doi: 10.1161/hh1301.093826
© 2001 American Heart Association, Inc.
Editorial |
On Genetics of Dilated Cardiomyopathy and Transgenic Models
All Is Not Crystal Clear in Myopathic Hearts
From Baylor College of Medicine, Department of Medicine, Section of Cardiology, Houston, Tex.
Correspondence to A.J. Marian, MD, Associate Professor of Medicine, Section of Cardiology, One Baylor Plaza, 543E, Houston, TX 77030. E-mail amarian{at}bcm.tmc.edu
Key Words: genetics • dilated cardiomyopathy • transgenic animal models • heat-shock proteins
The
B-crystallin protein, the predominant structural protein of the
ocular lens, is a member of the small heat shock proteins that is also
expressed abundantly in the heart and skeletal
muscle.1 The B-crystallin
was initially discovered in the vertebrate ocular lens and was dubbed
crystallin because of its role in maintenance of lens
transparency.2 It is also
essential for maintenance of microtubular integrity in striated
muscles.3 In the heart, as in
the ocular lens, B-crystallin forms soluble multimeres that
function as chaperone molecules, facilitating protein folding and
translocation.4 Thus, the
principal function of B-crystallin protein is to prevent unfolding
of cellular proteins damaged by all forms of stress. Ischemia
and oxidative stress increase the expression of B-crystallin in the
heart.5 In response to
stress, intracellular kinases phosphorylate
B-crystallin,6 leading to
its translocation from the cytosolic pool to Z lines and intercalated
disks. Translocated B-crystallin binds to the components of the
intermediary filaments and cytoskeletal proteins, such as actin and
desmin, and prevents their
aggregation.3 7 The
protective role of B-crystallin in the maintenance of
cytoskeletal integrity has been confirmed in gene transfer studies in
cultured cardiac myocytes8
and in transgenic mice.9
Overexpression of B-crystallin protects cardiac myocytes against
apoptosis and reperfusion
injury.8 9
Interest in B-crystallin has been heightened because of
recent elucidation of the genetic basis of desmin-related myopathy
(DRM), a familial muscular disorder characterized by skeletal myopathy,
heart failure, conduction defect, and arrhythmias.
Pathologically, DRM is characterized by the presence of protein
aggregates containing desmin in the cytoplasm of striated muscles. For
this reason, the initial genetic studies in families with DRM focused
on identification of mutations in the desmin gene
(DES).10 11
However, genetic heterogeneity of DRM was soon
recognized and substantiated by the discovery of the R120G mutation in
the CRYAB gene in patients with
DRM and lens cataract.12 In
this issue of Circulation
Research, Wang et
al13 describe a transgenic
mouse model whereby cardiac-restricted expression of the
B-crystallin–R120G protein leads to a phenotype similar to
DRM in humans.12 The
B-crystallin–R120G mice exhibit early mortality, aberrant desmin
and B-crystallin aggregation in the heart, and cardiac
hypertrophy and dysfunction. The observed
phenotypes are similar to those reported in the mutant desmin
transgenic mice,14 which
emphasize the intricate interactions between B-crystallin and
desmin. In contrast to the mutant protein, overexpression of wild-type
B-crystallin in the heart did not produce a significant
phenotype. This finding corroborates the results of studies in
a previous B-crystallin transgenic mouse
model9 and in transgenic mice
overexpressing wild-type
desmin.14 A striking feature
of the B-crystallin–R120G mice is the high rate of premature death
from congestive heart failure leading to 100% fatality at 32 weeks of
age. In humans, the R120G mutation has been described only in a single
large family with an apparently low rate of premature death. This
apparent discrepancy may reflect the relatively higher than natural
levels of the mutant protein in the hearts of transgenic mice. The
effects of the excess mutant protein on survival could also explain the
lower copy number and expression levels observed in the mutant
transgenic mice compared with the wild-type mice, ie, conferring an
embryonic survival disadvantage. Other factors, such as the genetic
background, known to affect the phenotypic expression of
cardiomyopathies in humans, also could account for
the observed differences in survival rates. The mutant B-crystallin
mice, despite having deposits of desmin/B-crystallin aggregates in
the heart, exhibited an enhanced +dP/dt at 3 months of age. The latter
may reflect the presence of left ventricular
hypertrophy, although it should be recognized that +dp/dt
is a load-dependent index, which varies significantly from mouse to
mouse. It is interesting that the initial description of the
phenotype in the family with the B-crystallin R120G mutation
was that of hypertrophic
cardiomyopathy.15
Despite apparent dissociation of the contractile function and cardiac
hypertrophy, it is likely that hypertrophy, as
in other forms of cardiomyopathy, is a secondary
phenotype because of impaired myocyte function and activation
of stress-responsive transcription machinery.
The primary purpose of transgenesis is to develop models
that provide opportunity to delineate the pathogenesis of the disease
of interest so that new targets for treatment and prevention of human
disease might be developed. In this regard, the report by Wang et
al12 provides some insight
into the pathogenesis of DRM and supports the results of previous in
vitro cell culture
studies.12 16 17
The results in transgenic mice suggest that the B-crystallin–R120G
is less soluble and its expression leads to formation of protein
aggregates, comprised, at least, of desmin and B-crystallin.
Previous in vitro studies provide sparse information that could only
partially explain the basis for the observed phenotype in mice.
The R120G mutation affects a highly conserved amino acid and alters
secondary, tertiary, and quaternary structure of B-crystallin. As a
result, the mutation increases susceptibility of B-crystallin to
heat-induced denaturation and reduces its chaperone activity and the
ability to prevent filament-filament
interaction.17 16
The precise mechanisms by which the mutant B-crystallin–R120G leads
to deposits of aggregates in the myocardium and DRM are
unknown. Similarly, regulation of chaperone activity of
B-crystallin, the target motifs, and the substrate binding sites
remain to be explored. Factors that determine the solubility of the
B-crystallin, the basis for the formation of aggregates, and their
complete composition are also unknown. In addition, the effects of
exercise, pH, temperature, mechanical stress, ischemia, and
oxidative stress on the solubility and formation of aggregates require
additional studies. The effects of the mutation on oligomerization,
autophosphorylation, phosphorylation by
stress-responsive kinases, nuclear translocation, and chaperone
activity of B-crystallin require additional investigation.
Furthermore, target proteins affected by the altered chaperone activity
of the mutant B-crystallin and the impact of the mutation on
the affinity of the B-crystallin for cytoskeletal proteins are not
fully understood. It is expected that the B-crystallin–R120G mouse
model will provide an opportunity to decipher some aspects of the
molecular pathogenesis of DRM caused by the mutant
B-crystallin–R120G protein.
Identification of the
CRYAB as a causal gene for
dilated cardiomyopathy (DCM) is also in
accord with the notion that the primary defect in DCM is the integrity
of the cytoskeleton, as initially proposed after identification of the
first mutation in cardiac -actin in patients with familial
DCM.18 It is now clear that
mutations in a variety of proteins with diverse structure and function
can lead to DCM
(Table
).
A unifying theme has emerged, which suggests that DCM occurs as a
consequence of a common functional defect in the integrity of the
cytoskeleton. Impaired function of the cytoskeleton, as a result of
mutations in its primary components, intermediary filaments, or the
supporting proteins, could lead to cardiac dilatation and heart
failure. The B-crystallin is considered essential for maintaining
the integrity of the cytoskeleton. Therefore, delineation of the
mechanism by which the mutant B-crystallin loses its ability to
support the cytoskeleton and causes DRM could provide significant
insight into the pathogenesis of heart failure from all
causes.
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Acknowledgments
This work was supported by grants from the National Heart, Lung, and Blood Institute, Specialized Centers of Research (P50-HL42267-01), and an Established Investigator Award (9640133N) from the American Heart Association, National Center, Dallas, Texas.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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