Abnormal expression of dystrophin-associated proteins in Fukuyama-type congenital muscular dystrophy

Molecular mechanisms underlying selective synapse formation of vertebrate retinal photoreceptor cells

In vertebrate central nervous systems (CNSs), highly diverse neurons are selectively connected via synapses, which are essential for building an intricate neural network. The vertebrate retina is part of the CNS and is comprised of a distinct laminar organization, which serves as a good model system to study developmental synapse formation mechanisms. In the retina outer plexiform layer, rods and cones, two types of photoreceptor cells, elaborate selective synaptic contacts with ON- and/or OFF-bipolar cell terminals as well as with horizontal cell terminals. In the mouse retina, three photoreceptor subtypes and at least 15 bipolar subtypes exist. Previous and recent studies have significantly progressed our understanding of how selective synapse formation, between specific subtypes of photoreceptor and bipolar cells, is designed at the molecular level. In the ON pathway, photoreceptor-derived secreted and transmembrane proteins directly interact in trans with the GRM6 (mGluR6) complex, which is localized to ON-bipolar cell dendritic terminals, leading to selective synapse formation. Here, we review our current understanding of the key factors and mechanisms underlying selective synapse formation of photoreceptor cells with bipolar and horizontal cells in the retina. In addition, we describe how defects/mutations of the molecules involved in photoreceptor synapse formation are associated with human retinal diseases and visual disorders.

Zebrafish Fukutin family proteins link the unfolded protein response with dystroglycanopathies

Allelic mutations in putative glycosyltransferase genes, fukutin and fukutin-related protein (fkrp), lead to a wide range of muscular dystrophies associated with hypoglycosylation of α-dystroglycan, commonly referred to as dystroglycanopathies. Defective glycosylation affecting dystroglycan–ligand interactions is considered to underlie the disease pathogenesis. We have modelled dystroglycanopathies in zebrafish using a novel loss-of-function dystroglycan allele and by inhibition of Fukutin family protein activities. We show that muscle pathology in embryos lacking Fukutin or FKRP is different from loss of dystroglycan. In addition to hypoglycosylated α-dystroglycan, knockdown of Fukutin or FKRP leads to a notochord defect and a perturbation of laminin expression before muscle degeneration. These are a consequence of endoplasmic reticulum stress and activation of the unfolded protein response (UPR), preceding loss of dystroglycan–ligand interactions. Together, our results suggest that Fukutin family proteins may play important roles in protein secretion and that the UPR may contribute to the phenotypic spectrum of some dystroglycanopathies in humans.

Changes in skeletal muscle expression of AQP1 and AQP4 in dystrophinopathy and dysferlinopathy patients

Transmembrane water transport is mediated by aquaporins (AQPs), of which AQP1 and AQP4 are expressed in skeletal muscle. AQP4 expression is reduced in Duchenne muscular dystrophy (DMD) patients, and is reported to correlate with decreased alpha1-syntrophin and altered osmotic permeability. In this study, we assessed the relationship between AQP1, AQP4, dystrophin and alpha1-syntrophin in dystrophinopathy and dysferlinopathy patients. Muscle biopsies of patients with DMD (n = 8) and limb-girdle muscular dystrophy type 2B (LGMD2B; n = 5) were screened for AQP1 and AQP4 expression by real-time quantitative RT-PCR or Western blot and immunohistochemistry. AQP expression was further analyzed in primary myotubes derived from DMD and LGMD2B patients by cell culture and immunohistochemistry. AQP1 transcript and protein expression was significantly elevated in DMD biopsies, and was localized to the sarcolemma of muscle fibers and endothelia of muscle capillaries. AQP4 was significantly reduced despite normal dystrophin and alpha1-syntrophin in dysferlinopathy patients, while expression of AQP1 was variably upregulated. Expression of AQP1 and AQP4 was normal in patient-derived primary myotubes, suggesting that altered AQPs observed in biopsies are likely secondary to the dystrophic process. Our study shows that AQP4 downregulation can occur in muscular dystrophies with either normal or disrupted expression of dystrophin-associated proteins, and that this might be associated with upregulation of AQP1.

Abnormalities in alpha-dystroglycan expression in MDC1C and LGMD2I muscular dystrophies

We recently identified mutations in the fukutin related protein (FKRP) gene in patients with congenital muscular dystrophy type 1C (MDC1C) and limb girdle muscular dystrophy type 2I (LGMD2I). The sarcolemma of these patients typically displays an immunocytochemical reduction of alpha-dystroglycan. In this report we extend these observations and report a clear correlation between the residual expression of alpha-dystroglycan and the phenotype. Three broad categories were identified. Patients at the severe end of the clinical spectrum (MDC1C) were compound heterozygote between a null allele and a missense mutation or carried two missense mutations and displayed a profound depletion of alpha-dystroglycan. Patients with LGMD with a Duchenne-like severity typically had a moderate reduction in alpha-dystroglycan and were compound heterozygotes between a common C826A (Leu276Ileu) FKRP mutation and either a missense or a nonsense mutation. Individuals with the milder form of LGMD2I were almost invariably homozygous for the Leu276Ile FKRP mutation and showed a variable but subtle alteration in alpha-dystroglycan immunolabeling. Our data therefore suggest a correlation between a reduction in alpha-dystroglycan, the mutation and the clinical phenotype in MDC1C and LGMD2I which supports the hypothesis that dystroglycan plays a central role in the pathogenesis of these disorders.

Congenital muscular dystrophies

The number of new syndromes, loci, and genes responsible for CMD forms has dramatically increased in the last few years, and it has become increasingly evident that the classification of the different forms of CMD is a difficult task. A recent classification separated the forms of CMD that have been mapped (CMD diseases) from the ones with clearly defined clinical and pathologic features that have not been mapped yet (CMD syndromes). Eight CMD forms have been mapped up to now, and the genes responsible for three of them have been identified. This review describes an update of clinical, pathologic, and genetic findings in the different CMD forms.

Molecular basis of muscular dystrophies

Muscular dystrophies represent a heterogeneous group of disorders, which have been largely classified by clinical phenotype. In the last 10 years, identification of novel skeletal muscle genes including extracellular matrix, sarcolemmal, cytoskeletal, cytosolic, and nuclear membrane proteins has changed the phenotype-based classification and shed new light on the molecular pathogenesis of these disorders. A large number of genes involved in muscular dystrophy encode components of the dystrophin-glycoprotein complex (DGC) which normally links the intracellular cytoskeleton to the extracellular matrix. Mutations in components of this complex are thought to lead to loss of sarcolemmal integrity and render muscle fibers more susceptible to damage. Recent evidence suggests the involvement of vascular smooth muscle DGC in skeletal and cardiac muscle pathology in some forms of sarcoglycan-deficient limb-girdle muscular dystrophy. Intriguingly, two other forms of limb-girdle muscular dystrophy are possibly caused by perturbation of sarcolemma repair mechanisms. The complete clarification of these various pathways will lead to further insights into the pathogenesis of this heterogeneous group of muscle disorders.

The Fukuyama congenital muscular dystrophy story

Fukuyama congenital muscular dystrophy is one of the most common autosomal recessive disorders in the Japanese population, characterized by congenital muscular dystrophy in combination with cortical dysgenesis (micropolygyria). Recently, we have identified the gene responsible for fukuyama congenital muscular dystrophy on 9q31, which encodes a novel 461-amino-acid protein termed fukutin. Most Fukuyama congenital muscular dystrophy-bearing chromosomes are derived from a single ancestral founder (87%), and a 3 kb-retrotransposal insertion into the 3' untranslated region of this gene was found to be a founder mutation. Two independent point mutations causing premature termination confirmed that that this gene is responsible for Fukuyama congenital muscular dystrophy. Fukuyama congenital muscular dystrophy is the first human disease to be caused by an ancient retrotransposal integration. Fukutin contains an amino-terminal signal sequence, which together with results from transfection experiments suggests that it is an extracellular protein. Discovery of the Fukuyama congenital muscular dystrophy gene represents an important step toward greater understanding of the pathogenesis of muscular dystrophies and also of normal brain development.

Fukuyama-type congenital muscular dystrophy: close relation between changes in the muscle basal lamina and plasma membrane

Despite the recent advance in genetic study of Fukuyama-type congenital muscular dystrophy (FCMD), the mechanism of muscle degeneration in the disease remains unclear. To clarify it, muscle biopsies from six cases of FCMD were subjected to immunohistochemical and ultrastructural studies. On the muscle cell surface, decreased expression of laminin alpha2 subunit was seen along with aberrant expression of laminin alpha5 and neural cell adhesion molecule. Electron microscopy revealed breach of muscle basal lamina. The electron density of plasma membrane was significantly lower at the places without identifiable basal lamina. Thus in FCMD changes of laminin and other proteins on the cell surface involve a process common to developing muscles, and loss of normal structure of the basal lamina is closely associated with changes of the plasma membrane. This suggests that the primary cause of FCMD is related to formation and maintenance of the basal lamina.

Dystroglycan in development and disease

Our understanding of the structure and function of dystroglycan, a cell surface laminin/agrin receptor, has increased dramatically over the past two years. Structural studies, analysis of its binding partners, and targeted gene disruption have all contributed to the elucidation of the biological role of dystroglycan in development and disease. It is now apparent that dystroglycan plays a critical role in the pathogenesis of several muscular dystrophies and serves as a receptor for a human pathogen as well as being involved in early development, organ morphogenesis, and synaptogenesis.

An ancient retrotransposal insertion causes Fukuyama-type congenital muscular dystrophy

Fukuyama-type congenital muscular dystrophy (FCMD), one of the most common autosomal recessive disorders in Japan (incidence is 0.7-1.2 per 10,000 births), is characterized by congenital muscular dystrophy associated with brain malformation (micropolygria) due to a defect in the migration of neurons. We previously mapped the FCMD gene to a region of less than 100 kilobases which included the marker locus D9S2107 on chromosome 9q31. We have also described a haplotype that is shared by more than 80% of FCMD chromosomes, indicating that most chromosomes bearing the FCMD mutation could be derived from a single ancestor. Here we report that there is a retrotransposal insertion of tandemly repeated sequences within this candidate-gene interval in all FCMD chromosomes carrying the founder haplotype (87%). The inserted sequence is about 3 kilobases long and is located in the 3' untranslated region of a gene encoding a new 461-amino-acid protein. This gene is expressed in various tissues in normal individuals, but not in FCMD patients who carry the insertion. Two independent point mutations confirm that mutation of this gene is responsible for FCMD. The predicted protein, which we term fukutin, contains an amino-terminal signal sequence, which together with results from transfection experiments suggests that fukutin is a secreted protein. To our knowledge, FCMD is the first human disease to be caused by an ancient retrotransposal integration.

Laminin-alpha2 (merosin), beta-dystroglycan, alpha-sarcoglycan (adhalin), and dystrophin expression in congenital muscular dystrophies: an immunohistochemical study

Muscle biopsies of 13 congenital muscular dystrophy (CMD) patients were investigated for the expression of laminin-alpha2 (merosin), beta-dystroglycan, alpha-sarcoglycan (adhalin) and dystrophin. Expression of these proteins was normal in six out of eight patients with pure-CMD, in three non-Japanese patients clinically resembling Fukuyama-CMD (F-CMD), and in two patients with Walker-Warburg syndrome (WWS). The two 'pure'-CMD patients with white matter hypodensity showed severely decreased laminin-alpha2 expression and normal expression of the other proteins. Our findings in the non-Japanese patients, clinically resembling F-CMD, are different from those in Japanese cases with F-CMD in the literature. Consequently, our patients suffer from WWS or from another yet undetermined form of CMD.

Early ultrastructural changes in the central nervous system in Fukuyama congenital muscular dystrophy

Electron microscopy of the central nervous system surface structure is described in two fetuses with Fukuyama congenital muscular dystrophy (FCMD). In addition to relatively large surface defects, many minute defects less than several micrometers in size associated with protrusion of glial cytoplasm were observed in the cerebrum. These findings were considered to represent early changes prior to cortical dysplasia. The basement membrane adjacent to the defects showed amorphous, wavy, or whorled configurations, and gradually disappeared. The glial cytoplasmic membrane seemed to be relatively well preserved in some areas where the basement membrane disappeared. On the other hand, both the basement membrane and cytoplasmic membrane became indistinct irregularly in areas without defects, including the spinal cord; similar lesions were found in the skeletal muscle. These observations confirm previous observations concerning defects of the pial-glial barrier of the brain surface, and may suggest the involvement of abnormal basement membrane or related structures, or both, in the genesis of the brain lesions of FCMD.

Increased cerebral choline-compounds in Duchenne muscular dystrophy

We investigated the hypothesis that cell membrane function is abnormal in brains of subjects with Duchenne muscular dystrophy (DMD) using proton-nuclear magnetic resonance (NMR) spectroscopy of human brain extracts. The total amount of choline-containing compounds was significantly higher (about three times) than in normal controls and patients with other myopathies, while N-acetyl-L-aspartic acid and creatine were within the normal range. These findings indicate that abnormal cell membrane function may be correlated with the abnormal dystrophin or lack of dystrophin in the brain of patients with DMD.

Congenital muscular dystrophy with severe retrocollis and mental retardation: a report of two siblings

Two siblings with a congenital muscular dystrophy and severe mental retardation which was not due to dystrophin, merosin, or adhalin deficiency are described. These cases overlap with congenital muscular dystrophy of the Fukuyama-type but are less severe. Atypical features include limited facial involvement, retained ambulation, and severe retrocollis.

YAC and cosmid contigs encompassing the Fukuyama-type congenital muscular dystrophy (FCMD) candidate region on 9q31

Fukuyama-type congenital muscular dystrophy (FCMD), the second most common form of childhood muscular dystrophy in Japan, is an autosomal recessive severe muscular dystrophy associated with an anomaly of the brain. We had mapped the FCMD gene to an approximately 5-cM interval between D9S127 and D9S2111 on 9q31-q33 and had also found evidence for linkage disequilibrium between FCMD and D9S306 in this candidate region. Through further analysis, we have defined another marker, D9S172, which showed stronger linkage disequilibrium than D9S306. A yeast artificial chromosome (YAC) contig spanning 3,5 Mb, which includes this D9S306-D9S172 interval on 9q31, has been constructed by a combination of sequence-tagged site, Alu-PCR, and restriction mapping. Also, cosmid clones subcloned from the YAC were assembled into three contigs, one of which contains D9S2107, which showed the strongest linkage disequilibrium with FCMD. These contigs also allowed us to order the markers as follows: cen-D9S127-(approximately 800 kb)-D9S306 (identical to D9S53)-(approximately 700 kb)-A107XF9-(approximately 500 kb)-D9S172-(approximately 30 kb)-D9S299 (identical to D9S774)-(approximately 120 kb)-WI2269-tel. Thus, we have constructed the first high-resolution physical map of the FCMD candidate region. The YAC and cosmid contigs established here will be a crucial resource for identification of the FCMD gene and other genes in this region.

Congenital muscular dystrophy: a review of the literature

Congenital muscular dystrophy (CMD) is a condition in which there are already at birth, marked hypotonia, generalized muscle weakness and frequently multiple contractures. CMD has recently been classified into four categories: CMD I, the classical or "pure' CMD without severe impairment of intellectual development; CMD II, the Fukuyama type CMD with muscle and structural brain abnormalities; CMD III and IV with muscle, eye and brain abnormalities; the milder Finnish type CMD (CMD III) and the severe Walker-Warburg syndrome (CMD IV). Data of the literature concerning those different CMD types have been reviewed and are presented with emphasis on signs and symptoms, clinical course, laboratory, neurophysiological, radiological, morphological and genetic characteristics.

alpha-Sarcoglycan (adhalin) deficiency: complete deficiency patients are 5% of childhood-onset dystrophin-normal muscular dystrophy and most partial deficiency patients do not have gene mutations

alpha-Sarcoglycan (adhalin), a 50-kDa component of the dystrophin-associated complex of proteins, participates in the stabilization of the myofiber plasma membrane in the membrane cytoskeleton. Deficiencies of alpha-sarcoglycan cause a subset of childhood-onset muscular dystrophy (SCARMD) cases. However, secondary deficiencies of alpha-sarcoglycan are common. To begin to establish the rates of false positives (secondary deficiencies), we used immunofluorescence to screen 30 Italian dystrophin-normal muscular dystrophy patient biopsies and identified 4 patients with partial alpha-sarcoglycan deficiency and 2 patients with complete deficiency. The entire alpha-sarcoglycan gene was screened for mutations using RT-PCR and SSCP of messenger RNA isolated from muscle biopsies in each of the six patients. Aberrant SSCP conformers and novel mutations were found only in the two complete immunohistochemical deficient patients. One patient was homozygous for a R34H amino acid substitution, while the other was a compound heterozygote (R77C, D97G). These three missense mutations, with additional mutations we and others have previously described, are all localized in the extracellular domain of alpha-sarcoglycan, and most result in the loss or gain of a positively charged amino acid. These data have strong implications for structure/function maps of the alpha-sarcoglycan molecule. Our results suggest that most patients showing partial alpha-sarcoglycan deficiency exhibit this as a secondary consequence of genetically distinct disorders. In support of this, we show biochemical data indicating that secondary deficiency patients show decreased immunostaining with antibodies directed against alpha-sarcoglycan, while having nearly normal quantities of alpha-sarcoglycan protein on immunoblot. This data also suggests that approximately 5% of childhood-onset dystrophin-normal muscular dystrophy patients will show a primary alpha-sarcoglycan deficiency.

Investigation of muscle disease

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Congenital muscular dystrophy with cerebral white matter hypodensity. Correlation of clinical features and merosin deficiency

We report clinical and pathological findings in 9 children affected by congenital muscular dystrophy with normal or borderline intelligence and hypodensity of cerebral white matter (CMD-HWM), also frequently called 'occidental or western form of cerebro-muscular dystrophy' (OCMD). Our patients have uniform, distinct, clinical presentation that includes: normal or subnormal intelligence, severe, slowly progressive motor disability, high rate of facial involvement and dysmorphic aspect, increased creatine kinase levels and variable degrees of abnormal, radiographic, cerebral white matter pattern. By comparing our cases with previous reports we suggest that this subtype of CMD is not uncommon in Brazil and it is represented by a particularly severe and homogeneous clinical picture with important motor disability. The immunohistochemical staining for merosin, performed on the muscle biopsy of 6 among 9 patients, showed that all are merosin negative.

Congenital muscular dystrophy and severe central nervous system atrophy in two siblings

Severe degenerative features of the nervous system of a hitherto unknown kind, associated with a neuromuscular disorder with histopathological features of congenital muscular dystrophy, are reported in two female siblings. The clinical profile was characterized by generalized hypotonia followed by spastic tetraplegia, contractures, polyneuropathy, lack of cognitive development and progressive microcephaly. There as no involvement of the eyes. Neuropathological examination of the brain of one sibling, who died at the age of 30 months, revealed subtotal loss of neurons in the cerebral and cerebellar cortex and in the ventral pons, and secondary loss of myelin in the cerebral and cerebellar subcortical white matter. Sural nerve biopsy in the other sibling, who had a similar neurological affection, showed a lack of large myelinated fibers.

Immunohistochemical alterations of dystrophin in congenital muscular dystrophy

The dystrophin distribution in the plasma muscle membrane using immunohystochemistry was studied in 22 children with congenital muscular dystrophy. The dystrophin was detected by immunofluorescence in muscle biopsy through a polyclonal antibody. All the cases had patchy interruptions of the fluorescence in the plasma membrane. A large patchy interruption of the sarcolemma was found in 17 cases, small interruption in 12, and a combination of large and small patchy discontinuity in 7. Small gaps around the fiber like a rosary were found in 15 cases. The frequency of these abnormalities ranged cases from: all fibers in 5 cases, frequent in 8, occasional in 5, and rare in 4. Five cases had total absence of immunofluorescence. These results suggest that the dystrophin expression is abnormal in this group of children and that this type of abnormalities can not be differentiated from early Becker muscular dystrophy nor childhood autosomal recessive muscular dystrophy through immunohystochemistry alone.

Effect of the beta 2-adrenergic agonist clenbuterol on the growth of fast- and slow-twitch skeletal muscle of the dystrophic (C57BL6J dy2J/dy2J) mouse

Clenbuterol (4mg/kg in diet for 21 days) had no statistically significant effect on whole body growth. It did cause a significant increase (18.2%) in wet weight of the fast twitch muscle extensor digitorum longus (EDL) and a corresponding 14.9% increase in total muscle protein. In transverse sections through dystrophic muscle fibre sizes were more variable than in normal muscle. Clenbuterol treatment resulted in a reduction in the proportion of small diameter fibres, and therefore an increase in mean fibre diameter, in dystrophic EDL. Clenbuterol had no significant effect upon the slow twitch muscle soleus.

Expression of laminin subunits in congenital muscular dystrophy

The expression of laminin subunits M, A, B1 and B2 was studied immunocytochemically in 25 cases of classical congenital muscular dystrophy (CMD), 11 hypotonic infants, 20 cases of a variety of inherited and acquired neuromuscular disorders, and 11 controls. Merosin, as indicated by labelling for the M chain, was deficient in 12 (48%) of the cases of classical CMD. Seven cases had no detectable labelling for the M chain whereas five showed traces, including three cousins from the same family. This suggests that very low expression may relate to a possible difference in the molecular defect, compared with cases completely devoid of the M chain. The A chain was abundant in regenerating fibres and in immature fibres expressing fetal myosin. In all merosin-deficient cases the A chain was over-expressed but this was not due to immaturity. A secondary reduction in sarcolemmal expression of the B1 chain occurred in five merosin-deficient cases, whilst expression in vascular tissue was normal. B1 was also reduced in one merosin-positive case of CMD, suggesting that other subunits may be involved in other forms of CMD. No differences in the expression of the B2 chain were observed in any of the cases studied. No abnormality in laminin subunits was found in controls or other neuromuscular disorders.

Recent advances in muscular dystrophies and myopathies

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Is dystrophin always altered in Becker muscular dystrophy patients?

The differential diagnosis between autosomal recessive limb-girdle (LGMD) and X-linked Becker muscular dystrophy (BMD) is very important for genetic counseling. It has been hypothesized that all BMD patients would have dystrophin alterations and dystrophin analysis could identify the Xp21 MD. Qualitatively abnormal dystrophin is easily detectable, but it is generally associated with in-frame DNA deletions or duplications. In patients with no detectable DNA deletions, in which X-linked inheritance cannot be proved, dystrophin quantification is still the only available test for differential diagnosis. In order to assess the accuracy of dystrophin quantification test in delineating Becker patients, we analyzed dystrophin abundance in BMD patients with a positive history of X-linked inheritance and no DNA detectable mutation, as compared to patients from families with LGMD. We observed that patients from 2 among the 5 BMD families have nearly normal dystrophin, while alteration in dystrophin content was observed in patients from 2 among the 7 LGMD families studied (probably as a secondary effect of alteration in the whole dystrophin-glycoproteins complex). These results suggest that dystrophin quantification, as an isolated test is not helpful for differential diagnosis between BMD and LGMD.

Mutations in the proteolytic enzyme calpain 3 cause limb-girdle muscular dystrophy type 2A

Limb-girdle muscular dystrophies (LGMDs) are a group of inherited diseases whose genetic etiology has yet to be elucidated. The autosomal recessive forms (LGMD2) constitute a genetically heterogeneous group with LGMD2A mapping to chromosome 15q15.1-q21.1. The gene encoding the muscle-specific calcium-activated neutral protease 3 (CANP3) large subunit is located in this region. This cysteine protease belongs to the family of intracellular calpains. Fifteen nonsense, splice site, frameshift, or missense calpain mutations cosegregate with the disease in LGMD2A families, six of which were found within La Réunion island patients. A digenic inheritance model is proposed to account for the unexpected presence of multiple independent mutations in this small inbred population. Finally, these results demonstrate an enzymatic rather than a structural protein defect causing a muscular dystrophy, a defect that may have regulatory consequences, perhaps in signal transduction.

Muscular degeneration in Duchenne's dystrophy may be caused by a mitochondrial defect

Duchenne's dystrophy (DMD), a recessive chromosome X-related disease, is the most common and severe form of myopathy. The different theories (vascular, neurogenic, membraneous, calcic and auto-immune) formulated to account for this disease have not been swept away by the discovery of the DMD gene and the deficient protein, dystrophin, since the exact cellular role played by the latter is still unknown. Our work on skeletal muscle has demonstrated a mitochondrial deficiency of the calcium-specific protein, calmitine, in degenerating muscle of myopathic persons and animals. Considering its great affinity for calcium, this protein specific to skeletal muscle could be essential to mitochondrial calcium regulation and thus to the functioning of the entire muscle cell. Its deficiency in Duchenne's and Becker type muscular dystrophy could be due to a mitochondrial genome alteration solely accountable for muscular degeneration. This hypothesis challenges the supposedly essential but still undefined role that researchers have attributed to dystrophin.

The childhood muscular dystrophies: diseases sharing a common pathogenesis of membrane instability

New observations demonstrate that several childhood forms of muscular dystrophy share a common pathogenesis. In muscle, dystrophin occurs as part of a membrane complex (dystrophin-glycoprotein) linking the cytoskeleton to the basal lamina. In Duchenne muscular dystrophy, dystrophin deficiency disrupts the linkage of the integral glycoproteins of the sarcolemma and leads to muscle fiber necrosis. In severe childhood autosomal recessive muscular dystrophy, a selective deficiency of adhalin (50-kd glycoprotein) also causes dysfunction of the dystrophin-glycoprotein complex. Most recently, a form of congenital muscular dystrophy demonstrates deficiency of laminin M (merosin) further demonstrating that sarcolemmal instability results from defects in structural proteins of the basal lamina. Animal models have been identified also demonstrating defects in specific proteins linking the subsarcolemmal cytoskeleton to the extracellular matrix. The mdx mouse has a defect in the gene encoding dystrophin. The cardiomyopathic hamster shows a specific deficiency of adhalin in skeletal muscle. The dy/dy mouse has been found deficient in merosin. These animal models will help researchers to understand their human counterparts and provide a system for testing therapeutic strategies.

Limb-girdle muscular dystrophy: clinical and pathologic reevaluation

To better define limb-girdle muscular dystrophy (LGMD), we examined 58 patients clinically and pathologically who fulfilled the criteria for LGMD and had normal dystrophin expression in their muscle biopsies. Only 27.6% of patients had evidence of inheritance. The onset of disease varied from 2 to 58 years of age, averaging 17.2 years. The disease progression also differed from patient to patient. In addition to evidence of muscle fiber necrosis and regeneration, in all muscle biopsies there were fibers with architectural changes of disorganized intermyofibrillar networks including moth-eaten (100%), lobulated (40%), whorled (17%) and targetoid (8%) fibers. The lobulated fibers which have never been reported in Duchenne muscular dystrophy (DMD) were seen in the advanced stages of LGMD, although the significance of such fibers remains unknown. On immunohistochemical examination, dystrophin-associated proteins (DAPs) and laminin were normally expressed along the surface membrane of muscle fibers, including the lobulated fibers.

Freeze-fracture analysis of muscle plasma membrane in Becker's muscular dystrophy

The intramembranous particle (IMP), orthogonal array (OA) and orthogonal array subunit particle (OASP) densities in skeletal muscle plasma membranes from eight patients with Becker's muscular dystrophy (BMD) were analysed by the freeze-fracture technique. The results showed almost normal IMP density with the significant decrease of OA and OASP densities in BMD. The group mean densities +/- SE of IMPs on the protoplasmic faces with and without OASPs, and on extracellular faces/microns 2 were 2137 +/- 207, 1839 +/- 68 and 895 +/- 108, respectively in controls; whereas those of BMD were 1989 +/- 259, 1837 +/- 203 and 900 +/- 239, respectively (P > 0.1 by two-tailed t-test). The group median density of OAs and their pits/microns 2 was 4.89 with mid-ranges (25-75% values of the counts) of 2.66-10.18 in controls; whereas that in BMD was 2.15 with mid-ranges of 1.14-4.31 (P < 0.01 by Wilcoxon rank-sum test). The group mean density +/- SE of OASPs in controls was 15.99 +/- 1.83; whereas that in BMD was 13.47 +/- 1.07 (P < 0.01 by two-tailed t-test). However, the diminution of OA and OASP densities in BMD muscle plasma membranes was not as severe as in Duchenne's muscular dystrophy. There was a relationship between OA density and clinical severity in BMD patients; the decrease of OA density in a severe BMD patient was more marked than that in mildly affected BMD patients. Therefore, it seems that marked depletion of OA density may lead to the severe disability in muscular dystrophies.

Basal lamina assembly

From studies of the 'classical' components, models for the assembly and structure of an idealized basal lamina have been developed. In particular, the evidence supports the concept of enmeshed collagen and laminin polymers, in which nidogen/entactin acts as a bridge between these molecules and provides anchorage for diverse matrix components. Different basement membranes, however, possess different members of the basic basal lamina families, such as the newly described alpha 6 (IV) collagen, alpha 2 (merosin) laminin, and beta 3 laminin (in kalinin/nicein) chains. Even though these members share homologous domains and sequences, and are likely to share certain functions, they also possess unique characteristics that are expected to provide for basal lamina heterogeneity. A combination of genetic, recombinant and biochemical approaches are now being applied to elucidate the special roles of both old and new components.

Autosomal recessive distal muscular dystrophy: normal expression of dystrophin, utrophin and dystrophin-associated proteins in muscle fibers

We examined 19 muscle biopsies from 14 patients with autosomal recessive distal muscular dystrophy (DisMD) histochemically and immunohistochemically to characterize the histologic features of this disease and to determine whether dystrophin and dystrophin-associated proteins (DAPs) are normally present in the muscles of patients with this disorder. The common histologic features in DisMD were active muscle fiber necrosis and regeneration with variation in fiber size, predominantly in the gastrocnemius muscle. There were occasional disorganizations of the intermyofibrillar network such as moth-eaten appearance, lobulated, whorled and targetoid fibers. In one half of the patients, small angular fibers and scattered rimmed vacuoles were also found. On immunohistochemical examination, dystrophin, DAPs, spectrin and laminin were normally expressed along the surface membrane of muscle fibers, even in the advanced stages of the disease. In contrast, dystrophin was absent and DAPs reduced in the sarcolemma of Duchenne muscular dystrophy (DMD) muscles. The overall histochemical features in DisMD were similar to those seen in DMD, though dystrophin and DAPs were normally expressed even in severely affected gastrocnemius muscle. A defect in an as yet unidentified protein rather than in DAPs and dystrophin is probably responsible for the muscle fiber necrosis in DisMD.

Dystroglycan is a binding protein of laminin and merosin in peripheral nerve

alpha-Dystroglycan, a 156 kDa dystrophin-associated glycoprotein, binds laminin in skeletal muscle. Here we demonstrate that alpha-dystroglycan is a binding protein of laminin (A/B1/B2) and merosin (M/B1/B2) in peripheral nerve. Immunocytochemical analysis demonstrates the localization of alpha-dystroglycan and merosin surrounding myelin sheath of peripheral nerve fibers. Biochemical analysis demonstrates that the 120 kDa peripheral nerve alpha-dystroglycan binds merosin as well as laminin. The binding of laminin and merosin is Ca2+ dependent and is inhibited by NaCl and heparin. Recently, merosin was shown to be deficient in the peripheral nerve of dy mice which have defects in myelination. The interaction between alpha-dystroglycan and merosin may play a role in the regulation of Schwann cell myelination and/or maintenance of myelin sheath.

Increasing complexity of the dystrophin-associated protein complex

Duchenne muscular dystrophy is a severe X chromosome-linked, muscle-wasting disease caused by lack of the protein dystrophin. The exact function of dystrophin remains to be determined. However, analysis of its interaction with a large oligomeric protein complex at the sarcolemma and the identification of a structurally related protein, utrophin, is leading to the characterization of candidate genes for other neuromuscular disorders.

Muscle and brain disease: an update

A number of syndromes included under this rubric are considered, and their main features discussed. The congenital muscular dystrophy of the Fukuyama type as it occurs in Japan and in the western world are almost certainly the same condition. The muscle disorder is associated with cerebral lesions which may be due to an arrest of neural migration or to demyelination. Muscle, eye and brain disease, or Santavuori's syndrome, shows ocular abnormalities, as well as those of the muscle and brain, as does the Walker-Warburg syndrome. In the latter disorder the cerebral lesions tend to be more severe, and it is more rapidly fatal. The manifestations of all these syndromes undoubtedly overlap, but there has been controversy on the question of their identity. Are they separate entities, or are they different expressions of a similar genetic disorder? The genes for all these conditions will have to be isolated to see if the different phenotypes are alleles of the same gene, or not. Some of the arguments, for and against, are presented.

Expression of myosin isoforms and of desmin, vimentin and titin in Tunisian Duchenne-like autosomal recessive muscular dystrophy

Morphological, morphometrical, histoenzymological, immunocytochemical and biochemical analysis were performed on muscle biopsies taken from patients suffering from tunisian autosomal recessive Duchenne-like muscular dystrophy (TDLMD) selected both by Duchenne-like clinical criteria and by the presence of normal dystrophin. Data were compared to that obtained from DMD biopsies characterized by the absence of dystrophin. The distribution of myosin heavy chain isoforms, desmin, vimentin and titin were determined in type I and type II muscle fibers. The protein pattern appeared to be less affected in TDLMD than in DMD biopsies. The regenerating fibers were mainly but not exclusively type IIC; a noticeable percentage of both type I and type II fibers coexpressed fast and slow MHC isoforms in TDLMD. This percentage was lower than in DMD. The expression of embryonic, fetal, and fast/slow myosin isoforms in type IIC fibers in TDLMD and DMD suggest different fiber type transformations in these two diseases.

Characterization and localization of a 77 kDa protein related to the dystrophin gene family

The Duchenne muscular dystrophy gene gives rise to transcripts of several lengths. These mRNAs differ in their coding content and tissue distribution. The 14 kb mRNA encodes dystrophin, a 427 kDa protein found in muscle and brain, and the short transcripts described encode DP71, a 77 kDa protein found in various organs. These short transcripts have many features common to the deduced primary structure of dystrophin, especially in the cysteine-rich specific C-terminal domains. The dystrophin C-terminal domain could be involved in membrane anchorage via the glycoprotein complex, but such a functional role for these short transcript products has yet to be demonstrated. Here we report the first isolation of a short transcript product from saponin-solubilized cardiac muscle membranes using alkaline buffer and affinity chromatography procedures. This molecule was found to be glycosylated and could be easily dissociated from cardiac muscle and other non-muscle tissues such as brain and liver. DP71-specific monoclonal antibody helped to identify this molecule as being related to the dystrophin gene family. Immunofluorescence analysis of bovine or chicken cardiac muscle showed a periodic distribution of DP71 in transverse T tubules and this protein was co-localized with the dystrophin glycoprotein complex in the Z-disk area.

Dystrophin-glycoprotein complex: its role in the molecular pathogenesis of muscular dystrophies

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is associated with a large oligomeric complex of sarcolemmal glycoproteins, including dystroglycan which provides a linkage to the extracellular matrix component, laminin. In patients with DMD, the absence of dystrophin leads to the loss in all of the dystrophin-associated proteins, causing the disruption of the linkage between the subsarcolemmal cytoskeleton and the extracellular matrix. This may render the sarcolemma vulnerable to physical stress. These recent developments in the research concerning the function of the dystrophin-glycoprotein complex pave a way for the better understanding of the pathogenesis of muscular dystrophies.

Clinical genetics in neurological disease

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Localization of a gene for Fukuyama type congenital muscular dystrophy to chromosome 9q31-33

Fukuyama type congenital muscular dystrophy (FCMD) is an autosomal recessive severe muscular dystrophy associated with an anomaly of the brain. Twenty-one FCMD families, 13 of them with consanguineous marriages, were analysed by genetic linkage analyses with polymorphic microsatellite markers to map the FCMD gene. Significant lod scores were obtained with the markers D9S58 (Zmax = 5.81 at theta = 0.06), D9S59 (Zmax = 4.33 at theta = 0.02), and HXB (Zmax = 3.28 at theta = 0.09) on chromosome 9q31-33. Multipoint analysis placed FCMD between D9S58 and D9S59, with a maximum lod score of 16.93. These markers will be useful for presymptomatic, prenatal and carrier diagnosis of family members carrying FCMD, and they represent important resources for the identification of a gene responsible for FCMD.

Abnormal localization of laminin subunits in muscular dystrophies

To address potential involvement of muscle basal lamina and membrane cytoskeleton proteins in the etiology of non-dystrophinopathy muscular dystrophies, we examined the immunostaining intensity and distribution of laminin subunits (A, B1, B2 and M), type IV collagen, dystrophin and spectrin in skeletal muscle biopsies from 64 myopathic patients (17 Fukuyama congenital muscular dystrophy: FCMD, 13 congenital muscular dystrophy unrelated to FCMD: other CMD, 16 Duchenne muscular dystrophy: DMD, and 18 other neuromuscular diseases. In FCMD muscle, we found a significant reduction of laminin M (merosin; a striated muscle specific basal lamina-associated protein) with approximately 26% of levels seen in controls by quantitative immunofluorescence. Other CMD and DMD muscles showed less dramatic reductions (78%, 80%, respectively). The localization of laminin M was also abnormal in FCMD muscle. Laminin B1 and B2 showed abnormalities similar to those observed with laminin M, but were less marked. Laminin A was only detected in rare regenerating fibers in control biopsies, whereas it was seen around most muscle fibers in FCMD patients, and in dystrophin deficient muscle fibers from DMD patients and its carrier. Staining intensity of type IV collagen in FCMD muscle was not significantly different from the other diseases. These findings may implicate a primary or central role for the basal lamina in FCMD muscle.