Heterozygous germline mutations in BMPR2, encoding a TGF-beta receptor, cause familial primary pulmonary hypertension

Severe pulmonary hypertension after the discovery of the familial primary pulmonary hypertension gene

The recent discoveries of the familial primary pulmonary hypertension gene and somatic mutations in key cell growth and cell death regulatory genes in primary pulmonary hypertension have added a new dimension to severe pulmonary hypertension research. These findings have already impacted on how the disease is viewed, and ultimately, how severe pulmonary hypertension is diagnosed and treated. However, this new information raises several fundamental questions related to the role of bone morphogenetic protein receptor signalling in the control of lung vascular cell function. Furthermore, additional genes and gene products may also be involved in the pathogenesis of the disease. The way severe pulmonary hypertension is viewed and studied is on the verge of shifting from a vasoconstrictive to a cell growth paradigm.

Divergence and convergence of TGF-beta/BMP signaling

The transforming growth factor-beta (TGF-beta) superfamily includes more than 30 members which have a broad array of biological activities. TGF-beta superfamily ligands bind to type II and type I serine/threonine kinase receptors and transduce signals via Smad proteins. Receptor-regulated Smads (R-Smads) can be classified into two subclasses, i.e. those activated by activin and TGF-beta signaling pathways (AR-Smads), and those activated by bone morphogenetic protein (BMP) pathways (BR-Smads). The numbers of type II and type I receptors and Smad proteins are limited. Thus, signaling of the TGF-beta superfamily converges at the receptor and Smad levels. In the intracellular signaling pathways, Smads interact with various partner proteins and thereby exhibit a wide variety of biological activities. Moreover, signaling by Smads is modulated by various other signaling pathways allowing TGF-beta superfamily ligands to elicit diverse effects on target cells. Perturbations of the TGF-beta/BMP signaling pathways result in various clinical disorders including cancers, vascular diseases, and bone disorders.

Gene expression patterns in the lungs of patients with primary pulmonary hypertension: a gene microarray analysis

Primary pulmonary hypertension (PPH) is a disease of unknown etiology characterized by lumen-obliterating endothelial cell proliferation and vascular smooth muscle hypertrophy of the small precapillary pulmonary arteries. Because the vascular lesions are homogeneously distributed throughout the entire lung, we propose that a tissue fragment of the lung is representative of the whole lung. RNA extracted from the fragments is likely to provide meaningful information regarding the changes in gene expression pattern in PPH when compared with structurally normal lung tissue. We hypothesize that the lung tissue gene expression pattern of patients with PPH has a characteristic profile when compared with the gene expression pattern of structurally normal lungs and that this characteristic gene expression profile provides new insights into the pathobiology of PPH. Using oligonucleotide microarray technology, we characterized the expression pattern in the lung tissue obtained from 6 patients with primary pulmonary hypertension (PPH)-including 2 patients with the familial form of PPH (FPPH)-and from 6 patients with histologically normal lungs. For the data analysis, gene clusters were generated and the gene expression pattern differences between PPH and normal lung tissue and between PPH and FPPH lung tissue were compared. All PPH lung tissue samples showed a decreased expression of genes encoding several kinases and phosphatases, whereas several oncogenes and genes coding for ion channel proteins were upregulated in their expression. Importantly, we could distinguish by pattern comparison between sporadic PPH and FPPH, because alterations in the expression of transforming growth factor-beta receptor III, bone morphogenic protein 2, mitogen-activated protein kinase kinase 5, RACK 1, apolipoprotein C-III, and the gene encoding the laminin receptor 1 were only found in the samples from patients with sporadic PPH, but not in FPPH samples. We conclude that the microarray gene expression technique is a new and useful molecular tool that provides novel information pertinent to a better characterization and understanding of the pathobiology of the distinct clinical phenotypes of pulmonary hypertension.

Microsatellite instability of endothelial cell growth and apoptosis genes within plexiform lesions in primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is a frequently fatal disease whose pathobiology is poorly understood. Monoclonal endothelial cell growth is present within plexiform lesions of patients with PPH but not secondary PH because of congenital heart malformations. We hypothesized that endothelial cells within PPH plexiform lesions harbor mutations permissive for clonal cell growth. We found that endothelial cells in PPH plexiform lesions demonstrated microsatellite instability within the human MutS Homolog 2 gene (10 of 20 lesions) and displayed microsatellite site mutations and reduced protein expression of transforming growth factor-beta receptor type II (6 of 19 lesions) and Bax (4 of 19 lesions). These results suggest that, in PPH, proliferated endothelial cells have genetic alterations associated with microsatellite instability and concomitant perturbation of growth and apoptosis gene expression akin to neoplasia. The full text of this article is available at http://www.circresaha.org.

BMPR2 haploinsufficiency as the inherited molecular mechanism for primary pulmonary hypertension

Primary pulmonary hypertension (PPH) is a potentially lethal disorder, because the elevation of the pulmonary arterial pressure may result in right-heart failure. Histologically, the disorder is characterized by proliferation of pulmonary-artery smooth muscle and endothelial cells, by intimal hyperplasia, and by in situ thrombus formation. Heterozygous mutations within the bone morphogenetic protein type II receptor (BMPR-II) gene (BMPR2), of the transforming growth factor beta (TGF-beta) cell-signaling superfamily, have been identified in familial and sporadic cases of PPH. We report the molecular spectrum of BMPR2 mutations in 47 additional families with PPH and in three patients with sporadic PPH. Among the cohort of patients, we have identified 22 novel mutations, including 4 partial deletions, distributed throughout the BMPR2 gene. The majority (58%) of mutations are predicted to lead to a premature termination codon. We have also investigated the functional impact and genotype-phenotype relationships, to elucidate the mechanisms contributing to pathogenesis of this important vascular disease. In vitro expression analysis demonstrated loss of BMPR-II function for a number of the identified mutations. These data support the suggestion that haploinsufficiency represents the common molecular mechanism in PPH. Marked variability of the age at onset of disease was observed both within and between families. Taken together, these studies illustrate the considerable heterogeneity of BMPR2 mutations that cause PPH, and they strongly suggest that additional factors, genetic and/or environmental, may be required for the development of the clinical phenotype.

Molecular determinants of left and right outflow tract obstruction

Congenital heart defects represent the most common group of human birth defects; they occur in 0.8-1% of live births and in 10% of spontaneously aborted fetuses. Heart defects seen in newborns typically represent specific morphogenetic defects of individual chambers or regions of the heart, with the remaining portions of the heart developing relatively normally. These developmental defects are commonly compatible with the intrauterine circulation, where the pulmonary circulation and systemic circulation work in concert, resulting in adequate embryonic growth and development. After delivery, however, significant cardiac symptoms develop. In many of these disorders, cyanosis is the earliest feature, while in others, cardiovascular collapse occurs before diagnosis. In this review, obstruction of the left and right sides of the heart are discussed. In these disorders, ventricular hypoplasia resulting in single ventricle physiologic characteristics is typical. The unaffected ventricle in these cases is usually morphologically and physiologically normal. These conditions include hypoplastic left heart syndrome and aortic coarctation on the left side, pulmonary stenosis, tetralogy of Fallot, and other complex right ventricle obstructive disorders. Many of these disorders occur in association with genetic syndromes identifiable by dysmorphic features. In some cases, the gene(s) has been identified or the genetic pathway has been defined. The purpose of this review is to discuss the molecular determinants of these obstructive disorders.