The neural hypothesis of muscular dystrophy. A review of recent experimental evidence with particular reference to the Duchenne form

Animal models of muscular dystrophy--what can they teach us?

The discovery and characterization of the X-linked gene which is defective in Duchenne muscular dystrophy (DMD) and of its protein product, dystrophin, has led to the identification of biochemical homologues of this disease in the mouse, the dog and the cat. All three animal models resemble DMD in that they lack dystrophin and that their skeletal muscle fibres undergo spontaneous necrosis and regeneration. In the dog and man, the degenerative and fibrotic aspects predominate, leading to a progressive loss of muscle structure and function, and to severe clinical disability. By contrast, in the mouse and the cat there is little fibrosis and the regenerative process seems to overcompensate, producing a true muscle hypertrophy and little or no clinical deficit. This interspecies variation in pathological response limits the usefulness of these animals as models for therapeutic testing, calling into question the strength of linkage between a given biochemical lesion and a particular pattern of pathology. However, these differences do give a valuable perspective to the pathology of the dystrophin-deficiency diseases, permitting identification of the immediate and secondary consequences of the lack of dystrophin. Moreover, the dystrophic mouse and dog are readily bred as colonies, thus providing consistent material for investigating the function of dystrophin and for testing methods of replacing its function or compensating for the absence of this function in the muscles of DMD patients. The fact that a lack of dystrophin is compatible, in some species, with only minor muscle dysfunction, raises hopes for an effective therapy in man.

Invited review: motor unit estimation: methods, results, and present status

The renewed interest in motor unit estimation (counting) has coincided with the introduction of computer-based methodology and with the application of the technique to proximal as well as distal muscles. The advantages and disadvantages of the different methods are considered, together with the assumptions inherent in this type of examination. In normal subjects, the extensor digitorum brevis (EDB) muscle has approximately 200 motor units while each of the intrinsic muscles of the hand has about 100 units; larger muscles in the limbs contain greater numbers of units. Beyond the age of 60 years, there is a decline in the number of functioning motor units in both proximal and distal muscles. In denervating disorders, motor unit estimation is useful for diagnosis and assessment; abnormal values may often be observed in muscles judged clinically to be unaffected. Serial studies have enabled the rate of motor unit loss to be determined in ALS and in spinal muscular atrophy. Depletion of motor units has also been found following upper motoneuron lesions caused by injury to the spinal cord or by cerebral hemorrhage; trans-synaptic dysfunction has been presumed responsible. Rather surprisingly, reduced numbers of motor units have been observed in a variety of myopathic disorders; of these, the most consistent abnormalities have been reported in myotonic muscular dystrophy.

Expression of the putative Duchenne muscular dystrophy gene in differentiated myogenic cell cultures and in the brain

Duchenne muscular dystrophy (DMD), a sex-linked degenerative disorder of the muscle, is one of the most common lethal genetic diseases in man. It affects about one male in 3,500, with an estimated one-third of cases being caused by new mutations. A less severe disease, Becker's muscular dystrophy (BMD), maps to the same chromosomal locus and is most probably an allelic form of DMD. Both diseases are sometimes associated with various degrees of mental retardation; the molecular basis of these phenotypes is unknown (for review, see ref. 1). The giant DMD gene spans approximately 2,000 kilobases (kb) (0.05% of the human genome) and encodes a 14-kb mRNA. The tissue-specificity of its expression has not been precisely determined. Monaco et al., using Northern blots, reported expression of the gene in human fetal skeletal muscle and small intestine but not in human fetal brain, or in human cultured myoblasts and transformed B and T cells. More recently, expression was detected in mouse skeletal and cardiac muscle, but not in mouse brain. Here we show, using a ribonuclease protection assay, that the DMD gene is developmentally regulated in rat and mouse myogenic cell cultures, and that it is expressed in rat and mouse striated muscle, in mouse smooth muscle and in rat, mouse and rabbit brain. We could not detect transcripts in other non-muscle tissues.

The pathogenesis of Duchenne muscular dystrophy: significance of experimental observations

The pathogenesis of Duchenne muscular dystrophy (DMD) remains elusive, but as this is an inherited condition the primary manifestation of the disease is assumed to be in the regulatory control or biosynthesis of a protein. Current hypotheses attribute the pathological state of skeletal muscle in DMD to a defect in the nerve supply, or in the vasculature, or in the muscle itself. However, various tissues other than skeletal muscle are also affected; thus the current view of DMD requires reevaluation. The following possibilities should be considered: That the primary lesion is expressed in one of the major communication systems (nervous, vascular, or endocrine). That the primary lesion is expressed in a specific tissue: a) if in skeletal muscle, alterations in non-muscular tissues must be due to the release of muscle constituents into extracellular fluid; b) if in a non-muscular tissue, this might produce too much (or too little or none) of a constituent normally secreted into the extracellular fluid, or produce a "toxic" agent. That the primary lesion is expressed in a wide variety of tissues: the effect on a particular tissue will depend entirely upon the degree of requirement of the altered protein for function.

Duchenne muscular dystrophy: pathogenetic aspects and genetic prevention

Duchenne muscular dystrophy (DMD) is the most common sex linked lethal disease in man (one case in about 4000 male live births). The patients are wheelchair bound around the age of 8-10 years and usually die before the age of 20 years. The mutation rate, estimated by different methods and from different population studies, is in the order of 7 X 10(-5), which is higher than for any other X-linked genetic disease. Moreover, unlike other X linked diseases such as hemophilia A or Lesh-Nyhan's disease, there seems to be no sex difference for the mutation rates in DMD. Several observations of DMD in girls bearing X-autosomal translocations and linkage studies on two X chromosomal DNA restriction fragment length polymorphisms indicate that the DMD locus is situated on the short arm of the X chromosome, between Xp11 and Xp22. It may be of considerable length, and perhaps consisting of actively coding and non-active intervening DNA sequences. Thus unequal crossing over during meiosis in females could theoretically account for a considerable proportion of new mutations. However, there is no structurally or functionally abnormal protein known that might represent the primary gene product, nor has any pathogenetic mechanism leading to the observed biochemical and histological alterations been elucidated. Among the numerous pathogenetic concepts the hypothesis of a structural or/and functional defect of the muscular plasma membrane is still the most attractive. It would explain both the excess of muscular constituents found in serum of patients and carriers, such as creatine kinase (CK), as well as the excessive calcium uptake by dystrophic muscle fibres, which, prior to necrosis, could lead to hypercontraction, rupture of myofilaments in adjacent sarcomeres and by excessive Ca uptake to mitochondrial damage causing crucial energy loss. The results of studies on structural and functional membrane abnormalities in cells other than muscle tissue, e.g., erythrocytes, lymphocytes and cultured fibroblasts, indicate that the DMD mutation is probably demonstrable in these tissues. However, most of the findings are still difficult to reproduce or even controversial. DMD is an incurable disease; therefore most effort, in research as well as in practical medicine, is concentrated upon its prevention.(ABSTRACT TRUNCATED AT 400 WORDS)

Study of neurotrophism in normal/dystrophic parabiotic mice

The trophic influences of nerve and muscle on one another were studied in normal and dystrophic littermates of C57BL/6J dy2J mice parabiosed at 20 to 23 days after birth. Each parabiont had a soleus muscle cross-reinnervated by a tibial nerve of its partner. Ultrastructural abnormalities of muscle and endplate were quantified and compared 6 to 7 months postoperatively. The dystrophic nerve degenerated despite reinnervation to a normal muscle. The normal muscle did not prevent the dystrophic nerve from degenerating, and the dystrophic nerve induced degenerative changes in the reinnervated normal muscle. Normal nerve did not retard the genetically programmed degeneration of the dystrophic muscle. The dystrophic muscle, however, did not appear to cause normal nerve terminals to degenerate. We conclude that both nerve and muscle cells in dystrophic mice express characteristics of muscular dystrophy. Muscle fibers of a few motor units further suffer from abnormal neurotrophic influence because of the degeneration of the motor neurons. Myotrophic influence on nerve was not observed.

Pathology of skeletal muscle: principles of reaction patterns and histochemistry and experience with 195 biopsies

Technique and interpretation of muscle biopsies require the coordinated team work between referring clinician, surgeon, and pathologist so that optimal management of a patient with neuromuscular disease may be attained. Investigation of muscle diseases has advanced so much with an interdisciplinary approach that not only can accurate diagnosis be offered for the benefit of the patient in certain instances, but also genetic counselling provided and pre-natal diagnosis established. The principal reaction patterns and pathogenetic mechanisms of skeletal muscle as a contractile and metabolically active tissue are described; the diagnostic usefulness of enzyme histochemistry and the basic principles of the motor unit are discussed and illustrated. For the practicing pathologist, adequate tissue preparation of a muscle biopsy specimen requires interest and a willingness to dedicate time, effort and funds. While the paraffin-embedded section is still very valuable, enzyme histochemistry provides certain highly diagnostic information not otherwise obtainable. Likewise, there must be an interest in electron microscopy and appreciation for its value in depicting ultrastructural abnormalities when certain reaction patterns are apparent on light microscopic sections, enzyme histochemical stains, or Epon-embedded "thick" sections. Finally, concurrent sural nerve biopsy also requires optimal processing and interpretation.

Biochemistry of muscle membranes in Duchenne muscular dystrophy

In Duchenne muscular dystrophy, as in other genetic diseases, there must be a biochemical abnormality. This fundamental genetic fault has not been identified, but several indirect lines of evidence suggest that the surface membranes of skeletal muscle are affected. The biochemical evidence implies abnormal egress of soluble enzymes and other proteins from muscle, abnormal permeability, and altered properties of membrane-bound enzymes. As a result of the presumed genetic abnormality, functional properties are altered, and impaired regulation of intracellular calcium content could be responsible for the hallmarks of the disease--progressive weakness and degeneration of muscle. The evidence is by no means conclusive, however, and some of it is contradictory. Technical advances must be made before isolated membranes can be characterized biochemically. Other theories are also being evaluated.

Incidence, severity, and time-course of motoneurone dysfunction in myotonic dystrophy: their significance for an understanding of anticipation

The numbers of functioning motor units and the amplitudes of the maximum evoked muscle responses have been measured in 198 muscles of 102 patients with myotonic dystrophy. Losses of units could be demonstrated in most of the extensor digitorum brevis and thenar muscles but less commonly in the hypothenar groups. A more proximal limb muscle, the soleus, was also shown to be involved frequently. Investigation of two premature infants withmyotonic dystrophy also revealed reductions of functiong units; in one infant clinical improvement was associated with increased muscle innervation. Repeated examinations of 10 adult patients disclosed an abnormal decline in neuromuscular function below the age of 60 years. The reduction in functioning units amounted to approximately 3% of the mean control value per annum. Analysis of 19 families showed that the severity of neuromuscular involvement was nearly always greater in members of later generations. If the predicted deterioration was also taken into account, the results strongly suggested that anticipation was a true genetic phenomenon rather than an artefact of selection. The combined results are considered to strengthen the concept of motoneurone dysfunction as the major pathogenetic factor in this form of dystrophy.