|
 Jane
H. Morse, MD
Professor Emerita of Clinical Medicine and Special Lecturer
Department of Medicine
College of Physicians and Surgeons
Columbia University, New York, New York
Introduction
Recent advances in molecular genetics during the past four
years have defined two genes that causally underlie the development
of
pulmonary arterial hypertension. The first and perhaps most important
is bone morphogenetic protein (BMP)
receptor II gene (BMPR2),1,2 and the second is activin receptor
kinase-like 1 (ALK1).3,4 The specifics of the genetic relationships
between ALK1 mutations and pulmonary arterial hypertension
in hereditary hemorrhagic telangiectasia (HHT) are discussed
in an accompanying article. Another accompanying article
reviews the
relationship between serotonin and the serotonin
transporter in pulmonary arterial hypertension. Both
BMPR2 and ALK1 are receptors in the transforming growth factor
beta (TGF-beta) superfamily (Figure 1).5
 Figure
1. Potential roles of transforming growth factor-beta
(TGF-ß) superfamily in vascular remodeling. The TGF-ß superfamily
has diverse roles in a wide variety of physiological processes,
including cell proliferation, differentiation, immunity, and
inflammation. BMP=bone morphogenetic protein; GDF=growth and
differentiation factor. Reprinted with permission from Humbert
et al.5
|
They participate,
in ways yet to be fully elucidated, in preventing proliferation of cells
in small pulmonary arterioles that can cause pulmonary arterial hypertension.
Although the discovery of mutations in these two genes has provided previously
unrecognized experimental approaches to elucidating the pathobiology
of this disorder, ie, investigating the BMP/TGF-beta signaling pathway,
the main focus of this review is on the reported BMPR2 mutations and
their clinical relevance in pulmonary arterial hypertension.
Clinical Classification
The classification of pulmonary arterial hypertension includes
a
heterogeneous group of clinical entities, sharing similar pathological
changes in the pulmonary vasculature, that has evolved
within one rubric. The framework for this discussion of BMPR2
mutations will use the Venice 2003 World Health Organization
(WHO) clinical classification of the various subcategories of
pulmonary arterial hypertension.6 The subcategories are idiopathic
pulmonary arterial hypertension (formerly called sporadic
primary pulmonary hypertension), familial pulmonary
arterial hypertension, and pulmonary arterial hypertension related
to connective tissue diseases, congenital systemic to pulmonary
shunts, portopulmonary hypertension, HIV infection,
and drugs and toxins.
BMPR2 Mutations in Familial Pulmonary
Arterial Hypertension
Gene discovery. Familial pulmonary arterial
hypertension, by definition, has two family members with
the disease. Early
pedigree studies showed that familial pulmonary arterial
hypertension (formerly familial primary pulmonary hypertension)
was
inherited as an autosomal dominant disease with incomplete
penetrance; that is, not all family members who are obligate
carriers of a BMPR2 mutation will develop disease.7 Overall
penetrance is between 10% and 20%. Affected families also
had an increased female to male ratio (2.7:1) and display
genetic anticipation, eg,
the mean age of
death is younger
with
succeeding generations.8 The techniques of using
microsatellite
markers followed by linkage analyses pinpointed the physical
locus of the gene,
initially called PPH1, to a large
~25 cM
region on
 Figure
2. The process leading to the discovery
of mutations in bone morphogenetic protein receptor
type-2 (BMPR2) as the cause of familial primary
pulmonary hypertension is depicted. Collection
of deoxyribonucleic acid from families with sufficient
numbers of affected and unaffected members allowed
linkage studies using microsatellite markers
that led to identification of a chromosome interval
on chromosome 2, at q31–32. Candidate genes
known from the Human Genome Project (HGP) in
the interval were then identified and tested
by deoxyribonucleic acid sequencing. Point mutations
in exons of the BMPR2 gene were found that co-segregated
with affected individuals known from the family
pedigrees. Reprinted with permission from Newman
et al.25
|
chromosome 2 q31,32 (Figure 2).9,10
Fine mapping of more families reduced the initial size of
the locus to 3
cM.11,12 Subsequently, two groups working independently
simultaneously reported that heterozygous germline mutations
in BMPR2 caused familial pulmonary arterial hypertension.1,2
Both groups used positional cloning, a method that sequences
candidate genes at the relevant chromosomal locus, to find
the
disease-causing gene. Soon after, BMPR2 mutations were
reported in idiopathic pulmonary arterial hypertension.13 Types
of BMPR2 mutations, molecular
location, and clinical
aspects. One initial familial pulmonary arterial hypertension
study reported nine BMPR2 mutations, which segregated with
disease, in 19 affected families,1 and the International
Consortium reported seven mutations in eight affected families.
2 Qualitatively, the mutations consisted of missense, nonsense,
frameshift and splice-site alterations in the exons and
intron/exon boundaries of BMPR2. There was no single founder
and the mutations were private to each family. Although three
families had the same mutation, data available from
microsatellite markers showed that the haplotypes, patterns
of
surrounding markers, were different in each family.1 A spontaneous
mutation was reported in a family with familial pulmonary
arterial hypertension1 and spontaneous mutations were
subsequently reported in idiopathic pulmonary arterial hypertension.
13 Although not rigorously studied, the disease penetrance,
originally estimated at 10% to 20%, varied within families
and between families. There are still no published extensive
genotype-phenotype correlations in familial pulmonary
arterial hypertension. Consanguinity was not observed initially,
1,2 and has rarely been observed in affected families.
BMPR2. BMPR2 is
a receptor in the BMP/TGF-beta superfamily,
initially cloned in 1995, that has 13 exons, 4000 base
pairs, and 1038 amino acids.14-17 Exons 1 to 3 comprise
the
extracellular ligand-binding domain, which usually binds
ligands
BMP2 and BMP4 in the lung. Exon 4 comprises the
transmembrane domain, and exons 5 to 11 make up the serinethreonine
kinase domain. Exons 12 and 13 comprise the cytoplasmic
tail of unknown function. The initial report, and most
subsequent reports of BMPR2 mutations have not studied
the
introns, the regulatory sequences interspersed between
the
exons, and have not used quantitative polymerase chain
reaction
(PCR) or other techniques to ensure that deletions, especially
large ones, are present. BMPR2 mutations have been
reported in all exons except exon 13; the spectrum of
frameshift/nonsense mutations, concentrated in the extracellular
and kinase domains, are found throughout the molecule
whereas missense mutations have not been reported in large
exon 12 (Figure 3 and Table
1). A Web-based list of many
but
not all BMPR2 mutations is available (http://archive.uwcm.ac.uk/
uwcm/mg/search/642243.html).
The number of reported mutations is growing exponentially,
as illustrated by the
number
of symbols in Figure 3.
 Figure
3. BMPR2 domains and positions of reported
mutations. Truncating (frameshift/nonsense)
mutations reported in familial/idiopathic pulmonary
arterial hypertension1,2,13,20 are illustrated
above the
diagram of the BMPR2 gene by closed circles and
missense mutations below the gene by open circles.
Diamonds below the molecule also illustrate the
missense mutations in pulmonary arterial hypertension
associated with fenfluramine derivatives33 and
with congenital heart disease.34 Exons 1 to 3 comprise
the extracellular domain, exon 4 the transmembrane
region, exons 5 to 11 the kinase region, and exons
12 and 13 the cytoplasmic tail. (The exon 11 C483R
mutations, though illustrated as both an open
circle and a diamond, are actually from a single
person initially reported in idiopathic pulmonary
arterial
hypertension13 but later reclassified as pulmonary
arterial hypertension associated with fenfluramine
derivatives.33)
|
Each BMP and TGF-beta ligand assembles a specific transcriptional
complex of two type 1 and two type II receptors.18,19
The type II receptor phosphorylates the type I receptor,
and this
complex then phosphorylates a series of intracellular
mediators
or SMADs specific to each pathway. The SMAD-receptor
complex
in turn combines with co-SMAD 4, which is transported
into the nucleus where transcriptional responses are
initiated
(Figure 4). The outcome depends on the
target gene to which
the SMADs
 Figure
4. Proposed mechanism of action of bone
morphogenetic proteins on pulmonary circulatory
cells. Bone morphogenetic protein receptors I and
II (BMPR-I and BMPR-II) are adjacent on cell membranes.
Bone morphogenetic protein binds to the extracellular
domain (ligand binding) of BMPR-II, resulting in
the formation of a heteromeric complex with BMPR-I.
BMPR-II then phosphorylates the transmembrane region
of BMPR-I, activating the kinase domain. The activated
BMPR-I phosphorylates receptor Smad (R-Smad), thus
activating one or more receptor-dependent cytoplasmic
Smad proteins (Smad1, Smad5, and Smad8), which
bind with Smad4 and migrate to the nucleus. The
phosphorylated Smad complex attaches to a binding
factor in the nucleus, and the resulting assembly
either stimulates or represses gene transcription
by interacting with DNA. In patients with familial
primary pulmonary hypertension, changes caused
by mutations have been found along the entire span
of BMPR2. Reprinted with permission from Newman
et al.26
|
have gained access. Mutations in the extracellular
domains of BMPR2 are
predicted to interfere with heterodimer
formation or ligand binding and mutations in the kinase
domain
to interfere with phosphorylation; mutations in the long
cytoplasmic
tail have an unknown function.
Haploinsufficiency, a case in which an individual who
is
heterozygous for a specific gene mutation, is clinically
affected
because a single copy of the normal gene is incapable
of providing
sufficient protein production for normal function, has
been suggested as the molecular mechanism for pulmonary
arterial hypertension in patients with a BMPR2 mutation;20
however, a dominant negative effect, a mutation whose
gene
product adversely affects the normal, wild-type gene
product
within the same cell, has also been implicated. Two functional
in vitro studies have supported these predictions.21,22
In
these studies, missense mutations within the extracellular
and
kinase domains lost their signal transduction abilities
whereas
constructs with mutations that caused truncation of the
cytoplasmic
tail retained their ability to transduce BMP signals.
Since a deleterious effect, as opposed to a benign polymorphism,
is harder to document in missense mutations (without
functional studies), most genetic studies have shown
that the
missense mutations involved sites conserved in evolution
and
that these mutations have not been reported in large
numbers
of normal individuals. BMPR2 mutations, asymptomatic BMPR2 mutation-positive
carriers, and suggested locus heterogeneity. A
German
group reported that familial pulmonary arterial hypertension
may be a heterogeneous disease with a second locus, more
centromeric
than BMPR2, on chromosome 2q31.23 However, to
date, no mutations in any other genes have been found
in this
region. This group of investigators also reported that
the risk
haplotype can be identified in family members by an abnormal
pulmonary artery systolic response to exercise when compared
to the response in normals.22 Pulmonary hypertension
was also
diagnosed by echocardiography in several asymptomatic
family
members.24 A prospective European Union-sponsored study
to
evaluate the response during exercise using Doppler echocardiography
should provide more definitive data on the usefulness
of this test.25
Our 5-year unpublished observations
found that previously identified asymptomatic carriers
in familial pulmonary
arterial
hypertension families developed the disease. Eight
were identified
as carriers by microsatellite marker haplotype determinations,
five by BMPR2 mutations, and another was not tested
by
either method. Prospective longitudinal studies allowing
comparisons
between asymptomatic gene-positive family members and
those who develop disease,
hopefully will provide information
on the natural history of the disease,
identify risk factors and
gender differences, and provide
data on disease penetrance. Genetic
comparisons between both
unaffected and affected genepositive
cohorts should aid in
determining the role of candidate
modifier genes as either protective
or disease enhancing.
BMPR2 mutations and clinical
WHO familial/idiopathic pulmonary
arterial hypertension classification. Anecdotal
and published experience suggests that
idiopathic pulmonary arterial
hypertension, especially BMPR2 mutation-positive disease,
is
more frequently familial pulmonary arterial hypertension
than
originally appreciated.25 Incomplete penetrance and
skipped
generations as well as the difficulty of obtaining
complete family
histories and carrying out longitudinal monitoring
in our
mobile society all contribute to misclassification.
Two of our
patients, initially classified as having idiopathic
pulmonary
arterial hypertension, were recently reclassified as
familial
when a sibling developed pulmonary arterial hypertension.
Misclassification of entire families can occur as well.
A superfamily
consisting of five subfamilies initially felt to be
unrelated
was recently reported.26 This family spans seven generations
and has almost 400 members, of which 200 are at risk
or obligate
carriers for having a BMPR2 mutation.
Cases can also be erroneously classified as familial
pulmonary
arterial hypertension. For example, in families thought
to have familial pulmonary arterial hypertension, the
second
affected member had either a previously unrecognized
HIV
infection, congenital cardiac defect, connective tissue
disease,
or other cause of pulmonary arterial hypertension,
thus requiring
reclassification. To illustrate this, a child with
pulmonary
arterial hypertension and a repaired atrial septal
defect had a
frameshift BMPR2 mutation whereas the mother with pulmonary
arterial hypertension was mutation-negative. Genetic
classifications may be more informative and appropriate
as
more is learned about BMPR2-positive patients and their
family
members. BMPR2 Mutations in Idiopathic
Pulmonary Arterial Hypertension Initial
reports of BMPR2 mutations showed a 26% frequency in
50 unrelated patients with idiopathic pulmonary arterial
hypertension.
13 BMPR2 mutations were identified in 13 patients.
Although 3 patients had the same mutation, microsatellite
markers surrounding the mutation indicated that the
patients
were not related. The patients, with no identifiable
family history
of familial pulmonary arterial hypertension, were recruited
from the United States, the United Kingdom, and France.13
Mutations were detected by direct nucleic acid sequencing
of
the exons and intron/exon boundaries of BMPR2. Two
patients
had de novo mutations and 3 had paternally derived
mutations.
More recent observations, albeit mostly unpublished,
suggest
that the frequency of BMPR2 mutations in idiopathic
pulmonary
arterial hypertension is more realistically between
5%
and 10%.25,27,28 A German group found 11 of 99 (11%)
adults
with idiopathic pulmonary arterial hypertension had
BMPR2
mutations27 and found no mutations in 13 children with
idiopathic
disease.28 In a subset of children with presumed idiopathic
disease, there was an abnormal pulmonary artery systolic
response to exercise in a parent and/or other family
members.28
The authors hypothesized that idiopathic pulmonary
arterial
hypertension in children may have a different genetic
background
than that in adults. However, this is not the case
in
American families where BMPR2 mutation-positive children
with pulmonary arterial hypertension are present in
many familial
pulmonary arterial hypertension families.1,2
A more recent study reported
BMPR2 mutations in 4 of 66
(6%) adults and in 1 of 75 (1%) children with idiopathic
pulmonary
arterial hypertension.29 The adults had two frameshift
and two nonsense mutations and the child had a missense
mutation. All five mutations were found in patients
with thyroid
disease. Sixteen of the 66 adults (24%) had thyroid
disease
when compared to a 5% to 8% prevalence of thyroid disease
in
the normal population. The frequency of thyroid disease
in the
children with idiopathic pulmonary arterial hypertension
did not
differ significantly from that of
the normal population. The association
of thyroid disease, especially
autoimmune thyroiditis,
with idiopathic pulmonary arterial
hypertension is well described30
and not yet well understood;
whether BMPR2 mutations
predispose to thyroid disease
is unknown.
BMPR2 Mutations in Cases
Associated with Fenfluramine Derivatives
Use of appetite suppressants,
such as the fenfluramine derivatives,
has been associated with
the development of pulmonary
hypertension31 and with cardiac
valvular disease.32 A single study
reported BMPR2 mutations in 3
of 33 (9%) unrelated patients
and two sisters with pulmonary
arterial hypertension, all from
France, who used appetite suppressants.
33 Table
1 illustrates
the three different missense
BMPR2 mutations reported in
the 33 patients with idiopathic
pulmonary arterial hypertension
and the fourth nonsense mutation
in both sisters. BMPR2 mutations
were not found in a control
population of 130 normal
individuals from France. Unfortunately,
the authors did not have a cohort of patients available
for mutation analysis that used these drugs but did
not develop
pulmonary hypertension. The duration of appetite suppressant
use for each patient is included in Table
1. Of interest,
the
mutation-positive patients had a significantly shorter
duration
of drug exposure before the onset of disease than the
mutationnegative
patients. The difference in exposure time suggested
that these drugs could provide an additional risk factor
for pulmonary
arterial hypertension in those patients with mutations.
The authors concluded that the onset of disease required “two
events,” namely the presence of a heterozygous
germline mutation
followed by the use of a fenfluramine derivative. Since
BMPR2 mutations were found in only 9% of patients with
pulmonary
arterial hypertension related to fenfluramines, other
genes and mechanisms are more likely to be associated
with
the development of pulmonary arterial hypertension
in patients
with a history of appetite suppressant use.
Table
1. BMPR2 Mutations in Pulmonary Arterial Hypertension
Associated with Congenital Heart Disease and with Fenfluramine
Derivatives.
|
| Congenital Heart Disease34 |
Patient |
Sex, Age |
Type of congenital heart disease/ genetic syndrome |
Exon |
Nucleotide change |
Amino acid change |
Type of mutations |
| 1 |
F, Adult |
AVC-V, Down syndrome |
2 |
123A>G |
Q42R |
Missense (spontaneous) |
| 2 |
M, Child |
AW and VSD |
2 |
140G>A |
G47N |
Missense |
| 3 |
F, Adult |
AVC-C |
3 |
304A>G |
T102A |
Missense |
| 4 |
F, Adult |
AVC-C |
3 |
319T>C |
S107P |
Missense (spontaneous) |
| 6 |
M, Child |
ASD/PDA/PAPVR |
11 |
1509A>C |
E503D |
Missense |
Fenfluramine Derivatives33 |
Patient |
Sex, Age |
Type of congenital heart disease/ genetic syndrome |
Exon |
Nucleotide change |
Amino acid change |
Type of mutations |
| 1 |
F, Adult |
Dexfenfluramine,
5 months |
2 |
246A>C |
Q8sH |
Missense |
| 2 |
F, Adult |
Fenfluramine,
2 months |
5 |
545G>A |
G182D |
Missense |
| 3 |
F, Adult |
Fenfluramine,
1 month |
11 |
1447T>C |
C483R |
Missense |
| 4 |
F, Adult sisters |
Dexfenfluramine,
1 month and
2 months |
6 |
631C>T |
R211X |
Nonsense |
AVC-C, atrioventricular canal, type
C; AW, aortopulmonary window; VSD, ventricular
septal defect; ASD, atrial septal defect; PDA,
patent ductus arteriosus; PAPVR, partial anomalous
pulmonary venous return.
|
BMPR2 Mutations in Cases
Associated with Congenital Heart Disease
This category pertains to patients whose pulmonary
arterial
hypertension is due to abnormal systemic to pulmonary
shunts
as a result of congenital heart disease. A single study
reported
a 6% frequency of BMPR2 mutations in a mixed cohort
of 40 adults and 66 children with pulmonary arterial
hypertension/
congenital heart disease.34 The predominant defects in
both cohorts were atrial and ventricular septal defects
but
included a number of patients with more complex congenital
heart disease. The congenital heart disease was determined
echocardiographically and pulmonary arterial hypertension
was
confirmed by right heart catheterization.
Table 1 illustrates the six novel missense BMPR2 mutations
reported in 3 of 4 adults with complete atrioventricular
canal
defects and in 3 children, with complex congenital heart
disease.
35 The finding of BMPR2 mutations in 3 of the 4 adults
with atrioventricular canal defects contrasted with the
failure to
find mutations in six children with such defects. One
BMPR2
mutation-positive adult with an atrioventricular canal
defect
also had Down syndrome. Three of the six mutations were
de
novo, as they were not found in either parent (Table
1). The 6%
frequency of BMPR2 mutations in this combined cohort
is significantly
less than the 26% frequency reported for idiopathic
pulmonary arterial hypertension,13 but similar to the
9% frequency
of BMPR2 mutations reported for pulmonary arterial
hypertension associated with appetite suppressants.33
This
study requires replication and evaluation of BMPR2 mutations
in patients with these types of congenital heart disease
without
the presence of pulmonary hypertension. The
TGF-beta/BMP signaling pathway is very important in
vasculogenesis and in both embryonic heart and lung
development.
35,36 The types of congenital heart disease found in
the
BMPR2-positive patients are analogous to those reported
in
murine models, particularly those with defects in embryonic
heart development. Homozygous BMPR2 knock-out mice
die at
gastrulation whereas no abnormalities have been reported
for
the heterozygous mouse37 unless there is a second insult
such
as hypoxia. A mouse with a truncated extracellular
domain of
BMPR2 had absence of the septation of the outflow tract
and
aortic arch interruption, the anatomic correlate of
human persistent
truncus arteriosus type 4A.38 Inactivation or knock-out
of other members of the BMP signaling pathway also
leads to
cardiac defects. Mice with tissue-specific inactivation
of ALK3
(BMPR1a) have abnormal endocardial cushion morphogenesis.
35,39 Finally, a cardiac muscle BMP4 conditional knock-out
mouse model resulted in reduced atrioventricular septation
and
endocardial cushion formation.40 BMPR2 Mutations in Cases
Associated with Connective Tissue Disease
Published studies have not identified BMPR2 mutations
in two
small cohorts of patients with pulmonary arterial hypertension
associated with connective tissue disease.41,42 One report
studied
a mixed cohort of 12 patients with pulmonary arterial
hypertension
associated with connective tissue disease that included
nine with systemic sclerosis, two with lupus, and one
with
mixed connective tissue disease.41 Patients with thromboembolic
disease and pulmonary fibrosis were excluded. A single
nucleotide repeat polymorphism in exon 12 was identified
but
its frequency was the same in both patients and in controls.
The
second report determined BMPR2 mutations, lung involvement,
and ANA/autoantibodies in 24 adults and included 17
patients with systemic sclerosis with limited cutaneous
involvement,
6 with diffuse cutaneous involvement, and one with
mixed connective tissue disease.42 A single BMPR2 change
in
exon 13, 2948G>A [R983Q], was found in a patient with
limited
cutaneous systemic sclerosis. However, it was considered
a
polymorphism since the patient was a 59-year-old Ashkenazi
Jew, and this same change in BMPR2 has been found in
Ashkenazi Jews. Despite technical limitations and the
small
sample sizes of these two reports, it appears that other
genes
and/or mechanisms remain to be characterized in pulmonary
arterial hypertension associated with the scleroderma
spectrum
of disease as well as other connective tissue diseases.
BMPR2
Mutations in Cases Associated with HIV Infection
and Portal Hypertension
BMPR2 mutations were not found in a small cohort of
19
patients with pulmonary arterial hypertension associated
with
HIV infection.43 Viral transmission was mainly intravenous,
but
also via sexual contact and pregnancy. There are no
reported
studies of BMPR2 mutations in pulmonary arterial hypertension
associated with portal hypertension.
BMPR2
Mutations in Cases with Significant Venous or Capillary Involvement
This category of pulmonary arterial hypertension includes
two
diseases, pulmonary veno-occlusive disease and pulmonary
capillary hemangiomatosis. There are no reports of
BMPR2
mutations in pulmonary capillary hemangiomatosis. A
BMPR2
mutation in pulmonary veno-occlusive disease has been
reported.
A proband with pulmonary veno-occlusive disease had
a
heterozygous frameshift exon 1, del44C mutation, also
present
in her asymptomatic sister, which was inferred to be
inherited
from her deceased mother.44 Examination of the mutation
bearing
haplotype in the pedigree suggested that the inferred
mutation
in the mother was spontaneous and unlikely to be present
in her parents. However, without tissue, the type of
pulmonary
hypertension in the mother could not be documented.
This
report suggests that BMPR2 mutations may predispose
to pulmonary
venous as well as pulmonary arterial disease.
Genetic Testing and Counseling
To date, genetic testing for BMPR2 mutations has been
performed
within research studies, mostly at specialized pulmonary
hypertension centers. Hence, individual results were
not available to the participants or to their physicians.
However,
Clinical Laboratory Improvement Act–approved genetic
testing
for BMPR2 mutations will become available in 2005 at
selected
pulmonary hypertension centers. Patients must receive
genetic counseling. Patients, family members, and physicians
have expressed an interest in obtaining results of BMPR2
testing.
Most family members in a study of attitudes and understanding
of familial pulmonary arterial hypertension wished to
know individual test results.45 DNA-based testing in
a familial
pulmonary arterial hypertension family with a known BMPR2
mutation is feasible and can identify individuals at
risk for
developing disease as well as those who are not at increased
risk. DNA-based testing in a BMPR2 mutation-unknown person
or family would require more expensive testing for all
13 exons
of BMPR2. In this instance, the failure to find a BMPR2
mutation
would provide limited information, as the technology
currently
used for doing the test would not detect all BMPR2
mutations and there may be other genes for familial pulmonary
arterial hypertension susceptibility. Better technological
approaches for identifying most, if not all, BMPR2 mutations
and other familial pulmonary arterial hypertension genes
are in progress.
The pros and cons of genetic
testing for BMPR2 have been
well presented in the genetic section of the Venice
2003 WHO
Symposium on Pulmonary Arterial Hypertension.25 This
group
of experts advised that genetic testing was not ready
for broad
implementation in idiopathic pulmonary arterial hypertension
but was appropriate in families with familial pulmonary
arterial
hypertension where the mutation is known and genetic
counseling
is available.25 The limitations are that BMPR2 mutations
have been unique to each family and haplotype testing
is rarely
done except within research studies. Also, the variability
in disease
penetrance in familial pulmonary arterial hypertension
can
be as low as 10% to 20% but as high as 80%. The 5%
to 26%
reported frequencies of BMPR2 mutation in nonfamilial
pulmonary
arterial hypertension suggest that genetic testing
has
broader applications than just in familial pulmonary
arterial
hypertension. The number of mutations commercially
being
tested is growing exponentially. Hopefully, the participants
and
physicians obtaining the results of commercially available
tests
will share them with researchers as there is much to
learn about the genetic aspects
of each mutation, the specific risk factors,
whether environmental or genetic, and the pathobiology
and
pharmacogenetics of pulmonary arterial hypertension. Conclusions
The past 4 years have seen the discovery of BMPR2 mutations
in familial and idiopathic pulmonary arterial hypertension
and
in pulmonary arterial hypertension associated with fenfluramine
derivatives and with congenital heart diseases, as well
as one
case of pulmonary veno-occlusive disease, but not in
pulmonary
arterial hypertension associated with connective tissue
disease
or with HIV infection. BMPR2 mutations have not been
reported
in the other categories of diseases associated with pulmonary
arterial hypertension. Qualitatively, the reported truncating
mutations have been located throughout the gene in
familial/idiopathic pulmonary arterial hypertension,1,2, 13,20
whereas the missense mutations have not been found in
the
cytoplasmic tail in familial/idiopathic pulmonary arterial
hypertension
or in pulmonary arterial hypertension associated with
fenfluramine derivatives32 or with congenital heart diseases.33
The “two hit” theory of disease, originally
proposed in cancer,
has been advanced to explain the incomplete penetrance
of the
disease.46 The two hits can result from either genetic
or environmental
factors, or both. In the near future, investigators
should elucidate both the currently identified and unrecognized
genetic and environmental causes required for disease,
provide
a genetic classification, and identify biomarkers for
early disease
onset. These findings may allow clinicians to initiate
preventative
therapies and should provide new therapies and new,
currently unrecognized pathobiological approaches.
Acknowledgments
The author is indebted to her long-time collaborators,
Drs Robyn J Barst, James A Knowles,
and Susan E. Hodge, for their
continued support and intellectual stimulation. Our small
group
fortuitously had the unusually gifted participation of
several
wonderful postgraduate fellows, including Zemin Deng,
PhD,
Patrick Wong, PhD, and Kari E. Roberts, MD. These studies
could never have been done without the generous contributions
of the patients, their families, and their physicians.
The author
also appreciates the opportunity given by the Pulmonary
Hypertension Association to write this article.
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