Clarifying the boundaries between the inflammatory and dystrophic myopathies: insights from...

Clarifying the boundaries between the

inflammatory and dystrophic myopathies:

insights from molecular diagnostics

and microarrays

Eric P. Hoffman, PhDa,*, Deepak Rao, BSa,Lauren M. Pachman, MDb

aCenter for Genetic Medicine, Children’s National Medical Center,

Washington DC 20010, USAbDivision of Immunology/Rheumatology, Department of Pediatrics,

Northwestern University Medical School, Chicago, IL 60614, USA

Differential diagnosis of the inflammatory myopathies from the muscular

dystrophies, based upon histologic and clinical findings, has been considered

relatively straightforward. Despite careful evaluations, some patients cannot be

easily classified into one of these groups. Recent implementation of routine

molecular diagnostic testing for the dystrophies has illuminated considerable

clinical and histopathologic overlap between some types of muscular dystro-

phies and some of idiopathic inflammatory myopathies, especially the poly-

myositis syndromes. This article discusses some of the most problematic

dystropathies for differential diagnosis from the idiopathic inflammatory myo-

pathies, including: dysferlin deficiency (LGMD2B and Miyoshi myopathy),

dystrophinopathies (isolated female manifesting carriers, Becker dystrophy),

and merosin deficiency presenting as infantile polymyositis. Newly emerging

microarray technology, in which the transcriptional status of all genes in the

genome can be analyzed simultaneously in a small biopsy, is providing new

insights into the pathophysiology of muscle disease. Recent expression profil-

ing data in juvenile dermatomyositis and Duchenne muscular dystrophy are

presented as an example of how future molecular studies may alter our

classification criteria and therapies of many myopathies.

0889-857X/02/$ – see front matter D 2002, Elsevier Science (USA). All rights reserved.

PII: S0889 -857X(02 )00031 -5

* Corresponding author. Center for Genetic Medicine, Children’s National Medical Center, 111

Michigan Avenue NW, Washington DC 20010.

E-mail address: [email protected] (E.P. Hoffman).

Rheum Dis Clin N Am 28 (2002) 743–757

The muscular dystrophies are caused by inherited biochemical defects that

result in chronic degeneration and regeneration of muscle. The slow onset and

progression of the dystrophies, and the less focal pattern of inflammation in the

muscle, typically enable differential diagnosis from the inflammatory myopathies

(see other articles in this issue). Recent advances in the understanding of the

molecular basis for many of the dystrophies have blurred the histologic and

clinical distinctions between certain dystrophies and the idiopathic inflammatory

myopathies. A particularly problematic differential diagnostic dilemma is the

newly described dysferlin-deficiency (limb–girdle muscular dystrophy type 2B

(LGMD2B) and Miyoshi myopathy), in which onset can be late and relatively

sudden and patients can show extensive inflammation in their muscle. Anecdotal

observations suggest that these patients may worsen after receiving steroid

treatment; the loss of strength may not be regained after cessation of steroids.

Other overlap disorders include two of the dystrophinopathies (the isolated

female manifesting carrier of Duchenne dystrophy, and Becker muscular dys-

trophy), and laminin a2 (merosin) deficiency (infantile polymyositis). Newly

emerging microarray approaches are enabling genome-wide mRNA expression

profiling. Microarray data are beginning to show pathophysiologic pathways

shared between the inflammatory myopathies and the dystrophies and pathways

unique to each diagnostic category. Expression profiling may become a new form

of molecular diagnosis, and may suggest novel pathway-targeted approaches to

treat these disorders.

Dysferlin-deficiency

Miyoshi myopathy and LGMD2B are caused by recessively inherited muta-

tions of the dysferlin gene [1,2]. Most pedigrees show a single affected patient

(isolated cases). Patients show onset in late teens or early twenties, and serum

creatine kinase activity levels are typically 2,000 to 10,000 IU/L. Muscle

weakness can be predominantly proximal (LGMD2B) or distal (Miyoshi myo-

pathy). Patient muscle biopsy shows features of a chronic dystrophy, and an

inflammatory myopathy (Fig. 1).

Dysferlin is a transmembrane protein that seems to be involved in plasma

membrane homeostasis, although its function is inferred from its subcellular

localization and from a similar gene in round worms (Caenorhabditis elegans).

Specifically, a genetic abnormality affecting C elegans fertility was due to the

lack of a protein involved in a membrane fusion event during sperm maturation;

the identified gene was dubbed ‘‘fer-1’’ because of its importance in fertility [3].

A human orthologue of this gene was identified by genetic mapping and cloning

of the gene responsible for two types of recessive muscular dystrophy: Miyoshi

myopathy (a distally-presenting dystrophy characterized in Japan), and LGMD2B

(a proximally-presenting dystrophy with large recessive families in the Middle

East) [1,2,4]. The gene that causes Miyoshi myopathy and LGMD2B is very

similar to the fer-1 gene/protein of C elegans, and was dubbed dysferlin

E.P. Hoffman et al / Rheum Dis Clin N Am 28 (2002) 743–757744

(‘‘dys’’trophy, ‘‘fer’’-1, ‘‘lin’’ for protein). The apparent similarity between a

protein involved in worm sperm maturation and human muscular dystrophy was

even more surprising when a form of nonsyndromic hearing loss in humans was

found to be due to yet another orthologue of the C elegans fer-1 gene, namely

otoferlin [5]. Human hearing loss, human muscular dystrophy, and worm

infertility share related biochemical defects, presumably involving plasma

membrane homeostasis.

Patients with Miyoshi myopathy or LGMD2B can have impressive inflam-

mation in their muscles, and both diseases show relatively late onset (typically in

the teens or early 20s) [4,6,7]. There may be histologic distinctions between early

stage, mildly affected patients, and later stage patients with more symptoms [8].

Nonnecrotic fibers show extensive staining with membrane-attack complex in

later stage patients; substitution of regions of the plasma membrane with layers of

vesicles and membranous projections can be observed by electron microscopy in

the majority of fibers [8]. The inflammation in patients with dysferlin-deficiency

is often perivascular or endomysial, and contains CD4+ T cells, macrophages,

and some CD8+ T cells, but no B cells (Fig. 1) [9]. Preferential involvement of

the hamstrings, adductors, gastrocnemius, and soleus can distinguish dysferlino-

pathies from other dystrophies, although in many cases, imaging is needed to

observe distal muscle involvement [10].

The relatively late presentation and inflammatory infiltrate on muscle biopsy

often leads to an initial diagnosis of idiopathic inflammatory myopathy, with

subsequent prescription of corticosteroids. Although this phenomenon has not

been published, our experience with approximately 20 patients with dysferlin-

deficiency suggests that those patients who were initially diagnosed with

inflammatory myopathy and treated with corticosteroids show a decrease in

strength; strength loss may not be regained after cessation of corticosteroids. It is

important to differentially diagnose patients with dysferlin-deficiency before

prescribing corticosteroids.

Fig. 1. Histopathology of dysferlin-deficiency shows features of an inflammatory myopathy. Two

different views of hematoxylin-eosin–stained cryosections of a muscle biopsy from an 18-year-old

patient showing complete dysferlin-deficiency by immunoblot studies. Both panels show evidence of

perifascicular grouping of small myofibers, and perimysial inflammatory infiltrates. The inflammatory

cells appear most abundant around blood vessels. Panel A shows a more severely affected region;

while panel B shows less pathologic involvement.

E.P. Hoffman et al / Rheum Dis Clin N Am 28 (2002) 743–757 745

Testing for dysferlin-deficiency is done by biochemical analysis of frozen

biopsies, using immunoblot analyses for the relatively large (230 kDa) protein

(Fig. 1) [8]. Most or all patients who lack dysferlin in their muscle have mutations

of the corresponding dysferlin gene, although the number of studies are limited

[11,12]. Genetic testing for gene mutations is difficult because there are no

common mutations, and the gene is very large, which makes mutation screening

expensive and time consuming. A number of laboratories offer clinical biochem-

ical testing of frozen biopsies; however, mutation studies are only available on a

research basis (see http://www.genetests.org for lists of offering laboratories).

Immunostaining of frozen sections can be done, although secondary loss of

dysferlin at the membrane is a relatively common, nonspecific finding.

An intensively studied murine genetic model for experimentally induced

autoimmune disease, the SJL/J mice, has pathogenic mutations in the murine

orthologue of the same dysferlin gene [13]. This inbred strain, like other inbred

mouse strains, is maintained by brother–sister matings over dozens or hundreds

of generations, and thus carry a relatively high ‘‘genetic load’’ for certain

recessive conditions. SJL/J mice have been inbred for about 170 generations.

In addition to their susceptibility to autoimmune disease they exhibit reticulum

cell sarcomas (similar to Hodgkin’s disease), retinal degeneration, albinism, and a

muscular dystrophy (with the greatest pathology at approximately 6 months of

age). The autoimmune susceptibility has led to their widespread use as models for

multiple sclerosis, gastroenteritis, and other immune-mediated diseases. The

dysferlin gene mutation in SJL mice is a 141 bp deletion, resulting in a splicing

defect in the mRNA, and a resulting 57 amino acid loss in the dysferlin protein.

This mutation does not eliminate dysferlin, but dramatically reduces the quantity

and alters the quality (molecular weight). The relationship between increased

susceptibility to autoimmunity and the dystrophy caused by dysferlin-deficiency

has been directly addressed in a recent publication. The investigators showed that

immunization of SJL/J mice with rabbit myosin resulted in a strong increase in

CD8+ T cells in muscle, with induction of STAT-1 and interferon pathways [14].

These data suggest that dysferlin-deficient muscle is predisposed to inducing a

Th1 response, consistent with an immune-mediated myositis-like pathology.

Dystrophinopathies

The most common muscular dystrophy, Duchenne muscular dystrophy, results

from loss of the dystrophin protein from the myofiber plasma membrane [15,16;

see 17 for, review]. Duchenne muscular dystrophy is usually easily recognized;

presentation is proximal muscle weakness in young boys (aged 3–6 years)

associated with a severe dystrophic process on muscle biopsy and very high

serum creatine kinase activity levels. Thus, differential diagnosis between

Duchenne dystrophy and the inflammatory myopathies is not generally an issue.

Two milder forms of dystrophinopathy can present a diagnostic dilemma,

namely the isolated female manifesting carrier of Duchenne muscular dystrophy


(mosaicism for dystrophin expression), and Becker dystrophy (present but

abnormal dystrophin). In X-linked pedigrees of Duchenne dystrophy, females

are typically asymptomatic carriers of the disease, although 70% of carriers show

elevated serum creatine kinase levels, and many may have subclinical cardiac

involvement (a rare subset has overt heart disease). All female carriers have two

populations of cells, those with the abnormal X chromosome active (dystrophin-

negative), and those with the normal X chromosome active (dystrophin-positive).

In most women, X inactivation patterns are ‘‘random’’; approximately half of the

cells use the maternally-derived X chromosome, and half use the paternally-

derived X. In asymptomatic female carriers of Duchenne dystrophy, half of the

myonuclei are dystrophin-positive, and half are dystrophin-negative, with 50% of

dystrophin levels expected in muscle. There is a tendency for the dystrophin-

positive cells to increase in frequency with advancing age. This is due to

diffusion of dystrophin within syncitial myofibers (biochemical normalization),

and the necrotic dystrophin-negative fibers can be regenerated by dystrophin-

positive myogenic cells (genetic normalization) [18,19]. As a result of these

normalization processes, the muscle becomes progressively more dystrophin-

positive with age; the serum creatine kinase levels typically decline with age to

normal levels.

The implementation of dystrophin testing as a routine diagnostic procedure

resulted in the identification of a new subset of female carriers who showed

symptoms (muscle weakness; so-called manifesting carriers). The males in their

families typically did not have a positive history for Duchenne dystrophy [20].

These girls and women had marked dystrophin deficiency in their biopsies, and,

consistent with the biochemical findings, showed ‘‘skewed X inactivation’’; the

abnormal mutation-bearing X chromosome was preferentially used. The pref-

erential use of the mutant dystrophin gene lead to mosaicism in muscle, where

dystrophin-negative myofibers predominated; the biochemical and genetic nor-

malization processes were unable to overcome the progressive dystrophic

degeneration of the muscle.

Some of these girls and women were diagnosed with polymyositis, because of

the focal nature of histopathology in their muscle; some areas showed dystrophic

features, and others had normal histology. The dystrophic regions corresponded

with dystrophin-negative regions of the muscle, whereas the normal regions

corresponded with the dystrophin-positive regions [21]. The focal histopathology,

with the accompanying macrophage and T-cell infiltration, could be interpreted as

consistent with polymyositis. Additionally, some isolated female carriers showed

asymmetric involvement of limbs; this also again suggested a nongenetic

etiology. The asymmetric involvement is due to varying degrees of dystrophin-

negative and dystrophin-positive myofibers in the specific muscles.

Differential diagnosis of the isolated manifesting carriers is done by immunos-

taining of muscle biopsy cryosections for dystrophin protein, and the visualization

of clear mosaicism for dystrophin immunostaining. Other types of dystrophies can

show partial mosaicism of dystrophin in muscle. Sarcoglycanopathies and dysfer-

linopathies can show variations in dystrophin immunostaining within a biopsy,


however, the strict dystrophin-positive and dystrophin-negative pattern is consid-

erably more dramatic in true manifesting carriers. About 5% to 10% of female

dystrophy patients are isolated manifesting carriers [20]; there is a tendency to

overdiagnose these patients because of the aforementioned secondary dystrophin

immunostaining abnormalities that can be seen (Table 1). The dystrophin immu-

nostaining patterns should correlate with the histopathology; dystrophin-negative

regions should show a dystrophic myopathy, whereas dystrophin-positive regions

show a much less severe histopathology or even normal morphology of fibers.

The differential diagnosis of these patients is particularly important because of

the genetic and reproductive ramifications. If a woman is a carrier of Duchenne

dystrophy, half of male offspring may be affected with Duchenne dystrophy.

Genetic counseling and prenatal diagnosis is possible, and should be offered to the

correctly diagnosed patient [22]. Most of the dystrophin gene mutations in these

girls and women are derived from their father; however, female family members

should be counseled as possible carriers [18]. The mechanisms that underly the

skewed X inactivation in these women have been obscure. Very recent results

suggest that they may be carriers for a distinct X-linked lethal trait inherited from

their mother, who herself may show skewed X inactivation [22–24].

Becker muscular dystrophy is clinically defined as a proximal dystrophic

myopathy similar to Duchenne dystrophy, but has a later onset and is milder in

progression. With the advent of dystrophin-based molecular diagnostics, the

clinical spectrum of Becker dystrophy has widened considerably. Many different

subphenotypes have been discovered, including asymptomatic patients with

increases in their serum creatine kinase levels [25,26; see 27 for complete

literature review]. Although Becker dystrophy is an X-linked recessive disorder,

many patients are isolated cases due to the high spontaneous mutation rate (1 in

10,000 eggs and sperm). Dystrophin protein and gene testing is considered

routine gene mutations (deletions of one or more exons) can be accurately and

easily detected in approximately 75% of patients. Immunostaining for dystrophin

is less specific and sensitive for Becker dystrophy and should not be the sole

criteria for its diagnosis. Immunoblotting is more sensitive and specific; however,

it is technically demanding and is only performed by a limited number of referral

laboratories (see Table 1).

Laminin A2 (merosin) deficiency—infantile polymyositis

Laminin a2 (also called merosin) is a component of the myofiber basal

lamina, where it interacts with dystroglycan and integrins in the sarcolemma to

anchor myofibers to the extracellular matrix (see [27] for a review). The laminin

a2 protein complexes with laminin gamma1 and laminin b1 to form a trimer.

Patients with loss-of-function (null) mutations in the laminin a2 gene have a

severe congenital muscular dystrophy, presenting with floppiness at birth and

high serum creatine kinase levels. Approximately half of all patients with

congenital muscular dystrophy show merosin-deficiency on muscle biopsy; the


Table 1

Differential diagnosis of the dystrophies and inflammatory myopathies

Myopathy

Dystrophin

immunostaining

Dystrophin

immunoblot

Sarcoglycan

immunostaining

Dysferlin

immunoblot

Merosin

immunostaining

Common

gene mutations

Idiopathic inflammatory

myopathies

Normal (although

rare dystrophin-

negative fibers)

Normal Normal Normal Normal None

Dysferlin deficiency

(LGMD2B,

Miyoshi myopathy)

Variable, but not

clearly mosaic

Normal Normal Complete or

near-complete absence

Normal None (difficult to screen)

Isolated female

manifesting carriers of

Duchenne dystrophy

Mosaicism (clear

dystrophin-positive

and dystrophin-

negative fibers)

Normal size

Normal-to-reduced

quantities

Secondary

mosaicism

Normal Normal None, but skewed X

inactivation test of peripheral

blood DNA can be done as

confirmation

Becker muscular

dystrophy

Faint/variable,

although can

be normal

Abnormal molecular

weight or quantity

Secondary

deficiency, but

can be normal

Normal Normal None (difficult to screen)

Merosin deficiency

(infantile polymyositis)

Normal Normal Normal Normal Absent None (difficult to screen)

Boldface represents the most sensitive and specific diagnostic test result for the corresponding disease.

E.P.Hoffm

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DisClin

NAm

28(2002)743–757

749

majority of these have mutations of the corresponding gene [28]. Dramatic white

matter changes resembling a leukodystrophy on MRI of the brain is critical in the

differential diagnosis, although patients are nearly always cognitively normal.

The MRI changes are thought to be secondary to altered water distribution in the

brain and do not reflect a demyelinating process.

Muscle from patients with merosin-deficiency shows a number of distinct

histologic stages. Near the time of birth, such muscle often shows dramatic

infiltration with B lymphocytes, and CD4+ and CD8+ T cells (Fig. 2). The

inflammatory changes can include functioning B cell follicles within the muscle,

and can lead to the diagnosis of ‘‘infantile polymyositis’’ (Fig. 2) [29,30]. Most,

Fig. 2. Merosin (laminin a2) deficient muscle shows dramatic inflammation at birth. Three

histopathologic stages seen in neonates with complete loss of the laminin a2 gene product (merosin)

due to gene mutations. At birth, dramatic inflammation can be seen, including mature B cell follicles,

and numerous T cells (upper right panel ) [29]. This time point corresponds to the change-over from

the laminin a5 chain, to the laminin a2 chain (merosin) in normal muscle (left flow diagram). This

change-over does not occur in merosin-deficient congenital muscular dystrophy muscle, and seems to

signal for inflammation (center flow diagram). This neonatal inflammatory response resolves into an

aggressive dystrophic histologic picture (right, center panel ). The muscle has very poor regeneration

of necrotic fibers, leading to rapid fatty replacement of the muscle (right, lower panel ). The remaining

myofibers show high persistent expression of laminin a5 protein, which seems to functionally rescue

these fibers from further inflammation and destruction. From Hoffman EP, Scacheri C, Pegoraro E.

Congenital muscular dystrophy (Jan 2001). GeneClinics: clinical genetic information resource.

Available at: http://www.geneclinics.org; with permission.


if not all, patients who are diagnosed with infantile polymyositis actually have

primary merosin-deficiency as the cause of their disorder. Clinically, the patients

who survive the neonatal period will stabilize; however, they rarely achieve any

motor milestones.

Genome-wide pathway analyses: insights from expression profiling

The inflammatory myopathies probably represent a complex interplay

between environmental triggers (eg, infectious or noninfectious agents), genetic

predispositions (eg, HLA and TNF-a genotypes in juvenile dermatomyositis),

and tissue physiology (eg, immune response, ischemia) (see article by Reed and

Ytterberg in this volume). The use of microarrays (eg, gene chips) to assay the

mRNA expression levels of tens of thousands of genes simultaneously in a

patient muscle biopsy (mRNA expression profiling) is a novel experimental

approach that is beginning to provide new perspectives on the complex biology

underlying IIM. This approach has started to identify interrelated genes and

gene products (pathways), and generates many hypotheses and models con-

cerning cross-talk between pathways leading to tissue pathology. Critical to this

approach is the emerging ability to ‘‘dissect’’ the different pathophysiologic

pathways or genetic programs intrinsic to a specific pathology. For example,

JDM is probably a mix of antiviral programs, ischemic programs, and myofiber

degeneration/regeneration programs. One can use expression profiles from a

noninflammatory dystrophic myopathy with a known biochemical defect as a

filter for those changes associated with myofiber degeneration/regeneration.

Cell-based in vitro models of antiviral cascades can be used as a filter for

antiviral programs in patient muscle. We recently used this approach to begin to

dissect the thousands of gene expression changes seen in muscle biopsies of

patients with JDM [31].

The most extensive studies in muscle and muscle disease have been in

dystrophin-deficiency (Duchenne muscular dystrophy in humans, and mdx mice)

[32–36]. These data showed the expression responses that resulted from a known

biochemical defect affecting sarcolemmal membrane stability (Fig. 3). Dystrophin-

deficiency leads directly to episodic unrestricted influx of calcium into myofibers,

and efflux of cellular contents (such as creatine kinase then detected in the serum of

patients). Nevertheless, there are many secondary ‘‘downstream’’ consequences of

dystrophin-deficiency that probably dictate the progressive and debilitating nature

of the disease. Some of these changes are anticipated by previous knowledge

regarding histopathology and biochemistry of the disease; necrotic fibers are

infiltrated by macrophages, and evidence for mRNAs associated with macro-

phages can be seen in the expression profiles. The nonhypothesis driven, ‘‘wide

net’’ approach of expression profiling has resulted in many unexpected findings.

For example, infiltration of activated dendritic cells into Duchenne muscular

dystrophy (DMD) muscle were found by expression profiling, as was the

persistent overexpression of cardiac actin, which suggest activation of alternative


developmental programs [32]. The entire transcriptome of dystrophic (DMD) and

nondystrophic controls was recently published [34]. It is available at a searchable

Website so that the status of any gene can be assessed in muscle (see http://

microarray.cnmcresearch.org link to ‘‘programs in genomic applications’’).

One can assume that the expression profile of muscle from patients with

Duchenne dystrophy represents a pure ‘‘dystrophic process’’(albeit with a major

involvement of inflammatory cells such as macrophages, mast cells, and dendritic

cells), and can be compared with the profiles of patients with juvenile dermato-

myositis [31]. As expected, considerable overlap with the Duchenne dystrophy

profiles is found; most of the genes involved in myofiber degeneration and

regeneration are seen in patients with DMD or JDM. There were, however, a

Fig. 3. Duchenne muscular dystrophy mRNA and biochemical pathways shown by expression

profiling. Expression profiling of this disorder has begun to define the age-related chain of events

initiated by dystrophin-deficiency, as shown in this schematic diagram [32–34]. Those proteins where

the corresponding mRNAwas found altered in the expression profiles are indicated, with the relative

increase or decrease of mRNA levels indicated by an arrow. Cell autonomous changes (right) directly

result from cellular defects. Noncell autonomous changes (left) occur external to the abnormal cell, in

the tissue microenvironment. Dystrophin-deficiency has a direct effect on sarcolemmal stability,

leading to unrestricted influx of calcium (center). The calcium influx has a toxic effect on mitochondria,

and probably many other cellular processes. The efflux of cellular components and eventual necrosis of

fibers has an effect on the tissue microenvironment (left), with extensive mast cell and dendritic cell

infiltration, and subsequent release of immune mediators (cytokines, proteases), that exacerbate the

membrane defect, and lead to grouped necrosis. The abnormal state of regeneration is seen in

expression profiles (right), with genes expressed that are more characteristic of other cell types (eg,

heart); this likely contributes to the gradual failure of regeneration of myofibers. Adapted from Chen

YW, Zhao P, Borup R, et al. Expression profiling in the muscular dystrophies: identification of novel

aspects of molecular pathophysiology. J Cell Biol 2000;151:1321–36; with permission.


large number of gene expression changes seen in patients with JDM that were not

seen in patients with DMD; these reflect expression profiles more specifically

associated with the pathogenesis of JDM. Many of the JDM-specific changes

were shared with an in vitro cell-based model of the cellular response to viral

infection (Fig. 4). There are two interpretations of this finding: either an active

viral infection, and subsequent antiviral program, is present in muscle from

patients with JDM long after the initial clinically detected viral event, or the

muscle is self-perpetuating the antiviral response in the absence of an active

virus. The latter model is more consistent with the inability of investigators to

find signs of active virus in muscle biopsies from patients with JDM, and also

explains why immune suppressive agents are effective in stopping the disease

Fig. 4. JDM mRNA and biochemical pathways elucidated by expression profiling. Juvenile

dermatomyositis seems to be initiated by viral infection; however, the muscle and skin symptoms are

often far removed in onset from the actual viral insult. Gene expression profiling (genechips) of

muscle biopsies from patients with JDM showed that an antiviral gene expression program remains

strongly induced in muscle, long after the initial viral or other environmental trigger. Shown is a model

of disease pathogenesis based upon expression profiling, where the antiviral cascades in the

vasculature results in local ischemia in muscle. The ischemic insult induces TNF-a production as

required for vasculoneogenesis; however, the TNF-a feeds back upon the antiviral cascade and

augments this cascade. TNF-a has a direct cytotoxic effect on regenerating muscle, and local

production may be responsible for fiber atrophy and failed regeneration. Finally, the necrosis of

myofibers leads to influx of macrophages that also modulate and augment the inflammatory cascades.

Adapted from Tezak et al 2002 [31]; with permission.


process in most patients (whereas immune suppression of an active viral infection

might be expected to be counter-productive).

A hypothetical model that explained self-perpetuation of an antiviral response in

muscle has been described (Fig. 4) [31]. In this model, three different biochemical

pathways (antiviral, ischemic, and myofiber degeneration/regeneration) occur

simultaneously in the muscle microenvironment. This model hypothesizes that

certain key molecules are used by multiple pathways, but have different roles in

each pathway (see Fig. 4). The normal feedback mechanisms within a single

pathway (eg, regulation and limitation of the antiviral response after the virus is no

longer active or present), is compromised by the contribution of these regulatory

molecules by other pathways. Thus, the model proposes that the ischemic program

(promitotic angiogenesis), and the myofiber degeneration/regeneration program

(macrophage infiltration, pro-mitotic myofiber regeneration) feed back to the

antiviral program, perpetuating it in the absence of active virus (Fig. 4).

A key molecule in pathway cross-talk may be TNF-a (see Fig. 4). Mounting

evidence suggests that TNF-a has a role in ischemic responses in muscle (angio-

genesis) and muscle inflammation. Induction of TNF-a signaling is sufficient to

cause significant and chronic muscle inflammatory disease. This was demonstrated

in human patients who had a periodic fever syndrome called TRAPS (TNF-

receptor associated periodic syndrome), caused by gain-of-function mutations of

the TNF-a receptor (TNFR1) [37–40]. Normally, TNF-a binds one of its

membrane-bound receptors; after binding, the receptor can be cleaved by specific

proteases, releasing the bound ligand (TNF-a) and receptor fragment into the

extracellular space. The mutations harbored by patients with TRAPS inhibit this

cleavage event; this leads to inappropriate regulation of the ligand–receptor

complex, and oversignaling (constitutive activity) of the receptor. In addition to

a periodic fever syndrome, most patients show arthralgias, myalgias, and skin

lesions containing monocytes and lymphocytes [38,39]. By MRI, the local

inflammatory changes of patient muscle can be quite pronounced. Thus, over-

activity of TNF/TNF receptor is sufficient to cause muscle inflammatory disease.

Induction of TNF-a was recently shown to be a key component of the

arteriogenesis cascade [41]. It is likely that muscle ischemia induced by an

antiviral cascade (coagulopathy) induces TNF-a as part of the angiogenesis

cascade (Fig. 4). The ischemia-induced TNF-a production will also promote

maturation of T cells towards the Th1 lineage and autoimmunity via pathway

cross-talk [31]. In addition to its role in angiogenesis and inflammation, TNF-aplays an important role in muscle cytotoxicity. Muscle cachexia in tumor-bearing

rodent models seemed to largely be mediated by TNF-a [42,43].

In other experimental models, TNF-a was shown to be a major modulator of

inflammation syndromes. The development of experimental autoimmune myas-

thenia gravis in mice could be blocked by genetic deficiencies of TNF-areceptors [44]. Finally, the G to A polymorphism at the TNF a-308 position is

an important genetic determinant of disease chronicity in juvenile dermatomyo-

sitis [45]. The precise relationship between muscle TNF-a induction, the Th1

maturation of T cells, muscle ischemia, and muscle inflammation is being


integrated into overlapping pathways via expression profiling (Fig. 2) [31]. The

different pathways, and possible genetic and biochemical factors involved in

pathway cross-talk, will take considerable work to fully understand.

Summary

Clinical and histopathologic overlaps between the muscular dystrophies and

inflammatory myopathies are being increasingly recognized. Most patients with a

muscular dystrophy show improvement with prednisone treatment, although they

will not be cured; many patients with idiopathic inflammatory myopathies are

cured. Dysferlin-deficiency was recently recognized as a cause of late-onset

dystrophy with substantial inflammation in muscle. Corticosteroid usage by these

patients may result in nonrecoverable loss of strength. Therefore, it is important

to rule out dysferlin-deficiency before initiating a course of corticosteroids.

Newly emerging, genome-wide transcriptional profiling technology allows the

identification of the interacting pathways that are active in the muscle of patients

with inflammatory myopathies or dystrophies. There are several, complex mo-

lecular pathways; however, the comparison of expression profiles in patients with

different muscle disorders permits the delineation of disease-specific patterns. It

is hoped that novel approaches for treating the inflammatory myopathies and

dystrophies can be derived from intimate knowledge of the pathways involved in

each disease, and the key molecules that provide cross-talk between pathways.

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Clarifying the boundaries between the inflammatory and dystrophic myopathies: insights from...

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