Genes and Pulmonary Arterial HypertensionSztrymf B. · Yaïci A. · Girerd B. · Humbert M.
UPRES EA2705, INSERM U764, IFR13, Service de Pneumologie et Réanimation Respiratoire, Centre National de Référence de l’Hypertension Artérielle Pulmonaire, Hôpital Antoine-Béclère, Assistance Publique Hôpitaux de Paris, Université Paris-Sud, Clamart, France Corresponding Author
Familial pulmonary arterial hypertension (FPAH) was first described more than 50 years ago. Before the availability of modern genetic tools, studies of the genealogies demonstrated that these cases segregated as an autosomic dominant trait, with an incomplete penetrance and a genetic anticipation phenomenon by which age at onset of the disease is decreasing in the subsequent generations. Germline mutations in the gene coding for the bone morphogenetic protein receptor II (BMPR2) are present in more than 70% of FPAH and up to 26% of idiopathic, apparently sporadic cases (IPAH). Incomplete penetrance (around 20%) is a major pitfall because FPAH becomes ignored when the disease skips one or several generations. Genetic counseling is complex, with a significant number of BMPR2 mutation healthy carriers screened in some families. Incomplete penetrance puts them in the anxious situation of being potentially affected in the future by this devastating condition or to transmit this risk to their offspring. Nevertheless, genetic testing and counseling is about to become a standard in the management of PAH. Recent international guidelines on PAH state that genetic testing is recommended in FPAH and that IPAH patients have to be informed about the availability of such testing.
Copyright © 2007 S. Karger AG, Basel
Pulmonary arterial hypertension (PAH) is a rare disorder characterized by a sustained increase in mean pulmonary artery pressure (above 25 mm Hg at rest or 30 mm Hg during exercise) and a normal pulmonary capillary wedge pressure (<15 mm Hg), thus excluding left heart failure and other postcapillary processes . This condition leads to right-heart failure with exercise limitation, palpitations, syncope and death. The disease process mainly affects small pulmonary arteries (<500 μm in diameter) in which there is intense remodeling resulting in a narrowing of the lumen, because of an abnormal proliferation of smooth muscle and endothelial cells . Despite major advances in the understanding of cellular and molecular pathogenesis of PAH, the exact mechanisms of this condition remain poorly understood and current therapies are still limited [1,2,3].
PAH can be idiopathic (IPAH), familial (FPAH) or associated with drug exposure (including anorectic fenfluramine derivatives) and other conditions such as connective tissue diseases, human immunodeficiency virus infection, congenital heart diseases, and portal hypertension . Familial cases were first described more than 50 years ago . Before the availability of modern genetic tools, studies of the genealogies demonstrated that these cases segregated as an autosomic dominant trait, with an incomplete penetrance of around 20% (penetrance is defined by the frequency with which a genotype manifests itself by a given phenotype) and a genetic anticipation phenomenon by which age at onset of the disease is decreasing in the subsequent generations [5, 6].
Germline mutations in the gene coding for the bone morphogenetic protein receptor II (BMPR2) are present in more than 70% of FPAH and up to 26% of idiopathic, apparently sporadic cases (IPAH) [7, 8]. Incomplete penetrance is a major pitfall because FPAH becomes ignored when the disease skips one or several generations. Genetic counseling is complex, with a significant number of BMPR2 mutation in healthy carriers screened in some families . Incomplete penetrance put them in the anxious situation of being potentially affected in the future by this devastating condition or to transmit this risk to their offspring. Nevertheless, genetic testing and counseling is about to become a standard in the management of PAH. Recent international guidelines on PAH state that genetic testing is recommended in FPAH and that IPAH patients have to be informed about the availability of such testing .
Familial PAH, Idiopathic PAH and Gene Hunting
The first description of FPAH by Dresdale in 1954 included a description of 3 related subjects displaying severe PAH with hemodynamic confirmation . Since this first report, a large number of familial cases have been described [6, 10, 11]. Genealogies provided the first characteristics of FPAH which segregates as an autosomal dominant trait with a markedly reduced penetrance of about 20% . The incidence of FPAH was first believed to be extremely low but Loyd et al. have demonstrated that its true incidence was in fact underestimated because of its low penetrance . Indeed, some FPAH might be misclassified as IPAH because the disease skips one or several generations, making health professionals ignore other related cases. The incidence of FPAH has been evaluated in the 1980s by the National Institute of Health Registry of PAH. In this prospective study, 12 FPAH cases were identified out of 187 PAH patients from several centers across United States . More recently, the French Registry provided similar figures in PAH patients hospitalized in French expert centers in 2002–2003 . It is thus widely accepted that at least 5% of IPAH patients (formerly known as ‘primary pulmonary hypertension’) have a familial history of this disorder. In addition, genetic anticipation is another characteristic of FPAH . This phenomenon, previously identified in other diseases such as fragile X syndrome or Huntington’s disease, has been explained by trinucleotide repeat in causal genes. However, none of the currently identified PAH genes have such characteristics. Thus, the explanation of the genetic anticipation in PAH is still unknown. It ispossible that an alternative explanation for this phenomenonin PAH might reflect other genetic involvements. Alternatively, increased environmental exposure, ratherthan a genetic mechanism, might explain it.
FPAH as well as IPAH affect predominantly females, with a female/male ratio of 1.7, indicating that hormonal influences presumably play a role in this condition [6, 10, 11]. The sex bias at presentation of the disease may implicate either a hormonal component or a role for an X-linked locus in disease predisposition. Alternative suggestion for the observed female predominance was that the genetic defect(s) characteristic of the disease might lead to male fetus loss . This was supported by Loyd et al. [ 6] who showed that more females than males were born to subjects who were obligate carriers for the gene defect, suggesting selective wastage of male fetuses or a distorted primary sex ratio. Abnormal gender ratio of progeny was the first clinical finding which raised the possibility that the PAH gene may have a role in embryologic development.
Recognition of a genetic basis enabled chromosomal localization of the locus designated PPH1 on chromosome 2q33 using linkage analysis [12, 13]. Subsequently, mutations of the BMPR2 gene have been found to be responsible for FPAH [7, 8]. This gene encodes for a subunit receptor of the transforming growth factor-β (TGF-β) molecule superfamily, named BMPR-II, which is ubiquitously expressed. Its main ligands are bone morphogenetic proteins 2, 4 and 7 (BMPs). These growth factors were initially described as regulators of growth and differentiation of bones and cartilages , but more recent findings brought evidence of their involvement in the regulation of many other cell types including smooth muscle and endothelial cells . By direct sequencing of the BMPR2 gene, mutations such as non-sense, missense and frameshift mutations were initially detected in more than 50% of FPAH patients . In an attempt to understand why nearly half of the FPAH patients did not carry a mutation, it has been discovered that some large gene rearrangements, namely exonic deletions or duplications have been ignored by simple sequencing [17,18,19]. This has been shown using a multiplex ligation-dependent probe amplification which in brief rely on sequence-specific probe hybridization to genomic DNA, followed by amplification of the hybridized probe, and semiquantitative analysis of the resulting PCR products. The relative peak heights or band intensities from each target indicate their initial concentration. Considering those new tools in the screening of FPAH, it appears that more than 70% of FPAH and more than 15% of IPAH patients harbor a BMPR2 mutation [8,17,18,19].
One important issue is whether a clinical, hemodynamic or survival difference exists based on the familial/idiopathic subtype and/or the BMPR2 status. We have shown that familial and idiopathic cases have similar clinical, functional and survival characteristics (fig. 1) . The only difference was a younger age at onset of the disease in FPAH, probably explained at least in part by the genetic anticipation phenomenon (fig. 2) . When comparing patients according to their BMPR2 status, irrespective of the familial PAH history, it appears that patients harboring a BMPR2 mutation have a slightly more severe hemodynamic impairment at diagnosis, but a similar survival (all patients being treated according to the current guidelines at the time of referral) (fig. 3) . Another difference is that patients harboring a BMPR2 mutation are less likely to respond to acute vasodilator testing during right heart catheterization . This raises the question about the pathophysiologic characteristics of patients with a proven BMPR2 mutation. Further studies in well-genotyped populations, including systematic screening for large gene rearrangements (in order to avoid gross deletions ignored by simple sequencing) are necessary to provide a more accurate conclusion on this point.
|Fig. 1. Survival of patients displaying familial or matched idiopathic PAH (all patients being treated according to the current guidelines at the time of referral); adapted from Sztrymf et al. . The continuous line represents survival of FPAH patients and the dashed line survival of IPAH patients. There is no difference in survival between the two subgroups of patients (log rank test p = NS).|
|Fig. 2. Genetic anticipation in a French series of FPAH patients; adapted from Sztrymf et al. . Age at onset of FPAH is decreasing in the subsequent generations of affected families (ANOVA p < 0.05).|
|Fig. 3. Survival of BMPR2 mutant and BMPR2 wild-type PAH patients (all patients being treated according to the current guidelines at the time of referral; adapted from Sztrymf et al. . The red line represents survival of patients without identified BMPR2 mutation, the blue line represents survival of mutated patients. There is no difference in survival between the two subgroups of patients (log rank test p = NS).|
The reduced disease gene penetrance in FPAH indicates that other genetic and/or environmental factors may also be required for the clinical manifestation of disease. Of these, the serotonin pathway has been implicated as a major factor in PAH pathogenesis. The pulmonary circulation of mice deficient in BMPR-II (BMPR2+/– mice) was investigated by Long et al. [ 24], who showed that pulmonary hemodynamics and vascular morphometry of BMPR2+/– mice were similar to wild-type littermate controls under normoxic or chronic hypoxic (2- to 3-week) conditions. However, chronic infusion of serotonin caused increased pulmonary artery systolic pressure, right ventricular hypertrophy, and pulmonary artery remodeling in BMPR2+/– mice compared with wild-type littermates, an effect that was exaggerated under hypoxic conditions . In vitro and in vivo experiments suggested that serotonin inhibits BMP signaling via Smad proteins and the expression of BMP responsive genes. These findings support an interaction between BMPR-II-mediated signaling and the serotonin pathway, perturbation of which may be critical to the pathogenesis of PAH. Interestingly, BMPR2 mutations have also been found in at least 10% of patients displaying fenfluramine-associated PAH . In these patients, duration of exposure to fenfluramines was shorter in mutants as compared to BMPR2 wild-type patients, suggesting that harboring a BMPR2 mutation might induce a higher risk of developing PAH after exposure to these anorectic agents . Of note, fenfluramine-inducedalteration of the serotonin pathway might promote the occurrenceof PAH in predisposed individuals. Indeed, by interacting with the serotonin transporter, fenfluramine derivatives release serotonin from platelets andinhibit its reuptake into platelets and pulmonary endothelialcells. The possible relevance of the serotoninhypothesis in fenfluramine-induced PAH is supported by the factthat a decrease in platelet serotonin storage with enhancedblood concentration of free serotonin has also been reported in sporadiccases of IPAH, and fenfluramine derivatives induce valvular heartdisease very similar to carcinoid syndrome .
Serotonin pathway-related genes are believed to contribute to the clinical manifestation of PAH . The serotonin transporter is a possible inherited modifier of PAH development . The serotonin transporter is abundant in pulmonary vascular smooth muscle. A polymorphism of the serotonin promoter has been identified with a long (L) and short (S) allele. In a pioneer study by Eddahibi et al. [ 26], the L-allelic variant of the serotonin transporter gene, which is associated with serotonin transporter overexpression and increased pulmonary artery smooth muscle cell growth, was present in homozygous form in 65% of a small cohort of IPAH patients but in only 27% of controls. It was thus hypothesized that serotonin transporter activity might play a role in the pathogenesis of PAH and that serotonin transporter polymorphism could confer susceptibility to PAH . However, in adequately powered cohorts for association analyses to identify not only genetic determinants of disease susceptibility but also inherited modifiers for disease development, variation of the serotonin transporter gene appeared unlikely to confer significant susceptibility to PAH [27, 28]. In addition, serotonin transporter genotypes did not correlate with age at diagnosis or survival interval in IPAH . In contrast, in patients with FPAH the LL genotype correlated with an earlier age at diagnosis than SL or SS, although survival among the groups was similar, suggesting a possible relationship between the serotonin transporter and BMPR2 . Thus, further studies exploring the molecular pathways that connect BMPR2 mutant genotype and the serotonin pathway are warranted [26,27,28].
TGF-β Pathway and Genotype/Phenotype Relationship
BMPR2 encodes for a type II BMPR of the TGF-β cell signaling superfamily. TGF-β signaling controls a diverse set of cellular processes, including cell proliferation, differentiation and apoptosis in embryogenesis as well as in mature tissue . It has been formerly implicated in the homeostasis of bones and cartilages. The mature protein harbors four functional domains corresponding to an extracellular ligand-binding domain, a transmembrane domain, a serine threonine kinase domain and a cytoplasmic tail. The receptor is highly conserved among species. However, there is variability in the existence of the cytoplasmic tail, with one form of the receptor, named short isoform, lacking this part of the protein whose precise function is incompletely understood, despite evidence of interaction with cytoskeleton regulating proteins LIMK-1, Tctex-1 and serine threonine kinase protein c-Src [30,31,32].
The TGF-β superfamily signals through a well-documented cascade following ligand binding to a cell surface heteromeric complex composed of type I and type II receptors. This allows phosphorylation of type I by type II subunits. Once phosphorylated, type I receptor phosphorylates second messengers named Smad Proteins. There are eight distinct Smad proteins in three functional classes: receptor Smad (R-Smad), comediator Smad (Co-Smad) and inhibitory Smad. R-Smad are directly phosphorylated by type I receptor and then form a complex with Co-Smad and Smad 4. This complex translocates to the nucleus in which it modulates the transcription of target genes after binding nuclear cofactors  (fig. 4). This is in fact a simplified summary of this complex pathway, and there is evidence that many other coactivators or corepressors are involved at different levels, depending both on the cell type and the network of transcriptions factors . This could provide an additional explanation for the low penetrance of the disease. Mutations of the BMPR2 gene would correspond to a predisposing factor increasing susceptibility to other inherited or acquired abnormalities that triggers development of the PAH phenotype . PAH in this setting would follow the ‘two-hit’ model that has been implicated in cancer pathophysiology.
|Fig. 4. Schematic representation of the BMPR2 transduction pathway. After binding of the ligand to BMPR2, there is phosphorylation of the BMPR1 subunit and subsequent phosphorylation of Smad second messengers. R-Smad are directly phosphorylated by type I receptor and then form a complex with Co-Smad and Smad 4. This complex translocates to the nucleus in which it modulates the transcription of target genes after binding nuclear cofactors. NBF = Nuclear binding factors; P = phosphate.|
Haploinsufficiency is the common molecular mechanism of the dominant inherited disease. The consequences of the gene mutation on the protein explain this mechanism. Actually, most of the reported mutations lead to a truncated protein, with a decrease in the quantity of efficient receptors at the cell surface . Furthermore, it has been shown that BMPR2 mutations also impede receptor trafficking to the cell surface, with an accumulation of the receptor in the cytoplasm without availability for ligand fixation and activation of the downstream transduction pathway .
The precise cellular effects of BMPR2 mutations remains poorly understood. Abnormal proliferation of pulmonary smooth muscle cells in response to fixation of BMPs have been demonstrated in PAH cases . Such altered growth response seems to be partly explained by the decreased induction of apoptosis in pulmonary vascular smooth muscle cells of PAH patients by BMPs . More recently, evidence for pulmonary arterial endothelial cell survival by normal BMPR2 signaling raised the possibility that an increase in endothelial cell apoptosis might contribute to the basic cellular injuries leading to pathologic lesions and PAH phenotype in BMPR2 mutants . Whatever the primary cellular injuries are, experimental studies in heterozygote BMPR2-mutated rodents strongly support that the mutations provide susceptibility to pulmonary hypertension [24, 40].
There is evidence that distinct BMPR2 mutations have different impacts on the BMP signaling pathways in cell-based in vitro system . It is of interest to underline that no such difference has ever been found among BMPR2-mutated patients, meaning that precise type or localization on the gene functional domain of the mutations does not seem to influence age at onset or severity of the disease . This absence of direct link between genotype and phenotype emphasizes the plasticity of the TGF-β pathway, in which the same association between a ligand and its receptor can lead to several reactions, depending of the ‘genetic’ and cellular environment. It makes clinical applications such as the genetic counseling difficult and underlines the need for further investigation into this issue.
Genetic counseling is very appropriate when there is a direct relationship between the genotype and the phenotype of a genetic illness with full penetrance. It is easier when a limited number of mutations are clinically relevant and are therefore shared between a large number of affected subjects. Finding a mutation in an affected patient of such genetic disorder is then very relevant, and the screening of potentially affected relatives is crucial because of the awaited consequences. The genetic characteristics of FPAH make genetic counseling much more complex . As BMPR2 mutations are multiple and distinct between different families, geneticists need first to analyze the BMPR2 gene from the affected patient(s). This may be possible even if the affected proband is deceased, by using a stored blood or tissue sample. It is important to stress that the BMPR2 gene is very large with 13 exons, and screening for all exons and intron/exon boundaries is associated with specific technical difficulties. Incomplete penetrance remains probably the major difficulty for counseling.
When a mutated PAH patient is identified, at-risk relatives are offered a genetic testing. These individuals do not exhibit a PAH phenotype and the incomplete penetrance makes the probability of the onset of the disease as low as 15–20% in asymptomatic carriers of the BMPR2 mutation. Those individuals may nevertheless transmit the PAH predisposition gene to their children, who may in turn develop the disease. So prior to any predictive test, it is crucial that these subjects see a genetic counselor to consider the implications of a positive test. It is obvious that major psychological trouble might rise from the knowledge of carrying a predisposition to such a devastating incurable disease. Furthermore, there is no known prevention in this population comprising at least 80% of subjects who will never develop the disease. It should also be highlighted that according to the different countries and laws, healthy carriers of a BMPR2 mutation might also suffer from genetic discrimination, for instance in employment or insurance. Despite all these difficulties, genetic counseling in PAH is regarded as a major progress for affected patients and their families.
In FPAH, one can identify subjects who do not carry the familial genetic defect associated with PAH, and who are thus not at higher risk of developing or transmitting the disease. Conversely, in patients carrying a BMPR2 mutation, it allows screening by means of echocardiography and right-heart catheterization, if appropriate. This should allow earlier diagnosis of PAH and presumably earlier access to PAH therapy. Disease-free subjects carrying the BMPR2 mutation are informed about signs and symptoms suggestive of PAH. Genetic counseling also allows information on the mode of transmission of the mutation and the possibility to screen other probands that may have inherited the mutation. Genetic counseling is a major hope for patients and their families, and studies testing the improvement of survival or quality of life in patients are currently in progress to test the gain it could offer . A similar approach is proposed in so-called IPAH, when a BMPR2 mutation is detected in a ‘sporadic’ case.
With particular relevance to PAH, it is particularly important to better evaluate the clinical tests that may be undertaken to look for evidence of the disease in case of a positive genetic test. Beside clinical examination, these may include chest X-ray, electrocardiogram, echocardiography, stress echocardiography, rest and/or exercise right-heart catheterization. A preliminary study on two large families concluded that the excessive rise of exercise pulmonary arterial systolic pressure above 40 mm Hg in asymptomatic family members was linked to chromosome 2q31–32 and might represent an early sign of PAH, possibly useful in screening persons at risk before the onset of abnormal pulmonary artery pressure at rest . A large study conducted in several centers across Europe to test this has been ongoing recently. Results should be available later this year. The question of the frequency of the clinical and echocardiographic screening is also a matter of uncertainty. At the 1998 world pulmonary hypertension conference, it was concluded that first-degree relatives of a known FPAH patient should be screened every 3–5 years using clinical examination and echocardiography, and no further recommendations were provided at the 2003 world conference . The ACCP evidence practice guidelines in 2004 have emphasized that no clinical or echocardiographic screening program has proved its usefulness in the setting of FPAH early detection . This is also a major issue because of the expected amelioration of the prognosis in case of an earlier detection of the PAH. We currently propose yearly echocardiography in patients harboring a BMPR2 mutation. With regard to testing children, we would presently recommend that each situation be considered in detail with all parties involved in the care of such families. More data are urgently needed to generate a clear view on this important issue before providing guidelines. Lastly, it will be critical to obtain an expert agreement on recommendations regarding exposure to risk factors of PAH in individuals harboring BMPR2 mutations. This will be particularly difficult for situations known to induce additional risk of PAH, including pregnancy. Indeed, pregnancy combines the particular risk of destabilizing a latent pulmonary vascular condition, as well as of course transmitting the gene to the offspring.
PAH and Hereditary Hemorrhagic Telangiectasia
Mutations of members of the TGF-β receptor pathway have been implicated in hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu syndrome), another autosomal dominant vascular dysplasia characterized by mucocutaneous telangiectasia affecting the nasal and gastrointestinal mucosa, lips, tongue, fingertips, and macroscopic arteriovenous malformations . These lead to recurrent epistaxis and gastrointestinal blood loss, as well as macroscopic arteriovenous malformations, particularly in the pulmonary, hepatic, and cerebral circulation. Pulmonary arteriovenous malformations create clinically significant right-to-left shunts, causing hypoxemia, orthodeoxia, platypnea, chest murmurs, paradoxical embolism, stroke, and cerebral abscesses. In this setting, the occurrence of pulmonary hypertension is usually due to the hyperkinetic state promoting ‘high cardiac output heart failure’ (venous ‘postcapillary’ pulmonary hypertension) . However, PAH that is clinically and histologically indistinguishable from IPAH may occur in this patient population . Mutations in two genes encoding family members of the TGF-β receptor pathway underlie hereditary hemorrhagic telangiectasia [46,47,48]. These genes code for a type I receptor (activin-receptor-like kinase 1, ALK1, located on chromosome 12) and a type III receptor (accessory; endoglin, located on chromosome 9), respectively. Clinical analysis of five kindreds plus one individual patient with hereditary hemorrhagic telangiectasia led to the identification of 10 PAH cases . These subjects suffered from ‘precapillary’ pulmonary hypertension confirmed by right-heart catheterization demonstrating elevated mean pulmonary artery pressure, low pulmonary artery wedge (‘capillary’) pressure, low cardiac output and markedly elevated pulmonary vascular resistance. Family members spanning the whole spectrum of both conditions could be studied (i.e. unaffected family members, and family members with PAH, hereditary hemorrhagic telangiectasia, or both) . Analysis of genes encoding TGF-β receptor proteins, including ALK1, endoglin, and BMPR2 allowed the demonstration that PAH in association with hereditary hemorrhagic telangiectasia can involve mutations in ALK1 . Lastly, immunohistochemical analysis of paraffin-embedded lung sections from a subject with plexiform PAH and mutations in ALK1 demonstrated the presence of ALK1 protein product in diseased pulmonary vascular endothelium . This latter finding lends support in suggesting that endothelium cell dysfunction is crucial to the pathogenesis of both hereditary hemorrhagic telangiectasia and PAH.
Subsequently, further case series highlighted the fact that ALK1 mutations could predispose to PAH [49, 50]. It is believed that the net effect of ALK1 dysfunction may depend on local vascular interactions and other environmental or genetic factors. In addition, rare cases of PAH in endoglin mutants have been reported including a patient with hereditary hemorrhagic telangiectasia and a history of dexfenfluramine exposure, further supporting the probable involvement of the TGF-β signaling pathway in the pathophysiology of both PAH and HHT .
Preimplantation diagnosis refers to the removal of a single cell from an embryo generated in vitro for genetic testing to diagnose a recurrent, serious, heritable condition and thereby to avoid the implantation of affected embryos . In theory, this technique mainly concerns couples in which the father is the carrier of the BMPR2 mutation. Preimplantation diagnosis still raises major ethical issues in the setting of PAH, mainly because of the uncertainties linked to incomplete penetrance. Further ethical debates are still required before proposing this technique as a routine procedure.
Several attempts of gene therapy have been made in experimental models of the disease. After having observed that angiopoietin-1 was overexpressed in the lung of PAH patients , Kido et al. [ 54] transfected a TIE-2 gene inhibitor in a rodent model. They found that this gene transfer was effective in preventing monocrotaline-induced pulmonary hypertension. Recent data about the transfer of angiogenic genes such as eNO synthase and VEGF showed an improvement in microvascular architecture associated with a decrease in hemodynamic alteration in a rodent model of established monocrotaline-induced PAH . It has also been shown that intratracheal transfer of the human prostacycline synthase gene ameliorated monocrotaline-induced pulmonary hypertension in rats . Based on the haploinsufficiency, a study tested the efficacy of the BMPR2 gene transfer in the same model. Despite robust BMPR2 expression in all lung lobes of treated rats, BMPR2 did not lower mean pulmonary artery pressure, pulmonary vascular resistance index, or remodeling . This supports the concept that BMPR2 mutations are susceptibility factor for pulmonary hypertension but that other signals are needed to induce a complete phenotype. Even if these pioneer studies remain exclusively experimental, they indicate that better knowledge in molecular mechanisms may lead to the development of novel therapeutic strategies that are urgently needed for this devastating disease.
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Number of Figures : 4, Number of Tables : 0, Number of References : 57
Respiration (International Journal of Thoracic Medicine)
Vol. 74, No. 2, Year 2007 (Cover Date: February 2007)
Journal Editor: Bolliger, C.T. (Cape Town)
ISSN: 0025–7931 (print), 1423–0356 (Online)
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