(Chest. 2002;122:284S-286S.)
© 2002
American College of Chest Physicians
Genetics and Pulmonary Hypertension*
James E. Loyd, MD
* From the Department of Pulmonary and Critical Care Medicine, Vanderbilt Medical Center North, Nashville, TN.
Correspondence to: James E. Loyd, MD, Professor of Allergy, the Department of Pulmonary and Critical Care Medicine, Room T1219, Vanderbilt Medical Center North, Nashville, TN 37232; e-mail: Jim.Loyd{at}Vanderbilt.edu
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Abstract
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Primary pulmonary hypertension (PPH) is a serious pulmonary vascular disease occurring mostly in adult women. Although its occurrence in families was reported within a few years after the original clinical report, PPH was formerly believed rarely to have a genetic basis. Recent progress has not only clarified a basic molecular mechanism for PPH in families, but has also identified mutations of the same gene in many sporadic PPH patients, suggesting that its basis is commonly genetic. Extensive investigations in many centers are now in progress to provide a complete dissection of all the pathogenetic mechanisms of PPH.
Key Words: cell surface receptors • epidemiology • genetics • germ-line mutation • hemodynamics • linkage • protein-serine-threonine kinases • pulmonary hypertension
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Introduction
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Primary pulmonary hypertension (PPH) is a lethal disease that has a mean patient survival time of < 3 years. It occurs at all ages and affects women twice as commonly as men. Despite many important advances in different aspects of PPH during the past decade, including progress in understanding biological mediators, genetic causes, and new therapies, the possibility of prevention and cure seems elusive until all of the basic mechanisms have been fully characterized.
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Genetics
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The prevalence of familial PPH (FPPH) is unknown, but only 6% of PPH patients reported a family history of PPH in the large National Institutes of Health prospective trial of 187 PPH patients.1
Penetrance is variable and quite low in some families. It seems possible that many PPH patients who appear to have acquired the disease may in fact have a familial basis that is unrecognized, in part due to low penetrance. The clinical and pathologic manifestations are identical whether the disease is familial or acquired. Transmission studies of families who were collected through our national FPPH registry revealed vertical transmission of the disease in up to five generations, which is highly indicative of a single dominant gene. Other unique features of the transmission of FPPH include variable age of onset, genetic anticipation, and incomplete penetrance. Our database contains records of 207 cases of FPPH that have occurred among > 2,000 individuals in these 101 families. Recently, we learned that five families with PPH in Tennessee have common ancestors and together comprise the largest reported kindred with PPH.2
This one family contains 18 PPH cases (16 women and 2 men) as well as 22 obligate heterozygotes (9 women and 13 men), who are individuals that have transmitted the disease but not developed it themselves.
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Microsatellite Marker Search
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A microsatellite marker search3
using a set of highly polymorphic, short, tandem-repeat markers was performed in 19 affected individuals from six families and detected evidence for linkage with chromosome 2q markers. We subsequently genotyped 19 additional markers spanning approximately 40 centiMorgans (cM) on the long arm of chromosome 2 and obtained a maximum two-point LOD score of 6.97 at a theta of 0 with the marker D2S389. A multipoint linkage analysis yielded a maximum LOD score of 7.86 with the marker D2S311. A haplotype analysis established a minimum candidate interval of approximately 25 cM. Subsequent studies narrowed the region of interest for the FPPH gene to a region of 6 cM.
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Heterozygous Mutations in Bone Morphogenetic Protein Receptor Type II Cause FPPH
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Using a positional candidate gene approach, we showed that FPPH is caused by mutations in bone morphogenetic protein receptor type II (BMPR2), which is a member of the transforming growth factor (TGF)-ß superfamily.4
Members of the TGF-ß superfamily transduce signals by binding to heteromeric complexes of type I and II receptors and by activating serine/threonine kinases, leading to transcriptional regulation by phosphorylated Smads. We defined the genomic structure of BMPR2 and detected seven independent mutations in a cohort of eight PPH kindreds. The identified defects of BMPR2 in persons with FPPH are predicted to disrupt ligand binding, kinase activity, and heterodimer formation. These data demonstrate the molecular basis of FPPH and underscore the importance of the TGF-ß pathway in the maintenance of vessel integrity.
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BMPR2 Mutations Also Cause Sporadic PPH
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We next sought BMPR2 mutations in 50 unrelated patients5
with PPH but with no identifiable family history of pulmonary hypertension. We performed direct sequencing of the entire coding region and intron/exon boundaries of the gene. The study of DNA from available parents assessed the occurrence of spontaneous mutations contributing to sporadic PPH. We found a total of 13 heterozygous germline mutations of the BMPR2 gene among the 50 PPH subjects studied, including missense (n = 3), nonsense (n = 3), and frameshift (n = 7) mutations that each were predicted to alter the cell-signaling response to specific ligands. No differences in clinical features or disease progression were seen in PPH patients with or without the germline mutations. The sporadic form of PPH is associated with germline mutations of the gene encoding the receptor protein BMPR2 in at least 26% of cases. A molecular classification of PPH, based on the presence or absence of BMPR2 mutations, has important implications for patient management and the screening of relatives.
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BMPR2 Mutations in 45 Families With PPH
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We analyzed DNA samples from 45 additional unrelated families with PPH.6
The entire coding region of the BMPR2 gene and all intron/exon junctions were sequenced in all affected individuals. A total of 22 different mutations were identified in 23 of 45 families. All of these mutations were shown to segregate with disease, and none was detected in a panel of 150 normal chromosomes. Of the 22 mutations, 13 (59%) were either frameshift mutations (8 mutations) or nonsense mutations (5 mutations) that would be predicted to lack normal BMPR2 function. These mutations were dispersed across the whole gene with the most 5' frameshift mutations occurring in exon 1 and the most 3' frameshift mutations occurring in exon 12. The exon 1 frameshift mutation would predict a protein containing only the 15 N-terminal amino acids of BMPR2 with an additional 30 amino acids until the next stop codon. The frameshift mutations could be the result of slippage of the polymerase during replication because they occur in regions of small mononucleotide or dinucleotide repeats.
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Does Another Gene Also Cause PPH?
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Using stress echocardiography to detect asymptomatic obligate heterozygotes and then performing microsatellite analysis in 87 members of five large European families with PPH, these investigators tested for linkage to the BMPR2 region.7
The affected status included 28 individuals with either overt clinical PPH or an abnormal pulmonary vascular response during exercise (pulmonary artery systolic pressure increase to > 45 mm Hg). The results supported linkage to BMPR2 in only two of the five families. The other three families demonstrated linkage to a more proximal location on chromosome 2q31–32 (maximum LOD score, 4.7). Mutations of the BMPR2 gene were excluded by denaturating high-performance liquid chromatography (WAVE) and single-strand conformation polymorphism analysis (Multiphor; Amersham Biosciences; Piscataway, NJ) in these three families. This locus was designated PPH2, and maps in between the markers designated CHRNA1 and D2S2314. The authors concluded that PPH is a genetically heterogeneous disorder with at least two causative genes, but a definitive conclusion awaits the identification of the specific molecular basis for the PPH families in whom a mutation has not yet been discovered.
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Genetic Anticipation
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Genetic anticipation means the increase in severity of a disease in subsequent generations. This phenomenon manifests in FPPH as earlier age of death in subsequent generations8
(age at death was 45.6 years, 36.3 years, and 24.2 years in successive generations; p < 0.05). Although genetic anticipation can have an artifactual component, the data for the genetic anticipation in FPPH is robust and suggests that a biological explanation (not yet discovered) may be responsible. The expansion of microsatellite repeat sequences, for instance trinucleotides (which cause 14 different neurologic diseases) or pentanucleotides (which cause spinocerebellar ataxia 10) is the only known biological mechanism to cause genetic anticipation. Trinucleotide repeat expansion was first described for fragile-X syndrome and is the basic molecular mechanism of many diseases including Huntington disease, myotonic dystrophy, and several spinocerebellar ataxias. Although it remains speculative at the present time as to whether microsatellite repeat expansion is related to genetic anticipation in FPPH, this knowledge could become available soon because of the rapid progress in understanding the molecular causes.
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Current Directions
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Mutations in BMPR2 are reported for only half of the PPH families. The other families link to a similar region, and it is hoped that the molecular explanation for these other families will be recognized soon due to the dedicated efforts by many different teams. Some likely candidates are unrecognized mutations in BMPR2, perhaps in an intron or the promoter, or perhaps in another gene nearby.
Another major effort at present is to identify the determinants that modify the clinical expression of the disease when mutations of BMPR2 are present. Only 20% of those persons with a mutation actually develop PPH, so there are modifying genes or exposures that have a major impact on the clinical expression when a mutation is present. It is conceivable that these two most important features (ie, the unrecognized mutation of half the families and the modifying gene for clinical expression) could be completely independent of each other or could have a single mechanistic basis.
Another recent study9
concluded that PPH is predominantly a genetic disease. The authors analyzed 188 relatives of 32 PPH patients by prospective assessment using stress echocardiography. Overt clinical PPH was detected in at least one relative (n = 15) of 28% of the index patients. An abnormal response in pulmonary artery systolic pressure during exercise (ie, > 40 mm Hg) was present in at least one relative of another 58% of the index patients (37 relatives). In total, 81% of the PPH patients had strong evidence of familial disease. This opinion that sporadic PPH usually has a familial basis is supported by our study, which found BMPR2 mutations in a large percentage of sporadic PPH patients described earlier.
The results of other series of studies contrasts to the opinion that PPH usually has a familial basis, and they support the opinion that sporadic PPH and FPPH are distinctly different. These investigations studied the whole lung by gene expression patterns,10
as well as described evidence for monoclonality11
and microsatellite instability12
in microdissected plexiform lesions. In aggregate, the studies support the opinion that sporadic PPH and FPPH are distinct and separable conditions. A definitive clarification of the differences between the molecular basis of FPPH and sporadic PPH is not possible at present but should soon be greatly facilitated by the further clarification of the molecular pathogenesis that has yet to be described for FPPH.
Interesting information is also rapidly developing relevant to the breadth of clinical pulmonary vascular disease related to mutations in members of the TGF-ß superfamily, such as the recently described novel mutations in ALK1 in five families with both hereditary hemorrhagic telangiectasia and PPH.13
Similar new vistas of understanding are developing on the horizon for many vascular disorders.
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Footnotes
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This work was supported by National Institutes of Health grant HL 48164 from the National Heart and Blood Institute.
Abbreviations: BMPR2 = bone morphogenetic protein receptor type II; cM = centiMorgan; FPPH = familial primary pulmonary hypertension; LOD = logarithm of the odds; PPH = primary pulmonary hypertension; TGF = transforming growth factor
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References
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- Rich, SD, Dantzker, DR, Ayres, SM, et al (1987) Primary pulmonary hypertension: a national prospective study. Ann Intern Med 107,216-223[CrossRef][ISI][Medline]
- Newman, JH, Wheeler, L, Lane, KB, et al Mutation in the gene for bone morphogenetic protein receptor II as a cause of primary pulmonary hypertension in a large kindred. N Engl J Med 2001;345,319-324[Abstract/Free Full Text]
- Nichols, WC, Koller, DL, Slovis, B, et al Localization of the gene for familial primary pulmonary hypertension to chromosome 2q31–32. Nat Genet 1997;15,277-280[CrossRef][ISI][Medline]
- Lane, KB, Machado, RD, Pauciulo, MW, et al Heterozygous germline mutations in a TGF-beta receptor, BMPR2, are the cause of familial primary pulmonary hypertension. Nat Genet 2000;26,81-84[CrossRef][ISI][Medline]
- Thomson, JR, Machado, RD, Pauciulo, MW, et al Sporadic primary pulmonary hypertension is associated with germline mutations of the gene encoding BMPR-II, a receptor member of the TGF-beta family. J Med Genet 2000;37,741-745[Abstract/Free Full Text]
- Machado, RD, Pauciulo, MW, Thompson, JR, et al BMPR2 haplo-insufficiency as the inherited molecular mechanism for primary pulmonary hypertension. Am J Hum Genet 2001;68,92-102[CrossRef][ISI][Medline]
- Janssen, B, Rindermann, M, Barth, U, et al Linkage analysis in a large family with PPH: genetic heterogeneity and a second PPH Locus on 2 q31–32. Chest 2002;121(suppl),54S-56S[Free Full Text]
- Loyd, JE, Butler, MG, Foroud, TM, et al Genetic anticipation and abnormal gender ratio at birth in familial primary pulmonary hypertension. Am J Respir Crit Care Med 1995;152,93-97[Abstract]
- Grunig, E, Mereles, D, Arnold, K, et al Primary pulmonary hypertension is predominantly a hereditary disease. Chest 2002;121(suppl),81S-82S[Free Full Text]
- Geraci, MW, Moore, M, Gao, B, et al Gene expression patterns in the lungs of patients with primary pulmonary hypertension: a gene microarray analysis. Circ Res 2001;88,555-562[Abstract/Free Full Text]
- Lee, SD, Shroyer, KR, Markham, NE, et al Monoclonal endothelial cell proliferation is present in primary but not secondary pulmonary hypertension. J Clin Invest 1998;101,927-934[ISI][Medline]
- Yeager, ME, Halley, GR, Golpon, HA, et al Microsatellite instability of endothelial cell growth and apoptosis genes within plexiform lesions in primary pulmonary hypertension. Circ Res 2001;88,E2-E11[ISI][Medline]
- Trembath, RC, Thomson, JR, Machado, RD, et al Clinical and molecular genetic features of pulmonary hypertension in hereditary hemorrhagic telangiectasia. N Engl J Med 2001;345,325-334[Abstract/Free Full Text]
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Abstract
Introduction
