Genetic heterogeneity for Duchenne-like muscular dystrophy (DLMD) based on linkage and 50 DAG analysis

LGMD. Identification, description and classification

The term ‘limb girdle muscular dystrophy’ (LGMD) was first used in the seminal paper by Walton and Nattrass in 1954, were they identified LGMD as a separate clinical entity In LGMD description it is pointed out that the category of LGMD most likely comprises a heterogeneous group of disorders. After that the clinical entity was discussed but the LMGD nosography reached a permanent classification during two ENMC workshops held in 1995 and 2017, in the last one an operating definition of LGMD was agreed. This last classification included dystrophies with proximal or distal-proximal presentation with evidence at biopsy of fibre degeneration and splitting, high CK, MRI imaging consistent with degenerative changes, fibro-fatty infiltration present in individuals that reached independent walking ability. To be considered in this group at least two unrelated families should be identified.

A review is done of the first genetic characterisation of a number of LGMDs during the late twentieth century and a historical summary is given regarding how these conditions were clinically described and identified, the progresses done from identification of genetic loci, to protein and gene discoveries are reported. The LGMD described on which such historical progresses were done are the recessive calpainopathy (LGMD 2A/R1), dysferlinopathy (LGMD 2B/R2), sarcoglycanopathy (LGMD 2C-2F/R3-R6) types and the dominant type due to TPNO3 variants named transportinopathy (LGMD 1F/D2). Because of new diagnostic techniques such as exome and genome sequencing, it is likely that many other subtypes of LGMD might be identified in the future, however the lesson from the past discoveries can be useful for scientists and clinicians.

Our trails and trials in the subsarcolemmal cytoskeleton network and muscular dystrophy researches in the dystrophin era

In 1987, about 150 years after the discovery of Duchenne muscular dystrophy (DMD), its responsible gene, the dystrophin gene, was cloned by Kunkel. This was a new substance. During these 20 odd years after the cloning, our understanding on dystrophin as a component of the subsarcolemmal cytoskeleton networks and on the pathomechanisms of and experimental therapeutics for DMD has been greatly enhanced. During this paradigm change, I was fortunately able to work as an active researcher on its frontiers for 12 years. After we discovered that dystrophin is located on the cell membrane in 1988, we studied the architecture of dystrophin and dystrophin-associated proteins (DAPs) complex in order to investigate the function of dystrophin and pathomechanism of DMD. During the conduct of these studies, we came to consider that the dystrophin-DAP complex serves to transmembranously connect the subsarcolemmal cytoskeleton networks and basal lamina to protect the lipid bilayer. It then became our working hypothesis that injury of the lipid bilayer upon muscle contraction is the cause of DMD. During this process, we predicted that subunits of the sarcoglycan (SG) complex are responsible for respective types of DMD-like muscular dystrophy with autosomal recessive inheritance. Our prediction was confirmed to be true by many researchers including ourselves. In this review, I will try to explain what we observed and how we considered concerning the architecture and function of the dystrophin-DAP complex, and the pathomechanisms of DMD and related muscular dystrophies.

Diagnostic protein expression in human muscle biopsies

Using immunohistochemistry in diagnosing neuromuscular diseases is meant to enhance the diagnostic yield in two ways. The first application aims at visualizing molecules which are developmentally, neurally, and/or immunologically regulated and not expressed by normal muscle. They are upregulated in pathological conditions and may help assign a given muscular biopsy to one of the main diagnostic entities (muscular dystrophies, inflammatory myopathy, neurogenic atrophy). In the past, muscle-specific molecules with a defined expression pattern during fetal myogenesis served as antigens, with the rationale that the developmental program was switched on in new fibers. Recently, myofibers in diseased muscle are thought of as targets of stimuli which are released by macrophages in muscular dystrophy, by lymphocytes in inflammatory myopathies, or by a lesioned peripheral nerve in neurogenic atrophies. This has somewhat blurred the borders between the diagnostic groups, for certain molecules, e.g. cytokines, may be upregulated after experimental necrotization, denervation, and also in inflammatory myopathies. In the second part of this review we summarise the experiences of a Centre in the North of England that specialises in the diagnosis and clinical support of patients with muscular dystrophy. Emphasis is placed on the use of protein expression to guide mutation analysis, particularly in the limb-girdle muscular dystrophies (a group of diseases that are very difficult to differentiate on clinical grounds alone). We confirm that genetic analysis is essential to corroborate the results of protein analysis in certain conditions (particularly in calpainopathy). However, we conclude that analysing biopsies for abnormal protein expression is very useful in aiding the decision between alternative diagnoses.

Molecular and cell biology of the sarcoglycan complex

The original sarcoglycan (SG) complex has four subunits and comprises a subcomplex of the dystrophin-dystrophin-associated protein complex. Each SG gene has been shown to be responsible for limb-girdle muscular dystrophy, called sarcoglycanopathy (SGP). In this review, we detail the characteristics of the SG subunits, and the mechanism of the formation of the SG complex and various molecules associated with this complex. We discuss the molecular mechanisms of SGP based on studies mostly using SGP animal models. In addition, we describe other SG molecules, epsilon- and zeta-SGs, with special reference to their expression and roles in vascular smooth muscle, which are currently in dispute. We further consider the maternally imprinted nature of the epsilon-SG gene. Finally, we stress that the SG complex cannot work by itself and works in a larger complex system, called the transverse fixation system, which forms an array of molecules responsible for various muscular dystrophies.

Confocal analysis of the dystrophin protein complex in muscular dystrophy

The dystrophin protein complex (DPC), composed of at least 10 proteins that associate with dystrophin, is critical for the maintenance of normal muscle fiber structure and physiology. In this study, we used immunohistochemistry and confocal microscopy to examine the relative abundance and distribution of several of these proteins in muscle biopsies taken from patients with various muscular dystrophies. The optical sectioning capability of confocal microscopy allowed us to comprehensively analyze the semiquantitative expression of components of the DPC. Alpha-sarcoglycan-deficient patients displayed a marked reduction in membrane immunostaining of the sarcoglycan complex. Gamma-sarcoglycan-deficient patients showed variable decreased immunostaining of the sarcoglycan complex proteins. When beta-sarcoglycan was expressed appropriately at the sarcolemma of gamma-sarcoglycan-deficient patients, intracellular labeling of beta-sarcoglycan was also present. Beta-sarcoglycan-deficient patients showed poor localization of extracellular matrix proteins in addition to a complete absence of the sarcoglycans. Merosin-deficient patients showed relatively normal immunostaining levels of all other members of the DPC. Finally, dystrophin-deficient patients showed little or no change in the expression of extracellular matrix proteins; however, some sarcoglycans were significantly decreased. These data allowed us to suggest unique fundamental interactions between the members of the DPC.

Plasma membrane cytoskeleton of muscle: a fine structural analysis

The discovery of dystrophin and its definition as the causative molecule in Duchenne Muscular Dystrophy has led to a renewed interest in the molecular structure of the muscle fiber plasma membrane and its association with the extracellular basal lamina. The original identification of dystrophin gave credence to the possibility that the plasma membrane of the muscle fiber may be highly organized and involved in maintaining appropriate homeostasis in this actively contracting cellular system. In this review, we examine the currently known members of the muscle fiber plasma membrane cytoskeleton and the interactions that occur between the different members of this complex using histological, electron microscopic, and confocal methods. From our studies and others cited in this review, it is clear that the dystrophin cytoskeletal complex is not completely understood and component molecules continue to be discovered. Perhaps equally importantly, currently defined molecules (such as alpha-actinin or neuronal nitric oxide synthase) are being recognized as being specifically associated with the complex. What is striking from all of the studies, to date, is that while we are able to identify members of the dystrophin cytoskeletal complex and while we are able to associate mutations of individual molecules with disease(s), we are still unable to truly define the roles of each of the molecules in maintaining the normal physiology of the muscle fiber.

Sarcoglycans in muscular dystrophy

Muscular dystrophy is a heterogeneous genetic disease that affects skeletal and cardiac muscle. The genetic defects associated with muscular dystrophy include mutations in dystrophin and its associated glycoproteins, the sarcoglycans. Furthermore, defects in dystrophin have been shown to cause a disruption of the normal expression and localization of the sarcoglycan complex. Thus, abnormalities of sarcoglycan are a common molecular feature in a number of dystrophies. By combining biochemistry, molecular cell biology, and human and mouse genetics, a growing understanding of the sarcoglycan complex is emerging. Sarcoglycan appears to be an important, independent mediator of dystrophic pathology in both skeletal muscle and heart. The absence of sarcoglycan leads to alterations of membrane permeability and apoptosis, two shared features of a number of dystrophies. beta-sarcoglycan and delta-sarcoglycan may form the core of the sarcoglycan subcomplex with alpha- and gamma-sarcoglycan less tightly associated to this core. The relationship of epsilon-sarcoglycan to the dystrophin-glycoprotein complex remains unclear. Animals lacking alpha-, gamma- and delta-sarcoglycan have been described and provide excellent opportunities for further investigation of the function of sarcoglycan. Dystrophin with dystroglycan and laminin may be a mechanical link between the actin cytoskeleton and the extracellular matrix. By positioning itself in close proximity to dystrophin and dystroglycan, sarcoglycan may function to couple mechanical and chemical signals in striated muscle. Sarcoglycan may be an independent signaling or regulatory module whose position in the membrane is determined by dystrophin but whose function is carried out independent of the dystrophin-dystroglycan-laminin axis.

Sarcoglycanopathies are responsible for 68% of severe autosomal recessive limb-girdle muscular dystrophy in the Brazilian population

Sarcoglycanopathies (SGPs) constitute a subgroup of limb-girdle muscular dystrophies (LGMD), where the primary defect in one sarcoglycan (SG) glycoprotein (alpha-SG, beta-SG, gamma-SG or delta-SG) results in a deficiency of the whole complex. Four genes, at 17q, 4q, 13q and 5q, encode the four glycoproteins, and mutations in these genes cause diseases called LGMD2D, 2E, 2C and 2F. To estimate the prevalence, relative proportions and clinical features of SGPs, we have studied the SG proteins in muscle biopsies of 140 patients (from 115 unrelated Brazilian families) with a clinical diagnosis of LGMD. Alpha-SG immunofluorescence analysis showed a positive staining pattern in 70% (80/115) of the families, a patchy pattern in 14% (16/115) and a negative pattern in 16% (19/115) of the families. All the 19 alpha-SG negative, and four of the 16 alpha-SG patchy patients were also deficient for the other three SG proteins, confirming the diagnosis of SGP in 20% of the LGMD families. None of the positive alpha-SG patients were deficient for any of the other three SG proteins, supporting the view that the SG complex functions as a unit. DNA analysis for the four sarcoglycan genes showed that alpha-SG mutations accounted for 47%, beta-SG for 16%, gamma-SG for 16% and delta-SG for 21% of the cases. SG abnormalities were observed in only 8.5% of patients with milder LGMD forms, but were present in 68% of patients with a severe Duchenne-like course. The relatively high frequency of SGP among Brazilian people with LGMD may be due to the disproportionally high frequency of African Brazilian SGP patients with the same mutation (particularly among LGMD2C and 2F patients), suggesting a founder effect. Consanguinity is also common in our SGP families.

Seven autosomal recessive limb-girdle muscular dystrophies in the Brazilian population: from LGMD2A to LGMD2G

The autosomal recessive limb-girdle muscular dystrophies (AR-LGMDs) are a heterogeneous group of disorders of progressive weakness of the pelvic and shoulder girdle musculature. The clinical course is characterized by great variability, ranging from severe forms with onset in the first decade and rapid progression resembling clinically Xp21 Duchenne muscular dystrophy (DMD) to milder forms with later onset and slower course. Eight genes are mapped for the AR-LGMDs; they are: LGMD2A (CAPN3) at 15q, LGMD2B (dysferlin) at 2p, LGMD2C (gamma-SG) at 13q, LGMD2D (alpha-SG) at 17q, LGMD2E (beta-SG) at 4q, LGMD2F (6-SG) at 5q, LGMD2G at 17q, and more recently LGMD2H at 9q. The LGMD2F (delta-SG) and LGMD2G genes were mapped in Brazilian AR-LGMD families. Linkage analysis in two unlinked families excluded the eight AR-LGMD genes, indicating that there is at least one more gene responsible for AR-LGMD. We have analyzed 140 patients (from 40 families) affected with one of seven autosomal recessive LGMD loci, that is, from LGMD2A to LGMD2G. The main observations were: 1) all LGMD2E and LGMD2F patients had a severe condition, but considerable inter- and intra-familial clinical variability was observed among patients from all other groups; 2) serum CK activities showed the highest values in LGMD2D (alpha-SG) patients among sarcoglycanopathies and LGMD2B (dysferlin) patients among nonsarcoglycanopathies; 3) comparison between LGMD2A (CAPN3) and LGMD2B (dysferlin) showed that the first have on average a more severe course and have calf hypertrophy more frequently (86% versus 13%); and 4) inability to walk on toes was observed in approximately 70% of LGMD2B patients.

The sarcoglycan complex in limb-girdle muscular dystrophy

The involvement of the sarcoglycan complex in the pathogenesis of muscular dystrophy is becoming increasingly clear. Sarcoglycan gene mutations lead to four forms of autosomal recessive limb-girdle muscular dystrophy. Recent progress has been made with the identification of novel mutations and their correlations with disease. Through this research, a better understanding the molecular pathogenesis of limb-girdle muscular dystrophy has been gained. Finally, animal models are now being used to study viral-mediated gene transfer for the future treatment of this disease.

From dystrophinopathy to sarcoglycanopathy: evolution of a concept of muscular dystrophy

Duchenne and Becker muscular dystrophies are collectively termed dystrophinopathy. Dystrophinopathy and severe childhood autosomal recessive muscular dystrophy (SCARMD) are clinically very similar and had not been distinguished in the early 20th century. SCARMD was first classified separately from dystrophinopathy due to differences in the mode of inheritance. Studies performed several years ago clarified some immunohistochemical and genetic characteristics of SCARMD, but many remained to be clarified. In 1994, the sarcoglycan complex was discovered among dystrophin-associated proteins. Subsequently, on the basis of our immunohistochemical findings which indicated that all components of the sarcoglycan complex are absent in SCARMD muscles, and the previous genetic findings, we proposed that a mutation of any one of the sarcoglycan genes leads to SCARMD. This hypothesis explained and predicted various characteristics of SCARMD at the molecular level, most of which have been verified by subsequent discoveries in our own as well as various other laboratories. SCARMD is now called sarcoglycanopathy, which is caused by a defect of any one of four different sarcoglycan genes, and thus far mutations in sarcoglycan genes have been documented in the SCARMD patients. In this review, the evolution of the concept of sarcoglycanopathy separate from that of dystrophinopathy is explained by comparing studies on these diseases.

Absence of alpha-sarcoglycan and novel missense mutations in the alpha-sarcoglycan gene in a young British girl with muscular dystrophy

An 11-year-old white female presented with progressive proximal muscle weakness and marked calf hypertrophy. Muscle biopsy showed severe dystrophy with normal expression of dystrophin. There was complete absence of the 50kDa dystrophin-associated glycoprotein (alpha-sarcoglycan). DNA analysis showed novel point mutations (one missense and one splicing) in the alpha-sarcoglycan gene at chromosomal location 17q21, confirming the diagnosis of limb-girdle muscular dystrophy type 2D (LGMD-2D). We believe this is one of the first confirmed white cases of primary alpha-sarcoglycanopathy identified in the UK. This case supports the assumption of a wide geographic prevalence of severe childhood onset autosomal recessive muscular dystrophy and genetic heterogeneity. In the future, with improved diagnostic accuracy it is likely that more cases demonstrating primary or secondary deficiency of alpha-sarcoglycan will be identified. We would recommend staining for dystrophin-associated glycoproteins (sarcoglycans) in all new cases of muscular dystrophy with normal dystrophin, and confirmation with DNA analysis where possible.

Mutational diversity and hot spots in the alpha-sarcoglycan gene in autosomal recessive muscular dystrophy (LGMD2D)

Sarcoglycanopathies are a genetically heterogeneous group of autosomal recessive muscular dystrophies in which the primary defect may reside in any of the genes coding for the different partners of the sarcolemmal sarcoglycan (SG) complex: the alpha-SG (LGMD2D at 17q21.2), the beta-SG (LGMD2E at 4q12), the gamma-SG (LGMD2C at 13q12), and the delta-SG (LGMD2F at 5q33). We report a series of 20 new unrelated families with 14 different mutations in the alpha-SG gene. Along with the mutations that we previously reported this brings our cohort of patients with alpha-sarcoglycanopathy to a total of 31 unrelated patients, carrying 25 different mutations. The missense mutations reside in the extracellular domain of the protein. Five of 15 missense mutations, carried by unrelated subjects on different haplotype backgrounds and of widespread geographical origins, account for 58% of the mutated chromosomes, with a striking prevalence of the R77C substitution (32%). The severity of the disease varies strikingly and correlates at least in part with the amount of residual protein and the type of mutation. The recurrent R284C substitution is associated with a benign disease course.

Characteristic expression of cell adhesion molecules in adhalin deficiency

We have reported the reduction of the B1 subunit of laminin and that of heparan sulfate proteoglycan (HSPG) in two Japanese patients with adhalin deficiency. We here investigated immunohistochemically the expression of cell adhesion molecules, including intercellular adhesion molecule-1 (ICAM-1), neural cell adhesion molecule (NCAM), and CD44 (HCAM), in four Japanese patients with adhalin deficiency, compared to other types of muscular dystrophy. We found that NCAM was upregulated in a fair number of muscle fibers, regardless of the type of muscular dystrophy. ICAM-1 was detected on the rare muscle cell membrane in all patients. CD44 was barely detected on the muscle cell membrane in adhalin deficiency, in contrast to the strong expression of CD44 which was observed in other types of muscular dystrophy. These findings suggest that a different degenerative or regenerative process is involved in adhalin deficiency compared to other types of muscular dystrophy.

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.

Clinical and molecular pathological features of severe childhood autosomal recessive muscular dystrophy in Saudi Arabia

The clinical, biochemical and histochemical features of 14 patients (nine females and five males) with severe childhood autosomal recessive muscular dystrophy (SCARMD) seen at a tertiary hospital in Riyadh from 1982 to 1993 are described. Onset was at 3 to 9 (median 3) years and four of five children aged > 12 years lost ambulation. Five of the eight pairs of parents were closely consanguineous. The mean creatine kinase was 20 times the upper normal limit. Histochemistry of muscle showed dystrophic features in all cases, and dystrophin was positive in all cases examined (N = 6). Three patients (two girls and a boy) were deficient in adhalin, the 50-kDa dystorphin-associated glycoprotein. A boy aged 13 years had rapidly progressing disease. Another boy of the same age (from a family characterized by early onset and slower progression) had normal dystrophin and adhalin. The clinical features conformed with previous observations from Sudan, North Africa and Qatar in the Arabian Peninsula. The disease is common in Saudi Arabia and seems to be more prevalent than Duchenne muscular dystrophy.

Main clinical features of the three mapped autosomal recessive limb-girdle muscular dystrophies and estimated proportion of each form in 13 Brazilian families

Autosomal recessive limb-girdle muscular dystrophies (AR LGMD) represent a group of muscle diseases with a wide spectrum of clinical signs, varying from very severe to mild. Four different loci that when mutated cause the AR LGMD phenotype have been mapped or cloned or both: in two of them the linked families seem to have a relatively mild phenotype (LGMD2a and LGMD2b), in the third one the reported linked families show a more severe clinical course (LGMD2c), while mutations in the fourth locus may cause severe or mild phenotypes (LGMD2d). The relative proportion of each of these genetic forms among the LGMD families and whether there are other genes that when mutated cause this phenotype is unknown. The closest available informative markers for each of the mapped AR LGMD genes have been tested in 13 Brazilian families with at least three affected patients. The findings from the present report confirm non-allelic heterogeneity for LGMD and suggest that in our population about 33% of the LGMD families are caused by mutations in the 15q gene, 33% in the 2p gene, 17% by mutations in the adhalin gene, and less than 10% may be by mutations at the 13q locus. They also suggest that there is at least one other gene responsible for this phenotype. In addition, the main clinical features of the different forms are discussed.

Clinical heterogeneity of adhalin deficiency

We report adhalin deficiency in 8 patients with clinically diagnosed muscular dystrophy, dystrophic histopathological features, high plasma creatine kinase levels, normal expression of dystrophin, and marked variability of symptoms. Although the distribution of hyposthenia was similar in all 8 patients and predominantly involved muscles in the pelvic girdle, age at onset and rate of disease progression were highly variable: In 2 patients onset, at ages 24 and 25, was later than has been previously observed. We found no apparent relation between disease severity and the quantity of adhalin expressed. Two kinds of myopathy with adhalin deficiency have been reported: one caused by a mutation in the adhalin gene on chromosome 17 (primary adhalinopathy) and the other linked to chromosome 13. The product of the gene on chromosome 13 is probably associated with adhalin and its deficiency results in secondary adhalinopathy. The severity of clinical phenotypes in these adhalinopathies seems to relate more to the kind and site of the mutations than to the residual amount of the protein. We also detected a variable reduction in the laminin beta 1 subunit by immunohistochemistry in most patients, confirming that this is commonly associated with adhalin deficiency.

Beta-sarcoglycan (A3b) mutations cause autosomal recessive muscular dystrophy with loss of the sarcoglycan complex

The dystrophin associated proteins (DAPs) are good candidates for harboring primary mutations in the genetically heterogeneous autosomal recessive muscular dystrophies (ARMD). The transmembrane components of the DAPs can be separated into the dystroglycan and the sarcoglycan complexes. Here we report the isolation of cDNAs encoding the 43 kD sarcoglycan protein beta-sarcoglycan (A3b) and the localization of the human gene to chromosome 4q12. We describe a young girl with ARMD with truncating mutations on both alleles. Immunostaining of her muscle biopsy shows specific loss of the components of the sarcoglycan complex (beta-sarcoglycan, alpha-sarcoglycan (adhalin), and 35 kD sarcoglycan). Thus secondary destabilization of the sarcoglycan complex may be an important pathophysiological event in ARMD.

Adhalin gene mutations in patients with autosomal recessive childhood onset muscular dystrophy with adhalin deficiency

Homozygous adhalin gene mutations were found in three patients from two consanguineous families with autosomal recessive childhood onset muscular dystrophy. Muscle biopsies from patients in each family showed complete absence of adhalin. Sequencing of adhalin cDNA prepared from skeletal muscle by reverse transcription PCR demonstrated a cytosine to thymidine substitution at nt 229 in the patient in family 1 and an adenine to guanine substitution at nt 410 and a 15-base insertion between nt 408 and 409 in the two patients in family 2. Sequencing of genomic DNA prepared from peripheral blood leukocytes by PCR confirmed these mutations. The parents in each family were found to be heterozygous for the respective mutations. These adhalin gene mutations are presumed to be responsible for the absence of adhalin in the skeletal muscle. Adhalin deficiency likely causes disruption of the muscle cell membrane, resulting in dystrophic changes in the skeletal muscle similar to dystrophin deficiency in Duchenne muscular dystrophy.

Primary adhalinopathy: a common cause of autosomal recessive muscular dystrophy of variable severity

Marked deficiency of muscle adhalin, a 50 kDa sarcolemmal dystrophin-associated glycoprotein, has been reported in severe childhood autosomal recessive muscular dystrophy (SCARMD). This is a Duchenne-like disease affecting both males and females first described in Tunisian families. Adhalin deficiency has been found in SCARMD patients from North Africa Europe, Brazil, Japan and North America (SLR & KPC, unpublished data). The disease was initially linked to an unidentified gene on chromosome 13 in families from North Africa, and to the adhalin gene itself on chromosome 17q in one French family in which missense mutations were identified. Thus there are two kinds of myopathies with adhalin deficiency: one with a primary defect of adhalin (primary adhalinopathies), and one in which absence of adhalin is secondary to a separate gene defect on chromosome 13. We have examined the importance of primary adhalinopathies among myopathies with adhalin deficiency, and describe several additional mutations (null and missense) in the adhalin gene in 10 new families from Europe and North Africa. Disease severity varies in age of onset and rate of progression, and patients with null mutations are the most severely affected.

Becker and limb-girdle muscular dystrophies: a psychiatric and intellectual level comparative study

There are some indications that Becker muscular dystrophy (BMD) might be related to mental disorders and mental retardation (MR). To investigate this question, we made a standardized psychiatric and intellectual level assessment of 22 BMD patients in comparison with 22 limb-girdle muscular dystrophy (LGMD) patients. There were not significant differences between the two groups. Twelve patients (54.5%) in each group received at least one lifetime psychiatric diagnosis, the most frequent being depressive disorders. The intelligence quotient means for BMD was 85.9 and 87.8 for LGMD. There was one case of mild MR among BMD patients and two cases among LGMD patients.

Dp71 can restore the dystrophin-associated glycoprotein complex in muscle but fails to prevent dystrophy

Two lines of transgenic mdx mice have been generated that express a 71 kD non-muscle isoform of dystrophin (Dp71) in skeletal muscle. This isoform contains the cysteine-rich and C-terminal domains of dystrophin, but lacks the N-terminal actin-binding and central spectrin-like repeat domains. Dp71 was associated with the sarcolemma membrane, where it restored normal expression and localization of all members of the dystrophin-associated glycoprotein complex. However, the skeletal muscle pathology of the transgenic mdx mice remained severe. These results indicate that the dystrophin C terminus cannot function independently to prevent dystrophic symptoms and confirms predictions based on patient data that both the N and C-terminal domains are required for normal dystrophin function.

Human adhalin is alternatively spliced and the gene is located on chromosome 17q21

Mutations in the dystrophin gene cause the X chromosome-linked, recessive Duchenne and Becker muscular dystrophies. Dystrophin, a large cytoskeletal protein, copurifies with a complex of dystrophin-associated proteins which serve to anchor dystrophin to the sarcolemma. One of these associated proteins, adhalin, has been implicated as a candidate for severe childhood autosomal recessive muscular dystrophy (SCARMD) due to absence of anti-adhalin staining in muscle biopsy samples taken from SCARMD patients. Furthermore, the Duchenne-like dystrophic phenotype seen in the SCARMD families was shown to be tightly linked to chromosome 13 markers. To determine the genetic mutation responsible for autosomal dystrophy, we characterized the human adhalin gene. Contrary to our expectation, human adhalin was mapped to chromosome 17q21, excluding adhalin as the gene causing chromosome 13-associated SCARMD. Additionally, a splice form of adhalin message was found that predicts a 35-kDa nontransmembrane adhalin. The expression of both adhalin splice forms is exclusively restricted to striated muscle, unlike other components of the dystrophin-glycoprotein complex.

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.

Missense mutations in the adhalin gene linked to autosomal recessive muscular dystrophy

Adhalin, the 50 kDa dystrophin-associated glycoprotein, is deficient in skeletal muscle of patients having severe childhood autosomal recessive muscular dystrophy (SCARMD). In several North African families, SCARMD has been linked to chromosome 13q, but SCARMD has been excluded from linkage to this locus in other families. We have now cloned human adhalin cDNA and mapped the adhalin gene to chromosome 17q12-q21.33, excluding it from involvement in 13q-linked SCARMD. However, one allelic variant of a polymorphic microsatellite located within intron 6 of the adhalin gene cosegregated perfectly with the disease phenotype in a large family. Furthermore, missense mutations were identified within the adhalin gene that might cause SCARMD in this family. Thus, the adhalin gene is involved in at least one form of autosomal recessive muscular dystrophy.

Abnormal expression of laminin suggests disturbance of sarcolemma-extracellular matrix interaction in Japanese patients with autosomal recessive muscular dystrophy deficient in adhalin

Dystrophin is associated with several novel sarcolemmal proteins, including a laminin-binding extracellular glycoprotein of 156 kD (alpha-dystroglycan) and a transmembrane glycoprotein of 50 kD (adhalin). Deficiency of adhalin characterizes a severe autosomal recessive muscular dystrophy prevalent in Arabs. Here we report for the first time two mongoloid (Japanese) patients with autosomal recessive muscular dystrophy deficient in adhalin. Interestingly, adhalin was not completely absent and was faintly detectable in a patchy distribution along the sarcolemma in our patients. Although the M and B2 subunits of laminin were preserved, the B1 subunit was greatly reduced in the basal lamina surrounding muscle fibers. Our results raise a possibility that the deficiency of adhalin may be associated with the disturbance of sarcolemma-extracellular matrix interaction leading to sarcolemmal instability.

Limb girdle muscular dystrophy: reappraisal of a rejected entity

The term limb girdle muscular dystrophy (LGMD) has been introduced to delineate a distinct form of muscular dystrophy with predominantly proximal upper and lower extremity weakness. Families with evidence of both autosomal recessive and autosomal dominant modes of inheritance have been described. The recognition of other disorders presenting with weakness in a limb girdle distribution, such as the spinal muscular atrophies, dystrophinopathies, inflammatory and metabolic myopathies, casted doubt on the existence of LGMD as a separate entity. Recent linkage studies showing association between various forms of LGMD and loci on chromosome 15, 13 and 5 respectively, and the demonstration of 50K dystrophin associated glycoprotein deficiency in some cases of LGMD, strongly support the notion that limb girdle muscular dystrophy constitutes a separate group of phenotypically and genotypically distinct disorders. Further investigations are necessary to recognize the different subtypes of this disease and to identify the underlying mutations.

Expression of dystrophin-associated glycoproteins during human fetal muscle development: a preliminary immunocytochemical study

An immunocytochemical study was performed on quadriceps muscle from eight fetuses ranging from 12 weeks of gestation to term, using antibodies against the dystrophin-associated proteins, in order to evaluate the developmental expression of these proteins. For comparison, antibodies against dystrophin and utrophin were also used. The expression of the 59 kDa dystrophin-associated protein was simultaneous with that of dystrophin, which is also a subsarcolemmal protein. The extracellular glycoprotein of 156 kDa (alpha-dystroglycan) and the transmembrane glycoprotein of 43 kDa (beta-dystroglycan) appeared to be expressed later. The transmembrane glycoproteins of 50 kDa (adhalin) and 35 kDa were fully expressed at an even later stage of fetal muscle development. This study suggests that the subsarcolemmal proteins may have an essential role in the assembly of the transmembrane and extracellular components of the dystrophin-glycoprotein complex during fetal muscle development. The knowledge obtained from observing the developmental expression of these proteins may contribute to the understanding of the molecular mechanism of their different involvement in muscle disorders.

Assessment of the 50-kDa dystrophin-associated glycoprotein in Brazilian patients with severe childhood autosomal recessive muscular dystrophy

Recently, we have demonstrated the specific deficiency of the 50-kDa dystrophin-associated glycoprotein (50DAG) in severe childhood autosomal recessive muscular dystrophy with Duchenne-like phenotype (SCARMD or AR-DLMD), a disease first reported in Tunisia and now presumed to be prevalent in North Africa and the Middle East. Here we demonstrate the deficiency of the 50DAG in one caucasoid and 5 negroid Brazilian patients with severe muscular dystrophy, which confirms that AR-DLMD with the 50DAG deficiency is not confined to the Arab populations. Without the analysis of both dystrophin and 50DAG, isolated male patients with this condition could be undiagnosed or misdiagnosed as having Duchenne or severe Becker muscular dystrophy. We also report, for the first time, the normal expression of the 50DAG and other dystrophin-associated proteins in one negroid and 2 caucasoid Brazilian patients with a phenotype indistinguishable from that of AR-DLMD with 50DAG deficiency. This is consistent with the genetic heterogeneity for the phenotype of AR-DLMD.

Searching for the 1 in 2,400,000: a review of dystrophin gene point mutations

The past few years have seen a rapid increase in our knowledge of naturally occurring mutations in the dystrophin gene. Although earlier studies were limited to gross rearrangement mutations, we are now in a position to draw lessons on the molecular etiology of the remaining one-third of cases of Duchenne and Becker muscular dystrophy (DMD, BMD) which are associated with small mutations. This paper reviews 70 published and unpublished small mutations in the dystrophin gene and asks what we can learn about their nature, their distribution, and approaches to their characterisation. Strikingly for such a well-conserved gene, missense mutations are extremely rare, and the vast majority of DMD point mutations, like the gross rearrangements, result in premature translational termination. It seems increasingly likely that almost all cases of DMD arise solely as a result of a reduction in the level of dystrophin transcripts, and we argue that > 95% of DMD mutations contribute nothing to the functional dissection of the dystrophin protein. Most of the few BMD point mutations presented here are missense mutations in the N-terminal or C-terminal domains or are splice-site mutations that probably act, like BMD deletions, via the production of in-frame, interstitially deleted transcripts.