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Biochemical Society Transactions (2003) Volume 31, part 1
Glycogen storage disease as a unifying mechanism
of disease in the PRKAG2 cardiac syndrome
M. H. Gollob1
Division of Cardiology, University of Western Ontario, London Health Sciences Centre, London, Ontario, Canada N6A 5A5
Abstract
The AMP-activated protein kinase (AMPK) system was first discovered 30 years ago. Since that time,
knowledge of the diverse physiological functions of AMPK has grown rapidly and continues to evolve. Most
recently, the observation that spontaneously occurring genetic mutations in the γ regulatory subunits of
AMPK give rise to a skeletal and cardiac muscle disease emphasizes the critical importance of AMPK in the
maintenance of health and disease. The cardiac phenotype observed in humans harbouring genetic mutations
in the γ 2 regulatory subunit (PRKAG2) of AMPK is consistent with abnormal glycogen accumulation in the
heart. The perturbation of AMPK activity induced by genetic mutations in PRKAG2 and the resultant effect
on muscle cell glucose metabolism may be relevant to the issue of targeting AMPK in drug development for
insulin-resistant diabetes mellitus.
Introduction
The AMP-activated protein kinase (AMPK) is known to
regulate vital cellular metabolic cascades. A key function
is the preservation of cellular ‘energy homoeostasis’ by
the regulation of lipid and glucose metabolic pathways [1].
The α, γ and β subunits comprising AMPK are highly
conserved through evolution [2]. Homologues for each subunit may be identified in fungi and plant species, reflecting
the biological importance of AMPK in normal cellular
physiology. A perturbation in the conserved physiological
activity of AMPK, therefore, may, not surprisingly, result in
consequences pathological to the cell.
Indeed, genetic mutations in γ regulatory subunits of
AMPK have been identified in a breed of pigs and in
humans, and have been shown to result in a pathological
skeletal and heart muscle disease, respectively. The original
identification of these genetic defects relied on the ‘genetic
linking’ of the disease phenotype to a specific chromosomal
region (locus) in the pig and human genome [3,4]. In
each case, a gene encoding a γ regulatory subunit resided
at the locus. Milan et al. [3] described a single base-pair
(missense) mutation in the γ 3 regulatory subunit of AMPK
(PRKAG3) of the Hampshire pig, resulting in an amino acid
change of arginine to glutamine at residue 200 (Arg-200Gln)
in the protein [3]. Interestingly, the first human mutation
identified occurred at the equivalent position in PRKAG2
(Arg-302Gln) [4]. Presently, six genetic mutations have been
identified in the human PRKAG2 gene, giving rise to a
complex cardiac syndrome [4–7]. This review will summarize
the pathological features of the PRKAG2 cardiac syndrome
and hypothesize a unifying mechanism of disease for this
complex phenotype.
Key words: AMP-activated protein kinase, Wolff–Parkinson–White.
Abbreviations used: AMPK, AMP-activated protein kinase; WPW, Wolff–Parkinson–White; HCM,
hypertrophic cardiomyopathy.
1
e-mail [email protected]
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Disease phenotype in the PRKAG2
cardiac syndrome
In 1986, a novel familial cardiac syndrome was described
in a large, non-consanguineous French–Canadian family [8].
Disease phenotype among affected family members was
variable, and consisted of a triad of cardiac abnormalities.
The clinical triad observed in patients, either alone or
in combination, included ventricular pre-excitation [Wolff–
Parkinson–White (WPW)], progressive conduction system
disease and cardiac hypertrophy [9].
As the term implies, ventricular pre-excitation refers to
early or ‘pre-excited’ electrical depolarization of ventricular
myocardium before excitation would be expected to occur
following conduction through the normal conduction pathway. The normal conduction axis of the heart, following sinus
node depolarization in the right atrial chamber, proceeds
via the atrio-ventricular node, where the electrical impulse
is delayed before conducting to the ventricular chambers. This
delay is visible on a 12-lead electrocardiogram in humans,
showing distinct atrial and ventricular electrical activity. The
pathological substrate for ventricular pre-excitation has been
shown to be due to the existence of accessory conducting
fibres that connect atrial and ventricular myocardium outside
of the normal conduction axis of the heart. This condition
is readily identified on 12-lead surface electrocardiograms of
patients, which demonstrates merging of atrial and ventricular
electrical signals. The presence of accessory conducting fibres
provides a mechanism for episodes of ‘short-circuiting’ the
heart, whereby electrical depolarization may follow a circular
‘re-entry’ pattern, utilizing both the normal conduction axis
and the accessory conducting fibres. Circular re-entry leads
to excessive heart rates, commonly in excess of 200 beats/min.
The occurrence of episodic, rapid heart rates in the context of evidence for ventricular pre-excitation defines the
WPW syndrome [10]. This clinical phenotype is frequently
recognized during youth in patients with the PRKAG2
AMPK 2002 – 2nd International Meeting on AMP-activated Protein Kinase
cardiac syndrome and may be the sole manifestation of
this genetic disease. Paradoxically, as patients reach their
fourth decade of life, extremely slow heart rates may develop,
suggesting a myopathic process affecting the accessory
conducting fibres and the cells of the normal conduction axis
of the heart, including the natural heart pacemaker, the sinus
node. Many patients will require artificial pacemaker devices
to maintain a sustainable heart rate.
In addition to WPW and a progressive development of
conduction system disease, a significant number of patients
will be found to have abnormal thickening of ventricular
myocardium, as detected by ultrasound imaging. This
condition is commonly referred to as hypertrophic cardiomyopathy (HCM). The pathological term ‘hypertrophy’
refers to cellular enlargement. Thus, cardiac hypertrophy is
the gross manifestation of enlargement of cardiac cells. In a
paediatric population, HCM is most commonly caused by
genetic metabolic disorders, with an excess of 30 such
disorders having been described [11]. Myocyte enlargement in
this setting reflects a defect in the specific metabolic pathway
involved, leading to excessive cellular storage of materials,
for example lipid or glycogen. In an adult population, the
isolated phenotype of HCM is well-recognized to occur
secondary to genetic defects in sarcomeric (contractile) proteins, inherited as an autosomal dominant disease known
as familial HCM [12]. In this disease, the basis of cellular
enlargement results from increased numbers of sarcomeric
units and organelles. Thus although ultrasound imaging
will detect cardiac hypertrophy, whether the condition
is secondary to a metabolic storage disorder or classical
familial HCM cannot be differentiated in the absence of
tissue histology. In the PRKAG2 cardiac syndrome, the
observed phenotype of cardiac hypertrophy is most likely
secondary to the degree of abnormal glycogen storage in the
heart.
Linking disease phenotype, genetic defect
and protein function
Knowledge of the physiological role of AMPK in mammalian
cells has surged over the past 10 years. Of the multitude of
substrates for the kinase now known, it was never thought
that AMPK subunits would be candidate genes for inducing
hereditary cardiac disease. This is not surprising, given the
vast number proteins expressed in cardiac tissue and the rather
widespread tissue activity of AMPK.
Ideally, the triad of pathology observed in the PRKAG2
cardiac syndrome should be explained by a single dominant
cellular abnormality resulting from a perturbation in AMPK
activity. The alternative approach is to invoke separate
mechanisms for the diverse phenotype, a daunting task given
the multiple cellular cascades that AMPK may influence.
Therefore, the following hypothesis is an attempt to unify
the observed clinical phenotype in this genetic syndrome
with a single cellular pathology induced by altered AMPK
muscle activity. However, it is important to recognize that
variable expression of disease occurs in the PRKAG2 cardiac
syndrome, which may be influenced by environmental and/or
modifying genetic background factors.
The triad of clinical features described in the PRKAG2
cardiac syndrome strongly resembles the cardiac phenotype
of a well-recognized metabolic storage disease, known as
Pompe disease [13]. Pompe disease is an autosomal recessive
genetic disorder caused by mutations in the gene encoding
α-1,4-glucosidase [14]. The pathology of this disease is
characterized by excessive cellular glycogen storage, resulting
in severe organ hypertrophy [14]. Disease manifestation most
commonly occurs in childhood. In the adult form, the clinical
triad of WPW, progressive conduction system disease, and
cardiac hypertrophy has been reported [15]. In light of the
phenotypic similarity to a known glycogen storage disease,
and the known role of AMPK in regulating the glucose
metabolic pathway in muscle, focusing on abnormal glycogen
storage as a unifying mechanism for disease pathology in the
PRKAG2 cardiac syndrome appears reasonable.
First, it is necessary to hypothesize how abnormal cellular
glycogen accumulation could manifest the observed clinical
triad in patients with this genetic syndrome. In the context
of cardiac hypertrophy, the explanation appears straightforward. The detection of abnormal myocardial thickening
on ultrasound imaging is secondary to the enlargement
(hypertrophy) of individual cardiac myocytes, as documented
in known glycogen storage diseases [15,16]. The diagnosis of
cardiac hypertrophy is based on conventional criteria defining
normal myocardial wall thickness on imaging. Therefore,
while some patients clearly exceed the defined criteria, others
may not. However, it is likely that all patients with this
syndrome exhibit some degree of abnormal cellular glycogen
storage, the extent of accumulation defining the clinical
diagnosis of cardiac hypertrophy.
The association of cardiac hypertrophy with WPW
outside of the PRKAG2 cardiac syndrome and other storage
diseases is a rare finding. How might abnormal glycogen
accumulation in heart cells account for the pathological
substrate of WPW, namely accessory conducting fibres
outside of the normal conduction axis of the heart ? A
brief review of heart embryology may clarify this issue.
During early cardiac development, the heart exists as a
tubular structure with muscular continuity between atrial
and ventricular myocardium [17]. Atrio-ventricular septation
occurs between 7 and 12 weeks of fetal life, with myocardial
continuity between atrial and ventricular myocardium now
occurring primarily via the developed normal conduction axis
[17]. However, it has been well established by histology study
that remnants of atrio-ventricular muscle continuity outside
of the normal conduction axis are still observed in normal
fetal and neonatal hearts [17,18]. The observed accessory
fibres connecting atrial and ventricular myocardium are thin
in nature and would therefore not be expected to have the
‘cable’ capacity to conduct a depolarizing electrical wavefront
from atrial to ventricular tissue. In this respect, these
accessory fibres may be considered quiescent or subclinical
and are a normal finding in newborn hearts. The effect
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of glycogen accumulation may be postulated to promote
conduction through these otherwise quiescent accessory
fibres. First, the enlargement of these accessory fibres due
to glycogen accumulation may improve their ‘cable’ capacity
and ability to conduct a depolarizing wavefront. This concept
is consistent with the histological finding of abnormally
enlarged accessory fibres, known to exist in normal hearts,
giving rise to WPW in a patient with glycogen storage
disease [13]. The effect of increased glycogen in myoctes may
further alter electrical properties by other mechanisms. The
relationship between decreased cellular pH due to glycogen
accumulation [19] and the effect on myocyte conduction
properties is unknown. However, given the known sensitivity
of ion channel gating and current to intracellular pH
[20,21], a change in kinetics favouring conduction would
not be unexpected. The hypothesis presented to account for
the presence of WPW in the PRKAG2 cardiac syndrome
suggests that this phenotype is secondary to the effects
of glycogen on naturally occurring, quiescent, accessory
fibres. In this regard, implicating a specific, independent
function of AMPK as a direct cause of this phenotype is not
necessary.
A consistent feature of the PRKAG2 cardiac syndrome is
the progressive development of slow heart rates, indicating
loss of function of tissue in the normal conduction axis of the
heart. In normal circumstances, the distribution of glycogen
in the heart is known to exist to a greater extent in the
specialized conduction tissue as compared with contracting
myocardial cells [22]. Therefore, in the presence of a
pathological condition leading to abnormal cellular glycogen
storage, conduction tissue (sinus node, atrio-ventricular
node) would not be excluded from the pathological consequences. Presumably, chronic accumulation and long-term
exposure to high glycogen content exerts a degenerative and
toxic effect on cells of the normal conduction axis (as well as
on contracting myocytes), resulting in loss of function.
Biochemical and physiological studies presently support the hypothesis that the PRKAG2 cardiac syndrome
represents a novel glycogen storage disease of the heart.
Increased AMPK activity is observed in the presence
of the equivalent Arg-302Gln mutation made by sitedirected mutagenesis in the PRKAG1 isoform [23]. This
increased activity appears to be independent of AMP levels
and correlates with increased phosphorlyation of acetylCoA carboxylase, a major substrate of AMPK [23]. These
in vitro observations are of interest given that in vivo chronic
activation of AMPK results in increased glycogen content of
skeletal muscle in animal studies [24]. Consistent with this
observation is the finding of increased GLUT4 transcript and
membrane-bound protein levels in the presence of chronic
AMPK activation [24,25]. More recent data suggest that
the promotion of glycogen accumulation in muscle under
chronic AMPK activation is due to enhanced glucose uptake,
independent of AMPK’s effect of glycogen synthase or
glycogen phosphorylase [26]. Thus, our current breadth of
knowledge suggests that genetic defects in PRKAG2 may lead
to increased AMPK activity, and the consequence of increased
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activity is abnormal glycogen accumulation in muscle. The
fact that glycogen storage disease represents the pathology of
the PRKAG2 cardiac syndrome is further supported by the
recent data of Arad et al. [5].
Summary
Spontaneously occurring genetic mutations in the PRKAG2
subunit of AMPK cause a complex cardiac syndrome. This
clinical syndrome is characterized by a triad of clinical disease,
including WPW, progressive conduction system disease and
cardiac hypertrophy. The expression of these phenotypes may
be explained by abnormal glycogen accumulation within the
heart, consistent with the known role of AMPK in regulating
glucose metabolism. This rare genetic syndrome illustrates
the critical role of AMPK in normal cellular physiology and
demonstrates that a perturbation in the activity of AMPK
may have pathological consequences. This concept is relevant
to the targeting of AMPK in drug development for insulinresistant diabetes mellitus [27], a condition with impaired
muscle glucose uptake and glycogen stores. The altered
AMPK activity due to genetic defects in the PRKAG2 cardiac
syndrome may serve as a template in which to measure the
appropriate effect and potency of new molecules in drug
development.
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