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Previous Volume 329:921-925 September 23, 1993 Number 13 Next

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Several forms of hereditary dilated cardiomyopathy have been identified; with the exception of those resulting from mutations of mitochondrial DNA,^1,2 no pathological finding can be used to differentiate the conditions, so their distinction depends on the pattern of transmission. Autosomal recessive, autosomal dominant, and matrilinear forms have been reported; several families with X-linked dilated cardiomyopathy have also been described^3,4,5.

X-linked dilated cardiomyopathy is a progressive myocardial disease presenting as congestive heart failure in teenage boys without clinical signs of skeletal myopathy⁵. No information is available on the pathogenetic defect involved in this disorder, although cardiomyopathy is an associated feature of several X-linked diseases. Among these, Duchenne's and Becker's muscular dystrophies are caused by abnormalities of the dystrophin gene located at Xp21^6,7. Cardiac involvement is a very frequent occurrence in Duchenne's muscular dystrophy (>80 percent), although cardiac failure represents a terminal event in only a minority (10 percent) of affected boys^8,9. Female carriers of Duchenne's muscular dystrophy may have cardiac symptoms as well¹⁰. Moreover, cardiomyopathy has been the presenting symptom of Becker's muscular dystrophy in rare instances,^11,12,13 and cardiac failure is not necessarily a late feature of this disorder¹⁴.

We investigated a family with a severe form of X-linked dilated cardiomyopathy in which affected male family members had elevated serum levels of creatine kinase. We therefore carried out a detailed study to confirm the possibility that this cardiomyopathy was caused by a genetic abnormality of dystrophin.

Case Report

A pedigree was constructed for the family (Figure 1), and medical records were sought for three generations of family members. The proband (Subject II-6) was a 23-year-old man of normal intelligence who was found to have dilated cardiomyopathy at the age of 13. At that time he played competitive football. Because of the cardiomyopathy he was forced to abandon sports activities. His condition was quite stable over the following years; at the age of 23 he had an episode of dyspnea, asthenia, and weight loss. Clinical studies, including two-dimensional, M-mode, and Doppler echocardiography and cardiac catheterization, led to a diagnosis of dilated cardiomyopathy (left ventricular fractional shortening of 14 percent with an end-diastolic diameter of 8.8 cm) with moderate mitral-valve insufficiency (Table 1). The patient was also found to have elevated levels of serum creatine kinase, type MM (Table 1). On manual muscle testing he had normal muscle power in all muscle groups; no elbow or ankle contractures or rigidity of the spine was present. A muscle biopsy was performed, and he died of ventricular fibrillation two months later while on the waiting list for cardiac transplantation.

View larger version (12K): [in this window] [in a new window]   Figure 1. Pedigree of a Family with X-Linked Dilated Cardiomyopathy. Squares denote male family members, circles female family members, open symbols unaffected subjects, solid symbols affected subjects, shaded symbols subjects whose status was unknown, and circles with a dot carriers. Deceased family members are indicated by a slash. Subject II-6 was the proband.

 

View this table: [in this window] [in a new window]   Table 1. Selected Laboratory Findings in Various Family Members.

 
A study of the family history disclosed that two maternal uncles had died of cardiomyopathy at the ages of 29 and 44 (Subjects I-2 and I-3, respectively, in Figure 1); an analysis of their medical records indicated the presence of a dilated cardiomyopathy. These two subjects were believed by their relatives to be free from neuromuscular symptoms; no material usable for DNA analysis was available from them. On further investigation, we found that three brothers of the proband (Subjects II-1, II-2, and II-5) had electrocardiographic abnormalities and echocardiographic signs indicative of dilated cardiomyopathy (Table 1). All the affected male family members had elevated creatine kinase levels (Table 1). The examination of these subjects, including manual muscle testing, did not reveal any muscle weakness. There was no muscle hypertrophy or wasting of any muscle groups. Peroneal- and median-nerve conduction velocities were normal in the affected male subjects, including the proband; moreover, electromyography of the deltoid and vastus lateralis muscles revealed normal insertional and resting activity, and the duration and amplitude of the motor units and recruitment patterns were also within the normal range.

Informed consent was obtained from all subjects; this study was approved by the ethics committee of the two hospitals involved in the clinical evaluation of patients.

Methods

Immunohistochemical Study of Skeletal Muscle

A needle biopsy of the vastus lateralis muscle was performed in the proband and studied according to standard techniques,¹⁶ including immunocytochemistry. Unfixed cryostat sections (6 microm thick) were immunostained with a panel of six antidystrophin antibodies¹⁷.

DNA Studies

DNA was extracted from leukocytes by standard methods. DNA amplifications of dystrophin exons were carried out with the multiplex polymerase chain reaction (PCR) according to the methods of Chamberlain et al.¹⁸ and Beggs et al¹⁹. Primers used to amplify the first muscle exon and the brain promoter were those described by Muntoni and Strong²⁰ and Boyce et al.,²¹ respectively, whereas those used to amplify the second exon were derived from the complementary DNA (cDNA) sequence⁷. After digestion with the HindIII restriction enzyme,²² the DNA was hybridized with cDNA probes according to the Southern blotting method. Polymorphisms of microsatellite loci in the 5' end of the dystrophin gene were examined according to the methods of Oudet et al.²³ and Feener et al²⁴.

Results

Muscle Biopsy

Muscle biopsy revealed a mild variability in fiber size owing to the presence of hypertrophic and atrophic fibers; the numbers of internal nuclei were increased (18 percent; normal, <4 percent). No evidence of mitochondrial dysfunction was found on staining with Gomori's trichrome, succinic dehydrogenase, or cytochrome oxidase. The immunohistochemical staining with antidystrophin antibodies was less intense in the muscle fibers from the proband than in the control muscle and thus suggestive of a dystrophin abnormality (Figure 2). Nevertheless, staining of dystrophin was clearly visible in the sarcolemma with all the antibodies used, indicating that the antibody-recognition epitopes were intact.

View larger version (91K): [in this window] [in a new window]   Figure 2. Immunohistochemical Staining of Skeletal Muscle with Dys-II Antidystrophin Antibodies. Panel A shows normal muscle in which all fibers are equally and intensely stained (x270). Panel B shows muscle from the proband, which stained less intensely than control muscle, suggesting a dystrophin abnormality (x270).

 
DNA Analysis

All the exons investigated with multiplex PCR^18,19 and Southern blotting²² were present in the proband, whereas the muscle-promoter region showed a deletion (Figure 3). On further study, the first muscle exon²⁰ was also found to be deleted, whereas the second exon and the brain promoter²¹ were successfully amplified. Of the three polymorphic microsatellite loci studied, only DYS-III²³ (located 3.5 kb 3' to the brain promoter) and DYSMSA²⁴ were successfully amplified, whereas DYSMSB²⁴ was deleted (Figure 3). The deletion therefore specifically removed the muscle promoter and the first muscle exon, while leaving the upstream brain promoter and first brain exon intact (Figure 4).

View larger version (67K): [in this window] [in a new window]   Figure 3. PCR Analysis of Various Dystrophin Regions. The numbers above each lane correspond to the numbers of the family members in the pedigree in Figure 1. Panel A shows the alleles detected at microsatellite locus DYS-III. The amplification products were resolved on a polyacrylamide gel. Subject II-7 was the only family member who was heterozygous for this marker; all other subjects carried only one allele. Panel B shows the amplification of the DYSMSB microsatellite locus. The products were resolved on a polyacrylamide gel. This dinucleotide repeat was deleted in Subjects II-1, II-2, II-5, and II-6. Two female family members, Subjects II-3 and II-4, carried only one allele, whereas Subject II-7 was heterozygous for the repeat. Panel C shows the coamplification of the muscle promoter (upper dash) and the first muscle exon (lower dash), resolved on an agarose gel. Both sequences were deleted in the four male family members (Subjects II-1, II-2, II-5, and II-6), whereas both were present in the three female family members (Subjects II-3, II-4, and II-7). Panel D shows the amplification of the DYSMSA microsatellite locus. The products were resolved on a polyacrylamide gel. The four male family members (Subjects II-1, II-2, II-5, and II-6) carried one (top dash) of the three alleles detected, whereas two of their sisters (Subjects II-3 and II-4) also had an intermediate-size allele (middle dash). The youngest sister (Subject II-7) was also heterozygous but carried the intermediate-size and the small allele (middle and bottom dashes).

 

View larger version (19K): [in this window] [in a new window]   Figure 4. Appearance of the 5' End of the Dystrophin Gene. The approximate size of the region and the exons involved are shown. PMB denotes the brain promoter, 1B the first brain exon, PMM the muscle promoter, and 1M the first muscle exon; DYS-III, DYSMSB, and DYSMSA are the three polymorphic microsatellite loci analyzed. The bars beneath these regions indicate the fragments subjected to PCR analysis. The letters A, B, C, and D above the bars correspond to the regions for which the amplification results are shown in Figure 3 in Panels A, B, C, and D, respectively; the plus and minus signs indicate the results of amplification. The approximate extent of the deletion is indicated at the bottom of the figure.

 
Identical deletions were documented in the other affected brothers (Subjects II-1, II-2, and II-5) (Figure 3), whereas linkage analysis indicated that Subjects I-1, I-5, II-3, and II-4 were obligate carriers of the disorder.

Discussion

In our study a deletion involving the first muscle exon containing the muscle promoter of the dystrophin gene was found in all affected members of a family with X-linked dilated cardiomyopathy. This mutation apparently involved the cardiac muscle in a specific way. Indeed, no clinical evidence of skeletal-muscle weakness was found in four affected male family members ranging in age from 23 to 36 years; two more male family members, who died of dilated cardiomyopathy at the ages of 29 and 44, were believed to be free from neuromuscular symptoms. Since creatine kinase levels were elevated in the affected men and signs of mild myopathy were found in the skeletal-muscle biopsy of the proband, the possibility remains that muscular weakness may develop in these patients later in life (and thus lead to a phenotype for mild Becker's dystrophy); even if this were the case, however, there would still be a dramatic discrepancy between the degree of skeletal- and cardiac-muscle involvement in this family. Not all the affected men had the same degree of cardiac-muscle involvement. Phenotypic variations within a kindred have also been reported in other forms of dilated cardiomyopathy²⁵.

A deletion of the dystrophin muscle-promoter region, including the transcriptional start site and the first muscle exon, should theoretically preclude muscle-specific transcription. However, since relatively high levels of dystrophin were detected in the skeletal muscle of our patient with all antidystrophin antibodies, including one that recognizes epitopes partially encoded by exon 3, we must assume that the transcription is driven by another promoter. A second promoter, most active in the brain, has been located at least 90 kb upstream of the dystrophin muscle promoter,²¹ and alternative splicing of the first exon in these two tissues has been described²⁶. The dystrophin brain promoter was not affected by the deletion present in this family and should, therefore, be functional. In view of the molecular data, the most likely interpretation of the observed phenotype is that the brain promoter is driving relatively high levels of transcription in skeletal muscle but not in the heart. Transcripts derived from the dystrophin brain promoter and first brain exon have recently been detected not only in neurons, but also in skeletal muscle, although they were absent in cardiac tissue, including Purkinje fibers²⁷. This finding reinforces our interpretation that specific cardiac abnormalities could originate from mutations in this 5' region of dystrophin.

Cardiomyopathy is a common feature of Xp21 Duchenne's and Becker's muscular dystrophies, and deletions involving dystrophin exons 48, 45 through 53, and 48 and 49 have been reported in male patients who also had concomitant cardiomyopathy^13,28. Since patients with identical deletions but without cardiomyopathy have been described as well,²⁸ no firm correlation between the dystrophin genotype and (cardiac) phenotype has been established. Recently, the possibility that isolated X-linked cardiomyopathy was due to an abnormality of dystrophin has been investigated in a series of unrelated male subjects, but no deletion of the central region of the dystrophin gene was found²⁹.

The deletion of the first muscle exon and the muscle-promoter region in the present family is an unusual finding in patients with dystrophinopathy,³⁰ and to date only two patients have been identified with a deletion similar to this one^21,28. Both had very mild skeletal-muscle involvement: one was a 10-year-old boy with minimal muscle weakness,²¹ and the other was studied after the incidental finding of high creatine kinase levels at the age of 12 and only had occasional myalgias²⁸. No information is available on the presence of cardiomyopathy in these two patients. Alternatively, differences in the phenotype between the previously described patients and our own could be due to different-sized deletions, involving regulatory sequences implicated in the tissue specificity of dystrophin expression³¹. Nevertheless, all these data suggest that the lack of the first muscle exon involving the muscle promoter is compatible with relatively high levels of dystrophin expression in the skeletal muscle, but not in the cardiac muscle, at least in our patients.

We have demonstrated that dilated cardiomyopathy with no accompanying muscle weakness was associated with a dystrophin gene abnormality in one family; this deletion should therefore be added to the wide spectrum of dystrophinopathies^32,33. We do not know the proportion of X-linked cardiomyopathies that may be ascribed to dystrophin gene mutations, but it may be relatively small. An increased male:female (2.4:1) ratio of idiopathic dilated cardiomyopathy has been reported in some studies,³⁴ but not confirmed in others³⁵. It is possible that other families with X-linked cardiomyopathy in which elevated creatine kinase levels have been found⁵ have a dystrophinopathy.

On the basis of our results, we propose that immunohistochemical or Western blot analysis with various antidystrophin antibodies should be performed in families with X-linked cardiomyopathy in order to investigate a possible dystrophinopathy. The dystrophin gene, including the muscle-promoter region, should also be screened for mutations.

Supported by Telethon-Italy and the Regione Autonoma Sardegna (L.R.11, 1990). Ms. Cau is supported by a fellowship from the Unione Italiana Lotta Distrofia Muscolare.

Source Information

From the Istituto di Neuropsichiatria Infantile (F.M., A.M., M.G.M., C.C.) and the Istituto di Clinica e Biologia dell'Eta' Evolutiva (M.C., R.C., A.C., M.A.M.), Cagliari; and the Istituto di Clinica Medica Generale e Terapia Medica, Sassari (A.G., G.A., G.R.) -- all in Italy.

Address reprint requests to Dr. Muntoni at the Department of Paediatrics and Neonatal Medicine, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Rd., London W12 OHS, United Kingdom.

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Related Letters:

X-Linked Dilated Cardiomyopathy
Bies R. D., Roubicek M., Towbin J. A., Ortiz-Lopez R., Muntoni F.
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N Engl J Med 1994; 330:368-370, Feb 3, 1994. Correspondence

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