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Exploring the Role of BMPR2
as Key Genetic Marker in Pulmonary Vascular Disease
 Joseph
Loscalzo, MD, PhD
Whitaker Cardiovascular Institute and Evans Department
of Medicine
Boston University School of Medicine
Boston, Massachusetts
Introduction
The genetics of pulmonary arterial hypertension is a rapidly
growing investigative area that has witnessed significant
advances in the past few years. A heritable form of pulmonary
hypertension, familial primary pulmonary hypertension,
was first recognized in 1948.1
In the general population the incidence of primary pulmonary
hypertension ranges from 1 to 2 cases per million, and
its pattern of inheritance is autosomal dominant with
incomplete penetrance.
Genetic linkage analysis of individuals with familial
primary pulmonary hypertension first identified an allele
on chromosome 2q31-32 (later refined to 2q33).2
In 2000 this locus was recognized as containing the gene
for bone morphogenetic protein receptor 2 (BMPR2).3
Since that original observation, heterozygous mutations
in BMPR2 have been found to account for approximately
50% of familial primary pulmonary hypertension, and may
also account for approximately 25% of the so-called sporadic
form of the disease.
BMPR2 Mutations in Pulmonary
Hypertension
BMPR2 is a ubiquitously expressed receptor member of the
transforming growth factor-beta (TGF-beta) receptor family.
These receptors mediate cellular responses to TGF-beta
superfamily members, including TGF-beta, activins, inhibins,
and the bone morphogenetic proteins (BMPs). BMPs manifest
pleiotropic activities in various cell types, including
regulation of cell growth, apoptosis, and differentiation,
as well as tissue patterning and organogenesis in the
developing embryo. TGF-beta receptors comprise two classes
of serine/threonine kinase receptors, types I and II,
and BMPR2 is a member of the type II class. Upon ligand
binding, type II receptors phosphorylate type I receptors,
which, in turn, phosphorylate a restricted set of intracellular
signaltransducing molecules, the Smads, to regulate cell
function. BMPR2-dependent signaling modulates proliferative
responses of pulmonary vascular smooth muscle cells; in
particular, BMP2 and BMP7 inhibit vascular smooth muscle
cell proliferation. Thus, the current working hypothesis
holds that mutations in BMPR2 lead to proliferation of
pulmonary vascular smooth muscle cells, promoting an increase
in pulmonary vascular resistance and pulmonary hypertension.4
To date, 46 different germline mutations in the BMPR2
gene have been identified in familial and sporadic primary
pulmonary hypertension. These mutations span the majority
of the open reading frame of the BMPR2 gene. Missense,
nonssense, and frameshift mutations have been identified,
as have partial deletions. The majority of these mutations
(approximately 60%) cause premature truncation of the
transcript through nonssense-mediated accelerated mRNA
decay. Theoretically, BMPR2 mutations could lead to simple
haploid insufficiency, haploid insufficiency with a secondary
somatic mutational or regulatory event affecting a second
allele, or a dominant negative effect. Functional studies
suggest that truncations and point mutations in the kinase
domain of BMPR2 exert dominant-negative effects on receptor
function.5-7 A recent report
of pulmonary hypertension developing in a mouse model
in which a dominant negative BMPR2 transgene is expressed
is entirely consistent with this hypothesis.8
Other Pathogenic Factors
While the genetic associations studies and functional
genetic analyses strongly support BMPR2 as a key genetic
determinant of primary pulmonary hypertension, other genetic
factors undoubtedly play a role in the pathobiology of
the disease and its phenotypic expression.9
The incomplete penetrance of the disease and its genetic
anticipation in families bearing mutations suggest that
other genetic factors and/or environmental exposures contribute
to disease pathogenesis. Carrying a mutation in BMPR2
does not ensure that one will invariably develop the disease:
other factors, or Ahits@ appear to be required that are
determined by other genes or environmental factors.
5-Lipoxygenase
Our own work in this area stems from our interest in another
gene product that is upregulated in established pulmonary
hypertension, 5-lipoxygenase (5LO). This enzyme catalyzes
two consecutive reactions that convert arachidonic acid
to leukotriene A4, a central precursor for the synthesis
of leukotriene B4 and other downstream leukotrienes (collectively
called cysteinyl leukotrienes) that mediate inflammatory
responses in the vasculature and in the lung. Increased
expression of 5LO has been demonstrated in the lung tissue
of patients with primary pulmonary hypertension, within
infiltrating perivascular alveolar macrophages and in
small pulmonary artery endothelial cells.10
Owing to the vasoconstrictor and cell proliferative effects
of this inflammatory mediator, 5LO has been considered
a potentiator of pulmonary hypertension. In a rat model
of monocrotaline-induced pulmonary hypertension, we found
that the 5LO gene, administered in an adenoviral vector
by inhalation, potentiated the pulmonary hypertension
induced by the alkaloid. Furthermore, we showed that a
5LO inhibitor prevented the potentiation of the pulmonary
hypertension evoked by overexpression of 5LO, as well
as, interestingly, preventing the development of pulmonary
hypertension in the rats treated with moncrotaline alone.11
These results indicate the essential role that 5LO plays
in the pathogenesis of this form of inflammation-dependent
pulmonary hypertension.
In work presented at this meeting from our group by
Y. Song, we more recently showed that adenoviral-mediated
overexpression of 5LO in the lungs of mice with heterozygous
deficiency of BMPR2 (-/+), kindly provided by H. Beppu
(Massachusetts General Hospital), led to the development
of pulmonary hypertension in these mice. The increases
in pulmonary arterial pressures we measured were transient
and coincident with transgene expression. Measurement
of vasoactive mediators that may either be stimulated
by 5LO overexpression or coincident with it, including
cysteinyl leukotrienes, prostaglandin E2, and prostacyclin
metabolites showed no difference between groups of mice.
Interestingly, however, thromboxane B2 levels were considerably
higher in the 5LO-expressing BMPR2(-/+) mice compared
with wildtype mice (120,000 + 11,000 vs 60,000 + 5,500
pg/mg urinary creatinine on day 7 following administration
of the 5LO-containing adenovirus, P< .05). This prostanoid
is the stable metabolite of thromboxane A2, which has
vasoconstrictor, proliferative, and prothrombotic effects;
its synthesis has been found to be increased in individuals
with pulmonary hypertension by Christman and colleagues
in 1992.12
Thromboxane A2
Thromboxane A2 is synthesized by thromboxane synthase,
which is found in inflamed lung tissue; its expression
appears to be regulated by the GATA transcription factor,
which is induced by BMPs. GATA-1, in turn, induces the
expression of NF-E2, which is the key transcription factor
regulating expression of thromboxane synthase.13
Current work focuses on the molecular regulatory events
that may account for the interrelationships among BMPs
signaling through BMPR2, GATA, thromboxane synthase, and
thromboxane A2 and their potential links in the pathogenesis
of primary pulmonary hypertension.
As a platelet activator, thromboxane A2 can stimulate
the release of serotonin from platelets. Recent work on
the serotonin transporter and the serotonin 2B receptor
suggests another potential link among these pathways.
The L-allelic variant of the serotonin transporter, which
is associated with increased expression of the transporter
and increased pulmonary vascular smooth muscle cell proliferation,
is more prevalent among pulmonary hypertensives than controls.13
Furthermore, a study by Launay and colleagues14
showed that hypoxia-induced pulmonary hypertension in
mice is associated with an increase in the expression
of the 5HT2B serotonin receptor, which promotes serotonin-dependent
adverse vascular remodeling. This group also showed that
the main metabolite of dexfenfluramine, nor-dexfenfluramine,
promotes vascular smooth muscle cell growth via this receptor,
thus potentially linking anorexigenic pulmonary hypertension
to genetically determined signaling pathways in pulmonary
vascular smooth muscle cells.
Conclusion
These intriguing yet not fully substantiated links among
gene products suggest that the determinants of the pulmonary
hypertensive phenotype are complex. Future studies of
these complex interactions among genetic and environmental
determinants of pulmonary hypertension should continue
to shed light on the pathogenesis of this disorder.
References
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