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(Circulation. 1997;95:2344-2347.)
© 1997 American Heart Association, Inc.

Articles

Dystrophinopathy, The Expanding Phenotype

Dystrophin Abnormalities in X-Linked Dilated Cardiomyopathy

Alan H. Beggs, PhD

the Division of Genetics, Children's Hospital and Department of Pediatrics, Harvard Medical School, Boston, Mass.

Correspondence to Alan H. Beggs, PhD, Genetics Division, Children's Hospital, 300 Longwood Ave, Boston, MA 02115. E-mail beggs{at}rascal.med.harvard.edu

Key Words: Editorials • genes • cardiomyopathy • dystrophin

	Introduction

Top Introduction References

 
Advances in diagnostic methodologies are constantly changing our definitions of disease entities. In particular, the nosology of the muscular dystrophies has been subject to much change as molecular genetics has provided us with the tools to define these disorders on the basis of their underlying molecular defects. Identification of dystrophin as the protein mutated in DMD has led to the appreciation of a wide range of clinical presentations associated with dystrophin gene mutations.¹ The term "dystrophinopathy" is now commonly used to describe these allelic disorders that can range from classic DMD to clinically silent cases of elevated serum CK. Most recently, it has become clear that a significant proportion of cases of XLDCM may be caused by underlying dystrophin gene defects, thus further expanding the spectrum of dystrophin-related phenotypes.

Idiopathic dilated cardiomyopathy generally presents with congestive heart failure secondary to an increase in ventricular size and impairment of ventricular function. It is a leading cause of cardiovascular morbidity and mortality, with an estimated prevalence of 36.5/100 000 in the United States.² Traditionally, most cases have been considered to be sporadic, but recent studies³ have demonstrated that up to 20% of cases may be familial in nature, suggesting a strong genetic component for this group of diseases. Inheritance patterns vary and may be X-linked, autosomal dominant, or autosomal recessive. One remarkable pedigree with XLDCM was reported by Berko and Swift in 1987.⁴ Affected males presented in their late teens to early 20s with syncope and rapidly progressive congestive heart failure, leading to death or transplantation within 1 to 2 years. Females tended to be less affected, with onset later in life and a more indolent progression lasting 10 years. Although serum CK-MM levels were elevated in most affected members, there were no overt signs of skeletal myopathy. Several years later, Towbin et al⁵ demonstrated genetic linkage of XLDCM in both this and another similar family to the dystrophin gene at Xp21. In this issue of Circulation, Ortiz-Lopez et al⁶ report the probable mutation responsible for the striking cardiospecificity of the first family's dystrophin abnormality.

DMD is one of the most common pediatric neuromuscular disorders, with an incidence of 1 in 4000 male births.¹ Classically, it presents between 3 and 6 years of age with skeletal muscle weakness preferentially affecting the large proximal muscle groups. The disease is relentlessly progressive, leading to loss of ambulation by 11 to 13 years and death by the third decade, often due to respiratory insufficiency. Significant laboratory findings include grossly elevated serum CK-MM levels, and skeletal muscle biopsy samples invariably reveal a dystrophic pattern of muscle degeneration and regeneration with fiber-size variation, increased central nuclei, and progressive interstitial fibrosis. BMD was long considered to be a potentially allelic disorder because of its clinical similarities and common pattern of X-linked inheritance. This was confirmed after the identification of dystrophin, because both DMD and BMD patients were shown to have dystrophin gene mutations. Typically, patients with DMD lack any detectable dystrophin expression in their skeletal muscles, and this is correlated with deletion mutations that disrupt the translational reading frame or point mutations that create stop codons. In contrast, muscle from patients with BMD contains dystrophin of altered size and/or reduced abundance secondary to deletion mutations that maintain the reading frame.

Dystrophin is a large (427 kD) cytoskeletal protein that localizes to the inner face of the skeletal muscle plasma membrane, or sarcolemma.⁷ It is thought to assume a rod-shaped structure with an actin-binding domain at the amino terminus. The carboxy-terminal domains associate with a large transmembrane complex of glycoproteins termed the DAG complex, some members of which directly bind with elements of the extracellular matrix.⁸ In this manner, dystrophin likely plays a critical role in establishing connections between the internal, actin-based cytoskeleton and/or sarcomeric structure and the external basement membrane. Its absence may lead to increased membrane fragility, a loss of linkage and force transduction across the sarcolemma, and/or a lack of organization of the DAG complex, as well as possibly other specialized membrane-bound structures. The end result is myofibril necrosis, leading to cycles of degeneration and regeneration with eventual permanent loss of fibers and fibrotic replacement.

Early studies of dystrophin expression established that it is present in neurons, smooth muscle, and cardiac muscle in addition to skeletal muscle. Consequently, patients with DMD or BMD often have associated clinical findings due to dystrophin deficiency in some of these other tissues. For instance, the mean IQ of DMD patients is 85, with up to one third exhibiting significant cognitive deficits.¹ Cardiac abnormalities are an even more uniform consequence but may be masked clinically by the physical inactivity imposed by skeletal muscle weakness. By 5 years of age, most patients have a sinus tachycardia, and by 10, more than half exhibit conduction changes, including a shortened PQ segment, prolonged QT interval, and/or an increased QT/PQ ratio.⁹ Classic ECG changes include tall right precordial R waves and deep Q waves in the left precordial and limb leads. This anterior shift of the QRS complex likely reflects diffuse interstitial fibrosis of the posterobasal part of the outer free wall of the left ventricle.¹⁰ More than one third of patients develop signs of cardiac dysfunction by age 14, and virtually all DMD patients have a cardiomyopathy, most often dilated, by age 18.9 Melacini et al¹¹ also documented a high incidence (60% to 72%) of myocardial involvement in patients with BMD, and in cases with particularly mild skeletal muscle involvement, cardiomyopathy may be the primary clinical feature. In these cases, cardiac dysfunction can be rapidly progressive, leading to transplantation or death within 1 or 2 years of initial presentation. Although clinical data are lacking, there is some concern that in the absence of skeletal muscle weakness, intense exercise by these patients would cause pressure and/or volume overload on the left and right ventricles, inducing mechanical stress that might exacerbate the clinical course of their disease.

The availability of assays for dystrophin gene and protein abnormalities soon led to a new appreciation of the extreme variability in clinical presentations associated with dystrophin mutations.^{12 13} At the severe end of the spectrum, infants can present with congenital hypotonia, severe motor delay, and joint contractures.¹⁴ In other cases, the skeletal myopathy may be so mild as to escape clinical detection, with the only sign being moderate elevations of serum CK.¹³ Cramps and myalgia, often after exercise, may also be common complaints among patients with particularly mild cases of BMD or those with normal strength.^{13 15} In the absence of skeletal muscle weakness, dysfunction of other organs may become the predominant clinical feature. Some patients with X-linked mental retardation and/or psychiatric disturbances have been found to suffer from unrecognized dystrophin gene mutations.¹⁶ In these cases, elevated serum CK levels led to the evaluation of muscle biopsy samples that exhibited mild dystrophic changes despite clinically normal muscle strength.

Put in this context, perhaps it is not surprising that some patients with dystrophin gene mutations would present with dilated cardiomyopathy as their primary clinical feature.^{11 12 13} What is more remarkable is that in some families, the clinical picture is homogenous, with all affected members having predominantly cardiac dysfunction. This observation implies that either dystrophin expression in the heart and skeletal muscle must be under separate control or that there must be functional differences between dystrophin in the two tissues. There have now been a handful of reports on such cases involving dystrophin abnormalities in patients with XLDCM and normal skeletal muscle strength.^{5 6 17 18 19 20} Affected males presented with congestive heart failure in their 20s to 40s and invariably followed a rapidly progressive course leading to transplantation and/or death within several years. Heterozygous females tended to present later in life with atypical chest pain and a slower progression to heart failure (over a decade or more). Where available, cardiac muscle pathology was consistent with a dilated cardiomyopathy, with necrotic and fibrotic changes in the posterolateral free wall of the left ventricle. Although none of these cases were associated with overt skeletal muscle weakness, a number of patients had subtle signs of skeletal myopathy, including calf hypertrophy and/or exertional cramps and myalgias. Significantly, with the exception of one brother reported by Milasin et al,¹⁸ every affected patient had elevated serum CK-MM levels, and several of these patients' muscle biopsy samples exhibited mild dystrophic or myopathic changes. Dystrophin protein studies in many of these patients consistently demonstrated abnormalities of cardiac dystrophin that were absent or less severe in skeletal muscle biopsy samples.^{5 17 18 21} These findings are consistent with the idea that the cardiospecificity of these dystrophin abnormalities is apparently related to differences in dystrophin expression and/or function between cardiac and skeletal muscles.

Clues to the nature of the differences between cardiac and skeletal muscle dystrophin can come from knowledge of the specific gene mutations that preferentially affect only the myocardium. There are at least four different tissue-specific promoter elements responsible for expression of "full-sized" (427 kD) dystrophin isoforms.⁷ Several of the dystrophin-deficient XLDCM patients have known dystrophin gene mutations that affect the muscle promoter (P_M) and muscle-specific exon 1 through deletion^{19 20} or altered mRNA splicing.¹⁸ In each of these cases, cardiac dystrophin, when tested, was found to be undetectable, whereas levels of skeletal muscle dystrophin were normal or only slightly reduced. Two recent reports by Muntoni et al^{21 22} demonstrate that in their promoter-deletion cases, no dystrophin transcripts are produced in cardiac muscle. However, two alternative promoters normally expressed in brain (P_B) and Purkinje cells (P_P) were expressed in skeletal muscles of patients with the P_M deletions, resulting in compensatory expression of enough dystrophin to ameliorate the skeletal muscle weakness. The exact mechanism(s) of differential transcriptional control between cardiac and skeletal muscles is still unclear, in part because it is not obvious why P_B, which is normally active in cardiac but not skeletal muscle, should reverse this pattern after disruption of sequences surrounding P_M and exon 1. However, this may be mediated by transcriptional enhancer elements in the region, one of which has recently been characterized.²³ Confusion arising from the great clinical variability associated with deletions of P_M^{13 20 22} should be alleviated by the inference that each of these deletions includes variable amounts of the first intron and therefore likely deletes different combinations of transcriptional control elements.

The other potential mechanism for cardiospecificity of dystrophin mutations may be differences in function between cardiac and skeletal muscle dystrophin. The families reported by Towbin et al⁵ and Franz et al¹⁷ likely illustrate this phenomenon in that they all had a unique pattern of cardiac dystrophin immunoreactivity, in which antibodies specific for the amino terminal and/or rod regions failed to bind dystrophin despite the fact that carboxy-terminal antisera recognized full-sized protein. Thus, their mutations are hypothesized to be in a cardiac-specific exon of dystrophin, or they may alter epitopes in a region of the protein of particular functional importance in cardiac muscle. Ortiz-Lopez et al⁶ now confirm this latter supposition with the identification of a single amino acid substitution of alanine for threonine at position 279 (T279A) in a probable hinge portion of the protein (H1). H1 is a proline-rich spacer located at the boundary between the amino-terminal actin-binding domain and the central rod portion of the protein, and sequence analysis suggests that this structure may confer flexibility to the protein. Identification of the T279A mutation should stimulate a series of biochemical studies to determine what functional differences might exist for this region of dystrophin. Along these lines, it is interesting to note that Meng et al²⁴ recently demonstrated an association of dystrophin with Z-disc regions of cardiac muscle but not with the analogous Z lines of skeletal muscle. Furthermore, dystrophin localizes to the transverse tubules of cardiac but not skeletal muscle. Each of these localizations is presumably mediated by tissue-specific binding to other protein components of the relevant structures, and it is easy to imagine that amino acid substitutions at a critical site could alter such an interaction.

What are the clinical implications of these observations? Clearly, dystrophin abnormalities, although relatively rare, should be considered in the differential for isolated or X-linked idiopathic dilated cardiomyopathy. Elevated serum CK-MM levels should be a red flag that leads to a more detailed neuromuscular workup, including strength testing, assessment of possible calf hypertrophy, careful questioning about family histories of cramps and myalgias, and possibly a skeletal muscle biopsy for pathological and dystrophin analysis. Although the sensitivity may be low, dystrophin gene deletion analysis should be considered for all such cases because it is a relatively noninvasive test, requiring only a small amount of peripheral blood, and the specificity is virtually 100%. If available, the most sensitive and specific test is likely to be dystrophin protein analysis of a myocardial biopsy sample. At this point in time, identification of dystrophin protein abnormalities would likely lead to careful gene-mutation analysis by a research laboratory. In the future, such studies should be available by clinical DNA diagnostic laboratories on a fee-for-service basis. Identification of dystrophin gene mutations will allow for accurate presymptomatic diagnosis of family members at risk for cardiomyopathy and, given the rapidly fatal course of the disease in most patients, will likely increase the urgency with which transplantation might be considered. Finally, future clinical studies should address the possibility that exercise may hasten the onset and progression of the disease so that appropriate recommendations concerning activity and possible therapy can be formulated.

	Selected Abbreviations and Acronyms

 

BMD = Becker muscular dystrophy CK = creatine kinase DAG = dystrophin-associated glycoprotein DMD = Duchenne muscular dystrophy PB = promoter expressed in brain PM = muscle promoter PP = promoter expressed in Purkinje cells XLDCM = X-linked dilated cardiomyopathy

	Acknowledgments

 
The author is supported by an Independent Scientist Award from the NIAMSD (K02 AR02026) and by research grants from the Muscular Dystrophy Association and the NIAMSD (R01 AR44345).

	Footnotes

 
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

	References

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