The genetic basis of neuromuscular disorders

A transient protective effect of low-level laser irradiation against disuse-induced atrophy of rats

Satellite cells, a population of skeletal muscular stem cells, are generally recognized as the main and, possibly, the sole source of postnatal muscle regeneration. Previous studies have revealed the potential of low-level laser (LLL) irradiation in promoting satellite cell proliferation, which, thereby, boosts the recovery of skeletal muscle from atrophy. The purpose of this study is to investigate the beneficial effect of LLL on disuse-induced atrophy. The optimal irradiation condition of LLL (808 nm) enhancing the proliferation of Pax7^+ve cells, isolated from tibialis anterior (TA) muscle, was examined and applied on TA muscle of disuse-induced atrophy model of the rats accordingly. Healthy rats were used as the control. On one hand, transiently, LLL was able to postpone the progression of atrophy for 1 week through a reduction of apoptosis in Pax7^-veMyoD^+ve (myocyte) population. Simultaneously, a significant enhancement was observed in Pax7^+veMyoD^+ve population; however, most of the increased cells underwent apoptosis since the second week, which suggested an impaired maturation of the population. On the other hand, in normal control rats with LLL irradiation, a significant increase in Pax7^+veMyoD^+ve cells and a significant decrease of apoptosis were observed. As a result, a strengthened muscle contraction was observed. Our data showed the capability of LLL in postponing the progression of disuse-induced atrophy for the first time. Furthermore, the result of normal rats with LLL irradiation showed the effectiveness of LLL to strengthen muscle contraction in healthy control.

Engraftment of mesenchymal stem cells into dystrophin-deficient mice is not accompanied by functional recovery

Mesenchymal stem cell preparations have been proposed for muscle regeneration in musculoskeletal disorders. Although MSCs have great in vitro expansion potential and possess the ability to differentiate into several mesenchymal lineages, myogenesis has proven to be much more difficult to induce. We have recently demonstrated that Pax3, the master regulator of the embryonic myogenic program, enables the in vitro differentiation of a murine mesenchymal stem cell line (MSCB9-Pax3) into myogenic progenitors. Here we show that injection of these cells into cardiotoxin-injured muscles of immunodeficient mice leads to the development of muscle tumors, resembling rhabdomyosarcomas. We then extended these studies to primary human mesenchymal stem cells (hMSCs) isolated from bone marrow. Upon genetic modification with a lentiviral vector encoding PAX3, hMSCs activated the myogenic program as demonstrated by expression of myogenic regulatory factors. Upon transplantation, the PAX3-modified MSCs did not generate rhabdomyosarcomas but rather, resulted in donor-derived myofibers. These were found at higher frequency in PAX3-transduced hMSCs than in mock-transduced MSCs. Nonetheless, neither engraftment of PAX3-modified or unmodified MSCs resulted in improved contractility. Thus these findings suggest that limitations remain to be overcome before MSC preparations result in effective treatment for muscular dystrophies.

Generation of transplantable, functional satellite-like cells from mouse embryonic stem cells

Satellite cells are myogenic stem cells responsible for the postnatal regeneration of skeletal muscle. Here we report the successful in vitro induction of Pax7-positive satellite-like cells from mouse embryonic stem (mES) cells. Embryoid bodies were generated from mES cells and cultured on Matrigel-coated dishes with Dulbecco's modified Eagle medium containing fetal bovine serum and horse serum. Pax7-positive satellite-like cells were enriched by fluorescence-activated cell sorting using a novel anti-satellite cell antibody, SM/C-2.6. SM/C-2.6-positive cells efficiently differentiate into skeletal muscle fibers both in vitro and in vivo. Furthermore, the cells demonstrate satellite cell characteristics such as extensive self-renewal capacity in subsequent muscle injury model, long-term engraftment up to 24 wk, and the ability to be secondarily transplanted with remarkably high engraftment efficiency compared to myoblast transplantation. This is the first report of transplantable, functional satellite-like cells derived from mES cells and will provide a foundation for new therapies for degenerative muscle disorders.

Stem cells from umbilical cord blood differentiate into myotubes and express dystrophin in vitro only after exposure to in vivo muscle environment

BACKGROUND INFORMATION

Duchenne muscular dystrophy is a disease characterized by progressive and irreversible muscle degeneration for which there is no therapy. HUCB (human umbilical cord blood) has been considered as an important source of haematopoietic and mesenchymal stem cells, each having been shown to differentiate into distinct cell types. However, it remains unclear if these cells are able to differentiate into muscle cells.

RESULTS

We have showed that stem cells from HUCB did not differentiate into myotubes or express dystrophin when cultured in muscle-conditioned medium or with human muscle cells. However, delivery of GFP (green fluorescent protein)-transduced mononucleated cells from HUCB, which comprises both haematopoietic and mesenchymal populations, into quadriceps muscle of mdx (mouse dystrophy X-chromosome linked) mice resulted in the expression of human myogenic markers. After recovery of these cells from mdx muscle and in vitro cultivation, they were able to fuse and form GFP-positive myotubes that expressed dystrophin.

CONCLUSIONS

These results indicate that chemical factors and cell-to-cell contact provided by in vitro conditions were not enough to trigger the differentiation of stem cells into muscle cells. Nevertheless, we showed that the HUCB-derived stem cells were capable of acquiring a muscle phenotype after exposure to an in vivo muscle environment, which was required to activate the differentiation programme.

Hematopoietic stem cell transplantation does not restore dystrophin expression in Duchenne muscular dystrophy dogs

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene on the X-chromosome that result in skeletal and cardiac muscle damage and premature death. Studies in mice, including the mdx mouse model of DMD, have demonstrated that circulating bone marrow–derived cells can participate in skeletal muscle regeneration, but the potential clinical utility of treating human DMD by allogeneic marrow transplantation from a healthy donor remains unknown. To assess whether allogeneic hematopoietic cell transplantation (HCT) provides clinically relevant levels of donor muscle cell contribution in dogs with canine X-linked muscular dystrophy (c-xmd), 7 xmd dogs were given hematopoietic cell (HC) transplants from nonaffected littermates. Compared with the pretransplantation baseline, the number of dystrophin-positive fibers and the amount of wild-type dystrophin RNA did not increase after HCT, with observation periods ranging from 28 to 417 days. Similar results were obtained when the recipient dogs were given granulocyte colony-stimulating factor (G-CSF) after their initial transplantation to mobilize the cells. Despite successful allogeneic HCT and a permissive environment for donor muscle engraftment, there was no detectable contribution of bone marrow–derived cells to either skeletal muscle or muscle precursor cells assayed by clonal analyses at a level of sensitivity that should detect as little as 0.1% donor contribution.

Myogenic stem cells from the bone marrow: a therapeutic alternative for muscular dystrophy?

Differentiated muscle fibres can be formed by transplanted haematopoietic stem cells in models of acute or chronic muscle regeneration, including the dystrophin-deficient mdx mouse. Muscle-forming activity can be found in adult, foetal and embryonic haematopoietic tissues. The blood-to-muscle transition may be due to transdifferentiation of haematopoietic progenitors in response to local signals provided by the regenerating muscle. These signals are only poorly provided by the muscle of the mdx mouse, since transplantation into these mice of normal C57Bl/6 bone marrow gives rise only to a minimal number of muscle fibres expressing the normal dystrophin protein (<1%) throughout the animal life span. Expansion and active recruitment to myogenic differentiation of transplanted haematopoietic cells are therefore critical factors for a future use of bone marrow transplantation in cell/gene therapy of muscular dystrophy.

The validity of button fastening as a test of hand disability in myotonic dystrophy

This study evaluated the validity of testing button fastening ability as a measurement of hand disability in patients with myotonic dystrophy. Forty subjects with myotonic dystrophy were tested on their ability to fasten four shirt buttons. The association between button fastening ability, strength of selected proximal upper limb muscles and lateral pinch grip was determined. A significant association (F(4,35) = 7.55, p < 0.01) was demonstrated between proximal upper limb strength and button fastening ability, but there was no relationship with lateral pinch grip strength. Myotonia was not a factor affecting button fastening performance. Comparison of these results with those of a group of 13 subjects with other neuromuscular diseases showed button fastening disability in myotonic dystrophy could not be attributed solely to proximal upper limb weakness but is a valid measure of hand disability. A standard for normal button fastening performance was also determined.

Evolutionary changes in mutation rates and spectra and their influence on the adaptation of pathogens

The evolutionary tuning of mutational processes may play a key role in prokaryotic evolution, particularly among pathogens. In this paper we review the evidence that genetic systems controlling the rate and spectrum of heritable mutations have evolved to optimize levels of adaptive variation and rates of genetic change.

Delineation of genomic deletion in cardiomyopathic hamster

Cardiomyopathic hamster is a representative animal model for autosomal recessive cardiomyopathy. We have previously shown that the transcript of delta-sarcoglycan is missing in the heart of cardiomyopathic hamster due to genomic deletion. Here we define the normal genomic region deleted in cardiomyopathic hamster, which spans about 30 kb interval and includes the two first exons of the delta-sarcoglycan gene. RNA blot analysis using genomic DNA fragments covering the entire deletion as probes failed to detect any transcript other than delta-sarcoglycan in normal hamster heart, suggesting that delta-sarcoglycan is the only transcript defective in the heart of cardiomyopathic hamster.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

The diagnostic yield of the nerve-muscle skin biopsy in paediatric neurology practice. The Montreal Children's Hospital Neuromuscular Group

OBJECTIVE

To determine the diagnostic yield of the nerve-muscle-skin (NMS) biopsy in paediatric neurology practice.

STUDY DESIGN

A consecutive series of 98 paediatric NMS biopsies done 1989-1994 retrospectively reviewed in the context of pre-biopsy clinical and laboratory parameters. Bivariate associations based on chi-square test. Unconfounded associations between pre-biopsy variables and positive diagnostic yield (PDY) assessed by multiple logistic regression.

RESULTS

Fifty seven out of 98 patients central (global delay, seizures, abnormal CNS imaging) process; 41/98 patients peripheral (motor delay, weakness) process, electromyography-nerve conduction studies (EMG-NCS) 87/98 cases; abnormal 43/87. Positive diagnostic yield (PDY) in 42/98 (43%) biopsies. Statistically significant bivariate associations between PDY and pre-biopsy; age, presenting symptom, developmental delay, weakness, reflexes, CPK, lactate, EMG-NCS and process. Unconfounded associations demonstrated with PDY and age, reflexes and process. The presence of a peripheral process or an abnormal EMG-NCS strongly predictive of PDY: 34/41 (83%) peripheral process cases had PDY, 32/40 (80%) abnormal EMG-NCS cases had PDY, and 29/31 (93.5%) peripheral process and abnormal EMG-NCS cases had PDY. Abnormal EMG-NCS with central process improved PDY to 3/9 (33%) from 4/37 (11%) for normal EMG-NCS.

CONCLUSION

NMS biopsy is a valuable diagnostic tool, particularly in the context of a suspected peripheral process or a central processes with an abnormal EMG-NCS.

Molecular basis of genetic heterogeneity: role of the clinical neurologist

Advances in molecular genetics have disclosed many different explanations for allelic heterogeneity, how different clinical syndromes arise from mutations in the same gene. The converse, how similar clinical syndromes arise from mutations of different genes on different chromosomes is called locus heterogeneity. Both, however, give rise to some disease-defining mutations, as in childhood spinal muscular atrophy or Duchenne muscular dystrophy. Nevertheless, new problems have been created, including what might be called "diagnosis by the number," diverse syndromes from mutations in the same gene without current explanation, or siblings with different clinical syndromes. These discoveries have transformed the clinical neurology of heritable diseases. They also provide clinicians with new responsibilities and opportunities in defining clinical syndromes and influencing the evolution of our clinical language.

Spatial relationship of the C-terminal domains of dystrophin and beta-dystroglycan in cardiac muscle support a direct molecular interaction at the plasma membrane interface

Dystrophin and beta-dystroglycan are components of a complex of at least nine proteins (the dystrophin-glycoprotein complex) that physically link the membrane cytoskeleton in skeletal and cardiac muscle, through the plasma membrane, to the extracellular matrix. Mutations in the dystrophin gene, which result in an absence or a quantitative or qualitative alteration of dystrophin, cause a subset of familial dilated cardiomyopathies as well as Duchenne and Becker muscular dystrophy. Biochemical studies on isolated skeletal muscle molecules indicate that dystrophin is bound to the glycoprotein complex via beta-dystroglycan, with the C-terminus of beta-dystroglycan binding to the cysteine-rich domain and first half of the C-terminal domain of dystrophin. Ultrastructural labeling has demonstrated a close spatial relationship between dystrophin and beta-dystroglycan in intact skeletal muscle, but no previous ultrastructural labeling studies have examined the dystrophin/beta-dystroglycan interaction in cardiac muscle. In the present study, we have applied complementary immunoconfocal microscopy and double immunogold fracture-label, a freeze-fracture cytochemical technique that allows high-resolution visualization of labeled membrane components in thin section and in platinum-carbon replicas, to investigate the spatial relationship between dystrophin and beta-dystroglycan in rat cardiac muscle. When immunogold probes of two different sizes for the two proteins were used, "doublets" representing side-by-side antibody labeling were demonstrated in en face views at the level of the plasma membrane. The results support the conclusions that dystrophin and beta-dystroglycan directly interact at the cytoplasmic face of the rat cardiac muscle plasma membrane.

Genetic analysis of cholinergic nerve terminal function in invertebrates

Genetic analysis of nerve terminal function is proving fruitful and studies on invertebrates are making a substantial impact. In this survey, particular emphasis has been placed on cholinergic chemical synaptic transmission. The advanced genetics of Drosophila melanogaster and Caenorhabditis elegans with their rich diversity of behavioural and biochemical mutants is providing new insights into the functions of key molecular components of synapses. A 'space-invader' mutant of Periplaneta americana permits investigations of competition between neurons during synaptogenesis and its impact on neurotransmitter release. The growing importance of the C. elegans genome as a major research resource is emphasized in this survey.