Aminoglycoside antibiotics restore dystrophin function to skeletal muscles of mdx mice

Stop codon suppression via inhibition of eRF1 expression

In humans, recognition of a stop codon by protein release factor eRF1 leads to release of the nascent peptide from the ribosome. Although efficient eRF1 activity is usually desirable, numerous pathologies result from eRF1 recognition of premature stop mutations in essential genes. In these cases, decreased eRF1 activity could increase readthrough of the premature stop codon, thereby making full-length protein. To broaden the means available to beneficially decrease eRF1 activity, we have targeted eRF1 mRNA using siRNAs and antisense oligonucleotides. We show that both eRF1-targeted siRNA and antisense oligonucleotides decrease eRF1 mRNA and eRF1 protein concentrations, and increase UAG readthrough in cultured human cells.

Stretch-activated ion channels and c-fos expression remain active after repeated eccentric bouts

This study was undertaken to measure the response of stretch-activated ion channels (SAC) and transcript levels of the oncogene c-fos to separate bouts of eccentric contractions (EC). It was hypothesized that SAC in rat skeletal muscle would contribute to resting membrane potential depolarization after separate repeated bouts of EC. Blockage of SAC during an EC training regime also tested the necessity of SAC for a training response. It was also hypothesized that transcript levels of c-fos would be maximally elevated after the first exposure to EC and diminish with repeated exposures. The results indicate less depolarization after multiple bouts of EC, which could be reversed by blocking the SAC. Transcript levels of c-fos were elevated to a similar degree after either a single or multiple exposures to EC. EC training resulted in significant increases in contractile force and muscle wet and dry weights in nontreated animals. Training in the presence of the SAC-blocker streptomycin produced similar changes in contractile force without changes in muscle weight. SAC and c-fos are activated after several exposures to EC and therefore remain as possible signals in EC training responses.

Gentamicin fails to increase dystrophin expression in dystrophin-deficient muscle

A recent report that aminoglycoside antibiotics restored the expression of functional dystrophin to skeletal muscles of mdx mice, a model of Duchenne muscular dystrophy (DMD), raised hopes that DMD may be treatable by a conventional drug. Subsequently, several human trials were initiated for evaluating gentamicin therapy in selected DMD patients. An increase of dystrophin expression was not detected in one human trial that was fully reported. Here, we report that we were unable to replicate previously published beneficial results by gentamicin treatment in the mdx mouse. Therefore, we believe that additional animal experimentation is required to further evaluate the possibility of in vivo aminoglycoside therapy of DMD.

Pharmacological strategies for muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal, genetic disorder whose relentless progression underscores the urgency for developing a cure. Although Duchenne initiated clinical trials roughly 150 years ago, therapies for DMD remain supportive rather than curative. A paradigm shift towards developing rational therapeutic strategies occurred with identification of the DMD gene. Gene- and cell-based therapies designed to replace the missing gene and/or dystrophin protein have achieved varying degrees of success. However, pharmacological strategies not designed to replace dystrophin per se appear promising, and can circumvent many hurdles hampering gene- and cell-based therapy. Here, we will review present pharmacological strategies, in particular those dealing with functional substitution of dystrophin by utrophin and enhancing muscle progenitor commitment by myostatin blockade, with a view toward facilitating drug discovery for DMD.

Novel therapies for Duchenne muscular dystrophy

The development of therapeutic strategies that overcome the unique problems posed by Duchenne muscular dystrophy (DMD) has lead to the development of many contemporary approaches to human disease in general. Various treatment approaches have been explored--such as pharmacological therapies and cell-based, cytokine, and genetic therapies--that are all targeted to specific features of dystrophic DMD muscle pathology. In genetic therapies, the large size of the dystrophin gene has necessitated the development and use of novel functional minidystrophin and microdystrophin genes, muscle-specific promoter systems, and gutted adenoviral systems. In addition to these well defined viral strategies, plasmid vectors and the upregulation of utrophin (a dystrophin homologue) have potential. Various novel genetic approaches--such as antisense-mediated exon skipping, gene correction, and new cytokine approaches--are also being developed. Together these exciting developments bring an effective treatment for DMD closer than ever before.

Crystal structure of geneticin bound to a bacterial 16S ribosomal RNA A site oligonucleotide

Aminoglycosides are antibacterial molecules that decrease translation accuracy by binding to the decoding aminoacyl-tRNA site (A site) on 16S ribosomal RNA. We have solved the crystal structure of an RNA fragment containing the A site bound to geneticin at 2.40A resolution. Geneticin, also known as G418, is a gentamicin-related aminoglycoside: it contains three rings that are functionalized by hydroxyl, ammonium and methyl groups. The detailed comparison of the distinctive behaviour of geneticin (binding to pro- and eukaryotic A sites) with the crystallographic, biochemical and microbiological results obtained so far for aminoglycoside-A site complexes offers new insights on the system. The two sugar rings constituting the neamine part common to most of the aminoglycosides bind to the A site, as already observed in the crystal structures solved previously with paromomycin and tobramycin. The essential hydrogen bonds involving ring I (to A1408) and ring II (to the phosphate oxygen atoms of the bulged adenine bases 1492 and 1493 and to G1494) are conserved and additional contacts are observed from ring III (to phosphate oxygen atoms of G1405 and U1406). The present work illustrates a molecular basis of the range in sensitiveness exhibited by geneticin towards common point A site mutations associated to resistance phenotypes. In addition, analysis and comparisons of the structures cast light on the role played by the conserved U1406.U1495 pair in the recognition of the A site by aminoglycosides.

M40403, a superoxide dismutase mimetic, protects cochlear hair cells from gentamicin, but not cisplatin toxicity

Gentamicin, an aminoglycoside antibiotic, and cisplatin, a platinum-based anticancer drug, are two commonly used clinical drugs with ototoxic side effects. The ototoxicity of gentamicin and cisplatin has been linked to the production of reactive oxygen species (ROS), although the specific ROS pathways have not been identified. One ROS that might play a role in ototoxicity is the superoxide radical, which is enzymatically dismutated to molecular oxygen and hydrogen peroxide by endogenous superoxide dismutase (SOD) enzymes. M40403, a manganese-based nonpeptidyl molecule that mimics the activity of SOD, was tested for its ability to protect against gentamicin and cisplatin toxicity in cochlear organotypic cultures from neonatal C57BL/10J mice. Cultures were treated with gentamicin or cisplatin alone or in combination with M40403. M40403 alone had no effect on outer hair cell (OHC) or inner hair cell (IHC) survival at doses of 1, 5, and 10 microM, but a high dose of 30 microM reduced hair cell numbers by approximately 30%. Gentamicin alone and cisplatin alone killed OHCs and IHCs in a dose-dependent manner. The addition of M40403 to gentamicin-treated cultures significantly increased OHC and IHC survival in a dose-dependent manner, whereas M40403 failed to protect hair cells in cisplatin-treated cultures at any dose. The results suggest that the toxicity of gentamicin and cisplatin to cochlear hair cells are mediated by different pathways. Clinically, increased levels of SOD or SOD mimetics might provide significant protection against aminoglycoside ototoxicity.

Adeno-associated virus vector-mediated gene transfer into dystrophin-deficient skeletal muscles evokes enhanced immune response against the transgene product

Duchenne muscular dystrophy (DMD) is an X-linked, lethal muscular disorder caused by a defect in the DMD gene. AAV vector-mediated micro-dystrophin cDNA transfer is an attractive approach to treatment of DMD. To establish effective gene transfer into skeletal muscle, we examined the transduction efficiency of an AAV vector in skeletal muscles of dystrophin-deficient mdx mice. When an AAV vector encoding the LacZ gene driven by a CMV promoter (AAV-CMVLacZ) was introduced, beta-galactosidase expression markedly decreased in mdx muscle 4 weeks after injection due to immune responses against the transgene product. We also injected AAV-CMVLacZ into skeletal muscles of mini-dystrophin-transgenic mdx mice (CVBA3'), which show ameliorated phenotypes without overt signs of muscle degeneration. AAV vector administration, however, evoked substantial immune responses in CVBA3' muscle. Importantly, AAV vector using muscle-specific MCK promoter also elicited responses in mdx muscle, but at a considerably later period. These results suggested that neo-antigens introduced by AAV vectors could evoke immune reactions in mdx muscle, since increased permeability allowed a leakage of neo-antigens from the dystrophin-deficient sarcolemma of muscle fibers. However, resident antigen-presenting cells, such as myoblasts, myotubes and regenerating immature myofibers, might also play a role in the immune response.

Improved antisense oligonucleotide induced exon skipping in the mdx mouse model of muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is a fatal genetic disorder caused by dystrophin gene mutations that preclude synthesis of a functional protein. One potential treatment of the disorder has utilised antisense oligoribonucleotides (AOs) to induce removal of disease-associated exons during pre-mRNA processing. Induced in-frame mRNA transcripts encode a shorter but functional dystrophin. We have investigated and improved the design of AOs capable of removing exon 23, and thus the disease-causing nonsense mutation, from mRNA in the mdx mouse model of DMD.

METHODS

H-2K(b)-tsA58 mdx cultures were transfected with complexes of Lipofectin and AOs. Exon skipping was detected by RT-PCR and subsequent protein production was demonstrated by Western blotting. AOs were delivered at a range of doses in order to compare relative efficiencies.

RESULTS

We describe effective and reproducible exon 23 skipping with several AOs, including one as small as 17 nucleotides. Furthermore, the location of a sensitive exon 23 target site has been refined, whilst minimum effective doses have been estimated in vitro. These doses are significantly lower than previously reported and were associated with the synthesis of dystrophin protein in vitro.

CONCLUSIONS

These results demonstrate the increasing feasibility of an AO-based therapy for treatment of DMD. By refining AO design we have been able to reduce the size and the effective dose of the AOs and have dramatically improved the efficiency of the technique.

ADAM12 alleviates the skeletal muscle pathology in mdx dystrophic mice

Muscular dystrophy is characterized by muscle degeneration and insufficient regeneration and replacement of muscle fibers by connective tissue. New therapeutic strategies directed toward various forms of muscular dystrophy are needed to preserve muscle mass and promote regeneration. In this study we examined the role of the transmembrane ADAM12, a disintegrin and metalloprotease, which is normally associated with development and regeneration of skeletal muscle. We demonstrate that ADAM12 overexpression in the dystrophin-deficient mdx mice alleviated the muscle pathology in these animals, as evidenced by less muscle cell necrosis and inflammation, lower levels of serum creatine kinase, and less uptake of Evans Blue dye into muscle fibers. These studies demonstrate that ADAM12 directly or indirectly contributes to muscle cell regeneration, stability, and survival.

Metabolic profiles of dystrophin and utrophin expression in mouse models of Duchenne muscular dystrophy

Metabolic profiles from (1)H nuclear magnetic resonance spectroscopy have been used to describe both one and two protein systems in four mouse models related to Duchenne muscular dystrophy using the pattern recognition technique partial least squares. Robust statistical models were built for extracts and intact cardiac tissue, distinguishing mice according to expression of dystrophin. Using metabolic profiles of diaphragm, models were built describing dystrophin and utrophin, a dystrophin related protein, expression. Increased utrophin expression counteracted some of the deficits associated with dystrophic tissue. This suggests the method may be ideal for following treatment regimes such as gene therapy.

Experimental and therapeutic approaches to muscular dystrophies

PURPOSE OF REVIEW

Most patients suffering from muscular dystrophies can now obtain a precise diagnosis of their underlying molecular defect, but no efficient treatment to prevent disability and death. This review summarizes recent progress towards developing efficient treatments for these severe diseases.

RECENT FINDINGS

Different levels of progress have been achieved in three main approaches: gene therapy, cell therapy and pharmacological therapy. Gene therapy has progressed by improving different vectors for gene delivery. Adenoviruses (mainly high capacity versions) and adeno-associated viruses were the most explored viral vectors. Progress was made in understanding the factors needed for an efficient transfection of muscle. An understanding of protein structure and function in muscular dystrophies has allowed elegant examples of protein engineering as a way of gene therapy. Non-viral vectors for gene transfer, targeted gene modification and transcription modulation have also been explored recently. Cell therapy (myogenic-cell transplantation) progressed in understanding myoblast transplantation in primates for human applications, evaluating protocols for the control of graft rejection, understanding the biology of donor myogenic cells, and searching for alternative sources of donor cells. Three clinical trials using pharmacological approaches (anabolic agents and gentamicin) show very poor or negative results. Other pharmacological approaches (upregulation of alternative therapeutic proteins) are still being researched in mice.

SUMMARY

This panoply of experimental approaches covered all the current possibilities of attacking the problem of treating muscular dystrophies. It is expected that one or more will progress to provide efficient tools for the ultimate clinical goal: to prolong function and life in severe muscular dystrophy patients.

Duchenne muscular dystrophy: current knowledge, treatment, and future prospects

The cloning of the dystrophin gene has led to major advances in the understanding of the molecular genetic basis of Duchenne, Becker, and other muscular dystrophies associated with mutations in genes encoding members of the dystrophin-associated glycoprotein complex. The recent introduction of pharmaceutical agents such as prednisone has shown great promise in delaying the progression of Duchenne muscular dystrophy but there remains a need to develop more long-term therapeutic interventions. Knowledge of the nature of the dystrophin gene and the glycoprotein complex has led many researchers to think that somatic gene replacement represents the most promising approach to treatment. The potential use of this strategy has been shown in the mdx mouse model of Duchenne muscular dystrophy, where germ line gene transfer of either a full-length or a smaller Becker-type dystrophin minigene prevents necrosis and restores normal muscle function.

Comprehensive detection of genomic duplications and deletions in the DMD gene, by use of multiplex amplifiable probe hybridization

Duplications and deletions are known to cause a number of genetic disorders, yet technical difficulties and financial considerations mean that screening for these mutations, especially duplications, is often not performed. We have adapted multiplex amplifiable probe hybridization (MAPH) for the screening of the DMD gene, mutations in which cause Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy. MAPH involves the quantitative recovery of specifically designed probes following hybridization to immobilized genomic DNA. We have engineered probes for each of the 79 exons of the DMD gene, and we analyzed them by using a 96-capillary sequencer. We screened 24 control individuals, 102 patients, and 23 potential carriers and detected a large number of novel rearrangements, especially small, one- and two-exon duplications. A duplication of exon 2 alone was the most frequently occurring mutation identified. Our analysis indicates that duplications occur in 6% of patients with DMD. The MAPH technique as modified here is simple, quick, and accurate; furthermore, it is based on existing technology (i.e., hybridization, PCR, and electrophoresis) and should not require new equipment. Together, these features should allow easy implementation in routine diagnostic laboratories. Furthermore, the methodology should be applicable to any genetic disease, it should be easily expandable to cover >200 probes, and its characteristics should facilitate high-throughput screening.

Genetic diseases of muscle

In the last twenty years, the genetic basis for most of the inherited myopathies and muscular dystrophies has been unveiled. Diseases have been found to result from loss of function of structural components of the muscle basal lamina (e.g., MCD1A), sarcolemma (e.g., the sarcoglycanopathies), nucleus (e.g., EDMD) and sarcomere (e.g., the nemaline myopathies). A few have been associated with abnormalities in the genes for muscle enzymes (e.g., calpain and fukutin). Alternate mechanisms of pathogenesis have also recently been suggested by mutations lying outside of coding regions, such as the "field effect" of chromosomal mutations in DM2. In the future, we will likely identify the genes responsible for the remaining disorders, including many of the distal myopathies. In addition, we may also find skeletal muscle diseases associated with some of the presently non-implicated muscle proteins: syntropin, dystrobrevin, epsilon-sarcoglycan and sarcospan. The next steps may be to identify and understand the relationship of modifier genes producing the phenotypic heterogeneity of many of these diseases and to characterize those and other targets for therapeutic intervention, whether by gene therapy or by pharmacological treatment.

Defective assembly of sarcoglycan complex in patients with beta-sarcoglycan gene mutations. Study of aneural and innervated cultured myotubes

Mutations in the sarcoglycan (SG) genes cause autosomal recessive muscular dystrophies. The absence of each SG complex component in muscle impairs the proper assembly of the entire SG complex, resulting in sarcolemmal damage. We investigated the consequences of beta-SG gene mutations in cultured muscle from two beta-SG mutated patients, and analysed each individual SG protein expression by cross-sectional immunocytochemistry and Western blot in aneural and innervated myotubes. Patients' muscle biopsy showed total loss of SG complex; however, a limited amount of beta-SG was detected in aneural and innervated myotubes, where the protein was localized to the plasma membrane. This paradoxical beta-SG expression can be attributable to antibody cross-reaction or to the expression of an unknown SG isoform specific of immature muscle. In our cultured myotubes, the other components of the SG complex were absent, suggesting that beta-SG gene mutations result in a defective assembly of the entire SG complex in early stages of muscle development, and that the role of beta-SG is crucial for the normal structure and/or function of the SG complex in the sarcolemma.

Nemaline and myotubular myopathies

Nemaline myopathy is caused by mutations in one of at least six different genes. The clinical picture also varies widely, in terms of the grade and the distribution of muscle weakness. In familial cases, autosomal-recessive inheritance is more common than autosomal-dominant inheritance, and in some patients the disorder is caused by new dominant mutations. Because of the genetic heterogeneity and the large size of one of the genes commonly involved, that is, nebulin, no routine molecular genetic testing is yet available. Thus, the diagnosis often still rests on clinical and histologic criteria. Prenatal diagnosis can only reliably be performed in families where the causative mutation(s) have been identified. No clear-cut prognostic indicators are known, and treatment decisions can only be taken in casu. In the long-term management of patients with nemaline myopathy, respiratory capacity requires regular monitoring for early detection of insidious hypoventilation.

Duchenne muscular dystrophy

Duchenne muscular dystrophy is an X-linked disease of muscle caused by an absence of the protein dystrophin. Affected boys begin manifesting signs of disease early in life, cease walking at the beginning of the second decade, and usually die by age 20 years. Until treatment of the basic genetic defect is available, medical, surgical, and rehabilitative approaches can be used to maintain patient function and comfort. Corticosteroids, including prednisone and a related compound, deflazacort, have recently been shown to markedly delay the loss of muscle strength and function in boys with Duchenne muscular dystrophy. Surgical release of lower extremity contractures may benefit some patients. Approximately 90% of boys with Duchenne muscular dystrophy will develop severe scoliosis, which is not amenable to control by nonsurgical means such as bracing or adaptive seating. The most effective treatment for severe scoliosis is prevention by intervening with early spinal fusion utilizing segmental instrumentation as soon as curves are ascertained and before the onset of severe pulmonary or cardiac dysfunction.

The muscular dystrophies

The muscular dystrophies are inherited myogenic disorders characterised by progressive muscle wasting and weakness of variable distribution and severity. They can be subdivided into several groups, including congenital forms, in accordance with the distribution of predominant muscle weakness: Duchenne and Becker; Emery-Dreifuss; distal; facioscapulohumeral; oculopharyngeal; and limb-girdle which is the most heterogeneous group. In several dystrophies the heart can be seriously affected, sometimes in the absence of clinically significant weakness. The genes and their protein products that cause most of these disorders have now been identified. This information is essential to establish an accurate diagnosis and for reliable genetic counselling and prenatal diagnosis. There is, as yet, no way of greatly affecting the long-term course of any of these diseases. However, advances in gene manipulation and stem-cell therapy suggest cautious optimism for finding an effective treatment in the not-too-distant future.

Expression of CTNS alleles: subcellular localization and aminoglycoside correction in vitro

Mutations in CTNS result in one of three forms of cystinosis: benign, intermediate, or nephropathic. Homozygosity for a nonsense mutation in CTNS (753G -->A), encoding a premature termination codon (PTC) at amino acid 138 (W138X), results in nephropathic cystinosis. Gentamicin is known to induce PTC readthrough and hence full-length protein production. We demonstrate that addition of gentamicin (300 microg/ml) to cystinotic fibroblasts leads to depletion of intracellular cystine in cell lines with a premature termination codon, but not in those with a large deletion or a deletion leading to a frameshift mutation. Plasmids were constructed with GFP as a C-terminal or N-terminal fusion to CTNS. The normal CTNS protein fused with either N- or C-terminal GFP colocalized with Lysotracker red, a fluorescent stain which selectively accumulates in lysosomes. PTC-GFP, a construct with GFP fused to the C-terminus of CTNS containing a PTC, allowed GFP to serve as a reporter of PTC readthrough. No significant fluorescence was observed in PTC-GFP-transfected cells in the absence of gentamicin but was seen and localized to lysosomes in its presence. A patient with a splice site mutation (IVS11 + 2T -->C) that eliminates the GYDQL lysosomal targeting sequence of cystinosin on one allele, and a PTC mutation (753G -->A) on the other, displays the intermediate phenotype. Transfection of the splice site mutant allele into CTNS null fibroblasts produced cystine depletion. Plasmids with GFP fused to the N-terminus of CTNS containing the splice site mutation (GFP-SS) were constructed. While the normal CTNS-GFP fusion protein was found to colocalize with Lysotracker red almost exclusively, the GFP-SS fusion product was found in the plasma membrane and cytoplasm, as well as lysosomes. A second lysosomal targeting motif in CTNS is present in this sequence, just proximal to the mutation, accounting for the partial lysosomal localization.

Leupeptin protects cochlear and vestibular hair cells from gentamicin ototoxicity

Calpains, a family of calcium-activated proteases that breakdown proteins, kinases, phosphatases and transcription factors, can promote cell death. Since leupeptin, a calpain inhibitor, protected against hair cell loss from acoustic overstimulation, we hypothesized that it might protect cochlear and vestibular hair cells against gentamicin (GM) ototoxicity. To test this hypothesis, mouse organotypic cultures from the cochlea, maculae of the utricle and the crista of the semicircular canal (P1-P3) were treated with different doses of GM (0.1-3 mM) alone or in the presence of leupeptin (0.1-3 mM). The percentage of outer hair cells (OHCs) and inner hair cells (IHCs) decreased with increasing doses of GM between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced GM-induced damage to IHCs and OHCs; this protective effect was dose-dependent. GM also significantly reduced hair cell density in the crista and utricle in a dose-dependent manner between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced hair cell loss in the crista and utricle for GM concentrations between 0.1 and 3 mM. These results suggest that one of the early steps in GM ototoxicity may involve calcium-activated proteases that lead to the demise of cochlear and vestibular hair cells.

Mistranslation induced by streptomycin provokes a RecABC/RuvABC-dependent mutator phenotype in Escherichia coli cells

Translational stress-induced mutagenesis (TSM) refers to the mutator phenotype observed in Escherichia coli cells expressing a mutant allele (mutA or mutC) of the glycine tRNA gene glyV (or glyW). Because of an anticodon mutation, expression of the mutA allele results in low levels of Asp-->Gly mistranslation. The mutA phenotype does not require lexA-regulated SOS mutagenesis functions, and appears to be suppressed in cells defective for RecABC-dependent homologous recombination functions. To test the hypothesis that the TSM response is mediated by non-specific mistranslation rather than specific Asp-->Gly misreading, we asked if streptomycin (Str), an aminoglycoside antibiotic known to promote mistranslation, can provoke a mutator phenotype. We report that Str induces a strong mutator phenotype in cells bearing certain alleles of rpsL, the gene encoding S12, an essential component of the ribosomal 30 S subunit. The phenotype is strikingly similar to that observed in mutA cells in its mutational specificity, as well as in its requirement for RecABC-mediated homologous recombination functions. Expression of Str-inducible mutator phenotype correlates with mistranslation efficiency in response to Str. Thus, mistranslation in general is able to induce the TSM response. The Str-inducible mutator phenotype described here defines a new functional class of rpsL alleles, and raises interesting questions on the mechanism of action of Str, and on bacterial response to antibiotic stress.

Gene therapy for muscular dystrophies: current status and future prospects

Since the identification in 1987 of the gene for Duchenne muscular dystrophy (DMD), research on the molecular pathogenesis of muscular dystrophy has progressed extensively. In particular, discovery of the DMD gene product, dystrophin, led to the identification of dystrophin-associated proteins and, subsequently, the recognition of other types of muscular dystrophy caused by the defects in each of the sarcoglycan genes. On the other hand, effective therapy for DMD has not yet been established. Some of the viral vectors, such as adeno-associated virus vectors or lentiviral vector, have been proven to enable the long-term expression of the exogenous gene without overt host immune reactions. However, dystrophin cDNAs are too large (14kb) to be accommodated in these viral vectors. To solve this problem, we and other research groups succeeded in truncating full-length dystrophin cDNA to small dystrophin cDNA (4 to 5kb), the products of which protect dystrophin-deficient mdx muscle from contraction-induced membrane damage when introduced by viral vectors or as a transgene into mdx mice. The usefulness of these truncated dystrophin cDNAs should be confirmed using other animal models such as dystrophic dogs. To develop successful treatment of DMD, the authors believe that several different approaches should be used, such as cell transfer therapy, drug design to up-regulate utrophin, or a strategy to repair the mutation in vivo.

Import of amber and ochre suppressor tRNAs into mammalian cells: a general approach to site-specific insertion of amino acid analogues into proteins

A general approach to site-specific insertion of amino acid analogues into proteins in vivo would be the import into cells of a suppressor tRNA aminoacylated with the analogue of choice. The analogue would be inserted at any site in the protein specified by a stop codon in the mRNA. The only requirement is that the suppressor tRNA must not be a substrate for any of the cellular aminoacyl-tRNA synthetases. Here, we describe conditions for the import of amber and ochre suppressor tRNAs derived from Escherichia coli initiator tRNA into mammalian COS1 cells, and we present evidence for their activity in the specific suppression of amber (UAG) and ochre (UAA) codons, respectively. We show that an aminoacylated amber suppressor tRNA (supF) derived from the E. coli tyrosine tRNA can be imported into COS1 cells and acts as a suppressor of amber codons, whereas the same suppressor tRNA imported without prior aminoacylation does not, suggesting that the supF tRNA is not a substrate for any mammalian aminoacyl-tRNA synthetase. These results open the possibility of using the supF tRNA aminoacylated with an amino acid analogue as a general approach for the site-specific insertion of amino acid analogues into proteins in mammalian cells. We discuss the possibility further of importing a mixture of amber and ochre suppressor tRNAs for the insertion of two different amino acid analogues into a protein and the potential use of suppressor tRNA import for treatment of some of the human genetic diseases caused by nonsense mutations.

Three mouse models of muscular dystrophy: the natural history of strength and fatigue in dystrophin-, dystrophin/utrophin-, and laminin alpha2-deficient mice

To optimize and evaluate treatments for muscular dystrophy, it is important to know the natural history of the disease in the absence of therapeutic intervention. Here we characterized disease progression of three mutant mouse strains of muscular dystrophy: mdx mice, which lack dystrophin; mdx:utrn-/- mice, which also lack utrophin; and dy/dy mice, which are deficient in laminin alpha2. Normal mice show a marked increase in forelimb strength over the first 10 weeks of life and little fatigue (<5%) over five consecutive strength trials. Mdx and mdx:utrn-/- mice demonstrate less strength then normal mice and approximately 40% fatigue at each age. Mdx mice become obese but mdx:utrn-/- mice do not. Dy/dy mice remain small and are much weaker than mdx and mdx:utrn-/- mice at all ages even when normalized to weight; however, they show only minimal fatigue (10%). This work demonstrates a distinct pattern of disease progression in each model and provides a foundation for assessing strategies for improving strength in each model.

Therapies for improving muscle function in neuromuscular disorders

Muscle atrophy or wasting is a loss of muscle tissue resulting from disease or lack of use. This review examines recent pharmacologic or nutrition interventions for ameliorating wasting and improving muscle function in neuromuscular disorders. The information has application for treating the muscular dystrophies, cancer cachexia, weightlessness, immobilization, denervation, and disuse atrophy.

Molecular pathophysiology and targeted therapeutics for muscular dystrophy

Experimental therapeutics of the muscular dystrophies has made impressive advances on several fronts. Adeno-associated virus has emerged as the clear 'vector of choice' for muscle gene delivery, with successful functional rescue of dystrophic muscle in rodent models. Correction of the dystrophin gene mutation in a dog model has been reported, and several reports of progress on myogenic stem cell characterization are resurrecting cell transplantation as a possible therapeutic approach. The downstream consequences of dystrophin deficiency are being defined quickly using microarray experiments, and drugs targeting specific biochemical pathways are being tested rapidly in animal models. Such targeted drug discoveries, which are discussed in this article, have begun to be implemented in human clinical trials.

Evidence that systemic gentamicin suppresses premature stop mutations in patients with cystic fibrosis

Here we report the effects of gentamicin treatment on cystic fibrosis transmembrane regulator (CFTR) production and function in CF airway cells and patients with CF with premature stop mutations. Using immunocytochemical and functional [6-methoxy-N- (3-sulfopropyl) quinolinium (SPQ)-based] techniques, ex vivo exposure of airway cells from stop mutation CF patients led to the identification of surface-localized CFTR in a dose-dependent fashion. Next, five patients with CF with stop mutations and five CF control subjects were treated with parenteral gentamicin for 1 wk, and underwent repeated in vivo measures of CFTR function (nasal potential difference [PD] measurements and sweat chloride [Cl(-)] testing). During the treatment period, the number of nasal PD readings in the direction of Cl(-) secretion was increased approximately 3-fold in the stop mutation patient group compared with controls (p < 0.001), and four of five stop mutation patients with CF had at least one reading during gentamicin treatment with a Cl(-) secretory response of more than -5 mV (hyperpolarized). A response of this magnitude was not seen in any of the CF control subjects (p < 0.05). In an independent series of experiments designed to test the ability of repeat nasal PDs to detect wild-type CFTR function, evidence of Cl(-) secretion was seen in 88% of control (non-CF) nasal PDs, and 71% were more than -5 mV hyperpolarized. Together, these results suggest that gentamicin treatment can suppress premature stop mutations in airway cells from patients with CF, and produce small increases in CFTR Cl(-) conductance (as measured by the nasal PD) in vivo.

Novel approaches to treat muscular dystrophies

Muscular dystrophies (MD) are a clinically and genetically heterogeneous group of skeletal muscle-wasting diseases. Mutations in the dystrophin gene result in dystrophin deficiency, which constitutes the pathogenic basis of Duchenne and Becker MD (DMD and BMD). Several MD are caused by mutations in other recently identified genes coding for proteins linked to the sarcolemma, the nuclear envelope or the contractile apparatus. In addition, several MD have been mapped to different chromosomal loci and for most of them, the identification of the molecular defect is underway. The immediate result is an ongoing reclassification of the MD into disorders defined not by clinical characteristics but specific genetic mutations. At present, therapy of MD is based on symptomatic treatment and supportive care. Convincing evidence for clinical efficacy is only available for corticosteroids that also suffer from frequent and severe side effects. Up to now, curative therapy is not available, although promising new molecular therapies are under investigation in animal models of MD. Current treatment strategies are discussed and a perspective for effective molecular therapy is given.

Structural origins of aminoglycoside specificity for prokaryotic ribosomes

Aminoglycoside antibiotics, including paromomycin, neomycin and gentamicin, target a region of highly conserved nucleotides in the decoding region aminoacyl-tRNA site (A site) of 16 S rRNA on the 30 S subunit. Change of a single nucleotide, A1408 to G, reduces the affinity of many aminoglycosides for the ribosome; G1408 distinguishes between prokaryotic and eukaryotic ribosomes. The structures of a prokaryotic decoding region A-site oligonucleotide free in solution and bound to the aminoglycosides paromomycin and gentamicin C1a were determined previously. Here, the structure of a eukaryotic decoding region A-site oligonucleotide bound to paromomycin has been determined using NMR spectroscopy and compared to the prokaryotic A-site-paromomycin structure. A conformational change in three adenosine residues of an internal loop, critical for high-affinity antibiotic binding, was observed in the prokaryotic RNA-paromomycin complex in comparison to its free form. This conformational change is not observed in the eukaryotic RNA-paromomycin complex, disrupting the binding pocket for ring I of the antibiotic. The lack of the conformational change supports footprinting and titration calorimetry data that demonstrate approximately 25-50-fold weaker binding of paromomycin to the eukaryotic decoding-site oligonucleotide. Neomycin, which is much less active against Escherichia coli ribosomes with an A1408G mutation, binds non-specifically to the oligonucleotide. These results suggest that eukaryotic ribosomal RNA has a shallow binding pocket for aminoglycosides, which accommodates only certain antibiotics.

Recent advances in the molecular basis of inherited skin diseases

Over the last few years the molecular basis of several inherited skin diseases has been delineated. Some discoveries have stemmed from a candidate gene approach using clinical, biochemical, immunohistochemical, and ultrastructural clues, while others have arisen from genetic linkage and positional cloning analyses. Notable advances have included elucidation of specific gene pathology in the major forms of inherited skin fragility, ichthyosis, and keratoderma. These findings have led to a better understanding of the significance of individual structural proteins and regulatory enzymes in keratinocyte adhesion and differentiation. From a clinical perspective, the advances have led to better genetic counseling in many disorders, the development of DNA-based prenatal diagnosis, and a foundation for planning newer forms of treatment, including somatic gene therapy, in selected conditions.

Harnessing the potential of dystrophin-related proteins for ameliorating Duchenne's muscular dystrophy

Duchenne's muscular dystrophy (DMD) is a fatal disease caused by mutations in the DMD gene that lead to quantitative and qualitative disturbances in dystrophin expression. Dystrophin is a member of the spectrin superfamily of proteins. Dystrophin itself is closely related to three proteins that constitute a family of dystrophin-related proteins (DRPs): the chromosome 6-encoded DRP or utrophin, the chromosome-X encoded, DRP2 and the chromosome-18 encoded, dystrobrevin. These proteins share sequence similarity and functional motifs with dystrophin. Current attempts at somatic gene therapy of DMD face numerous technical problems. An alternative strategy for DMD therapy, that circumvents many of these problems, has arisen from the demonstration that the DRP utrophin can functionally substitute for the missing dystrophin and its overexpression can rescue dystrophin-deficient muscle. Currently, a promising avenue of research consists of identifying molecules that would increase the expression of utrophin and the delivery of these molecules to dystrophin-deficient tissues as a means of DMD therapy. In this review, we will focus on DRPs from the perspective of strategies and issues related to upregulating utrophin expression for DMD therapy. Additionally, we will address the techniques used for anatomical, biochemical and physiological evaluation of the potential benefits of this and other forms of DMD therapy in dystrophin-deficient animal models.

Pharmacologic and genetic therapy for childhood muscular dystrophies

The outstanding advances in the molecular characterization of muscle diseases, including muscular dystrophies, inflammatory myopathies, and ion channel disorders, have resulted in the identification of potential targets for pharmacologic and genetic therapy in the best characterized of these diseases. The most common myopathy in children, Duchenne muscular dystrophy (DMD), is the focus of active pharmacologic clinical trials. Genetic transfer therapy research for this and other dystrophies is rapidly moving forward. However, as new approaches for treatment are being actively investigated, the current modality of treatment for all myopathies is still in the realm of physical medicine and rehabilitation. The focus of this review is on the advances in pharmacologic and genetic therapy research in DMD and limb girdle muscular dystrophies.

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is not treatable; there is no cure. More than a decade ago, randomized trials demonstrated that oral steroid therapy was of benefit to DMD patients by prolonging ambulation. Although few significant side effects were reported, study patients were followed for 18 months or less. However, when treating DMD with steroids, the clinician must consider beginning treatment in mid-childhood and continuing until adolescence or longer, a total of at least 10 years. There is no evidence that steroids are associated with prolonged life or with improved pulmonary or cardiac function in DMD. It is clear that the risk of side effects increases with duration of use of oral steroids. Therefore, oral steroids are not recommended for treatment of DMD on a routine basis. If, in certain cases, one does institute therapy, the patient should be monitored carefully for side effects, maintain dietary restriction, and exercise regularly.

Premature termination mutations in exon 3 of the SMN1 gene are associated with exon skipping and a relatively mild SMA phenotype

Autosomal recessive spinal muscular atrophy (SMA) is a common motor neuron disease caused by absence or mutation in the survival motor neuron (SMN1) gene. SNM1 and a nearly identical copy, SMN2, encode identical proteins, but SMN2 only produces a little full length protein due to alternative splicing. The level of functional SMN protein and the number of SMN2 genes correlate with the clinical phenotype ranging from severe to very mild. Here, we report on premature termination mutations in SMN1 exon 3 (425del5 and W102X) which induce skipping of the mutated exon. The novel nonsense mutation W102X was detected in two patients with a relatively mild phenotype who had only two copies of the SMN2 gene, a number that has previously been found associated with the severe form of SMA. We show that the shortened transcripts are translated into predicted in frame protein isoforms. Aminoglycoside treatment suppressed the nonsense mutation in cultured cells and abolished exon skipping. Fibroblasts from both patients show a high number of nuclear structures containing SMN protein (gems). These findings suggest that the protein isoform lacking the exon 3 encoded region contributes to the formation of the nuclear protein complex which may account for the milder clinical phenotype.

Problems and solutions in myoblast transfer therapy

Duchenne muscular dystrophy is a severe X-linked neuromuscular disease that affects approximately 1/3500 live male births in every human population, and is caused by a mutation in the gene that encodes the muscle protein dystrophin. The characterization and cloning of the dystrophin gene in 1987 was a major breakthrough and it was considered that simple replacement of the dystrophin gene would ameliorate the severe and progressive skeletal muscle wasting characteristic of Duchenne muscular dystrophy. After 20 years, attempts at replacing the dystrophin gene either experimentally or clinically have met with little success, but there have been many significant advances in understanding the factors that limit the delivery of a normal dystrophin gene into dystrophic host muscle. This review addresses the host immune response and donor myoblast changes underlying some of the major problems associated with myoblast-mediated dystrophin replacement, presents potential solutions, and outlines other novel therapeutic approaches.

Progress in gene therapy for Duchenne muscular dystrophy

Gene transfer research for Duchenne muscular dystrophy (DMD) has brought the goal of successful treatment of this devastating, inherited disease closer to being a reality. Although gene therapeutic approaches for DMD patients are not yet in clinical use, recent advances using DMD animal models are encouraging. Progress in vector design, such as high-capacity adenoviral vectors, targeted adenoviral vectors, and heterodimerization of DNA delivered by adeno-associated virus (AAV) vectors have advanced the field considerably. The recent studies into the pharmacologic-induced read-through of stop codons, the increased study of utrophin and its upregulation, and the introduction of point mutation correction using chimeric oligonucleotides have expanded the field, providing new avenues of inquiry.

Aminoglycoside-mediated suppression of nonsense mutations in late infantile neuronal ceroid lipofuscinosis

The ability of aminoglycoside antibiotics to promote readthrough of eukaryotic stop codons has attracted interest in these drugs as potential therapeutic agents in human disorders caused by nonsense mutations. One disease for which such a therapeutic strategy may be viable is classical late infantile neuronal ceroid lipofuscinosis (LINCL), a fatal childhood neurodegenerative disorder with currently no effective treatment. Premature stop codon mutations in the gene CLN2 encoding the lysosomal tripeptidyl-peptidase 1 (TPP-I) are associated with disease in approximately half of children diagnosed with LINCL. The aim of this study was to examine the ability of the aminoglycoside gentamicin to restore TPP-I activity in LINCL cell lines. In one patient-derived cell line that was compound heterozygous for a commonly seen nonsense mutation, Arg208Stop and a different rare nonsense mutation, approximately 7% of normal levels of TPP-I were maximally restored with gentamicin treatment. In other cell lines from patients that were compound heterozygous for Arg208Stop and a splice junction mutation, approximately 0.5% of maximal activity was restored. These results suggest that pharmacological suppression of nonsense mutations by aminoglycosides or functionally similar pharmaceuticals may have therapeutic potential in LINCL.

Gene therapy and molecular approaches to the treatment of hereditary muscular disorders

Gene therapy for inherited muscle disease is an active area of research and development. Initial emphasis has been on gene replacement but alternative approaches are increasingly being considered in order to overcome difficulties, such as the immune rejection of transduced cells, the need for appropriate and tissue-specific control of expression, and the requirement for systemic spread in some conditions. However, the most significant obstacles to the clinical success of gene therapy are still the lack of efficiency and accuracy of gene medicine delivery.

Aminoglycoside antibiotics mediate context-dependent suppression of termination codons in a mammalian translation system

The translation machinery recognizes codons that enter the ribosomal A site with remarkable accuracy to ensure that polypeptide synthesis proceeds with a minimum of errors. When a termination codon enters the A site of a eukaryotic ribosome, it is recognized by the release factor eRF1. It has been suggested that the recognition of translation termination signals in these organisms is not limited to a simple trinucleotide codon, but is instead recognized by an extended tetranucleotide termination signal comprised of the stop codon and the first nucleotide that follows. Interestingly, pharmacological agents such as aminoglycoside antibiotics can reduce the efficiency of translation termination by a mechanism that alters this ribosomal proofreading process. This leads to the misincorporation of an amino acid through the pairing of a near-cognate aminoacyl tRNA with the stop codon. To determine whether the sequence context surrounding a stop codon can influence aminoglycoside-mediated suppression of translation termination signals, we developed a series of readthrough constructs that contained different tetranucleotide termination signals, as well as differences in the three bases upstream and downstream of the stop codon. Our results demonstrate that the sequences surrounding a stop codon can play an important role in determining its susceptibility to suppression by aminoglycosides. Furthermore, these distal sequences were found to influence the level of suppression in remarkably distinct ways. These results suggest that the mRNA context influences the suppression of stop codons in response to subtle differences in the conformation of the ribosomal decoding site that result from aminoglycoside binding.