Cloning of the Duchenne/Becker muscular dystrophy locus

Molecular and cellular basis of genetically inherited skeletal muscle disorders

Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue - encompassing the muscle cells (myofibres) themselves and their extracellular matrix - or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder - mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy - the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability - processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.

Ixazomib, an oral proteasome inhibitor, exhibits potential effect in dystrophin-deficient mdx mice

Dystrophin deficiency makes the sarcolemma fragile and susceptible to degeneration in Duchenne muscular dystrophy. The proteasome is a multimeric protease complex and is central to the regulation of cellular proteins. Previous studies have shown that proteasome inhibition improved pathological changes in mdx mice. Ixazomib is the first oral proteasome inhibitor used as a therapy in multiple myeloma. This study investigated the effects of ixazomib on the dystrophic muscle of mdx mice. MDX mice were treated with ixazomib (7.5 mg/kg/wk by gavage) or 0.2 mL of saline for 12 weeks. The Kondziela test was performed to measure muscle strength. The tibialis anterior (TA) and diaphragm (DIA) muscles were used for morphological analysis, and blood samples were collected for biochemical measurement. We observed maintenance of the muscle strength in the animals treated with ixazomib. Treatment with ixazomib had no toxic effect on the mdx mouse. The morphological analysis showed a reduction in the inflammatory area and fibres with central nuclei in the TA and DIA muscles and an increase in the number of fibres with a diameter of 20 µm2 in the DIA muscle after treatment with ixazomib. There was an increase in the expression of dystrophin and utrophin in the TA and DIA muscles and a reduction in the expression of osteopontin and TGF-β in the DIA muscle of mdx mice treated with ixazomib. Ixazomib was thus shown to increase the expression of dystrophin and utrophin associated with improved pathological and functional changes in the dystrophic muscles of mdx mice.

Current Translational Research and Murine Models For Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked genetic disorder characterized by progressive muscle degeneration. Mutations in the DMD gene result in the absence of dystrophin, a protein required for muscle strength and stability. Currently, there is no cure for DMD. Since murine models are relatively easy to genetically manipulate, cost effective, and easily reproducible due to their short generation time, they have helped to elucidate the pathobiology of dystrophin deficiency and to assess therapies for treating DMD. Recently, several murine models have been developed by our group and others to be more representative of the human DMD mutation types and phenotypes. For instance, mdx mice on a DBA/2 genetic background, developed by Fukada et al., have lower regenerative capacity and exhibit very severe phenotype. Cmah-deficient mdx mice display an accelerated disease onset and severe cardiac phenotype due to differences in glycosylation between humans and mice. Other novel murine models include mdx52, which harbors a deletion mutation in exon 52, a hot spot region in humans, and dystrophin/utrophin double-deficient (dko), which displays a severe dystrophic phenotype due the absence of utrophin, a dystrophin homolog. This paper reviews the pathological manifestations and recent therapeutic developments in murine models of DMD such as standard mdx (C57BL/10), mdx on C57BL/6 background (C57BL/6-mdx), mdx52, dystrophin/utrophin double-deficient (dko), mdxβgeo, Dmd-null, humanized DMD (hDMD), mdx on DBA/2 background (DBA/2-mdx), Cmah-mdx, and mdx/mTRKO murine models.

Current Perspectives in Autism Spectrum Disorder: From Genes to Therapy

Autism spectrum disorder (ASD) is a constellation of neurodevelopmental presentations with high heritability and both phenotypic and genetic heterogeneity. To date, mutations in hundreds of genes have been associated to varying degrees with increased ASD risk. A better understanding of the functions of these genes and whether they fit together in functional groups or impact similar neuronal circuits is needed to develop rational treatment strategies. We will review current areas of emphasis in ASD research, starting from human genetics and exploring how mouse models of human mutations have helped identify specific molecular pathways (protein synthesis and degradation, chromatin remodeling, intracellular signaling), which are linked to alterations in circuit function and cognitive/social behavior. We will conclude by discussing how we can leverage the findings on molecular and cellular alterations found in ASD to develop therapies for neurodevelopmental disorders.

Long-term administration of the TNF blocking drug Remicade (cV1q) to mdx mice reduces skeletal and cardiac muscle fibrosis, but negatively impacts cardiac function

Duchenne muscular dystrophy (DMD) is a degenerative skeletal muscle disease caused by mutations in the gene encoding dystrophin (DYS). Tumor necrosis factor (TNF) has been implicated in the pathogenesis since short-term treatment of mdx mice with TNF blocking drugs proved beneficial; however, it is not clear whether long-term treatment will also improve long-term outcomes of fibrosis and cardiac health. In this investigation, short and long-term dosing studies were carried out using the TNF blocking drug Remicade and a variety of outcome measures were assessed. Here we show no demonstrable benefit to muscle strength or morphology with 10mg/kg or 20mg/kg Remicade; however, 3mg/kg produced positive strength benefits. Remicade treatment correlated with reductions in myostatin mRNA in the heart, and concomitant reductions in cardiac and skeletal fibrosis. Surprisingly, although Remicade treated mdx hearts were less fibrotic, reductions in LV mass and ejection fraction were also observed, and these changes coincided with reductions in AKT phosphorylation on threonine 308. Thus, TNF blockade benefits mdx skeletal muscle strength and fibrosis, but negatively impacts AKT activation, leading to deleterious changes to dystrophic heart function. These studies uncover a previously unknown relationship between TNF blockade and alteration of muscle growth signaling pathways.

Mammalian artificial chromosomes: A review

A mammalian artificial chromosome (MAC) may be assembled through the juxtapposition of three kinds of DNA elements: a centromere, several DNA replication origins, and two telomeric repeats. The resulting structure should be able to carry and express one or more selected genes (transgenes), introduced for specific purposes. The minimal length is unknown, but may be of several Mb.Of its basic elements, the telomeres may present lesser problems, in view of their simple composition and organization. Centromeres could be an issue, given their many unknowns. Mammalian DNA replication origins are at present poorly characterized, but it is expected that at least one may be contained within the MAC components, especially the transgene. Their overall assembly may require a combination of in vivo and in vitro approaches.A promising strategy aims at constructing two telomeric arms of a MAC, one of which may include the transgene. The two novel arms could acquire a functional centromere through recombination with the two arms of a resident chromosome. Alternatively, if the two telomeric constructs are also endowed with properly placed and oriented centromeric sequences, a centromere may be rescued in vivo by homologous recombination with the external parts of the centromere of the resident chromosome. Positive selection for the artificial arms and counterselection against the resident arms should facilitate the assembly process.The assembly of such construct would not change the ploidy number of the host cell. After loading of a transgene, however, the resulting MAC may be isolated and transferred into an expression cell, where it may represent a novel chromosomal element. In this case untoward effects to the host cell may derive from an ensuing dosage effect for the transgene(s) rather than from the presence of a MAC per se.A MAC may contribute to a deeper understanding of the structural requirements for chromosomal function and evolution as well as the mechanism of chromatin formation. It should also help in the development of second generation vectors for transfer of Mb-long DNA sequences, as required for properly regulated mammalian gene function as well as, possibly, for therapy.

Carbamylated erythropoietin does not alleviate signs of dystrophy in mdx mice

Erythropoietin promotes myoblast proliferation and inhibits fibrosis and thus it could impede the pathogenesis of muscle degenerative diseases. However, its stimulation of erythropoiesis limits its use as a therapeutic agent. An erythropoietin analog, carbamylated erythropoietin (C-EPO), retains these protective actions, yet it does not interact with the erythropoietin receptor. To determine whether treatment with C-EPO alleviates the signs of muscular dystrophy in an animal model of Duchenne muscular dystrophy, we treated mdx mice with intraperitoneal injections of 50 μg/kg and 100 μg/kg C-EPO for 4 and 12 weeks, and we monitored weight, serum creatine kinase levels, and changes in muscle histology. Moderate histological improvement was observed at 4 weeks, which did not translate into a significantly decreased level of serum creatine kinase. At the doses tested, C-EPO is not an effective therapeutic for the treatment of a mouse model of Duchenne muscular dystrophy.

Molecular, cellular, and pharmacological therapies for Duchenne/Becker muscular dystrophies

Although the molecular defect causing Duchenne/Becker muscular dystrophy (DMD/BMD) was identified nearly 20 years ago, the development of effective therapeutic strategies has nonetheless remained a daunting challenge. Over the years, a variety of different approaches have been explored in an effort to compensate for the lack of the DMD gene product called dystrophin. This review not only presents some of the most promising molecular, cellular, and pharmacological strategies but also highlights some issues that need to be addressed before considering their implementation. Specifically, we describe current strategies being developed to exogenously deliver healthy copies of the dystrophin gene to dystrophic muscles. We present the findings of several studies that have focused on repairing the mutant dystrophin gene using various approaches. We include a discussion of cell-based therapies that capitalize on the use of myoblast or stem cell transfer. Finally, we summarize the results of several studies that may eventually lead to the development of appropriate drug-based therapies. In this context, we review our current knowledge of the mechanisms regulating expression of utrophin, the autosomal homologue of dystrophin. Given the complexity associated with the dystrophic phenotype, it appears likely that a combinatorial approach involving different therapeutic strategies will be necessary for the appropriate management and eventual treatment of this devastating neuromuscular disease.

Study of human SP-A, SP-B and SP-D loci: allele frequencies, linkage disequilibrium and heterozygosity in different races and ethnic groups

BACKGROUND

SP-A, SP-B, and SP-D are pulmonary surfactant proteins. Several linkage and association studies have been done using these genes as markers to locate pulmonary disease susceptibility genes, but few have studied the markers systematically in different ethnic groups. Here we studied eight markers in SP-A, SP-B, and SP-D genes in seven ethnic groups from three races (Caucasian, Black and Hispanic). We measured the similarity of the marker distribution among the ethnic groups in order to see whether people in different ethnic groups or races could be mixed together for linkage and association studies. To evaluate the usefulness of these markers, we estimated the informativeness of each marker loci in the seven ethnic groups by assessing their heterozygosity and PIC values. We also conducted linkage disequilibrium (LD) analysis to identify associated marker loci and to estimate the haplotype frequencies in each of the seven ethnic groups in an attempt to find valuable haplotypes so that the level of polymorphism of the "markers" could be increased.

RESULTS

Our findings indicate that allele and genotype frequencies may be different between different ethnic groups, especially between ethnic groups from different races. The markers are in general polymorphic in a variety of study groups, especially for the two SP-A1 and SP-A2 markers. Two-locus LD analysis reveals that three pairs of loci are strongly associated together: B-18(A/C) with B1013(A/C), DA11(C/T) with DA160(A/G), SP-A1 with SP-A2. Three-locus LD analysis suggests that B-18(A/C), B1013(A/C) and B1580(C/T) are strongly associated with each other.

CONCLUSIONS

Allele and genotype frequency differences imply that different ethnic groups should be mixed with extreme caution before performing linkage and association studies. The associated markers could be used together to increase the level of polymorphism and the informativeness of the "markers".

High-resolution methylation analysis of the human hypoxanthine phosphoribosyltransferase gene 5' region on the active and inactive X chromosomes: correlation with binding sites for transcription factors

DNA methylation within GC-rich promoters of constitutively expressed X-linked genes is correlated with transcriptional silencing on the inactive X chromosome in female mammals. For most X-linked genes, X chromosome inactivation results in transcriptionally active and inactive alleles occupying each female nucleus. To examine mechanisms responsible for maintaining this unique system of differential gene expression, we have analyzed the methylation of individual cytosine residues in the 5' CpG island of the human hypoxanthine phosphoribosyltransferase (HPRT) gene on the active and inactive X chromosomes. Methylation analysis of 142 CpG dinucleotides by genomic sequencing was carried out on purified DNA using the cytosine-specific Maxam and Gilbert DNA sequencing reaction in conjunction with ligation-mediated PCR. These studies demonstrate the 5' CpG islands of active and 5-azacytidine-reactivated alleles are essentially unmethylated while the inactive allele is hypermethylated. The inactive allele is completely methylated at nearly all CpG dinucleotides except in a 68-bp region containing four adjacent GC boxes where most CpG dinucleotides are either unmethylated or partially methylated. Curiously, these GC boxes exhibit in vivo footprints only on the active X chromosome, not on the inactive X. The methylation pattern of the inactive HPRT gene is strikingly different from that reported for the inactive X-linked human phosphoglycerate kinase gene which exhibits methylation at all CpG sites in the 5' CpG island. These results suggest that the position of methylated CpG dinucleotides, the density of methylated CpGs, the length of methylated regions, and/or chromatin structure associated with methylated DNA may have a role in repressing the activity of housekeeping promoters on the inactive X chromosome. The pattern of DNA methylation on the inactive human HPRT gene may also provide insight into the process of inactivating the gene early in female embryogenesis.

Expression of utrophin (dystrophin-related protein) and dystrophin-associated glycoproteins in muscles from patients with Duchenne muscular dystrophy

We examined whether the dystrophin-associated glycoprotein complex (GPC), which serves to fix dystrophin to cell membranes, is present at the sarcolemma in Duchenne muscular dystrophy (DMD) muscles using an immunohistochemical method. Antibodies against 50DAG (A2) and 43DAG (A3a), the components of GPC, were used for the detection of GPC. We found that, although the amount of GPC was reduced in DMD muscles where utrophin but not dystrophin was distinctly present, 43DAG (A3a) was fairly heavily and 50DAG (A2) was lightly but distinctly stained on the cell surfaces. It is likely that the capability of utrophin to preserve 50DAG (A2) is less than that of dystrophin, although utrophin has been reported to bind to GPC. We also found that 43DAG (A3a) but not 50DAG (A2) was detected in the peripheral nerves where utrophin was detected. Therefore, it is likely that 43DAG (A3a) is essential for the fixation of utrophin to cell membranes, as in the case of dystrophin. 50DAG (A2) may play other important roles in the pathogenesis of DMD.

High-resolution methylation analysis of the human hypoxanthine phosphoribosyltransferase gene 5' region on the active and inactive X chromosomes: correlation with binding sites for transcription factors

DNA methylation within GC-rich promoters of constitutively expressed X-linked genes is correlated with transcriptional silencing on the inactive X chromosome in female mammals. For most X-linked genes, X chromosome inactivation results in transcriptionally active and inactive alleles occupying each female nucleus. To examine mechanisms responsible for maintaining this unique system of differential gene expression, we have analyzed the methylation of individual cytosine residues in the 5' CpG island of the human hypoxanthine phosphoribosyltransferase (HPRT) gene on the active and inactive X chromosomes. Methylation analysis of 142 CpG dinucleotides by genomic sequencing was carried out on purified DNA using the cytosine-specific Maxam and Gilbert DNA sequencing reaction in conjunction with ligation-mediated PCR. These studies demonstrate the 5' CpG islands of active and 5-azacytidine-reactivated alleles are essentially unmethylated while the inactive allele is hypermethylated. The inactive allele is completely methylated at nearly all CpG dinucleotides except in a 68-bp region containing four adjacent GC boxes where most CpG dinucleotides are either unmethylated or partially methylated. Curiously, these GC boxes exhibit in vivo footprints only on the active X chromosome, not on the inactive X. The methylation pattern of the inactive HPRT gene is strikingly different from that reported for the inactive X-linked human phosphoglycerate kinase gene which exhibits methylation at all CpG sites in the 5' CpG island. These results suggest that the position of methylated CpG dinucleotides, the density of methylated CpGs, the length of methylated regions, and/or chromatin structure associated with methylated DNA may have a role in repressing the activity of housekeeping promoters on the inactive X chromosome. The pattern of DNA methylation on the inactive human HPRT gene may also provide insight into the process of inactivating the gene early in female embryogenesis.

Mapping of 386 kb of genomic DNA in the human dystrophin-encoding gene (DYS) using an ordered phage lambda sublibrary of a YAC clone containing the DYS region

An integrated restriction map for HindIII and EcoRI has been constructed for 386 kb of the human dystrophin-encoding gene by partial digest mapping of 35 overlapping lambda EMBL3cosW phage clones derived from a yeast artificial chromosome containing this region. Map construction was simplified in two ways. Firstly, the sequence arrangement of lambda EMBL3cosW is such that only map data from cloned inserts are generated using partial digests of lambda phage DNA asymmetrically labelled at the left cos end with a complementary 32P-labelled oligodeoxyribonucleotide. Secondly, the degree of partial digestion was standardised for each restriction enzyme by using ultraviolet light-induced formation of thymine dimers in the recognition sequence to partially block the cleavage reaction. The map provides the basis for work on the analysis of chromosomal rearrangements in this region which give rise to Duchenne muscular dystrophy, and for studies of chromosome structure and function.

Heterozygotic gene expression in endomyocardial biopsies: a new diagnostic tool confirms the Duchenne carrier status

Identification of the defective gene and the absent gene product dystrophin can substantiate the clinical evidence for manifesting X-linked Duchenne type muscular dystrophy (DMD). It is not always possible, however, to rule out definitely a clinically asymptomatic carrier status in question, since even in the proven carrier DNA analysis is often inconclusive, and multinucleated skeletal muscle fibers express a basically normal membrane dystrophin. To substantiate the value of endomyocardial biopsy as a new tool for detection of the DMD carrier status we examined an endomyocardial biopsy of a volunteer who met the clinical criteria of a DMD carrier. Dystrophin immunohistochemistry and western blot of her skeletal muscle biopsy were inconclusive, and polymerase chain reaction and cDNA analysis failed to locate directly the X-chromosomal defect. We observed a clearcut mosaic of dystrophin-positive and -negative mononucleated cardiac muscle cells, reflecting a heterozygote carrier status in her endomyocardial biopsy, whereas 20 controls were uniformely positive. The incidence of DMD (1:3000 males) and especially the 30% spontaneous mutation rate, still the major pitfall in DNA analysis, show the need for an additional diagnostic tool.

Prediction of dystrophin phenotype by DNA analysis in Duchenne/Becker muscular dystrophy

Allele-specific molecular diagnosis of Duchenne and Becker muscular dystrophies (DMD and BMD) has been largely dependent upon muscle biopsy for dystrophin protein assay. We performed lymphocyte DNA mutation analysis by polymerase chain reaction on 14 boys presenting with a clinical picture compatible with DMD or BMD. DNA analysis revealed that 12 of 14 boys had a deletion of the dystrophin gene, thus establishing the diagnosis of DMD/BMD. Furthermore, genotypes for 9 of 12 deletion patients permitted prediction of the specific allelic disorder (i.e., DMD or BMD). Subsequent dystrophin testing confirmed all of the DNA-based diagnoses. We propose that DNA mutation analysis be included in the initial evaluation of patients suspected of having DMD/BMD, thus potentially eliminating the need for muscle biopsy in the majority of patients.

Distal transcript of the dystrophin gene initiated from an alternative first exon and encoding a 75-kDa protein widely distributed in nonmuscle tissues

A transcript generated by the distal part of the Duchenne Muscular Dystrophy (DMD) gene was initially detected in cells where the full size 14-kilobase (kb) messenger RNA is not found at a significant level. This transcript, approximately 4.5 kb long, corresponds to the cysteine-rich and carboxyl-terminal domains of dystrophin. It begins with a novel 80- to 100-nucleotide exon containing an ATG start site for a new coding sequence of 17 nucleotides in-frame with the consecutive dystrophin cDNA sequence from exon 63. This result suggests the existence of a third promoter that would be localized about 8 kilobases upstream from exon 63 of the DMD gene. The distal transcript is widely distributed but is absent in adult skeletal and myometrial muscle. It is much more abundant in fetal tissues. With an antibody directed against the dystrophin carboxyl terminus, the protein corresponding to this transcript was detected as a 70- to 75-kDa entity on Western blots. It was found in all tissues analyzed except in skeletal muscle. It was not found in lymphoblastoid cells from a Duchenne patient with a complete deletion of the dystrophin gene. The role and subcellular localization of this protein is not known. It may explain extramuscular symptoms exhibited by some Duchenne patients.

Expression of the N-terminal domain of dystrophin in E. coli and demonstration of binding to F-actin

The N-terminal head domain of human dystrophin has been expressed in soluble form and high yield in E. coli, allowing us to test the previously unconfirmed assumption that dystrophin binds actin. DMD246, the first 246 amino acid residues of dystrophin, binds F-actin in a strongly co-operative manner with a Hill constant of 3.5, but does not bind G-actin. Dystrophin heads are thus functionally competent actin-binding proteins. This result opens the way to identifying critical residues in the actin-binding site and encourages us that the other domains of dystrophin might also be treated as functionally autonomous modules, accessible to a similar approach.

Duchenne muscular dystrophy: gene and gene product; mechanism of mutation in the gene

The X-linked gene responsible for Duchenne muscular dystrophy encodes dystrophin, a high-molecular-weight cytoskeletal protein. Studies in several laboratories have revealed deletion of one or more exons in 60% of affected boys; quantitative analysis in our laboratory has detected duplication of exons in another 6%. The severe Duchenne phenotype is associated with deletions or duplications that shift the reading frame of the message, whereas the milder Becker muscular dystrophy is associated with deletions or duplications that maintain the reading frame. Patients who have neither deletion nor duplication may have nonsense mutations, one of which has been detected by predicting the site of the mutation from the size of the truncated protein. Rare females with the disease have a translocation that disrupts the dystrophin gene on one X chromosome and causes non-random inactivation of the normal X, resulting in the expression of the disease. The high frequency of new mutation provides an opportunity to study the mechanism of chromosomal rearrangement that is characteristic of the disease. Our laboratory has focused on the translocations in females and on duplications in affected males. The X-autosome translocations of affected females are all de novo events that originated in the paternal set of chromosomes. Molecular characterization of the translocation junctions revealed reciprocal translocation with both deletion and addition of nucleotides at the junction, suggestive of a breakage and reunion mechanism. Duplications studied to date are all tandem in nature and sequence analysis of duplication junctions has revealed both homologous and non-homologous recombination. Marker segregation analysis has revealed that five out of five duplications originated in a single X chromosome of one of the maternal grandparents, suggesting that the recombination event is unequal sister chromatid exchange.

Replication errors may contribute to the generation of large deletions and duplications in the dystrophin gene

Frequent recurrent mutations of the human dystrophin gene lead to Duchenne and Becker muscular dystrophies. Although the approximately 2.5 Mb size of the gene may form a large target for mutations it is not clear to date which mechanisms promote the observed high frequency of spontaneous mutants (1 in 10,000 X-chromosomes) of which a high percentage (> 70%) are gross structural aberrations (deletions/duplications). In order to gain insight into possible molecular mechanisms we have cloned and sequenced the deletion junction fragments from two unrelated Duchenne patients. Our data, together with a short review on other cases from the literature, present evidence that errors of DNA replication may contribute to the generation of submicroscopic chromosome rearrangements.

Genotype-phenotype correlation and germline mosaicism in DMD/BMD patients with deletions of the dystrophin gene

The molecular analysis of 127 DMD/BMD patients showed that 73 of them (57%) had deletions in the dystrophin gene. Two different methods were used in this study: (a) hybridization of HindIII-digested genomic DNA with nine cDNA probes corresponding to the entire 14kb cDNA of the DMD gene; and (b) simultaneous amplification of nine exons of the DMD gene (multiplex DNA amplification) by the polymerase chain reaction (PCR). When the deletion breakpoints of the intragenic deletions were analyzed with regard to their phenotypic consequences, nine patients were found to represent exceptions to the reading-frame hypothesis. Information regarding mental development was also available for 61 of the 73 deleted patients and for 34 of the 54 non-deleted ones. The proportion of mentally retarded patients was found to be similar in the two groups (deleted, 15%; non-deleted, 18%). Finally, in one family, a junction fragment present in the patient was not found in the peripheral blood DNA of the mother but was present in the sister, thus indicating germline mosaicism in the mother.

Dystrophin diagnosis: comparison of dystrophin abnormalities by immunofluorescence and immunoblot analyses

Immunoblot characterization and immunofluorescence localization of dystrophin are presented for 76 human patients with various neuromuscular diseases. Normal dystrophin (shown by immunoblotting) was invariably visualized as a continuous, peripheral membrane immunostaining of myofibers. Biochemical abnormalities of dystrophin (either lower or higher molecular weight dystrophin) resulted in patchy, discontinuous immunostaining, suggesting that the abnormal dystrophin proteins are not capable of creating a complete membrane cytoskeleton network. There was a very strong correlation of clinical diagnoses with the type of dystrophin abnormality; all Duchenne muscular dystrophy patient muscle contained no detectable dystrophin, Becker muscular dystrophy patient muscle had clearly abnormal dystrophin, and unrelated diseases showed normal dystrophin. However, a single patient of five carrying the diagnosis of Fukuyama dystrophy showed no detectable dystrophin and thus appeared to be a Duchenne dystrophy patient by the biochemical assays. We know of no other case of a patient with a disease thought to be unrelated to Duchenne/Becker dystrophy yet demonstrating dystrophin deficiency. Based on the data presented, we conclude that immunofluorescence is the best technique for the detection of female carriers of Duchenne dystrophy, whereas immunoblotting appears superior for the prognostic diagnosis of Becker muscular dystrophy.

Hexose transport in human myoblasts

The present investigation reports on the hexose transport properties of human myoblasts isolated from normal subjects and from patients with Duchenne muscular dystrophy (DMD). Similar to rat myoblast L6, normal human myoblasts possess a high- (HAHT) and a low- (LAHT) affinity hexose transport system. The non-metabolizable hexose analogue, 2-deoxyglucose, is preferentially taken up by HAHT. The transport of this analogue is the rate-limiting step in the uptake process. This human myoblast HAHT is also similar to that of the rat myoblast in its substrate specificity and in response to the energy uncouplers, cytochalasin B and phloretin. The human myoblast LAHT resembles that of rat myoblast in its insensitivity to energy uncouplers, and in its transport affinity and capacity for 3-O-methyl-D-glucose. Although DMD myoblasts resemble their normal counterpart in their ability to differentiate, they differ significantly in their hexose transport properties. In addition to HAHT and LAHT present in normal human myoblast, DMD myoblasts contain a super-high-affinity hexose transport system (SHAHT). SHAHT can be detected only at very low substrate concentrations. It differs from HAHT not only in its much higher transport affinity, but also in its response to the traditional hexose transport inhibitors. For example, SHAHT can be activated by cytochalasin B and phlorizin, whereas it is more sensitive to inhibition by phloretin. Unlike HAHT, energy uncouplers are found to be ineffective in inhibiting SHAHT. It should be mentioned that SHAHT cannot be detected in myoblasts isolated from patients with other types of myopathy. The present study serves to demonstrate that more than one hexose transport system is operating in human skeletal muscle cells, as found in other cell types.