A distinctive type of infantile inflammatory myopathy with abnormal myonuclei

Four infants developed progressive muscle weakness after a normal initial postnatal development. All patients had a moderate elevation of serum creatine kinase (CK) activity. Muscle biopsies revealed, in addition to myopathic features, endomysial and perivascular inflammation. Electron microscopy disclosed prominent myonuclear abnormalities. Corticosteroids in 3 patients were moderately beneficial. This appears to be a clinicopathologically distinct form of inflammatory myopathy of infants.

The principles of gene therapy for the nervous system

Research pertaining to gene transfer into cells of the nervous system is one of the fastest growing fields in neuroscience. An important application of gene transfer is gene therapy, which is based on introducing therapeutic genes into cells of the nervous system by ex vivo or in vivo techniques. With the eventual development of efficient and safe vectors, therapeutic genes, under the control of a suitable promoter, can be targeted to the appropriate neurons or glial cells. Gene therapy is not only applicable to the treatment of genetic diseases of the nervous system and the control of malignant neoplasia, but it also has therapeutic potential for acquired degenerative encephalopathies (Alzheimer's disease, Parkinson's disease), as well as for promoting neuronal survival and regeneration in various pathological states.

Dystrophin expression in muscles of mdx mice after adenovirus-mediated in vivo gene transfer

We have generated high-titer adenoviral recombinants (AVR) expressing a 6.3-kb partial dystrophin cDNA insert under the control of either the Rous sarcoma virus (RSV) or cytomegalovirus (CMV) promoter. These AVR preparations were free of both E1-containing AVR and AVR with a nonfunctional dystrophin expression cassette. With these optimal AVR preparations, we have obtained a high degree of short-term (10 days) expression of a truncated (approximately 200 kD) dystrophin in dystrophin-deficient mdx muscles injected in the neonatal period; a lesser degree of expression of dystrophin was found in muscles injected in the young adult age and in old animals. Microscopic indices of muscle damage revealed that the truncated dystrophin provided a significant protection of the transduced muscle fibers. However, by 60 days post-injection, a substantial reduction of the number of dystrophin-positive fibers was noted, even in the neonatally injected muscles, and near-total elimination of dystrophin-positive fibers occurred in muscles injected in the adult age. These effects appeared to be brought about by the activity of CD8+ cytotoxic lymphocytes directed against the transduced cells, leading to their eventual elimination. In severe combined immunodeficiency (SCID) mice, lacking both humoral and cellular immune competence, muscles transduced (either in the neonatal or adult age) by AVR containing a CMV-LacZ expression cassette maintained the early (10 day) transduction level up to 30 days post-injection. Systemic administration of AVR (i.e., into the left ventricle of the heart) led in 5 days to a high number of dystrophin-positive fibers in heart, diaphragm, and intercostal muscles but not in limb muscles.

Characterization of cDNA and genomic DNA encoding SERCA1, the Ca(2+)-ATPase of human fast-twitch skeletal muscle sarcoplasmic reticulum, and its elimination as a candidate gene for Brody disease

Genomic DNA and cDNA encoding human SERCA1, the Ca(2+)-ATPase of fast-twitch skeletal muscle sarcoplasmic reticulum (the ATP2A1 gene on chromosome 16p12), were isolated and characterized. The cDNA encodes 994 amino acids. The genomic DNA is 26 kb long and contains 23 exons, one of which can be alternatively spliced. The locations of each of the exon/intron boundaries are the same as those previously identified in the rabbit ATP2A1 gene. Brody disease is an inherited disorder of skeletal muscle, characterized by exercise-induced impairment of muscle relaxation. It has been postulated to result from a deficiency in SERCA1. In a search for the genetic basis of Brody disease, the coding sequence of the ATP2A1 gene in one Brody patient and the full-length sequences of two SERCA1 cDNAs in two other, unrelated Brody patients were compared with normal ATP2A1 sequences. In all three cases, the coding and splice junction sequences were normal, indicating that the forms of Brody disease manifested in these three patients are not caused by mutations in the coding or splice junction regions of the ATP2A1 gene.

Efficiency and functional consequences of adenovirus-mediated in vivo gene transfer to normal and dystrophic (mdx) mouse diaphragm

The protein dystrophin is absent in muscles of patients with Duchenne muscular dystrophy (DMD) as well as in mdx mice. The mdx mouse diaphragm closely resembles the human DMD phenotype and should serve as an appropriate model for future studies of dystrophin gene replacement. In this regard, recombinant adenovirus (AV) holds great promise as a vector for delivering a functional dystrophin gene to muscle. However, the use of AV is hampered by the development of an immune response against transduced cells, resulting in short-lived transgene expression as well as possible adverse effects on organ function. In the present study, sensitive reporter genes were employed to determine the efficiency and functional consequences of AV-mediated gene transfer to the diaphragm in both normal and mdx adult mice. One week after direct intramuscular injection of AV into the diaphragm, the level of transgene expression was significantly increased in mdx compared with normal diaphragms. In addition, small-caliber fibers (< 500 microns2) demonstrated preferential transduction in both groups of mice. Normal diaphragms receiving AV exhibited a substantial reduction in maximal twitch and tetanic force generation, whereas no significant effect on diaphragm contractility was found in the mdx group at 1 wk after injection. At 1 mo after AV administration, however, there was a significant decrease in force production by both normal and mdx diaphragms. Immunosuppression with cyclosporine A over 1 mo did not augment the level of transgene expression, but a beneficial effect on diaphragm force-generating capacity was observed in both groups of animals. We conclude the following: (1) short-term transduction of the diaphragm is more efficient in mdx than in normal mice; (2) AV leads to reduced force production by the diaphragm, with this effect being more pronounced in normal than in mdx in the early (but not the late) postinjection period; and (3) immunosuppressive therapy with cyclosporine has a partially protective effect on muscle function after AV administration, which is apparently unrelated to sparing of transduced fibers from elimination by the host immune system. These findings have important implications for the application of AV-mediated dystrophin gene transfer to the treatment of DMD.

Immunosuppression by FK506 markedly prolongs expression of adenovirus-delivered transgene in skeletal muscles of adult dystrophic [mdx] mice

Adenovirus-mediated gene transfer into skeletal muscles of adult immune competent animals has been limited by the fact that a cellular immune attack of the host against transduced muscle fibers prevented long-term transgene expression. In this study we treated adult dystrophic [mdx] mice with daily subcutaneous injections of the immunosuppressive drug FK506 (tacrolimus) over 10 and 30 days after adenovirus-mediated reporter gene transfer and compared the transduction level to saline-injected controls. After 30 days, transgene expression was no longer demonstrable in the control group, whereas it remained at about 70% of the 10-day transduction value in the FK506 treated group. In addition, we demonstrated a reduction in the number of CD3 and CD8 positive T-lymphocytes in the muscles of the immunosuppressed group compared to controls.

Short-term dichloroacetate treatment improves indices of cerebral metabolism in patients with mitochondrial disorders

We performed a short-term, double-blind, placebo-controlled, crossover trial of sodium dichloroacetate (DCA) therapy in 11 patients affected by various primary mitochondrial disorders. Independent measures of oxidative metabolism (venous blood metabolites, exercise testing, phosphorus magnetic resonance [MR] spectroscopy of muscle, and proton MR spectroscopy of brain) were used in order to monitor metabolic responses to the drug. One week of DCA treatment produced significant decreases (p < 0.05) in blood lactate, pyruvate, and alanine at rest and after bicycle exercise. Proton MR spectra collected from a supraventricular volume of interest in brain of seven of 11 patients also showed significant changes. Brain lactate/creatine ratio decreased by 42% during DCA treatment (p < 0.05). Brain choline/creatine ratio (which is low in patients with myelinopathies) increased by 18% (p < 0.01) after therapy. N-Acetylaspartate/creatine ratio (an index of neuronal damage or loss) increased by 8% after treatment (p < 0.05). Proton MR spectra collected in two of 11 patients from a volume of interest including the basal ganglia showed similar results (decrease of 36.6% in lactate/creatine; increases of 16% in choline/creatine and 4.5% in N-acetylaspartate/creatine). Phosphorus MR spectroscopy of muscle and self-assessed clinical disability were unchanged. Our study indicates that short-term DCA treatment not only lowers blood lactate but also improves indices of both brain oxidative metabolism and neuronal and glial density or function.

Differential short-term transduction efficiency of adult versus newborn mouse tissues by adenoviral recombinants

We demonstrated different transduction efficiency in several major organs of the immature (newborn) versus mature (adult) mice using adenoviral recombinants containing expression cassettes for either firefly luciferase or bacterial beta-galactosidase reporter genes. The studied tissues included skeletal muscle, heart, brain, lung, kidney, and liver. The transduction efficiency in all tissues, especially skeletal muscle, was significantly less in adults than in newborns, with two exceptions. In the heart, transduction efficiency was the same in newborns and adults, while in brain, it was greater in the adult than in the newborn. The cited differences in transduction efficiencies between newborn and adult tissues applied approximately equally to both reporter genes. The alpha v integrin level showed the same trend as the transduction efficiency in all tissues, except the heart. Polymerase chain reaction showed a specific adenoviral product in proportion to the reporter gene expression in muscle, heart, and brain. The results of this study should be considered in designing gene therapy strategies in genetic diseases.

The route of administration is a major determinant of the transduction efficiency of rat tissues by adenoviral recombinants

One of the key factors that determines the efficacy of adenovirus-mediated gene therapy in genetic diseases, is the degree and extent of transduction of the target cells by adenovirus (AV)-recombinants carrying the therapeutic gene or cDNA. In this paper we provide experimental evidence which indicates that the route of administration of the AV-recombinants has a major influence on the transduction of various tissues in young rats. The heart, diaphragm, intercostal muscles and thymus show high transduction after intra-arterial (left cardiac ventricle) injection. By contrast, the liver shows a high transduction after intravenous injection. A substantial viremia develops within 2 h of gastric-rectal, intraperitoneal and intracardiac administration of AV recombinants. The number of adenoviral DNA copies per nucleus of transduced cells ranged from one to three in most tissues. These numbers correlated well with the overall transduction efficiency of the tissue determined by reporter gene expression. The various factors that determine which route of administration favors a high transduction rate in a particular tissue can be analyzed and this can lead to an improved efficiency of gene therapy in targeting a particular tissue in a disease.

Familial myopathy with conspicuous depletion of mitochondria in muscle fibers: a morphologically distinct disease

Three patients (two of them siblings) presented with easy fatiguability and prominent postexercise pain. Muscle biopsy showed that large areas of about one third of the type II fibers were completely devoid of mitochondria. The remaining mitochondria were unusually large in size, but otherwise normal ultrastructurally. In two patients, 31P in vivo MRS showed low phosphocreatine (PCr), high ADP, low phosphorylation potential at rest and slow ADP and PCr recovery after aerobic exercise. This appears to be a pathologically unique form of metabolic myopathy. The cause of the focal mitochondrial depletion is not known. It should be distinguished from the mtDNA depletion syndrome in which muscle mitochondria are not reduced, but proliferate.

The effect of glucocorticoids on the accumulation of utrophin by cultured normal and dystrophic human skeletal muscle satellite cells

Human muscle cultures undergo a long-term loss of myotubes and a decline in dystrophin content, which can be prevented by glucocorticoid treatment of the cultures. We confirmed these findings and extended them to show that the utrophin content of control and dexamethasone-treated normal myotube cultures is not significantly different. In contrast to normal cultures, the utrophin content of long-term dexamethasone-treated DMD myotube cultures was significantly greater than that of the corresponding untreated cultures. Utrophin mRNA transcript levels normalized to total poly (A) were unaffected by dexamethasone treatment of either normal or DMD myotube cultures, suggesting the effect of dexamethasone on utrophin accumulation by DMD cultures is mediated post-transcriptionally. A combination of an increase in myotube numbers and lack of competition with dystrophin for membrane-binding sites in DMD myotubes may explain the distinct effects of dexamethasone on utrophin levels in normal and DMD cultures.

Late-onset mitochondrial myopathy

In the majority of patients with mitochondrial encephalomyopathies, signs and symptoms appear in the first three decades of life. Here we report on a group of 9 older patients (> 69 years old) with late-onset skeletal myopathy characterized by focal accumulations of deleted mitochondrial DNAs (mtDNAs) and altered muscle energy status, suggestive of a primary mitochondrial disease. The clinical phenotype was somewhat variable. However, all patients shared a common feature of insidious moderate proximal muscle weakness; some also showed fatigability and axial muscle weakness. In situ hybridization analysis demonstrated accumulations of messenger RNAs transcribed from deleted mtDNAs in a relatively large number of muscle fibers in the patient group. These fiber segments appeared as ragged red with the modified Gomori trichrome stain and hyperreactive with a modified succinate dehydrogenase stain. Most were negative for cytochrome c oxidase activity. On transverse sections their mean frequency was 0.69% (trichrome) and 1.97% (succinate dehydrogenase) significantly above control levels. Multiple mtDNA deletions were demonstrated by the polymerase chain reaction in both the patients and an age-matched control group, but not in younger control subjects. Phosphorus 13 magnetic resonance spectroscopy of resting muscle showed a decreased phosphocreatine-inorganic phosphate ratio in the patient group. The myopathy in this group of patients appears to result from mitochondrial dysfunction related to the clonal expansion of different mtDNA deletions in individual fiber segments. While the origin of the mtDNA mutations is not clear, the phenotype seems to represent an exaggerated form of what is observed in the normal aging process.

Emergence of early region 1-containing replication-competent adenovirus in stocks of replication-defective adenovirus recombinants (delta E1 + delta E3) during multiple passages in 293 cells

Early region 1 (E1)-deleted human adenovirus (AV) recombinants have been shown to be powerful tools of gene transfer in vivo and in vitro and are considered for application in human gene therapy. We could detect increasing titers of E1-containing adenovirus in two independent E1 + E3-deleted recombinant AV stocks during multiple passages in 293 cells, most likely due to a recombinant event with the host cell genome. We show the deleterious effects of this E1-containing, mostly replication-competent AV subpopulation in vivo and compare different screening methods of AV stocks for its detection. These considerations are important for the safety of human gene therapy trials.

The principles of gene therapy in Duchenne muscular dystrophy

Somatic cell gene therapy (GT) for genetic disease such as Duchenne muscular dystrophy entails the introduction of normal, or at least functionally adequate, alleles of a gene into target cells for correction or mitigation of deleterious consequences of the disease's characteristic mutation. The following factors have a major impact upon the efficiency of GT: the artificial gene construct, the promoter, the delivery system, and the mode of dissemination of the therapeutic genes. For skeletal muscles, replication-defective adenovirus represents an efficient delivery system, but only if immature muscle cells are abundant in the muscle. The major drawback of adenoviruses is that the maximal insert capacity is only about 7.5 kb, which is only sufficient to accommodate a dystrophin minigene (6.3 kb) with a constitutive promoter. These and many other problems still require solution in experimental animals before single-muscle pilot studies of GT can be undertaken for such human muscle diseases as Duchenne muscular dystrophy.

Cultured human myoblasts and myotubes show markedly different transducibility by replication-defective adenovirus recombinants

Human adenovirus (AV) is a favored vector for delivery of therapeutic genes into certain target cells, such as skeletal muscle cells for gene therapy. Here we show that replication-defective (E1 + E3 deleted) human type 5 adenovirus (AV) recombinants containing a reporter gene insert (RSV-luciferase or RSV-Lux) can very efficiently transduce cultured human myoblasts. However, transduction efficiency is about one order of magnitude less in cultured myotubes 16 days postfusion. The high transduction of myoblasts by AV-RSV-Lux could be effectively blocked by an arginine-glycine-asparagine (RGD) oligopeptide that serves as a ligand for the natural internalization receptor of AV. The normalized level of beta 3/beta 5-integrin, the main component of the internalization receptor for AV is about three times as abundant in myoblasts than in myotubes. This could contribute, among other things, to the relatively high susceptibility of myoblasts to AV infection and AV-mediated gene transduction.

Gene transfer into skeletal muscles by isogenic myoblasts

The best way to overcome immunorejection in heterologous myoblast transfer (HMT) is by the use of immunodeficient and/or highly immunosuppressed mice as hosts. The same may be attained by autologous myoblast transfer (AMT). In this paper, we describe myoblast transfer in mdx and normal mice where the donor myogenic cells originated from highly inbred litter mates that are considered to be isogenic and thus the procedure is analogous to AMT. The myoblasts were marked in vitro with Rous Sarcoma Virus (RSV)-luciferase (Lux) or RSV-beta-galactosidase (LacZ) reporter genes through transduction mediated by an autonomously replication-defective recombinant human adenovirus. This permitted us to follow their fate after transplantation. mdx and normal mice were irradiated with 20 Gray gamma rays; necrosis and regeneration were induced by intramuscular notexin prior to myoblast injection. In both mdx and normal mice, the expression of luciferase rapidly declined after the injection implying that a large portion of the injected myoblasts were lost by 48 hr, due to undetermined cause(s). The surviving, injected myoblasts well-mosaicized large groups of host fibers but only in the immediate vicinity of the injection. Substantial expression of the reporter gene continued up to 1 month post-transplantation in normal mice, but there was a gradual decline and eventual disappearance of the reporter gene expression in mdx mice. This latter phenomenon was due to the ongoing intense necrosis of muscle fibers in mdx. There was no evidence of immunorejection. These experiments indicate that even in the absence of immunorejection, myoblast transfer suffers from important negative features: major loss of myoblasts within 48 hr after the injection and lack of significant spread of the injected cells from the injection site in the host muscle. These factors, plus the limited proliferative and fusion capacity of Duchenne muscular dystrophy (DMD) myoblasts, make them less than an ideal vector for the dystrophin cDNA for dystrophin gene replacement therapy in DMD.

Conspicuous accumulation of a single-stranded DNA binding protein in skeletal muscle fibers in inclusion body myositis

In muscle biopsies from patients with inclusion body myositis (IBM), multiple sites were found in many muscle fibers that bound single-stranded but not double-stranded DNA without sequence specificity, as exemplified by several different cDNA probes. This activity was attributable to a protein, because it was abolished by proteases but not by RNAse. Most of the sites of binding were myonuclei, whereas some were rimmed vacuoles, which probably result from nuclear breakdown. No comparable binding was seen in 27 control biopsies. A number of human and viral single-stranded DNA binding proteins exist but our data does not identify the protein responsible for DNA binding in IBM. Our findings reinforce the supposition that nuclear damage plays a basic role in the pathogenesis of IBM.

Expression of NCAM and its polysialylated isoforms during mdx mouse muscle regeneration and in vitro myogenesis

In order to understand the mechanism of the muscular regenerative process which occurs in mdx mice, the expression of neural cell adhesion molecule (NCAM) isoforms and their polysialylated (PSA) derivatives were studied during the postnatal development of normal and mdx mice in relation to the stage of the regeneration of muscle fibres, in the quadriceps. NCAM expression was also examined during in vitro differentiation of satellite cells isolated from both mdx and normal muscles. The immunohistochemical and biochemical analyses were done using antibodies for the different isoforms. The data presented here suggest that before the onset of necrosis and regeneration, the expression of NCAM isoforms in the quadriceps of mdx mice was similar to normal mice. Later, NCAM and PSA-NCAM expression in mdx mice increased and was related to the muscular regenerative process, and the overall level of NCAM expression can be considered as a good index of muscle regeneration. Young regenerative fibres expressed NCAM and PSA-NCAM, while mature regenerative fibres, in which myonuclei remained centrally located, did not express either NCAM or the PSA isoforms. Therefore, in terms of NCAM expression, the fibres in mdx muscle with centrally located nuclei appeared similar to mature fibres found in normal adult muscle. A major form of 145 kDa and a minor form of 115 kDa were detected in mdx regenerative muscle. The 145 kDa NCAM was sialylated, as demonstrated by its sensibility to exoneuraminidase which generates a desialoform of 125 kDa, but not polysialylated since it was not recognized by the anti-MenB antibody, specific for PSA-NCAM. In contrast, the molecular forms of NCAM migrating as a broad band from 160 kDa to 220 kDa were identified as PSA-NCAM. The comparison of in vitro differentiation of normal and mdx satellite cells showed that the expression of NCAM isoforms by mdx cells was similar to that expressed by normal cells. Both our in vivo and in vitro data concerning NCAM expression show that regeneration in mdx mice does not differ from that observed in other necrotic diseases. In other words, NCAM is unlikely to be a dystrophin-associated molecule since lack of dystrophin does not affect its expression.

A differential efficiency of adenovirus-mediated in vivo gene transfer into skeletal muscle cells of different maturity

High titre (10(11)-10(12) pfu/ml) suspensions of autonomously replication-defective type 5 human adenovirus (AV) recombinants with different reporter gene inserts (CMV-Luciferase (Lux), CMV-beta-galactosidase (Lac Z), RSV-Lux and RSV-Lac Z) were injected into intact quadriceps muscles of 1-5 day old (Group 1) or 35-45 day old (Group 2) normal mice, as well as regenerating adult mouse muscles (Group 3) and 35 day old mdx muscles (Group 4). The expression of the reporter genes was quantitated 10 days and 2 months later. At 10 days postinjection all reporter gene expression was very high in the neonatally injected (Group 1) muscles. In Group 2 muscles the transduction was markedly less. In Group 3 muscles the gene expression was significantly better than in the Group 2 muscles. In adult mdx muscles (Group 4) where spontaneous regeneration is usually present, the results were similar to those in Group 3 animals. At 2 months post-injection in Group 1 animals, the RSV-Lux expression was even higher than at 10 days postinjection. The cell surface density of alpha v-integrin-containing molecules including the internalization receptor for AV in Groups 1, 2, 3 and 4 showed a positive correlation with AV transducibility. We conclude that adenovirus vector in high titre (10(10) pfu/ml or above) is capable of efficiently transducing only immature muscle cells but not mature muscle fibers in vivo and this appears to correlate with a higher surface density of the available AV internalization receptor in immature muscle cells and lower level in mature muscle fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton MR spectroscopic characterization of differences in regional brain metabolic abnormalities in mitochondrial encephalomyopathies

Localized brain proton MR spectra were acquired from patients with different mitochondrial encephalomyopathies (myoclonus epilepsy with ragged-red fibers [MERRF], Kearns-Sayre syndrome [KSS], and mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes [MELAS]). The regional brain metabolic abnormalities in patients with these syndromes showed different features consistent with the distinct phenotypes. In MERRF, only one of four patients showed an increase in the lactate/creatine resonance intensity ratio (an index of impairment of oxidative metabolism) in spectra from central (supraventricular) or occipital brain volumes, and this was small. There were significant decreases in N-acetylaspartate/creatine (a measure of neuronal loss or dysfunction) in central cerebral volumes of demented patients and, more prominently, in occipital volumes. In the one patient in whom it was studied, the cerebellum also showed a decreased N-acetylaspartate/creatine. Spectra from two patients with KSS both showed large (four- to sevenfold) increases in lactate/creatine and large decreases in N-acetylaspartate/creatine in central brain volumes. Yet another pattern of regional metabolic abnormality was present in the MELAS syndrome, where proton spectroscopic imaging demonstrated focal localization of abnormally increased lactate/creatine and decreased N-acetylaspartate/creatine to the regions of the stroke-like lesions on conventional MR images. Serial studies emphasized that the regional metabolic abnormalities in MELAS are highly variable as the stroke-like lesions appear and evolve.

Pathology of the inflammatory myopathies

Careful examination of muscle biopsy specimens is indispensable in the diagnostic work-up of patients whose clinical picture is suggestive of or consistent with an inflammatory myopathy. Pathological diagnosis can, in the vast majority of cases, determine the specific type of inflammatory myopathy which is of paramount importance for prognosis and therapy. The pathological study of muscle has also been useful to reveal information pertaining to the pathogenesis of the different entities and ultimately it may also lead us to the precise aetiology.

The potential for gene therapy in Duchenne muscular dystrophy and other genetic muscle diseases

Dystrophin cDNAs have been introduced into skeletal muscle fibers of dystrophin-deficient mice (mdx) through direct DNA injection in plasmid expression vectors and by replication-defective recombinant adenovirus vectors. The introduced genes appear to protect those muscle fibers from necrosis in which they become expressed. By direct injection of dystrophin cDNA in plasmid expression vector, only 1-2% of adult mdx muscle fibers of the injected muscle expressed dystrophin. On the other hand, by recombinant adenovirus injection into very young mdx muscle, a better efficiency has been reported. We have discussed several putative and proven factors that may contribute to the thus far demonstrated relatively low efficiency of dystrophin gene transfer. These include poor uptake of gene constructs by muscle fibers, degradation of the injected DNA, and poor access of gene constructs to the nuclear compartment. Neutralization or elimination of these factors could improve the efficiency of gene transfer so that it might, in the future, qualify as an effective therapy for DMD and some other genetic diseases of muscle.