Functional heterogeneity of bone morphogenetic protein receptor-II mutants found in patients with primary pulmonary hypertension

Germline mutations in the BMPR2 gene encoding bone morphogenetic protein (BMP) type II receptor (BMPR-II) have been reported in patients with primary pulmonary hypertension (PPH), but the contribution of various types of mutations found in PPH to the pathogenesis of clinical phenotypes has not been elucidated. To determine the biological activities of these mutants, we performed functional assays testing their abilities to transduce BMP signals. We found that the reported missense mutations within the extracellular and kinase domains of BMPR-II abrogated their signal-transducing abilities. BMPR-II proteins containing mutations at the conserved cysteine residues in the extracellular and kinase domains were detected in the cytoplasm, suggesting that the loss of signaling ability of certain BMPR-II mutants is due at least in part to their altered subcellular localization. In contrast, BMPR-II mutants with truncation of the cytoplasmic tail retained the ability to transduce BMP signals. The differences in biological activities among the BMPR-II mutants observed thus suggest that additional genetic and/or environmental factors may play critical roles in the pathogenesis of PPH.     

Bone morphogenetic proteins, genetics and the pathophysiology of primary pulmonary hypertension

Several recent papers have shown that both familial primary pulmonary hypertension (FPPH) and sporadic primary pulmonary hypertension (PPH) may have a common etiology that is associated with the inheritance and/or spontaneous development of germline mutations in the bone morphogenetic protein receptor (BMPR) type II gene. Because BMPR-II is a ubiquitously expressed receptor for a family of secreted growth factors known as the bone morphogenetic proteins (BMPs), these findings suggest that BMPs play an important role in the maintenance of normal pulmonary vascular physiology. In the present commentary we discuss the implications of these findings in the context of BMP receptor biology, and relate these data to the genetics and pulmonary pathophysiology of patients with PPH.     

Chronic allergic inflammation causes vascular remodeling and pulmonary hypertension in BMPR2 hypomorph and wild-type mice

Loss-of-function mutations in the bone morphogenetic protein receptor type 2 (BMPR2) gene have been identified in patients with heritable pulmonary arterial hypertension (PAH); however, disease penetrance is low, suggesting additional factors play a role. Inflammation is associated with PAH and vascular remodeling, but whether allergic inflammation triggers vascular remodeling in individuals with BMPR2 mutations is unknown. Our goal was to determine if chronic allergic inflammation would induce more severe vascular remodeling and PAH in mice with reduced BMPR-II signaling. Groups of Bmpr2 hypomorph and wild-type (WT) Balb/c/Byj mice were exposed to house dust mite (HDM) allergen, intranasally for 7 or 20 weeks to generate a model of chronic inflammation. HDM exposure induced similar inflammatory cell counts in all groups compared to controls. Muscularization of pulmonary arterioles and arterial wall thickness were increased after 7 weeks HDM, more severe at 20 weeks, but similar in both groups. Right ventricular systolic pressure (RVSP) was measured by direct cardiac catheterization to assess PAH. RVSP was similarly increased in both HDM exposed groups after 20 weeks compared to controls, but not after 7 weeks. Airway hyperreactivity (AHR) to methacholine was also assessed and interestingly, at 20 weeks, was more severe in HDM exposed Bmpr2 hypomorph mice versus WT. We conclude that chronic allergic inflammation caused PAH and while the severity was mild and similar between WT and Bmpr2 hypomorph mice, AHR was enhanced with reduced BMPR-II signaling. These data suggest that vascular remodeling and PAH resulting from chronic allergic inflammation occurs independently of BMPR-II pathway alterations.     

Bone morphogenetic protein receptor-II mutation Arg491Trp causes malignant phenotype of familial primary pulmonary hypertension

A four-generation pedigree of familial primary pulmonary hypertension (FPPH) with 14 alive members was collected. In the family, three of the 14 alive familial members were diagnosed as FPPH. Mutations in bone morphogenetic protein receptor-II (BMPR-II) gene were screened by using sequencing analysis. A C-to-T transition at position 1471 in exon 11 of the BMPR-II gene was identified, resulting in an Arg491Trp mutation. We confirmed segregation of the mutation within the family and excluded the presence of the mutations in a panel of 240 chromosomes from normal individuals. No mutations were found in BMPR-II gene in other 10 patients with sporadic primary pulmonary hypertension. The Arg491Trp mutation is located in the kinase domain and predicted to disturb the kinase activity of BMPR-II. Total 7 familial members died at age 8-45 years with various symptoms, indicating other genetic or environmental modifiers involved in the modification of the clinical phenotype.     

Familial primary pulmonary hypertension (gene PPH1) is caused by mutations in the bone morphogenetic protein receptor-II gene

Familial primary pulmonary hypertension is a rare autosomal dominant disorder that has reduced penetrance and that has been mapped to a 3-cM region on chromosome 2q33 (locus PPH1). The phenotype is characterized by monoclonal plexiform lesions of proliferating endothelial cells in pulmonary arterioles. These lesions lead to elevated pulmonary-artery pressures, right-ventricular failure, and death. Although primary pulmonary hypertension is rare, cases secondary to known etiologies are more common and include those associated with the appetite-suppressant drugs, including phentermine-fenfluramine. We genotyped 35 multiplex families with the disorder, using 27 microsatellite markers; we constructed disease haplotypes; and we looked for evidence of haplotype sharing across families, using the program TRANSMIT. Suggestive evidence of sharing was observed with markers GGAA19e07 and D2S307, and three nearby candidate genes were examined by denaturing high-performance liquid chromatography on individuals from 19 families. One of these genes (BMPR2), which encodes bone morphogenetic protein receptor type II, was found to contain five mutations that predict premature termination of the protein product and two missense mutations. These mutations were not observed in 196 control chromosomes. These findings indicate that the bone morphogenetic protein-signaling pathway is defective in patients with primary pulmonary hypertension and may implicate the pathway in the nonfamilial forms of the disease.     

Primary pulmonary hypertension: molecular basis and potential for therapy

Primary pulmonary hypertension (PPH) is defined clinically by sustained elevation of pulmonary arterial pressure without a demonstrable cause, and is a progressive, often-fatal disease. PPH can be associated with ingestion of appetite suppressants, human immunodeficiency virus infection and certain autoimmune diseases. Familial PPH is known to account for 6% of all cases. Mutations in the gene encoding the bone morphogenetic protein (BMP) type II receptor have been identified in 72% of affected families and 26% of apparently sporadic cases. BMPs are members of the transforming growth factor b superfamily and affect intracellular signalling via Smads and mitogen-activated protein kinases. Evidence supports a 'two-hit' hypothesis in which PPH is triggered by accumulation of genetic and environmental insults in a susceptible individual. Elucidation of the precise molecular and cellular mechanisms underlying PPH will provide a powerful basis for the development of novel therapeutic strategies in the treatment of this devastating condition.     

Generation of a floxed allele of the mouse BMP type II receptor gene

Bone morphogenetic proteins (BMPs) regulate a wide range of cellular functions that contribute to embryonic development from mesoderm formation to organogenesis. BMP type II receptor (BMPR-II) transduces BMP signals by forming heteromeric complexes with and phosphorylating BMP type I receptors. Heterozygous germline mutations of BMPR-II gene have been identified in patients with familial and sporadic primary pulmonary hypertension, indicating that BMPR-II may contribute to the maintenance of normal pulmonary vascular structure and function. Since embryos homozygous for a null BMPR-II allele died during gastrulation, precluding further studies of BMPR-II function in organ formation and in adult tissues, we generated mice carrying a conditional mutant BMPR-II allele in which exons 4 and 5 were flanked by loxP sequences. We anticipate that studies of mice carrying a floxed BMPR-II allele and a Cre transgene (under the control of a tissue-specific promoter) will enable characterization of the role of BMPR-II in specific cell types during development and in the pathogenesis of cardiovascular diseases.     

Mendelian forms of Parkinson's disease

Over the last few years, genetic findings have changed our views on Parkinson's disease (PD), as mutations in a growing number of genes are found to cause monogenic forms of the disorder. Point mutations in the gene for α-synuclein, as well as duplications and triplications of the wild-type gene cause a dominant form of PD in rare families, pointing towards mishandling of this protein as a crucial step in the molecular pathogenesis of the disorder. Mutations in the gene for leucine-rich repeat kinase 2 (LRRK2) have recently been identified as a much more common cause for dominant PD, while mutations in the parkin gene, in DJ-1, PINK1 and ATP13A2 all cause autosomal-recessive parkinsonism of early onset. Mutations in recessive genes probably are pathogenic through loss-of-function mechanisms, suggesting that their wild-type products protect dopaminergic cells against a variety of insults. Evidence is emerging that at least some of these genes may play a direct role in the etiology of the common sporadic form of PD. Further, it is likely that the cellular pathways identified in rare monogenic variants of the disease also shed light on the molecular pathogenesis in typical sporadic PD.     

BMPR-II heterozygous mice have mild pulmonary hypertension and an impaired pulmonary vascular remodeling response to prolonged hypoxia

Heterozygous mutations of the bone morphogenetic protein type II receptor (BMPR-II) gene have been identified in patients with primary pulmonary hypertension. The mechanisms by which these mutations contribute to the pathogenesis of primary pulmonary hypertension are not fully elucidated. To assess the impact of a heterozygous mutation of the BMPR-II gene on the pulmonary vasculature, we studied mice carrying a mutant BMPR-II allele lacking exons 4 and 5 (BMPR-II(+/-) mice). BMPR-II(+/-) mice had increased mean pulmonary arterial pressure and pulmonary vascular resistance compared with their wild-type littermates. Histological analyses revealed that the wall thickness of muscularized pulmonary arteries (<100 mum in diameter) and the number of alveolar-capillary units were greater in BMPR-II(+/-) than in wild-type mice. Breathing 11% oxygen for 3 wk increased mean pulmonary arterial pressure, pulmonary vascular resistance, and hemoglobin concentration to similar levels in BMPR-II(+/-) and wild-type mice, but the degree of muscularization of small pulmonary arteries and formation of alveolar-capillary units were reduced in BMPR-II(+/-) mice. Our results suggest that, in mice, mutation of one copy of the BMPR-II gene causes pulmonary hypertension but impairs the ability of the pulmonary vasculature to remodel in response to prolonged hypoxic breathing.     

Cytoskeletal defects in Bmpr2-associated pulmonary arterial hypertension

The heritable form of pulmonary arterial hypertension (PAH) is typically caused by a mutation in bone morphogenic protein receptor type 2 (BMPR2), and mice expressing Bmpr2 mutations develop PAH with features similar to human disease. BMPR2 is known to interact with the cytoskeleton, and human array studies in PAH patients confirm alterations in cytoskeletal pathways. The goal of this study was to evaluate cytoskeletal defects in BMPR2-associated PAH. Expression arrays on our Bmpr2 mutant mouse lungs revealed cytoskeletal defects as a prominent molecular consequence of universal expression of a Bmpr2 mutation (Rosa26-Bmpr2(R899X)). Pulmonary microvascular endothelial cells cultured from these mice have histological and functional cytoskeletal defects. Stable transfection of different BMPR2 mutations into pulmonary microvascular endothelial cells revealed that cytoskeletal defects are common to multiple BMPR2 mutations and are associated with activation of the Rho GTPase, Rac1. Rac1 defects are corrected in cell culture and in vivo through administration of exogenous recombinant human angiotensin-converting enzyme 2 (rhACE2). rhACE2 reverses 77% of gene expression changes in Rosa26-Bmpr2(R899X) transgenic mice, in particular, correcting defects in cytoskeletal function. Administration of rhACE2 to Rosa26-Bmpr2(R899X) mice with established PAH normalizes pulmonary pressures. Together, these findings suggest that cytoskeletal function is central to the development of BMPR2-associated PAH and that intervention against cytoskeletal defects may reverse established disease.     

Truncating and missense BMPR2 mutations differentially affect the severity of heritable pulmonary arterial hypertension

Background

Autosomal dominant inheritance of germline mutations in the bone morphogenetic protein receptor type 2 (BMPR2) gene are a major risk factor for pulmonary arterial hypertension (PAH). While previous studies demonstrated a difference in severity between BMPR2 mutation carriers and noncarriers, it is likely disease severity is not equal among BMPR2 mutations. We hypothesized that patients with missense BMPR2 mutations have more severe disease than those with truncating mutations.

Methods

Testing for BMPR2 mutations was performed in 169 patients with PAH (125 with a family history of PAH and 44 with sporadic disease). Of the 106 patients with a detectable BMPR2 mutation, lymphocytes were available in 96 to functionally assess the nonsense-mediated decay pathway of RNA surveillance. Phenotypic characteristics were compared between BMPR2 mutation carriers and noncarriers, as well as between those carriers with a missense versus truncating mutation.

Results

While there was a statistically significant difference in age at diagnosis between carriers and noncarriers, subgroup analysis revealed this to be the case only for females. Among carriers, there was no difference in age at diagnosis, death, or survival according to exonic location of the BMPR2 mutation. However, patients with missense mutations had statistically significant younger ages at diagnosis and death, as well as shorter survival from diagnosis to death or lung transplantation than those with truncating mutations. Consistent with this data, the majority of missense mutations were penetrant prior to age 36 years, while the majority of truncating mutations were penetrant after age 36 years.

Conclusion

In this cohort, BMPR2 mutation carriers have more severe PAH disease than noncarriers, but this is only the case for females. Among carriers, patients with missense mutations that escape nonsense-mediated decay have more severe disease than those with truncating mutations. These findings suggest that treatment and prevention strategies directed specifically at BMPR2 pathway defects may need to vary according to the type of mutation.     

Bone morphogenetic protein receptor 2 in patients with idiopathic portal hypertension

In idiopathic portal hypertension (IPH) typical vascular lesions are present in the branches of the portal vein or in the perisinusoidal area of the liver. Similar histological alterations have been reported in the pulmonary vasculature of patients with idiopathic pulmonary artery hypertension (IPAH). As IPAH is associated with mutations of the bone morphogenetic protein receptor 2 (BMPR2) gene, the aim of this study was to investigate whether this association might also be found in patients with IPH. Twenty-three samples belonging to 21 unrelated caucasian patients with IPH followed in the hepatic haemodynamic laboratory of the Hospital Clinic in Barcelona were included in the study. All patients were studied for the entire open reading frame and splice site of the BMPR2 gene by direct sequencing and multiple ligation probe amplification (MLPA) in order to detect large deletions/duplications. None of the 23 patients had pulmonary artery hypertension. Four patients presented one single nucleotide polymorphism (SNP) in intron 5, four patients had a SNP in exon 12 and a SNP in exon 1 was found in two cases. Two patients had both intron 5 and exon 12 polymorphisms. All SNPs were previously described. Except for these three SNPs, neither mutations nor rearrangements have been identified in the BMPR2 gene in this population. We did not detect mutations or rearrangements in the coding region of the BMPR2 gene in our patients with IPH. These findings suggest that, in contrast to IPAH, mutations in BMPR2 are not involved in the pathogenesis of IPH.     

Heterozygous Null Bone Morphogenetic Protein Receptor Type 2 Mutations Promote SRC Kinase-dependent Caveolar Trafficking Defects and Endothelial Dysfunction in Pulmonary Arterial Hypertension

Hereditary pulmonary arterial hypertension (HPAH) is a rare, fatal disease of the pulmonary vasculature. The majority of HPAH patients inherit mutations in the bone morphogenetic protein type 2 receptor gene (BMPR2), but how these promote pulmonary vascular disease is unclear. HPAH patients have features of pulmonary endothelial cell (PEC) dysfunction including increased vascular permeability and perivascular inflammation associated with decreased PEC barrier function. Recently, frameshift mutations in the caveolar structural protein gene Caveolin-1 (CAV-1) were identified in two patients with non-BMPR2-associated HPAH. Because caveolae regulate endothelial function and vascular permeability, we hypothesized that defects in caveolar function might be a common mechanism by which BMPR2 mutations promote pulmonary vascular disease. To explore this, we isolated PECs from mice carrying heterozygous null Bmpr2 mutations (Bmpr2^+/−) similar to those found in the majority of HPAH patients. We show that Bmpr2^+/− PECs have increased numbers and intracellular localization of caveolae and caveolar structural proteins CAV-1 and Cavin-1 and that these defects are reversed after blocking endocytosis with dynasore. SRC kinase is also constitutively activated in Bmpr2^+/− PECs, and localization of CAV-1 to the plasma membrane is restored after treating Bmpr2^+/− PECs with the SRC kinase inhibitor 3-(4-chlorophenyl)-1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine (PP2). Late outgrowth endothelial progenitor cells isolated from HPAH patients show similar increased activation of SRC kinase. Moreover, Bmpr2^+/− PECs have impaired endothelial barrier function, and barrier function is restored after treatment with PP2. These data suggest that heterozygous null BMPR2 mutations promote SRC-dependent caveolar trafficking defects in PECs and that this may contribute to pulmonary endothelial barrier dysfunction in HPAH patients.     

Mutations in ANKH cause chondrocalcinosis

Chondrocalcinosis (CC) is a common cause of joint pain and arthritis that is caused by the deposition of calcium-containing crystals within articular cartilage. Although most cases are sporadic, rare familial forms have been linked to human chromosomes 8 (CCAL1) or 5p (CCAL2) (Baldwin et al. 1995; Hughes et al. 1995; Andrew et al. 1999). Here, we show that two previously described families with CCAL2 have mutations in the human homolog of the mouse progressive ankylosis gene (ANKH). One of the human mutations results in the substitution of a highly conserved amino acid residue within a predicted transmembrane segment. The other creates a new ATG start site that adds four additional residues to the ANKH protein. Both mutations segregate completely with disease status and are not found in control subjects. In addition, 1 of 95 U.K. patients with sporadic CC showed a deletion of a single codon in the ANKH gene. The same change was found in a sister who had bilateral knee replacement for osteoarthritis. Each of the three human mutations was reconstructed in a full-length ANK expression construct previously shown to regulate pyrophosphate levels in cultured cells in vitro. All three of the human mutations showed significantly more activity than a previously described nonsense mutation that causes severe hydroxyapatite mineral deposition and widespread joint ankylosis in mice. These results suggest that small sequence changes in ANKH are one cause of CC and joint disease in humans. Increased ANK activity may explain the different types of crystals commonly deposited in human CCAL2 families and mutant mice and may provide a useful pharmacological target for treating some forms of human CC.     

Downregulation of type II bone morphogenetic protein receptor in hypoxic pulmonary hypertension

Heterozygous mutations in the type II receptor for bone morphogenetic protein (BMPR-II) and dysfunction of BMPR-II have been implicated in patients with primary pulmonary hypertension (PH). To clarify the possible involvement of BMP and BMPR-II in the development of hypoxic PH, the expression of BMP-2, BMPR-II, and their downstream signals were investigated in rat lung under normal and hypoxic conditions by RT-PCR, immunoblot, and immunohistochemical methods. In rats under normal conditions, BMP-2 is localized in the endothelium of the pulmonary artery, whereas BMPR-II is abundantly expressed in the endothelium, smooth muscle cells, and adventitial fibroblasts. After 0.5 and 3 days of exposure to hypoxia, upregulation of BMP-2 was observed in the intrapulmonary arteries. The change was accompanied by activation of its downstream signaling, p38 MAPK, and Erk1/2 MAPK, and the apoptotic process, measured by caspase-3 activity and TdT-mediated dUTP nick end labeling-positive cells. In contrast, a significant decrease in the expression of BMPR-II and inactivation of p38 MAPK and caspase-3 were observed in the pulmonary vasculature after 7-21 days of hypoxia exposure. Because BMP-2 is known to inhibit proliferation of vascular smooth muscle cells and promote cellular apoptosis, disruption of BMP signaling pathway through downregulation of BMPR-II in chronic hypoxia may result in pulmonary vascular remodeling due to the failure of critical antiproliferative/differentiation programs in the pulmonary vasculature. These results suggest abrogation of BMP signaling may be a common molecular pathogenesis in the development of PH with various pathophysiological events, including primary and hypoxic PH.     

The genetic basis of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare disease characterized by distinctive changes in pulmonary arterioles that lead to progressive elevation of pulmonary artery pressure, pulmonary vascular resistance, right ventricular failure, and a high mortality rate. The etiology of PAH is heterogeneous and incompletely understood. Based on clinical classification, WHO Group 1 PAH includes sporadic disease (idiopathic PAH), inherited PAH (heritable PAH), and association with certain medical conditions (associated PAH). Genes play an important role in idiopathic and heritable PAH. Mutations in bone morphogenetic protein receptor 2 (BMPR2), a member of the transforming growth factor β (TGFβ) superfamily of receptors, have been identified in 70 % of cases of familial PAH, as well as in 10–40 % of cases of idiopathic PAH. Mutations in ALK-1, ENG, SMAD4 and SMAD8, other TGFβ family members, are additional rare causes of PAH. CAV1 regulates SMAD2/3 phosphorylation, and mutations in CAV1 are a rare cause of PAH. KCNK3 is a member of the two-pore domain potassium channels expressed in pulmonary artery smooth muscle cells, and mutations in KCNK3 are a rare cause of both familial and IPAH. The genetics of PAH are complex due to incomplete penetrance and genetic heterogeneity. In addition to rare mutations as a monogenic cause of HPAH, common variants in cerebellin 2 (CBLN2) increase the risk of PAH by approximately twofold. PAH in children is much more heterogeneous than in adults and can be associated with several genetic syndromes, specifically syndromes with congenital heart disease, vascular disease, and hepatic disease. Clinical genetic testing is available for PAH and should be considered in families to allow for more definitive risk stratification and allow for reproductive planning.     

Juvenile polyposis: massive gastric polyposis is more common in MADH4 mutation carriers than in BMPR1A mutation carriers

Juvenile polyposis syndrome (JPS) is an autosomal dominant predisposition to multiple juvenile polyps in the gastrointestinal tract. Germline mutations in the MADH4 or BMPR1A genes have been found to be causative of the disease in a subset of JPS patients. So far, no genotype-phenotype correlation has been reported. We examined 29 patients with the clinical diagnosis of JPS for germline mutations in the MADH4 or BMPR1A genes and identified MADH4 mutations in seven (24%) and BMPR1A mutations in five patients (17%). A remarkable prevalence of massive gastric polyposis was observed in patients with MADH4 mutations when compared with patients with BMPR1A mutations or without identified mutations. This is the first genotype-phenotype correlation observed in JPS.     

Identification of microdeletions spanning the Diamond-Blackfan anemia locus on 19q13 and evidence for genetic heterogeneity.

Diamond-Blackfan anemia (DBA) is a rare pure red-cell hypoplasia of unknown etiology and pathogenesis. A major DBA locus has previously been localized to chromosome 19q13.2. Samples from additional families have been collected to identify key recombinations, microdeletions, and the possibility of heterogeneity for the disorder. In total, 29 multiplex DBA families and 50 families that comprise sporadic DBA cases have been analyzed with polymorphic 19q13 markers, including a newly identified short-tandem repeat in the critical gene region. The results from DNA analysis of 29 multiplex families revealed that 26 of these were consistent with a DBA gene on 19q localized to within a 4.1-cM interval restricted by loci D19S200 and D19S178; however, in three multiplex families, the DBA candidate region on 19q13 was excluded from the segregation of marker alleles. Our results suggest genetic heterogeneity for DBA, and we show that a gene region on chromosome 19q segregates with the disease in the majority of familial cases. Among the 50 families comprising sporadic DBA cases, we identified two novel and overlapping microdeletions on chromosome 19q13. In combination, the three known microdeletions associated with DBA restrict the critical gene region to approximately 1 Mb. The results indicate that a proportion of sporadic DBA cases are caused by deletions in the 19q13 region.