is extremely difficult to predict due to the very similar symptomology of the two conditions and confounding of common screening tests for PH in patients with ILD. It is not clear what invasive and non-invasive variables predict mortality in ILD-PH patients, or whether existing mortality prediction tools used in ILD patients are valid in ILD-PH. I hypothesised that the prediction of PH occurring in ILD patients was possible using non invasive screening tests, and that baseline and longitudinal change in non-invasive variables would predict mortality in ILD-PH patients. The integration of echocardiographic, brain natriuretic peptide (BNP), pulmonary function tests and CT variables showed that prediction of PH occurring in ILD patients was possible, although false positives were common. Echocardiographic variables best correlated with invasive right heart catheter (RHC) pressures. A score was developed to predict severe PH using echocardiographic variables, and was effective even when blinding the most powerful predictor which is commonly unavailable in patients with ILD-PH. CT is commonly employed in suspected ILD-PH patients to exclude co-existent pulmonary emboli and assess parenchymal disease progression. The right ventricle to left ventricle measured at CT pulmonary angiography was superior to both echocardiographic and RHC derived variables at predicting mortality. The presence of PH confounds commonly used mortality prediction tools in ILD. A multi modality mortality prediction model was developed to predict mortality using baseline demographics, lung function and ILD diagnosis. Longitudinal change in pulmonary function tests and BNP were shown to predict mortality. A longitudinal model using demographics and change in gas transfer was developed. External validation of the mortality prediction tools is necessary before its utility is demonstrated. Open Access

Skeletal muscle is a major tissue in the body, and, as a consequence of chronic illness or ageing, can be lost in a process known as wasting or atrophy. Muscle wasting individuals have a worse quality of life and increased mortality. The molecular mechanisms responsible for the reduction in mass are not well understood. MicroRNAs (miRNAs) are small non-coding RNAs that reduce mRNA half-life and translation. Previous work by the group identified miR-424-5p and miR-1-5p as being up-regulated and down-regulated, respectively, in a cohort of patients with chronic obstructive pulmonary disease (COPD). In this thesis, the expression of miR-424-5p was determined in cohorts of individuals who are sarcopenic and those with intensive care unit-acquired weakness (ICUAW), and found to be up-regulated compared to controls. miR-424-5p associated negatively with measures of disease severity in all cohorts, and muscle mass and function in COPD. In vitro studies revealed miR-424-5p inhibited the expression of components required to transcribe ribosomal RNA (rRNA), and reduced maximal translational capacity. Targeting of insulin-like growth factor 1 (IGF-1) by miR-424-5p was also confirmed. In vivo, overexpression of this miRNA caused atrophy in tibialis anterior (TA) muscles of mice and downregulation of mature rRNAs, suggesting that miRNA induced ribosomal stress may contribute to the atrophy. miR-424-5p expression was down-regulated in the hypertrophied right ventricle (RV) of rats with pulmonary arterial hypertension (PAH), while IGF-1 protein and rRNA was up-regulated, suggesting targets of this miRNA may promote hypertrophy following miR-424-5p downregulation. Additionally, miR-1-5p was shown to target TGF-βR1 (alk5), and these components were down-regulated and up-regulated respectively in the RV of PAH rats, suggesting another mechanism of hypertrophy propagation. Together, these data present novel mechanisms through which skeletal and cardiac muscle mass may be regulated in chronic disease and ageing. Open Access

Eisenmenger syndrome (ES) represents the extreme manifestation of pulmonary arterial hypertension in patients with congenital heart disease and is associated with significant morbidity and mortality. Yet, survival prospects and exercise performance vary significantly among ES patients. Those at higher risk of death may benefit from closer follow-up, more intense conservative management and may be considered for heart and lung or, occasionally, lung-only transplantation. Yet, mortality risk assessment is challenging and there were in 2013 no established mortality risk stratification methods. I explore in this thesis the pathophysiology of ES and the prognostic significance of commonly used clinical markers of disease progression. Chapters III – VI present analyses performed on a large cohort of patients with ES at the Royal Brompton Hospital in London while Chapter VII presents results of the largest multicentre study on mortality risk stratification in ES. Chapter III explores the exercise physiology and methods for assessment of exercise performance in ES. I demonstrate that six-minute walk test (6MWT) distance and arterial oxygen saturation (SO2) at rest significantly contribute to mortality risk stratification. Chapter IV and V discuss the role of inflammation and dysproteinaemia in ES and demonstrate that assessment of serum C-reactive protein and albumin levels may contribute to prognostication. Chapter VI discusses the challenges associated with multivariable mortality risk stratification in ES and the pathophysiologic rationale for collection of selected parameters in the multicentre study. Chapter VII presents the results of a multicentre study for assessment of mortality and predictors of death in contemporary ES patients. I demonstrate that five simple, non-invasive predictors of death, including age, SO2, type of shunt, 6MWT distance and presence or absence of pericardial effusion on echocardiography, enable accurate mortality risk stratification. These predictors identify a high-risk subgroup which may benefit from optimization of therapy or transplantation. Open Access

Muscle wasting is an important complication of a wide range chronic diseases. Pulmonary arterial hypertension (PAH) is no different. Novel targets and treatments are required to help improve outcomes in patients with PAH complicated by muscle wasting. GDF-15 is prognostic marker in PAH and has been shown to cause muscle wasting in vitro and in vivo. The pathway through which GDF-15 acts in muscle cells is not established. I used a human study, animal models of PAH; the monocrotaline (MCT) rat; and Sugen/hypoxia mouse, as well as C2C12 muscle cells to determine whether GDF-15 and its downstream signals may be important in the development of muscle wasting and low physical activity in PAH. Muscle wasting and low physical activity were associated with poor outcomes in a population of patients with PAH. Circulating GDF-15 levels were associated with markers of muscle strength and size in 2 animal models and in patients with PAH. In the MCT rat the pulmonary vasculature was an important site of production of GDF-15. GDF-15 acted by increasing the expression of ubiquitin ligases, which are involved in muscle wasting. GDF-15 decreased the activity of SMAD 1, 5 whilst increasing transforming growth factor β activated kinase 1 (TAK1) phosphorylation in muscle cells. Antagonising TAK1 with 5(Z)-7-oxozeaenol partially prevented muscle loss and rises in ubiquitin ligase expression in muscle cells and prevented weight loss in some MCT rats treated for 9 days with the drug. Muscle wasting and low physical activity are potentially modifiable risk factors for poor prognosis in PAH. GDF-15 may be a useful biomarker of muscle loss in PAH. Antagonising GDF-15 through its downstream mediator TAK1 requires further evaluation in animal models and patients with PAH to establish its usefulness in clinical practice. Open Access