Dystrophin abnormalities in Duchenne/Becker muscular dystrophy

The structural and functional diversity of dystrophin

Duchenne and Becker muscular dystrophies are caused by defects of the dystrophin gene. Expression of this large X-linked gene is under elaborate transcriptional and splicing control. At least five independent promoters specify the transcription of their respective alternative first exons in a cell-specific and developmentally controlled manner. Three promoters express full-length dystrophin, while two promoters near the C terminus express the last domains in a mutually exclusive manner. Six exons of the C terminus are alternatively spliced, giving rise to several alternative forms. Genetic, biochemical and anatomical studies of dystrophin suggest that a number of distinct functions are subserved by its great structural diversity. Extensive studies of dystrophin may lead to an understanding of the cause and perhaps a rational treatment for muscular dystrophy.

The 87K postsynaptic membrane protein from Torpedo is a protein-tyrosine kinase substrate homologous to dystrophin

Postsynaptic peripheral membrane proteins at the neuromuscular junction have been proposed to participate in the immobilization of the nicotinic acetylcholine receptor at the synapse. An 87 kd cytoplasmic peripheral membrane protein has been demonstrated to colocalize with the nicotinic acetylcholine receptor in the Torpedo electric organ and at the mammalian neuromuscular junction. We have cloned the cDNA encoding the 87K protein from Torpedo electric organ, and the predicted protein sequence is homologous to the C-terminal domains of dystrophin, the protein product of the Duchenne muscular dystrophy gene. The 87K protein displays a restricted pattern of expression detected only in electric organ, brain, and skeletal muscle. Analysis of the in vitro and in vivo phosphorylation of the 87K protein indicates that it is multiply phosphorylated on serine, threonine, and tyrosine residues. The 87K protein is in a complex with other proteins associated with the postsynaptic membrane, including dystrophin and a 58 kd protein. These results suggest that the 87K protein is involved in the formation and stability of synapses and is regulated by protein phosphorylation.

Efficient adenovirus-mediated transfer of a human minidystrophin gene to skeletal muscle of mdx mice

Duchenne progressive muscular dystrophy is a lethal and common X-linked genetic disease caused by the absence of dystrophin, a 427K protein encoded by a 14 kilobase transcript. Two approaches have been proposed to correct the dystrophin deficiency in muscle. The first, myoblast transfer therapy, uses cells from normal donors, whereas the second involves direct intramuscular injection of recombinant plasmids expressing dystrophin. Adenovirus is an efficient vector for in vivo expression of various foreign genes. It has recently been demonstrated that a recombinant adenovirus expressing the lac-Z reporter gene can infect stably many mouse tissues, particularly muscle and heart. We have tested the ability of a recombinant adenovirus, containing a 6.3 kilobase pair Becker-like dystrophin complementary DNA driven by the Rous sarcoma virus promoter to direct the expression of a 'minidystrophin' in infected 293 cells and C2 myoblasts, and in the mdx mouse, after intramuscular injection. We report here that in vivo, we have obtained a sarcolemmal immunostaining in up to 50% of fibres of the injected muscle.

Predicted and observed sizes of dystrophin in some patients with gene deletions that disrupt the open reading frame

Among 85 patients with Duchenne and Becker muscular dystrophy, 29 were found to have mutations which disrupted the open reading frame for dystrophin. Thus any dystrophin detected in this group of patients should consist of the severely truncated polypeptides that represent prematurely terminated translation products. Dystrophin was detected in blots from 17/29 biopsies and the observed sizes of the polypeptides were compared with predicted sizes calculated in two ways: if translation was terminated at the stop codon generated by each frameshifting deletion, and if the reading frame was restored and translation proceeded. In every case the observed size matched the size predicted on the basis of a restored reading frame. This was in accord with immunocytochemical labelling of scattered dystrophin positive fibres which were found on serial sections labelled with antibodies to both the rod and C-terminal domains. Thus analysis at the protein level supports genetic evidence of exon skipping as a mechanism which restores frameshifting mutations in some fibres.

Detection of dystrophin in the postsynaptic density of rat brain and deficiency in a mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a common, lethal, chromosome X-linked inherited disease. Moderate cognitive impairment is a feature of DMD, but the underlying mechanisms are unknown. DMD is characterized by a defect in a protein, dystrophin, that is located predominantly in muscle but has been detected in brain. We sought to directly localize dystrophin within the complex synaptic structure of the cerebral cortex by focusing on the postsynaptic density (PSD), which appears to be central to synaptic function. We report that a specific anti-dystrophin antibody (anti 6-10) recognizes three distinct proteins in the purified PSD: the 400-kDa dystrophin and two previously unidentified dystrophin-related proteins of 120 and 110 kDa. These proteins exhibited differential regional expression in PSDs from cerebral cortex, cerebellum, and olfactory bulb. In the cortical PSD, the 400-kDa dystrophin was predominant, whereas the 120-kDa protein was the major species in cerebellum and olfactory bulb PSDs. The three proteins were differentially expressed in the PSD during cortical development: the 400-kDa protein exhibited a selective 9-fold increase during postnatal days 7 to 10, suggesting a normal physiological role in synaptic maturation. The PSD from the mdx mouse, a model of human DMD, contained no detectable 400-kDa dystrophin but expressed the two dystrophin-related proteins. Our results indicate that brain dystrophins are localized to the PSD, potentially as three isoforms, and raise the possibility that cognitive abnormalities in DMD are attributable to synaptic dysfunction associated with deficits in brain dystrophin molecules.

Direct visualization of the dystrophin network on skeletal muscle fiber membrane

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene locus, is expressed on the muscle fiber surface. One key to further understanding of the cellular function of dystrophin would be extended knowledge about its subcellular organization. We have shown that dystrophin molecules are not uniformly distributed over the humen, rat, and mouse skeletal muscle fiber surface using three independent methods. Incubation of single-teased muscle fibers with antibodies to dystrophin revealed a network of denser transversal rings (costameres) and finer longitudinal interconnections. Double staining of longitudinal semithin cryosections for dystrophin and alpha-actinin showed spatial juxtaposition of the costameres to the Z bands. Where peripheral myonuclei precluded direct contact of dystrophin to the Z bands the organization of dystrophin was altered into lacunae harboring the myonucleus. These lacunae were surrounded by a dystrophin ring and covered by a more uniform dystrophin veil. Mechanical skinning of single-teased fibers revealed tighter mechanical connection of dystrophin to the plasma membrane than to the underlying internal domain of the muscle fiber. The entire dystrophin network remained preserved in its structure on isolated muscle sarcolemma and identical in appearance to the pattern observed on teased fibers. Therefore, connection of defined areas of plasma membrane or its constituents such as ion channels to single sarcomeres might be a potential function exerted by dystrophin alone or in conjunction with other submembrane cytoskeletal proteins.

Localization of dystrophin gene transcripts during mouse embryogenesis

The spatial and temporal expression of the dystrophin gene has been examined during mouse embryogenesis, using in situ hybridization on tissue sections with a probe from the 5' end of the dystrophin coding sequence. In striated muscle, dystrophin transcripts are detectable from about 9 d in the heart and slightly later in skeletal muscle. However, there is an important difference between the two types of muscle: the heart is already functional as a contractile organ before the appearance of dystrophin transcripts, whereas this is not the case in skeletal muscle, where dystrophin and myosin heavy chain transcripts are first detectable at the same time. In the heart, dystrophin transcripts accumulate initially in the outflow tract and, at later stages, in both the atria and ventricles. In skeletal muscle, the gene is expressed in all myocytes irrespective of fiber type. In smooth muscle dystrophin transcripts are first detectable from 11 d post coitum in blood vessels, and subsequently in lung bronchi and in the digestive tract. The other major tissue where the dystrophin gene is expressed is the brain, where transcripts are clearly detectable in the cerebellum from 13 d. High-level expression of the gene is also seen in particular regions of the forebrain involved in the regulation of circadian rhythms, the endocrine system, and olfactory function, not previously identified in this context. The findings are discussed in the context of the pathology of Duchenne muscular dystrophy.

Prediction of dystrophin phenotype by DNA analysis in Duchenne/Becker muscular dystrophy

Allele-specific molecular diagnosis of Duchenne and Becker muscular dystrophies (DMD and BMD) has been largely dependent upon muscle biopsy for dystrophin protein assay. We performed lymphocyte DNA mutation analysis by polymerase chain reaction on 14 boys presenting with a clinical picture compatible with DMD or BMD. DNA analysis revealed that 12 of 14 boys had a deletion of the dystrophin gene, thus establishing the diagnosis of DMD/BMD. Furthermore, genotypes for 9 of 12 deletion patients permitted prediction of the specific allelic disorder (i.e., DMD or BMD). Subsequent dystrophin testing confirmed all of the DNA-based diagnoses. We propose that DNA mutation analysis be included in the initial evaluation of patients suspected of having DMD/BMD, thus potentially eliminating the need for muscle biopsy in the majority of patients.

The subcellular distribution of chromosome 6-encoded dystrophin-related protein in the brain

Chromosome 6-encoded dystrophin-related-protein (DRP) shows significant structural similarities to dystrophin at the carboxyl terminus, though the two proteins are encoded on different chromosomes. Both DRP and dystrophin are expressed in muscle and brain and show some similarity in their subcellular localization. For example, in skeletal muscle both are expressed at neuromuscular and myotendinous junctions. However, while dystrophin is absent or severely reduced in Duchenne/Becker muscular dystrophy, DRP continues to be expressed. Within the brain, dystrophin is enriched at the postsynaptic regions of specific subsets of neurons, while the distribution of DRP is yet to be described. In this study we demonstrate a distinct though highly specific pattern of distribution of DRP in the brain. DRP is enriched in the choroid plexus, pia mater, intracerebral vasculature, and ependymal lining. Within the parenchyma proper, DRP is located at the inner plasma face of astrocytic foot processes at the abluminal aspect of the blood-brain barrier. The distribution of DRP is conserved across a large evolutionary distance, from mammals to elasmobranchs, suggesting that DRP may play a role in the maintenance of regional specializations in the brain.

Induction of dystrophin localization in cultured Xenopus muscle cells by latex beads

The distribution of dystrophin in Xenopus myotomal muscle cells was examined in conventional and confocal immunofluorescence microscopy. By labeling dissociated single muscle fibers with a monoclonal or a polyclonal antibody against dystrophin, we found that dystrophin is ten times more concentrated at the myotendinous junction (MTJ) than at the extrajunctional sarcolemma. At the MTJ, dystrophin lines the membrane invaginations where myofibrils attach to the membrane. It is colocalized with talin, but is not related to the distribution of acetylcholine receptors (AChRs) which are clustered at the postsynaptic membrane in the vicinity of the MTJ in these fibers. We found that the localization of dystrophin can be induced in cultured Xenopus myotomal muscle cells by treating them with polystyrene latex beads. Dystrophin is discretely localized at the bead-muscle contacts. With electron microscopy, a sarcolemma specialization with all the salient features of the MTJ, including basal lamina-lined membrane invaginations along which myofibrils make attachment. Although these beads also induce clustering of AChRs, the patterns of dystrophin and AChR localization are distinct. The appearance of dystrophin at the bead-contacted sarcolemma is coincident with the development of the membrane invaginations. This, together with its concentration along membrane invaginations at the MTJ in vivo, suggests a role for dystrophin in the formation of this junctional specialization. Since the signal for MTJ development can be presented to cultured muscle cells in a temporally and spatially controlled manner by beads, this system offers a simple model for analyzing the mechanism of this sarcolemma specialization.

Differential expression of muscular dystrophy in diaphragm versus hindlimb muscles of mdx mice

Contractile properties of diaphragm (DIA) from mdx and control mice were compared with those of hindlimb muscles [soleus (SOL) and extensor digitorum longus (EDL)] in vitro. Mice ranged in age from 2 weeks to 1.5 years. Muscles were directly stimulated and properties measured were: contraction time, half-relaxation time, active tension per unit area, fatigue index, and maximal velocity of shortening (Vmax). Active tension decreased significantly with age in mdx DIA but not in control DIA. SOL and EDL active tensions were less in mdx than control over the whole age range and did not decrease with age. Vmax was decreased in mdx DIA, but not in mdx SOL or EDL. These results demonstrate that DIA is more affected by muscular dystrophy than hindlimb muscles. Since many Duchenne patients exhibit respiratory distress, this differential expression of dystrophy in diaphragm, as compared to limb muscles, may have important clinical implications.

Additional dystrophin fragment in Becker muscular dystrophy may result from proteolytic cleavage at deletion junctions

Becker muscular dystrophy is usually caused by intragenic dystrophin gene deletions that result in production of an internally deleted protein. Previous studies have detected what appears to be a unique dystrophin degradation product that appears only in muscle biopsies from patients with Becker muscular dystrophy. This dystrophin fragment is always seen in addition to the "full-size" dystrophin of the expected size for a given gene deletion. It is only found in biopsies from patients with mutations in the deletion-prone region encompassing exons 45-53, but it does not appear to correlate with any observable phenotype at the clinical level. By correlating the size and locations of dystrophin gene deletions with the size of this degradation product, together with use of region-specific dystrophin antisera, we find that proteolytic cleavage may occur at the deletion breakpoints, perhaps due to alterations of the secondary and/or tertiary structures of the protein. This cleavage results in loss of the carboxy-terminal domains that are thought to be important for interactions between dystrophin and other membrane-bound proteins.

Dilating cardiomyopathy as the expression of Xp21 Becker type muscular dystrophy

A 35-year-old man with severe progressive dilating cardiomyopathy and no clinical signs of muscle disease underwent muscular investigations because of markedly increased serum creatine kinase. Muscle biopsy demonstrated Becker type muscular dystrophy with dystrophin of low molecular weight. Genetic analysis showed a deletion spanning from exon 45 to exon 46 in the Xp21 region. Xp21 Becker type muscular dystrophy must be considered in the differential diagnosis of dilating cardiomyopathy.

Glycoprotein-binding site of dystrophin is confined to the cysteine-rich domain and the first half of the carboxy-terminal domain

Dystrophin, a protein product of the Duchenne muscular dystrophy gene, is thought to associate with the muscle membrane by way of a glycoprotein complex which was co-purified with dystrophin. Here, we firstly demonstrate direct biochemical evidence for association of the carboxy-terminal region of dystrophin with the glycoprotein complex. The binding site is found to lie further inward than previously expected and confined to the cysteine-rich domain and the first half of the carboxy-terminal domain. Since this portion corresponds well to the region that, when missing, results in severe phenotypes, our finding may provide a molecular basis of the disease.

Dystrophin-like immunoreactivity in monkey and human brain areas involved in learning and motor functions

Two antidystrophin antibodies against different fragments of dystrophin were used to detect this polypeptide in monkey and human brains. Dystrophin was revealed by immunoperoxidase amplified with the biotin/avidin system and by immunoblotting. A dystrophin-like immunoreactivity was uniformly expressed in several brain regions implicated in learning and motor functions. Dystrophin function is not clear but our results raise the possibility that this protein may be involved in the cognitive impairment observed in several Duchenne muscular dystrophy (DMD) patients.

Dystrophin or a "related protein" in Duchenne muscular dystrophy?

Previously we have shown low levels of dystrophin immunoreactivity in muscle from patients with DMD. According to the "frame-shift hypothesis" DMD muscle should not synthesize any dystrophin through to the C-terminus and it has been suggested that the protein detected is not dystrophin, but a related autosomal homologue. We have labelled serial sections of DMD muscle with specific monoclonal antibodies to the amino, rod and C-terminal domains of dystrophin and find labelling on the same individual fibres, allowing us to conclude that the protein detected is Xp21-encoded dystrophin. This has an impact on the interpretation of myoblast transfer experiments. The abundance (on blots) of "C-terminal dystrophin" appears lower than "rod dystrophin" in both BMD and DMD.

Dystrophin deficiency causes lethal muscle hypertrophy in cats

Two 5-month-old male Domestic Shorthair littermates showed general skeletal muscle hypertrophy, multifocal submucosal lingual calcification with lingual enlargement, and excessive salivation. Both cats had a reduced level of activity, walked with a stiff gait, and tended to "bunny hop" when they ran. These clinical features were similar to those of previously reported dystrophin-deficient cats. Using multiple dystrophin antibodies, we found that the cats described in this report also showed marked dystrophin deficiency. The histopathology was remarkable for hypertrophy and splitting of fibers, and progressive accumulation of calcium deposits within the muscle. There was little or no endomysial fibrosis at 2 years of age. The natural history of dystrophin-deficiency in cats has not been described: both previous cats had been euthanized at 2 years of age prior to experiencing any life-threatening problems. At 6 months of age, one of the new cats developed megaesophagus because of severe progressive hypertrophy of the diaphragmatic muscles. The diaphragm completely occluded the esophagus, and the cat was euthanized for humane reasons. The second cat remained in good condition until age 18 months when it developed acute renal failure attributed to severe prolonged dehydration and hyperosmolality. The cat recovered after receiving supportive treatment but was unable to maintain fluid homeostasis. The insufficient water intake was attributed to glossal hypertrophy and dysfunction. At age 2 years, the cat received regular subcutaneous injections of low-sodium fluids to maintain proper hydration. The clinical consequence of dystrophin deficiency in cats is lethal muscle hypertrophy. We have called the feline disease "hypertrophic feline muscular dystrophy" (HFMD).

In vitro digestion of dystrophin by calcium-dependent proteases, calpains I and II

Dystrophin is a cytoskeletal protein which is thought to play an important role in membrane physiology since its absence (due to gene deficiency) leads to the symptoms of Duchenne muscular dystrophy (DMD). Some disruption in the regulation of intracellular free Ca2+ levels could lead to DMD-like symptoms. In this study, calpains, which are very active calcium-dependent proteases, were examined for their capacity to hydrolyse dystrophin in vitro. The results show that calpains are able to split dystrophin and produce breakdown products of different sizes (the degree of cleavage being dependent on the incubation time with proteases). The time-course of protease degradation was examined by Western immunoblot using three polyclonal sera which were characterized as being specific to the central (residues 1173-1728) and two distal parts of the molecule ie specific to the N-terminal (residues 43-760) or the C-terminal (residues 3357-3660) extremities of the dystrophin molecule. The cleavage patterns of dystrophin showed an accumulation of some major protease-resistant fragments of high relative molecular mass (250-370 kDa). These observations demonstrate that calpains digest dystrophin very rapidly when the calcium concentration is compatible with their activation. For instance, it is clear that calpains first give rise to large dystrophin products in which the C-terminal region is lacking. These observations suggest that dystrophin antibodies specific to the central domain of the molecule should be used to detect dystrophin for diagnostic purposes and before any conclusion as to the presence or absence of dystrophin can be deduced from results obtained using immunoanalyses of muscle biopsies.

Molecular genetic and immunological analysis of dystrophin of a young patient with X-linked muscular dystrophy

We examined the nucleotide sequence of deleted part of dystrophin mRNA and its translational product with immunoblot and immunohistochemical methods in a 6-year-old boy with a deleted DMD/BMD gene. On Southern blot analysis of his genomic DNA, we found a deletion of exons 10 to 37 in the DMD/BMD gene, which was expected to preserve the translational open reading frame (ORF). Dystrophin mRNA from his biopsy sample was amplified by polymerase chain reaction (PCR) and sequenced. The mRNA lacked the sequence corresponding to the gene from exons 10-37, and the translational ORF was preserved. The transcript was expected to code a 260 kDa protein. Dystrophin expressed in this patient was investigated with immunological methods. A 260 kDa protein was detected by immunoblot analysis with antidystrophin antiserum against nondeleted regions. These observations confirmed the preservation of the reading frame and the 260 kDa protein was produced as a mutant dystrophin. All these are compatible with the diagnosis of BMD. However, the immunohistochemical pattern of his muscle cells was peculiar. With deleted-region-directed antiserum, the membrane was not stained at all as in DMD patients. In contrast, with nondeleted-region-directed antiserum, all the muscle cell membrane was stained continuously as in non-DMD/BMD individuals. These are quite different from the staining pattern in most BMD patients where muscles are stained patchily or discontinuously.

Localization of dystrophin and dystrophin-related protein at the electromotor synapse and neuromuscular junction in Torpedo marmorata

The immunological identification of dystrophin isoforms at the neuromuscular junction and Torpedo marmorata electromotor synapse was attempted using various antibodies. A polyclonal antibody raised against electrophoretically purified dystrophin from T. marmorata electrocyte has been thoroughly investigated. This antibody recognized dystrophin in the electric tissue as well as sarcolemmal and synaptic neuromuscular junction dystrophin in all studies species (T. marmorata, rat, mice and human) at serum dilutions as high as 1:10,000. At variance, no staining of either the sarcolemma or neuromuscular junction was observed in Duchenne muscular dystrophy or mdx mice skeletal muscles. In these muscles, other members of the dystrophin superfamily, in particular the dystrophin-related protein(s) encoded by autosomal genes are present. These data thus demonstrate the specificity of our antibodies for dystrophin. Anti-dystrophin-related protein antibodies [Khurana et al. (1991) Neuromusc. Disorders 1, 185-194] which gave a strong immunostaining of the neuromuscular junction in various species, including T. marmorata, cross-reacted weakly with the postsynaptic membrane of the electrocyte. Taken together, these observations are in favor of the existence of a protein very homologous to dystrophin at the electromotor synapse in T. marmorata, whereas both dystrophin and dystrophin-related protein co-localize at the neuromuscular junction as in all species studied. The electrocyte thus offers the unique opportunity to study the interaction of dystrophin with components of the postsynaptic membrane.

Immunocytochemical study of dystrophin in cultured mouse muscle cells by the quick-freezing and deep-etching method

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is deficient in patients with DMD and in mdx mice. It is immunocytochemically localized in skeletal muscle sarcolemma. However, little is known about the three-dimensional ultrastructural localization of dystrophin and its relationship with other cytoskeletal proteins. We found that dystrophin is localized irregularly, just underneath the plasma membrane in normal cultured mouse myotubes, by using the quick-freezing and deep-etching (QF-DE) method; it was found to be closely linked to actin-like filaments (8-10 nm in diameter), most of which were decorated with myosin subfragment 1, and was attached to the cytoplasmic side of the plasma membrane. These results suggest that dystrophin might play an important role in the preservation of cell membrane stability by connecting actin cytoskeletons with the cytoplasmic side of the plasma membrane.

Clinical variability of type 1 neurofibromatosis: is there a neurofibromatosis-Noonan syndrome?

Detailed clinical, ophthalmological, and molecular studies were performed on a multigeneration family in which there were many subjects with type 1 neurofibromatosis, a common autosomal dominant disorder. Affected family members displayed a wide range of clinical findings including, in two subjects, features seen in Noonan syndrome (triangular facies, downward slanting palpebral fissures, micrognathia, short stature, and learning disability). Subjects have been described previously whose features have overlapped with neurofibromatosis and Noonan syndrome, and it has been suggested that these persons might represent a separate condition. DNA haplotype analysis showed linkage of the neurofibromatosis phenotype seen in this family to the proximal long arm of chromosome 17 in the region where the type 1 neurofibromatosis gene has been mapped. These results imply that the Noonan phenotype seen in some patients with type 1 neurofibromatosis might be the result of variable or variant expression of the neurofibromatosis gene on chromosome 17. The possible role of non-specific factors, such as fetal hypotonia, in producing the neurofibromatosis-Noonan phenotype needs further investigation. The availability of closely linked and intragenic molecular markers for neurofibromatosis could potentially be useful in the diagnosis and characterisation of patients and families with atypical forms of neurofibromatosis.

Normal dystrophin transcripts detected in Duchenne muscular dystrophy patients after myoblast transplantation

Gene delivery by transplantation of normal myoblasts has been proposed as a treatment of the primary defect, lack of the muscle protein dystrophin, that causes Duchenne muscular dystrophy (DMD), a lethal human muscle degenerative disorder. To test this possibility, we transplanted normal myoblasts from a father or an unaffected sibling into the muscle of eight boys with DMD, and assessed their production of dystrophin. Three patients with deletions in the dystrophin gene expressed normal dystrophin transcripts in muscle biopsy specimens taken from the transplant site one month after myoblast injection. Using the polymerase chain reaction we established that the dystrophin in these biopsies derived from donor myoblast DNA. These results show that transplanted myoblasts persist and produce dystrophin in muscle fibres of DMD patients.

Congenital muscular dystrophy with abnormal radiographic myelin pattern

We report 11 children with a homogeneous clinical syndrome affecting both sexes, characterized by weakness at birth, slowly improving course, weakness of all muscle groups, arreflexia, elevated blood creatine kinase, normal nerve conduction velocity, dystrophic changes on muscle biopsy, and diffuse periventricular cortical white-matter abnormalities with sparing of corpus callosum, internal capsule, and brain stem. We compare them to 48 other previously reported similar cases and designate them as altered myelin radiographic pattern congenital muscular dystrophy (CMD), which is the same as occidental CMD. We compare them to the other presently accepted phenotypes: progressive Fukuyama CMD, Walker-Warburg or cerebral-ocular CMD, and Santavuori or muscle-eye-brain CMD. We suggest that the different phenotypes are alleles of the same gene, which regulates or expresses a structural protein required for muscle integrity, myelination, and formation of the cortex. Such phenotypic diversity has been established for mutations of Xp21 in X-linked muscular dystrophies.

Congenital muscular dystrophy presenting with respiratory failure

Respiratory failure is an unusual initial manifestation of congenital muscular dystrophy. The authors describe a case of congenital muscular dystrophy in a patient presenting with rhabdomyolysis at birth. Despite an initially poor prognosis, aggressive respiratory therapy during the neonatal period permitted normal subsequent development. The muscular dystrophy predominantly involved the respiratory muscles.

Amplification of ten deletion-rich exons of the dystrophin gene by polymerase chain reaction shows deletions in 36 of 90 Japanese families with Duchenne muscular dystrophy

We analyzed DNA samples taken from 95 Duchenne muscular dystrophy (DMD) patients belonging to 90 different families in Japan using the polymerase chain reaction. Ten different regions at the 5' end or in the central region of the dystrophin cDNA gene that were previously shown to be prone to deletion were selected for amplification and analysis. Patients in 36 of the 90 families (40%) had deletions in at least one of these segments of the gene. Identical deletions were detected in the dystrophin gene of patients from the same family. The deletions were heterogeneous in size and location. One patient had deletions in 7 of the 10 amplified regions, while 19 patients from 18 families had a deletion in only one of the regions studied. Deletions at the 5' end were generally larger and more heterogeneous than those in the central region of the gene. One third of deletions had their proximal end breakpoints between exons 44 and 45. This region seems to be particularly vulnerable to gene breakage in DMD patients.

Somatic mosaicism for a deletion of the dystrophin gene in a carrier of Becker muscular dystrophy

Duchenne muscular dystrophy (DMD) and the allelic milder form of Becker muscular dystrophy (BMD) are caused by mutations of the dystrophin gene on the short arm of the X chromosome. One third of affected individuals are expected to result from de novo mutations. Genetic counselling of families with sporadic cases is complicated by the potential meiotic origin of the mutation in the mother resulting in germline mosaicism. Here we present direct evidence for combined somatic and germline mosaicism for a deletion of the dystrophin gene, thereby proving the mitotic origin of this deletion and pinpointing a further potential pitfall for genetic counselling. The mother of a BMD son and a BMD carrier daughter, both carrying a deletion of dystrophin cDNA 7 (0.5 kb Hind III fragment) and cDNA 8, was herself clinically healthy and had normal creatine kinase levels. A muscle specimen of the mother showed mild overall pathology as well as focal dystrophin deficiency. In contrast chromosomal in situ suppression (CISS) hybridization of metaphase chromosomes using a cosmid clone of the corresponding cDNA deleted in her son revealed no evidence of somatic mosaicism in their lymphocytes. These results emphasize the value of an approach correlating genetic and immunological data for the definition of a carrier state in BMD or DMD. The possibility of somatic mosaicism should be considered when genetic counselling of a family with a sporadic case of BMD or DMD is performed.

Possible influences on the expression of X chromosome-linked dystrophin abnormalities by heterozygosity for autosomal recessive Fukuyama congenital muscular dystrophy

Abnormalities of dystrophin, a cytoskeletal protein of muscle and nerve, are generally considered specific for Duchenne and Becker muscular dystrophy. However, several patients have recently been identified with dystrophin deficiency who, before dystrophin testing, were considered to have Fukuyama congenital muscular dystrophy (FCMD) on the basis of clinical findings. Epidemiologic data suggest that only 1/3500 males with autosomal recessive FCMD should have abnormal dystrophin. To explain the observation of 3/23 FCMD males with abnormal dystrophin, we propose that dystrophin and the FCMD gene product interact and that the earlier onset and greater severity of these patients' phenotype (relative to Duchenne muscular dystrophy) are due to their being heterozygous for the FCMD mutation in addition to being hemizygous for Duchenne muscular dystrophy, a genotype that is predicted to occur in 1/175,000 Japanese males. This model may help explain the genetic basis for some of the clinical and pathological variability seen among patients with FCMD, and it has potential implications for understanding the inheritance of other autosomal recessive disorders in general. For example, sex ratios for rare autosomal recessive disorders caused by mutations in proteins that interact with X chromosome-linked gene products may display predictable deviation from 1:1.

The fate of dystrophin during the degeneration and regeneration of the soleus muscle of the rat

Immunocytochemistry and Western blotting were used to monitor the fate of dystrophin in the soleus muscle of the rat during a cycle of degeneration and regeneration induced by inoculation of the muscle with the venom of Notechis scutatus scutatus (the Australian tiger snake). In control muscle dystrophin was localised close to the plasma membrane. Dystrophin began to break down 3-6 h after venom inoculation, giving a characteristic discontinuous labelling pattern. At 12 h dystrophin was absent from the plasma membrane, and by 1 day the architecture of the muscle fibers had completely broken down. By 2 days post inoculation regeneration had commenced. The regenerating myofibres possessed well-organised myofibrils and the plasma membrane was intact. Dystrophin was detected by Western blot at 3 days, but was not seen in sections until regeneration of the muscle was well advanced, at 4 days post inoculation. The results suggested that although dystrophin was present in the myofibres at 3 days, it was not incorporated into the plasma membrane until 4 days post inoculation. This may be due to the influence of the functional reinnervation of the regenerating fibres, which occurs at 4-5 days, or to the growing fibres reaching a critical diameter.

Dystrophin at the plasma membrane of human muscle fibers shows a costameric localization

We studied the distribution of dystrophin at the sarcolemma of normal human muscle fibers using high resolution immunofluorescence and confocal laser scanning optical microscopy (CLSOM). We found that the dystrophin lattice is organized at the muscle plasma membrane in an array of thick bands interconnected by a finer network. The bands encircle the muscle fiber perpendicular to the long axis of the fiber and they matched the sites of attachment of the sarcomeres to the plasma membrane. Dystrophin co-localized with vinculin, and dystrophin and vinculin co-localized with alpha-actinin at the region of the I-band. Dystrophin may be one of the proteins involved in the linkage of the sarcomeres to the extracellular matrix.

Mosaic expression of dystrophin in the cerebellum of heterozygote dystrophic (mdx) mice

The monoclonal NCLDys1 revealed the presence of dystrophin in the Purkinje cells of normal mice but not of mdx mice and a mosaic staining in Purkinje cells of heterozygote mdx mice. Dystrophin was present in the soma and the dendrites of the dystrophin positive Purkinje cells and was absent in both regions of the dystrophin negative Purkinje cells. However, the polyclonal antibody d10 produced a uniform labeling of all Purkinje cells not only in the normal mice but also in mdx and heterozygote mdx mice. This staining was attributed to a reaction of this antibody not only with dystrophin but also with a different isoform of dystrophin or with a dystrophin related protein present even in mdx mice.

Mechanical properties of normal and mdx mouse sarcolemma: bearing on function of dystrophin

The tensile strength of the muscle fibre surface membrane was estimated (1) from the suction required to burst membrane patches and (2) by aspiration of sarcolemmal vesicles into micropipettes of uniform bore. Each method gave an average value close to 60 microN cm-1 for the maximum tension sustainable by normal mouse sarcolemma and only slightly lower values for sarcolemma from mdx mice which lack dystrophin. The elastic modulus of area expansion, as measurable by pipette aspiration of sarcolemmal vesicles, was found to have an average value of 3160 microN cm-1 for normal and 2770 microN cm-1 for mdx mouse sarcolemma. The tensile strength of the sarcolemma is much too small for any differences in it to be the basis for the different osmotic behaviour of normal and mdx muscle fibres reported recently (Menke & Jockusch, 1991). By analogy with the better understood origin of the osmotic fragility of different types of red blood cells, the higher osmotic fragility of mdx muscle fibres is suggested to be of morphological origin. We postulate that dystrophin functions as an element of the submembrane cytoskeleton so as to maintain the normal folding which safeguards the sarcolemma against mechanical damage.

Developmental study of the expression of dystrophin in cultured human muscle aneurally and innervated with fetal rat spinal cord

So far there have been no developmental studies including the influences of innervation and contractile activity on the expression of dystrophin in cultured human muscle. We performed immunocytochemical studies of the localization of dystrophin on aneurally cultured non-contracting (AMs) and innervated continuously contracting cross-striated human muscle fibers (ICMs) with fetal rat spinal cord from normal and Duchenne muscular dystrophy (DMD) biopsied muscles. In normal AMs, myoblasts and some immature AMs showed negative staining of dystrophin, but many AMs had a patchy (discontinuous) distribution of dystrophin in the subplasmalemmal region and with some granularity near the sarcolemma and in the deeper cytoplasm. In normal ICMs, dystrophin was localized continuously at the inner aspect of the sarcolemmal membrane and some periodic dense patterns were detected in some areas. Both AMs and ICMs from DMD had negative staining of dystrophin. To investigate the muscle contractile activity on the distribution of dystrophin, we paralyzed ICMs with tetrodotoxin (TTX) for two weeks from the first appearance of muscle contractions. In paralyzed innervated muscles (PIMs), dystrophin remained in a patchy (discontinuous) pattern at the inner aspect of the plasmalemma similar to that in AMs. It is strongly suggested that muscle contractile activity plays an important role in the continuous and even distribution of dystrophin at the sarcolemma during development.

Muscle histology in Becker muscular dystrophy

Twenty patients with Becker muscular dystrophy (BMD), confirmed by dystrophin tests, were studied histologically. There were several morphological differences between younger (less than or equal to 15-year-old) and older (greater than 15-year-old) patients. In the younger patients, active muscle fiber necrosis followed by a regenerating process was conspicuous. In the older patients, the active degenerative changes appeared less prominent and, instead, more chronic myopathic changes such as moth-eaten fibers, fiber splitting, and hypertrophic fibers were evident. These age-dependent differences in the pathology of BMD were irrespective of the duration of clinical symptoms, i.e., BMD patients of a similar age showed a similar morphological feature regardless of age at onset. Although the presence of mild fiber type grouping and some small angulated atrophic fibers suggested a certain degree of neurogenic involvement, none of biopsies showed significant grouped atrophy as seen in neuropathic disorders. There was no correlation between the histological changes and the specific dystrophin abnormality.

Human dystrophin gene transfer: production and expression of a functional recombinant DNA-based gene

The identification and cloning of the gene responsible for Duchenne muscular dystrophy (DMD) and characterization of the protein product of the gene, dystrophin, has led to major advances in diagnostic and genetic counselling procedures for this inherited disorder. Due to its high mutation rate, however, individuals affected by DMD will continue to arise in large proportion by de novo mutations, and the search for direct therapies remains a high priority. In this respect direct genetic correction of dystrophin deficiency via grafting of healthy myoblast stem cells or direct introduction of functional DNA into diseased muscle tissue have both been proposed as potential therapeutic approaches. We describe here, the first example of the engineering and cloning of a synthetic gene encoding recombinant human dystrophin and its stable transfer to and expression in mammalian cells. This DMD gene construction represents a primary step towards evaluating direct DNA-mediated gene transfer as a potential treatment for this debilitating disorder.

Recent advances in dystrophin research

Evidence suggesting that dystrophin is a component of the membrane cytoskeleton of excitable cells continues to accumulate. Whereas the specific mechanisms leading to muscle pathology in Duchenne muscular dystrophy are still being debated it is apparent that the progressive weakness that occurs in this disease is the result of a chronic process that is initiated by dystrophin deficiency.

Is the carboxyl-terminus of dystrophin required for membrane association? A novel, severe case of Duchenne muscular dystrophy

Duchenne muscular dystrophy is a lethal X-linked recessive disorder caused by the deficiency of a component of the muscle fiber membrane cytoskeleton called dystrophin. Becker muscular dystrophy, a clinically milder disorder, results from dystrophin abnormalities rather than deficiency. We identified the first patient who is clearly an exception to these established clinical and biochemical correlates. The patient described clinically had particularly severe Duchenne dystrophy. Biochemically, his muscle contained substantial amounts of abnormal dystrophin (Becker-like). Characterization of the dystrophin protein and gene revealed a unique intragenic gene deletion resulting in a dystrophin protein missing the carboxyl-terminal domain. This patient's dystrophin seemed to have a deleterious "dominant" effect on his muscle: The presence of this abnormal protein was more damaging to the myofibers than the absence of dystrophin would have been. This patient challenges the current hypothesis that dystrophin associates with the plasma membrane solely via its carboxyl-terminus, yet supports the hypothesis that an intact carboxyl-terminus is crucial for correct dystrophin function.

Developmental regulation of mechanosensitive calcium channels in skeletal muscle from normal and mdx mice

Single-channel activity was recorded from cell-attached membrane patches on flexor digitorum brevis fibres acutely isolated from normal and mdx mice at different stages of postnatal development. Recordings from cell-attached patches on both normal and mdx fibres were dominated by the activity of mechanosensitive ion channels with a conductance of approximately 17 pS with 110 mM Ba2+ in the patch electrode. In a small fraction of the patches on mdx fibres from young mice, channels showed very high levels of activity. Channel activity recorded from mdx fibres from older mice was higher than in age-matched normal fibres and the level of activity decreased during development. Channel density decreased in normal fibres, whereas it remained relatively constant in mdx fibres, as if channels are down-regulated in normal, but not mdx, fibres during postnatal development. An early step in the dystrophic process may be an alteration of the mechanisms that regulate the expression of functional channels.

Negative halothane-caffeine contracture test in mdx (dystrophin-deficient) mice

The genetics of malignant hyperthermia (MH) are ill-understood; however, the association of Duchenne muscular dystrophy (DMD) with MH is well known. A deficiency of dystrophin is common to both the DMD and mdx mouse, an animal model for DMD. Using muscle contracture tests for MH, we have shown that in the mdx mouse there is no MH susceptibility, suggesting the lack of a direct role of the dystrophin in the development of MH syndrome.