A second promoter provides an alternative target for therapeutic up-regulation of utrophin in Duchenne muscular dystrophy

Endogenous bioluminescent reporters reveal a sustained increase in utrophin gene expression upon EZH2 and ERK1/2 inhibition

Duchenne muscular dystrophy (DMD) is an X-linked disorder caused by loss of function mutations in the dystrophin gene (Dmd), resulting in progressive muscle weakening. Here we modelled the longitudinal expression of endogenous Dmd, and its paralogue Utrn, in mice and in myoblasts by generating bespoke bioluminescent gene reporters. As utrophin can partially compensate for Dmd-deficiency, these reporters were used as tools to ask whether chromatin-modifying drugs can enhance Utrn expression in developing muscle. Myoblasts treated with different PRC2 inhibitors showed significant increases in Utrn transcripts and bioluminescent signals, and these responses were independently verified by conditional Ezh2 deletion. Inhibition of ERK1/2 signalling provoked an additional increase in Utrn expression that was also seen in Dmd-mutant cells, and maintained as myoblasts differentiate. These data reveal PRC2 and ERK1/2 to be negative regulators of Utrn expression and provide specialised molecular imaging tools to monitor utrophin expression as a therapeutic strategy for DMD.

Identifying FDA-Approved Drugs that Upregulate Utrophin A as a Therapeutic Strategy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a neuromuscular disease caused by mutations and deletions within the DMD gene, which result in a lack of dystrophin protein at the sarcolemma of skeletal muscle fibers. The absence of dystrophin fragilizes the sarcolemma and compromises its integrity during cycles of muscle contraction, which, progressively, leads to reductions in muscle mass and function. DMD is thus a progressive muscle-wasting disease that results in a loss of ambulation, cardiomyopathy , respiratory impairment, and death. Although there is presently no cure for DMD, recent advances have led to many promising treatments. One such approach entails increasing expression of a homologous protein to dystrophin, named utrophin A, which is endogenously expressed in both healthy and DMD muscle fibers. Upregulation of utrophin A all along the sarcolemma of DMD muscle fibers can, in part, compensate for the absence of dystrophin. Over the years, our laboratory has focused a significant portion of our efforts in identifying and characterizing drugs and small molecules for their ability to target utrophin A and cause its overexpression. As part of these efforts, we have recently developed a novel ELISA-based high-throughput drug screen, to identify FDA-approved drugs that increase the expression of utrophin A in muscle cells in culture as well as in dystrophic mice. Here, we describe our overall strategy to identify and characterize several FDA-approved drugs that upregulate utrophin A expression and provide details on all experimental approaches. Such strategy has the potential to lead to the rapid development of novel therapeutics for DMD.

Utrophin modulator drugs as potential therapies for Duchenne and Becker muscular dystrophies

Utrophin is an autosomal paralogue of dystrophin, a protein whose deficit causes Duchenne and Becker muscular dystrophies (DMD/BMD). Utrophin is naturally overexpressed at the sarcolemma of mature dystrophin‐deficient fibres in DMD and BMD patients as well as in the mdx Duchenne mouse model. Dystrophin and utrophin can co‐localise in human foetal muscle, in the dystrophin‐competent fibres from DMD/BMD carriers, and revertant fibre clusters in biopsies from DMD patients. These findings suggest that utrophin overexpression could act as a surrogate, compensating for the lack of dystrophin, and, as such, it could be used in combination with dystrophin restoration therapies. Different strategies to overexpress utrophin are currently under investigation. In recent years, many compounds have been reported to modulate utrophin expression efficiently in preclinical studies and ameliorate the dystrophic phenotype in animal models of the disease. In this manuscript, we discuss the current knowledge on utrophin protein and the different mechanisms that modulate its expression in skeletal muscle. We also include a comprehensive review of compounds proposed as utrophin regulators and, as such, potential therapeutic candidates for these muscular dystrophies.

PMO-based let-7c site blocking oligonucleotide (SBO) mediated utrophin upregulation in mdx mice, a therapeutic approach for Duchenne muscular dystrophy (DMD)

Upregulation of utrophin, a dystrophin related protein, is considered a promising therapeutic approach for Duchenne muscular dystrophy (DMD). Utrophin expression is repressed at the post-transcriptional level by a set of miRNAs, among which let-7c is evolutionarily highly conserved. We designed PMO-based SBOs complementary to the let-7c binding site in UTRN 3′UTR, with the goal of inhibiting let-7c interaction with UTRN mRNA and thus upregulating utrophin. We used the C2C12UTRN5′luc3′ reporter cell line in which the 5′- and 3′-UTRs of human UTRN sequences flank luciferase, for reporter assays and the C2C12 cell line for utrophin western blots, to independently evaluate the site blocking efficiency of a series of let-7c PMOs in vitro. Treatment of one-month old mdx mice with the most effective let-7c PMO (i.e. S56) resulted in ca. two-fold higher utrophin protein expression in skeletal muscles and the improvement in dystrophic pathophysiology in mdx mice, in vivo. In summary, we show that PMO-based let-7c SBO has potential applicability for upregulating utrophin expression as a therapeutic approach for DMD.

Targeting IRES-dependent translation as a novel approach for treating Duchenne muscular dystrophy

Internal-ribosomal entry sites (IRES) are translational elements that allow the initiation machinery to start protein synthesis via internal initiation. IRESs promote tissue-specific translation in stress conditions when conventional cap-dependent translation is inhibited. Since many IRES-containing mRNAs are relevant to diseases, this cellular mechanism is emerging as an attractive therapeutic target for pharmacological and genetic modulations. Indeed, there has been growing interest over the past years in determining the therapeutic potential of IRESs for several disease conditions such as cancer, neurodegeneration and neuromuscular diseases including Duchenne muscular dystrophy (DMD). IRESs relevant for DMD have been identified in several transcripts whose protein product results in functional improvements in dystrophic muscles. Together, these converging lines of evidence indicate that activation of IRES-mediated translation of relevant transcripts in DMD muscle represents a novel and appropriate therapeutic strategy for DMD that warrants further investigation, particularly to identify agents that can modulate their activity.

Genome Editing-Mediated Utrophin Upregulation in Duchenne Muscular Dystrophy Stem Cells

Utrophin upregulation is considered a promising therapeutic strategy for Duchenne muscular dystrophy (DMD). A number of microRNAs (miRNAs) post-transcriptionally regulate utrophin expression by binding their cognate sites in the 3′ UTR. Previously we have shown that miRNA: UTRN repression can be alleviated using miRNA let-7c site blocking oligonucleotides (SBOs) to achieve utrophin upregulation and functional improvement in mdx mice. Here, we used CRISPR/Cas9-mediated genome editing to delete five miRNA binding sites (miR-150, miR-296-5p, miR-133b, let-7c, miR-196b) clustered in a 500 bp inhibitory miRNA target region (IMTR) within the UTRN 3′ UTR, for achieving higher expression of endogenous utrophin. Deleting the UTRN IMTR in DMD patient-derived human induced pluripotent stem cells (DMD-hiPSCs) resulted in ca. 2-fold higher levels of utrophin protein. Differentiation of the UTRN edited DMD-hiPSCs (UTRNΔIMTR) by MyoD overexpression resulted in increased sarcolemmal α-sarcoglycan staining consistent with improved dystrophin glycoprotein complex (DGC) restoration. These results demonstrate that CRISPR/Cas9-based UTRN genome editing offers a novel utrophin upregulation therapeutic strategy applicable to all DMD patients, irrespective of the dystrophin mutation status.

Identification of therapeutics that target eEF1A2 and upregulate utrophin A translation in dystrophic muscles

Up-regulation of utrophin in muscles represents a promising therapeutic strategy for the treatment of Duchenne Muscular Dystrophy. We previously demonstrated that eEF1A2 associates with the 5’UTR of utrophin A to promote IRES-dependent translation. Here, we examine whether eEF1A2 directly regulates utrophin A expression and identify via an ELISA-based high-throughput screen, FDA-approved drugs that upregulate both eEF1A2 and utrophin A. Our results show that transient overexpression of eEF1A2 in mouse muscles causes an increase in IRES-mediated translation of utrophin A. Through the assessment of our screen, we reveal 7 classes of FDA-approved drugs that increase eEF1A2 and utrophin A protein levels. Treatment of mdx mice with the 2 top leads results in multiple improvements of the dystrophic phenotype. Here, we report that IRES-mediated translation of utrophin A via eEF1A2 is a critical mechanism of regulating utrophin A expression and reveal the potential of repurposed drugs for treating DMD via this pathway.

One potential approach for the treatment of Duchenne muscular dysrophy is to increase expression of the dystrophin homolog utrophin. Here, the authors show that eEF1A2 regulates utrophin expression, and show that 2 FDA-approved drugs upregulate eEIF1A2 and utrophin level in mice, leading to improvement of the dystrophic phenotype.

High-throughput identification of post-transcriptional utrophin up-regulators for Duchenne muscle dystrophy (DMD) therapy

Upregulation of endogenous utrophin offers great promise for treating DMD, as it can functionally compensate for the lack of dystrophin caused by DMD gene mutations, without the immunogenic concerns associated with delivering dystrophin. However, post-transcriptional repression mechanisms targeting the 5′ and 3′ untranslated regions (UTRs) of utrophin mRNA significantly limit the magnitude of utrophin upregulation achievable by promoter activation. Using a utrophin 5′3′UTR reporter assay, we performed a high-throughput screen (HTS) for small molecules capable of relieving utrophin post-transcriptional repression. We identified 27 hits that were ranked using a using an algorithm that we designed for hit prioritization that we call Hit to Lead Prioritization Score (H2LPS). The top 10 hits were validated using an orthogonal assay for endogenous utrophin expression. Evaluation of the top scoring hit, Trichostatin A (TSA), demonstrated utrophin upregulation and functional improvement in the mdx mouse model of DMD. TSA and the other small molecules identified here represent potential starting points for DMD drug discovery efforts.

Alternative utrophin mRNAs contribute to phenotypic differences between dystrophin-deficient mice and Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disorder caused by absence of functional dystrophin protein. Compensation in dystrophin‐deficient (mdx) mice may be achieved by overexpression of its fetal paralogue, utrophin. Strategies to increase utrophin levels by stimulating promoter activity using small compounds are therefore a promising pharmacological approach. Here, we characterise similarities and differences existing within the mouse and human utrophin locus to assist in high‐throughput screening for potential utrophin modulator drugs. We identified five novel 5′‐utrophin isoforms (A′,B′,C,D and F) in adult and embryonic tissue. As the more efficient utrophin‐based response in mdx skeletal muscle appears to involve independent transcriptional activation of conserved, myogenic isoforms (A′ and F), elevating their paralogues in DMD patients is an encouraging therapeutic strategy.

In Vivo Target Gene Activation via CRISPR/Cas9-Mediated Trans-epigenetic Modulation

Current genome-editing systems generally rely on inducing DNA double-strand breaks (DSBs). This may limit their utility in clinical therapies, as unwanted mutations caused by DSBs can have deleterious effects. CRISPR/Cas9 system has recently been repurposed to enable target gene activation, allowing regulation of endogenous gene expression without creating DSBs. However, in vivo implementation of this gain-of-function system has proven difficult. Here, we report a robust system for in vivo activation of endogenous target genes through trans-epigenetic remodeling. The system relies on recruitment of Cas9 and transcriptional activation complexes to target loci by modified single guide RNAs. As proof-of-concept, we used this technology to treat mouse models of diabetes, muscular dystrophy, and acute kidney disease. Results demonstrate that CRISPR/Cas9-mediated target gene activation can be achieved in vivo, leading to measurable phenotypes and amelioration of disease symptoms. This establishes new avenues for developing targeted epigenetic therapies against human diseases. VIDEO ABSTRACT.

Spell Checking Nature: Versatility of CRISPR/Cas9 for Developing Treatments for Inherited Disorders

Clustered regularly interspaced short palindromic repeat (CRISPR) has arisen as a frontrunner for efficient genome engineering. However, the potentially broad therapeutic implications are largely unexplored. Here, to investigate the therapeutic potential of CRISPR/Cas9 in a diverse set of genetic disorders, we establish a pipeline that uses readily obtainable cells from affected individuals. We show that an adapted version of CRISPR/Cas9 increases the amount of utrophin, a known disease modifier in Duchenne muscular dystrophy (DMD). Furthermore, we demonstrate preferential elimination of the dominant-negative FGFR3 c.1138G>A allele in fibroblasts of an individual affected by achondroplasia. Using a previously undescribed approach involving single guide RNA, we successfully removed large genome rearrangement in primary cells of an individual with an X chromosome duplication including MECP2. Moreover, removal of a duplication of DMD exons 18-30 in myotubes of an individual affected by DMD produced full-length dystrophin. Our findings establish the far-reaching therapeutic utility of CRISPR/Cas9, which can be tailored to target numerous inherited disorders.

Cis-Acting Sequence Elements and Upstream Open Reading Frame in Mouse Utrophin-A 5'-UTR Repress Cap-Dependent Translation

Utrophin, the autosomal homologue of dystrophin can functionally compensate for dystrophin deficiency. Utrophin upregulation could therefore be a therapeutic strategy in Duchenne Muscular Dystrophy (DMD) that arises from mutation in dystrophin gene. In contrast to its transcriptional regulation, mechanisms operating at post-transcriptional level of utrophin expression have not been well documented. Although utrophin-A 5'-UTR has been reported with internal ribosome entry site (IRES), its inhibitory effect on translation is also evident. In the present study we therefore aimed to compare relative contribution of cap-independent and cap-dependent translation with mouse utrophin-A 5'-UTR through m7G-capped and A-capped mRNA transfection based reporter assay. Our results demonstrate that cap-independent translation with utrophin-A 5'-UTR is not as strong as viral IRES. However, cap-independent mode has significant contribution as cap-dependent translation is severely repressed with utrophin-A 5'-UTR. We further identified two sequence elements and one upstream open reading frame in utrophin-A 5'-UTR responsible for repression. The repressor elements in utrophin-A 5'-UTR may be targeted for utrophin upregulation.

Identification of a secondary promoter of CASP8 and its related transcription factor PURα

Caspase-8 (CASP8) is an essential initiator of apoptosis and is associated with many diseases in humans including esophageal squamous cell carcinoma. CASP8 produces a variety of transcripts, which might perform distinct functions. However, the cis and trans transcriptional determinants that control CASP8 expression remain poorly defined. Using a series of luciferase reporter assays, we identified a novel secondary promoter of CASP8 within chr2: 202,122,236 to 202,123,227 and 25 kb downstream of the previously described CASP8 promoter. ENCODE ChIP-seq data for this novel promoter region revealed several epigenetic features, including high levels of histone H3 lysine 27 acetylation and lysine 4 methylation, as well as low levels of CpG island methylation. We developed a mass spectrometry based strategy to identify transcription factors that contribute to the function of the secondary promoter. We found that the transcription activator protein PURα is specifically involved in the transcriptional activation of the secondary promoter and may exert its function by forming a complex with E2F-1 and RNA polymerase II. PURα can bind to both DNA and RNA, and functions in the initiation of DNA replication, regulation of transcription. We observed that knockdown of PURα expression decreased the transcriptional activity of the secondary promoter and mRNA expression of CASP8 isoform G. Although the physiologic roles of this secondary promoter remain unclear, our data may help explain the complexity of CASP8 transcription and suggest that the various caspase 8 isoforms may have distinct regulations and functions.

Duchenne muscular dystrophy drug discovery - the application of utrophin promoter activation screening

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a devastating genetic muscle wasting disease caused by mutations in the DMD gene that in turn lead to an absence of dystrophin. Currently, there is no definitive therapy for DMD. Gene- and cell-based therapies designed to replace dystrophin have met some degree of success, as have strategies that seek to improve the dystrophic pathology independent of dystrophin.

AREAS COVERED

In this review the authors focus on utrophin promoter activation-based strategies and their implications on potential therapeutics for DMD. These strategies in common are designed to identify drugs/small molecules that can activate the utrophin promoter and would allow the functional substitution of dystrophin by upregulating utrophin expression in dystrophic muscle. The authors provide an overview of utrophin biology with a focus on regulation of the utrophin promoter and discuss current attempts in identifying utrophin promoter-activating molecules using high-throughput screening (HTS).

EXPERT OPINION

The characterisation of utrophin promoter regulatory mechanisms coupled with advances in HTS have allowed researchers to undertake screens and identify a number of promising lead compounds that may prove useful for DMD. In principle, these pharmacological compounds offer significant advantages from a translational viewpoint for developing DMD therapeutics.

A cell-based high-throughput screening assay for posttranscriptional utrophin upregulation

Duchenne muscular dystrophy (DMD) is a devastating muscle-wasting disease caused by mutations in the dystrophin gene. Utrophin is a homologue of dystrophin that can compensate for its absence when overexpressed in DMD animal models. Utrophin upregulation is therefore a promising therapeutic approach for DMD. Utrophin is regulated at both transcriptional and posttranscriptional levels. Transcriptional regulation has been studied extensively, and assays have been described for the identification of utrophin promoter-targeting molecules. However, despite the profound impact that posttranscriptional regulation has on utrophin expression, screening assays have not yet been described that could be used to discover pharmaceuticals targeting this key phase of regulation. We describe the development and validation of a muscle cell line-based assay in which a stably expressed luciferase coding sequence is flanked by the utrophin 5'- and 3'-untranslated regions (UTRs). The assay was validated using the posttranscriptional regulation of utrophin by miR-206. The assay has a Z' of 0.7, indicating robust performance in high-throughput format. This assay can be used to study utrophin regulatory mechanisms or to screen chemical libraries for compounds that upregulate utrophin posttranscriptionally via its UTRs. Compounds identified via this assay, used alone or in a synergistic combination with utrophin promoter-targeting molecules, would be predicted to have therapeutic potential for DMD.

Genome-wide genetic associations with IFNγ response to smallpox vaccine

Smallpox is a deadly and debilitating disease that killed hundreds of millions of people in the past century alone. The use of Vaccinia virus-based smallpox vaccines led to the eradication of smallpox. These vaccines are remarkably effective, inducing the characteristic pustule or "take" at the vaccine site in >97 % of recipients, and inducing a wide spectrum of long-lasting humoral and cellular immune responses. The mechanisms behind inter-individual vaccine-response variability are likely to involve host genetic variation, but have not been fully characterized. We report here the first smallpox vaccine response genome-wide association study of over 1,000 recent recipients of Dryvax(®). The data presented here focus on cellular immune responses as measured by both production of secreted IFNγ and quantitation of IFNγ secreting cells by ELISPOT assay. We identified multiple significant SNP associations in genes (RASA1, ADRA1D, TCF7L1, FAS) that are critical components of signaling pathways that directly control lymphocyte IFNγ production or cytotoxic T cell function. Similarly, we found many associations with SNPs located in genes integral to nerve cell function; findings that, given the complex interplay between the nervous and immune systems, deserve closer examination in follow-up studies.

Translational regulation of utrophin by miRNAs

BACKGROUND

Utrophin is the autosomal homolog of dystrophin, the product of the Duchenne Muscular Dystrophy (DMD) locus. Its regulation is of therapeutic interest as its overexpression can compensate for dystrophin's absence in animal models of DMD. The tissue distribution and transcriptional regulation of utrophin have been characterized extensively, and more recently translational control mechanisms that may underlie its complex expression patterns have begun to be identified.

METHODOLOGY/PRINCIPAL FINDINGS

Using a variety of bioinformatic, molecular and cell biology techniques, we show that the muscle isoform utrophin-A is predominantly suppressed at the translational level in C2C12 myoblasts. The extent of translational inhibition is estimated to be ~99% in C2C12 cells and is mediated by both the 5'- and 3'-UTRs of the utrophin-A mRNA. In this study we identify five miRNAs (let-7c, miR-150, miR-196b, miR-296-5p, miR-133b) that mediate the repression, and confirm repression by the previously identified miR-206. We demonstrate that this translational repression can be overcome by blocking the actions of miRNAs, resulting in an increased level of utrophin protein in C2C12 cells.

CONCLUSIONS/SIGNIFICANCE

The present study has identified key inhibitory mechanisms featuring miRNAs that regulate utrophin expression, and demonstrated that these mechanisms can be targeted to increase endogenous utrophin expression in cultured muscle cells. We suggest that miRNA-mediated inhibitory mechanisms could be targeted by methods similar to those described here as a novel strategy to increase utrophin expression as a therapy for DMD.

Drug discovery for Duchenne muscular dystrophy via utrophin promoter activation screening

BACKGROUND

Duchenne muscular dystrophy (DMD) is a devastating muscle wasting disease caused by mutations in dystrophin, a muscle cytoskeletal protein. Utrophin is a homologue of dystrophin that can functionally compensate for its absence when expressed at increased levels in the myofibre, as shown by studies in dystrophin-deficient mice. Utrophin upregulation is therefore a promising therapeutic approach for DMD. The use of a small, drug-like molecule to achieve utrophin upregulation offers obvious advantages in terms of delivery and bioavailability. Furthermore, much of the time and expense involved in the development of a new drug can be eliminated by screening molecules that are already approved for clinical use.

METHODOLOGY/PRINCIPAL FINDINGS

We developed and validated a cell-based, high-throughput screening assay for utrophin promoter activation, and used it to screen the Prestwick Chemical Library of marketed drugs and natural compounds. Initial screening produced 20 hit molecules, 14 of which exhibited dose-dependent activation of the utrophin promoter and were confirmed as hits. Independent validation demonstrated that one of these compounds, nabumetone, is able to upregulate endogenous utrophin mRNA and protein, in C2C12 muscle cells.

CONCLUSIONS/SIGNIFICANCE

We have developed a cell-based, high-throughput screening utrophin promoter assay. Using this assay, we identified and validated a utrophin promoter-activating drug, nabumetone, for which pharmacokinetics and safety in humans are already well described, and which represents a lead compound for utrophin upregulation as a therapy for DMD.

Progress in therapy for Duchenne muscular dystrophy

Duchenne muscular dystrophy is a devastating muscular dystrophy of childhood. Mutations in the dystrophin gene destroy the link between the internal muscle filaments and the extracellular matrix, resulting in severe muscle weakness and progressive muscle wasting. There is currently no cure and, whilst palliative treatment has improved, affected boys are normally confined to a wheelchair by 12 years of age and die from respiratory or cardiac complications in their twenties or thirties. Therapies currently being developed include mutation-specific treatments, DNA- and cell-based therapies, and drugs which aim to modulate cellular pathways or gene expression. This review aims to provide an overview of the different therapeutic approaches aimed at reconstructing the dystrophin-associated protein complex, including restoration of dystrophin expression and upregulation of the functional homologue, utrophin.

Effective myotube formation in human adipose tissue-derived stem cells expressing dystrophin and myosin heavy chain by cellular fusion with mouse C2C12 myoblasts

Stem cell therapy for muscular dystrophies requires stem cells that are able to participate in the formation of new muscle fibers. However, the differentiation steps that are the most critical for this process are not clear. We investigated the myogenic phases of human adipose tissue-derived stem cells (hASCs) step by step and the capability of myotube formation according to the differentiation phase by cellular fusion with mouse myoblast C2C12 cells. In hASCs treated with 5-azacytidine and fibroblast growth factor-2 (FGF-2) for 1 day, the early differentiation step to express MyoD and myogenin was induced by FGF-2 treatment for 6 days. Dystrophin and myosin heavy chain (MyHC) expression was induced by hASC conditioned medium in the late differentiation step. Myotubes were observed only in hASCs undergoing the late differentiation step by cellular fusion with C2C12 cells. In contrast, hASCs that were normal or in the early stage were not involved in myotube formation. Our results indicate that stem cells expressing dystrophin and MyHC are more suitable for myotube formation by co-culture with myoblasts than normal or early differentiated stem cells expressing MyoD and myogenin.

Monitoring duchenne muscular dystrophy gene therapy with epitope-specific monoclonal antibodies

Several molecular approaches to Duchenne muscular dystrophy (DMD) therapy are at or near the point of clinical trial and usually involve attempts to replace the missing dystrophin protein. Although improved muscle function is the ultimate measure of success, assessment of dystrophin levels after therapy is essential to determine whether any improved function is a direct consequence of the treatment or, in the absence of improved function, to determine whether new dystrophin is present, though ineffective. The choice of a monoclonal antibody (mAb) to distinguish successful therapy from naturally occurring "revertant" fibres depends on which dystrophin exons are deleted in the DMD patient. Over the past 20 years, we have produced over 150 "exon-specific" mAbs, mapped them to different regions of dystrophin and made them available through the MDA Monoclonal Antibody Resource for research and for clinical trials tailored to individual patients. In this protocol, we describe the use of these mAb to monitor DMD gene therapy.

Dynamic functional modules in co-expressed protein interaction networks of dilated cardiomyopathy

Background

Molecular networks represent the backbone of molecular activity within cells and provide opportunities for understanding the mechanism of diseases. While protein-protein interaction data constitute static network maps, integration of condition-specific co-expression information provides clues to the dynamic features of these networks. Dilated cardiomyopathy is a leading cause of heart failure. Although previous studies have identified putative biomarkers or therapeutic targets for heart failure, the underlying molecular mechanism of dilated cardiomyopathy remains unclear.

Results

We developed a network-based comparative analysis approach that integrates protein-protein interactions with gene expression profiles and biological function annotations to reveal dynamic functional modules under different biological states. We found that hub proteins in condition-specific co-expressed protein interaction networks tended to be differentially expressed between biological states. Applying this method to a cohort of heart failure patients, we identified two functional modules that significantly emerged from the interaction networks. The dynamics of these modules between normal and disease states further suggest a potential molecular model of dilated cardiomyopathy.

Conclusions

We propose a novel framework to analyze the interaction networks in different biological states. It successfully reveals network modules closely related to heart failure; more importantly, these network dynamics provide new insights into the cause of dilated cardiomyopathy. The revealed molecular modules might be used as potential drug targets and provide new directions for heart failure therapy.

Molecular characterization of a de novo 6q24.2q25.3 duplication interrupting UTRN in a patient with arthrogryposis

Chromosome 6q duplications have been documented repeatedly, allowing the delineation of a "6q duplication syndrome," characterized by hypertelorism, downslanting palpebral fissures, tented upper lip, short neck, severe mental and growth retardation, and joint contractures. Most reported cases result from malsegregation of a reciprocal translocation leading to a terminal 6q duplication and partial monosomy of another chromosome. Only 11 cases of de novo pure duplication have been reported so far. The breakpoints do not appear to be recurrent, but in most cases they have not been characterized molecularly, precluding genotype-phenotype correlation. We report on an 8-year-old girl with a phenotype consistent with mild 6q duplication syndrome, including characteristic physical findings, mild mental retardation, and joint contractures. She carries a 13 Mb de novo 6q24.2q25.3 duplication, diagnosed by high-resolution karyotype and confirmed by array-CGH. Molecular characterization of the duplicated segment with quantitative PCR showed that the proximal breakpoint is localized within the UTRN gene, encoding utrophin, the autosomal homologue of dystrophin. We discuss the possible implication of UTRN in arthrogryposis associated with duplications spanning the 6q23q26 region.

Dystrophins, utrophins, and associated scaffolding complexes: role in mammalian brain and implications for therapeutic strategies

Two decades of molecular, cellular, and functional studies considerably increased our understanding of dystrophins function and unveiled the complex etiology of the cognitive deficits in Duchenne muscular dystrophy (DMD), which involves altered expression of several dystrophin-gene products in brain. Dystrophins are normally part of critical cytoskeleton-associated membrane-bound molecular scaffolds involved in the clustering of receptors, ion channels, and signaling proteins that contribute to synapse physiology and blood-brain barrier function. The utrophin gene also drives brain expression of several paralogs proteins, which cellular expression and biological roles remain to be elucidated. Here we review the structural and functional properties of dystrophins and utrophins in brain, the consequences of dystrophins loss-of-function as revealed by numerous studies in mouse models of DMD, and we discuss future challenges and putative therapeutic strategies that may compensate for the cognitive impairment in DMD based on experimental manipulation of dystrophins and/or utrophins brain expression.

Exon-specific dystrophin antibodies for studies of Duchenne muscular dystrophy

Exon-specific anti-dystrophin antibodies are used to monitor the success of treatments for Duchenne muscular dystrophy that aim to restore the missing dystrophin protein. Dystrophin is a large cytoskeletal protein encoded by 79 exons and expressed mainly in muscle. Most cases of Duchenne and Becker muscular dystrophies are caused by genetic deletion of one or more exons. In-frame deletions permit some synthesis of internally-deleted dystrophin and cause the milder Becker form, while out-of-frame deletions in the severe Duchenne form result in early stop-codons and no functional dystrophin synthesis. In this study, we describe the production of ten new monoclonal antibodies against a rod region encoded by exons 55–59 and their mapping to specific dystrophin exons, thus filling a major gap in the spectrum of available antibodies. The antibodies have already been applied in a published clinical trial of a drug treatment for Duchenne muscular dystrophy.

Cis-acting elements responsible for dopaminergic neuron-specific expression of zebrafish slc6a3 (dopamine transporter) in vivo are located remote from the transcriptional start site

The purpose of this study was to analyze the transcriptional regulation of the zebrafish solute carrier family 6 member 3 gene (slc6a3, dopamine transporter, dat), as a first step towards isolating regulatory sequences useful for driving transgene expression within dopaminergic neurons of the zebrafish CNS in vivo. We found that the 3.0 kb slc6a3 mRNA is expressed in each of the major groups of dopaminergic neurons previously identified in the zebrafish CNS. The slc6a3 gene spans >20 kb of genomic DNA and contains 15 exons. The genomic organization of slc6a3 is highly conserved with respect to its human orthologue, including the presence of an untranslated first exon. The promoter lacks a canonical TATA box and there are multiple transcriptional start sites. Functional analysis of cis-acting elements responsible for the expression pattern of slc6a3 was carried out by generating stable transgenic zebrafish lines expressing fluorescent reporters under transcriptional control of fragments of slc6a3 genomic sequence. The region between -2 kb and +5 kb with respect to the transcriptional start site contains the core slc6a3 promoter, in addition to neuronal enhancers and/or non-neuronal repressors that restrict expression to the CNS, but this region lacks cis-acting elements responsible for slc6a3 expression in dopaminergic neurons. The upstream sequence between -6 kb and -2 kb contains an enhancer element that drives slc6a3 expression in dopaminergic neurons of the pretectal region, and additional sequences that partially repress expression in non-dopaminergic neurons. However, expression of slc6a3 in dopaminergic neurons of the ventral diencephalon and telencephalon is dependent on elements that lie outside the region -6 kb to +5 kb. These data provide a detailed analysis of the slc6a3 gene and show that its expression in different populations of dopamine neurons is driven by discrete enhancers, rather than a single target sequence for a terminal factor involved in specifying neurochemical phenotype.

Differential expression of utrophin-A and -B promoters in the central nervous system (CNS) of normal and dystrophic mdx mice

Utrophin (Utrn) is the autosomal homolog of dystrophin, the Duchene Muscular Dystrophy (DMD) locus product and of therapeutic interest, as its overexpression can compensate dystrophin's absence. Utrn is transcribed by Utrn-A and -B promoters with mRNAs differing at their 5' ends. However, previous central nervous system (CNS) studies used C-terminal antibodies recognizing both isoforms. As this distinction may impact upregulation strategies, we generated Utrn-A and -B promoter-specific antibodies, Taqman Polymerase chain reaction (PCR)-based absolute copy number assays, and luciferase-reporter constructs to study CNS of normal and dystrophic mdx mice. Differential expression of Utrn-A and -B was noted in microdissected and capillary-enriched fractions. At the protein level, Utrn-B was predominantly expressed in vasculature and ependymal lining, whereas Utrn-A was expressed in neurons, astrocytes, choroid plexus and pia mater. mRNA quantification demonstrated matching patterns of differential expression; however, transcription-translation mismatch was noted for Utrn-B in caudal brain regions. Utrn-A and Utrn-B proteins were significantly upregulated in olfactory bulb and cerebellum of mdx brain. Differential promoter activity, mRNA and protein expressions were studied in cultured C2C12, bEnd3, neurons and astrocytes. Promoter activity ranking for Utrn-A and -B was neurons > astrocytes > C2C12 > bEnd3 and bEnd3 > astrocytes > neurons > C2C12, respectively. Our results identify promoter usage patterns for therapeutic targeting and define promoter-specific differential distribution of Utrn isoforms in normal and dystrophic CNS.

The value of mammalian models for duchenne muscular dystrophy in developing therapeutic strategies

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy. There is no effective treatment and patients typically die in approximately the third decade. DMD is an X-linked recessive disease caused by mutations in the dystrophin gene. There are three mammalian models of DMD that have been used to understand better the pathogenesis of disease and develop therapeutic strategies. The mdx mouse is the most widely used model of DMD that displays some features of muscle degeneration, but the pathogenesis of disease is comparatively mild. The severity of disease in mice lacking both dystrophin and utrophin is similar to DMD, but one has to account for the discrete functions of utrophin. Canine X-linked muscular dystrophy (cxmd) is the best representation of DMD, but the phenotype of the most widely used golden retriever (GRMD) model is variable, making functional endpoints difficult to ascertain. Although each mammalian model has its limitations, together they have been essential for the development of several treatment strategies for DMD that target dystrophin replacement, disease progression, and muscle regeneration.

Targeting artificial transcription factors to the utrophin A promoter: effects on dystrophic pathology and muscle function

Duchenne muscular dystrophy is caused by a genetic defect in the dystrophin gene. The absence of dystrophin results in muscle fiber necrosis and regeneration, leading to progressive muscle fiber loss. Utrophin is a close analogue of dystrophin. A substantial, ectopic expression of utrophin in the extrasynaptic sarcolemma of dystrophin-deficient muscle fibers can prevent deleterious effects of dystrophin deficiency. An alternative approach for the extrasynaptic up-regulation of utrophin involves the augmentation of utrophin transcription via the endogenous utrophin A promoter using custom-designed transcriptional activator proteins with zinc finger (ZFP) motifs. We tested a panel of custom-designed ZFP for their ability to activate the utrophin A promoter. Expression of one such ZFP efficiently increased, in a time-dependent manner, utrophin transcript and protein levels both in vitro and in vivo. In dystrophic mouse (mdx) muscles, administration of adenoviral vectors expressing this ZFP led to significant enhancement of muscle function with decreased necrosis, restoration of the dystrophin-associated proteins, and improved resistance to eccentric contractions. These studies provide evidence that specifically designed ZFPs can act as strong transcriptional activators of the utrophin A promoter. These may thus serve as attractive therapeutic agents for dystrophin deficiency states such as Duchenne muscular dystrophy.

Downstream utrophin enhancer is required for expression of utrophin in skeletal muscle

BACKGROUND

Duchenne muscular dystrophy is caused by the absence of the muscle cytoskeletal protein dystrophin. Utrophin is an autosomal homologue of dystrophin, and overexpression of utrophin is expected to compensate for the dystrophin deficit. We previously reported that the 5.4-kb 5'-flanking region of the utrophin gene containing the A-utrophin core promoter did not drive transgene expression in heart and skeletal muscle. To clarify the regulatory mechanism of utrophin expression, we generated a nuclear localization signal-tagged LacZ transgenic (Tg) mouse, in which the LacZ gene was driven by the 129-bp downstream utrophin enhancer (DUE) and the 5.4-kb 5'-flanking region of the utrophin promoter.

METHODS

Two Tg lines were established. The levels of transgene mRNA expression in several tissues were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitative RT-PCR. Cryosections of several tissues were stained with haematoxylin and eosin and X-gal.

RESULTS

The transgene expression patterns were consistent with endogenous utrophin in several tissues including heart and skeletal muscle. Transgene expression was also up-regulated more in regenerating muscle than in nonregenerating muscle. Moreover, utrophin expression was augmented in the skeletal muscle of DUE Tg/dystrophin-deficient mdx mice through cross-breeding experiments. We finally established cultures of primary myogenic cells from this Tg mouse and found that utrophin up-regulation during muscle differentiation depends on the DUE motif.

CONCLUSIONS

Our results showed that DUE is indispensable for utrophin expression in skeletal muscle and heart, and primary myogenic cells from this Tg mice provide a high through-put screening system for drugs that up-regulate utrophin expression.

RNA analysis by nuclease protection

Nuclease protection assays (S1 nuclease protection and RNase protection) are extremely sensitive procedures for detection and quantitation of mRNA species in complex mixtures of total cellular RNA. These assays are well suited for mapping positions of external and internal junctions in RNA, such as transcription initiation and termination sites and intron/exon boundaries, and to discriminate between closely related targets by using probes designed to span the regions where the related genes differ the most. Also, because the size of the probes used in nuclease protection assays is a variable chosen by the investigator, probes may be designed to protect fragments of different sizes. This feature permits the simultaneous analysis of several different mRNAs in the same total RNA sample. In this unit, a method is included for RNase protection of target mRNA sequences, including hybridization of the probe to the target sequence, details of the actual protection assay, and detection of reaction products. An alternative method is provided for performing the RNase protection assay on a microvolume scale, which is useful when there are many samples to be analyzed. Support protocols describe synthesis and gel purification of labeled RNA probes; preparation of RNase-free yeast RNA, which acts as an aid in the quantitative precipitation of newly synthesized probe; and quantitation of target mRNA. A method describing S1 nuclease protection of target mRNA using either RNA or DNA probes is also included. Additional support protocols provide instructions for the preparation of radiolabeled DNA probes by primer-extension of double-stranded plasmid or PCR product using Klenow fragment of E. coli DNA polymerase I or Taq or Tth polymerase in a thermal cycler. Another radiolabeling method details 5' end labeling of oligodeoxynucleotides and oligoribonucleotides using T4 polynucleotide kinase. Additionally, a method is described for mapping transcription start sites using the S1 nuclease protection assay.

Ca2+/calmodulin-based signalling in the regulation of the muscle fibre phenotype and its therapeutic potential via modulation of utrophin A and myostatin expression

Ca2+ signalling plays an important role in excitation-contraction coupling and the resultant force output of skeletal muscle. It is also known to play a crucial role in modulating both short- and long-term muscle cellular phenotypic adaptations associated with these events. Ca2+ signalling via the Ca2+/calmodulin (CaM)-dependent phosphatase calcineurin (CnA) and via Ca2+/CaM-dependent kinases, such as CaMKI and CaMKII, is known to regulate hypertrophic growth in response to overload, to direct slow versus fast fibre gene expression, and to contribute to mitochondrial biogenesis. The CnA- and CaMK-dependent regulation of the downstream transcription factors nuclear factor of activated T cells (NFAT) and myocyte-specific enhancer factor 2 are known to activate muscle-specific genes associated with a slower, more oxidative fibre phenotype. We have also recently shown the expression of utrophin A, a cytoskeletal protein that accumulates at the neuromuscular junction and plays a role in maturation of the postsynaptic apparatus, to be regulated by CnA-NFAT and Ca2+/CaM signalling. This regulation is fibre-type specific and potentiated by interactions with the transcriptional regulators and coactivators GA binding protein (also known as nuclear respiratory factor 2) and peroxisome proliferator-activated receptor-gamma coactivator 1 alpha. Another downstream target of CnA signalling may be myostatin, a transforming growth factor-beta family member that is a negative regulator of muscle growth. While the list of the downstream targets of CnA/NFAT- and Ca2+/CaM-dependent signalling is emerging, the precise interaction of these pathways with the Ca2+-independent pathways p38 mitogen-activated protein kinase, extracellular signal-regulated kinases 1 and 2, phosphoinositide-3 kinase, and protein kinase B (Akt/PKB) must also be considered when deciphering fibre responses and plasticity to altered contractile load.

Generation of a transgenic zebrafish model of Tauopathy using a novel promoter element derived from the zebrafish eno2 gene

The aim of this study was to isolate cis-acting regulatory elements for the generation of transgenic zebrafish models of neurodegeneration. Zebrafish enolase-2 (eno2) showed neuronal expression increasing from 24 to 72 h post-fertilization (hpf) and persisting through adulthood. A 12 kb eno2 genomic fragment, extending from 8 kb upstream of exon 1 to exon 2, encompassing intron 1, was sufficient to drive neuronal reporter gene expression in vivo over a similar time course. Five independent lines of stable Tg(eno2 : GFP) zebrafish expressed GFP widely in neurons, including populations with relevance to neurodegeneration, such as cholinergic neurons, dopaminergic neurons and cerebellar Purkinje cells. We replaced the exon 2-GFP fusion gene with a cDNA encoding the 4-repeat isoform of the human microtubule-associated protein Tau. The first intron of eno2 was spliced with high fidelity and efficiency from the chimeric eno2-Tau transcript. Tau was expressed at approximately 8-fold higher levels in Tg(eno2 : Tau) zebrafish brain than normal human brain, and localized to axons, neuropil and ectopic neuronal somatic accumulations resembling neurofibrillary tangles. The 12 kb eno2 promoter drives high-level transgene expression in differentiated neurons throughout the CNS of stable transgenic zebrafish. This regulatory element will be useful for the construction of transgenic zebrafish models of neurodegeneration.

Heregulin-induced epigenetic regulation of the utrophin-A promoter

Utrophin is the autosomal homolog of dystrophin, the product of the Duchenne's muscular dystrophy (DMD) locus. Utrophin is of therapeutic interest since its over-expression can compensate dystrophin's absence. Utrophin is enriched at neuromuscular junctions due to heregulin-mediated utrophin-A promoter activation. We demonstrate that heregulin activated MSK1/2 and phosphorylated histone H3 at serine 10 in cultured C2C12 muscle cells, in an ERK-dependent manner. MSK1/2 inhibition suppressed heregulin-mediated utrophin-A activation. MSK1 over-expression potentiated heregulin-mediated utrophin-A activation and chromatin remodeling at the utrophin-A promoter. These results identify MSK1/2 as key effectors modulating utrophin-A expression as well as identify novel targets for DMD therapy.

Ets-2 repressor factor silences extrasynaptic utrophin by N-box mediated repression in skeletal muscle

Utrophin is the autosomal homologue of dystrophin, the protein product of the Duchenne's muscular dystrophy (DMD) locus. Utrophin expression is temporally and spatially regulated being developmentally down-regulated perinatally and enriched at neuromuscular junctions (NMJs) in adult muscle. Synaptic localization of utrophin occurs in part by heregulin-mediated extracellular signal-regulated kinase (ERK)-phosphorylation, leading to binding of GABPalpha/beta to the N-box/EBS and activation of the major utrophin promoter-A expressed in myofibers. However, molecular mechanisms contributing to concurrent extrasynaptic silencing that must occur to achieve NMJ localization are unknown. We demonstrate that the Ets-2 repressor factor (ERF) represses extrasynaptic utrophin-A in muscle. Gel shift and chromatin immunoprecipitation studies demonstrated physical association of ERF with the utrophin-A promoter N-box/EBS site. ERF overexpression repressed utrophin-A promoter activity; conversely, small interfering RNA-mediated ERF knockdown enhanced promoter activity as well as endogenous utrophin mRNA levels in cultured muscle cells in vitro. Laser-capture microscopy of tibialis anterior NMJ and extrasynaptic transcriptomes and gene transfer studies provide spatial and direct evidence, respectively, for ERF-mediated utrophin repression in vivo. Together, these studies suggest "repressing repressors" as a potential strategy for achieving utrophin up-regulation in DMD, and they provide a model for utrophin-A regulation in muscle.

A and B utrophin in human muscle and sarcolemmal A-utrophin associated with tumours

Utrophin is an autosomal homologue of dystrophin, abnormal expression of which is responsible for X-linked Duchenne and Becker muscular dystrophy. In normal mature muscle utrophin is confined to blood vessels, nerves and myotendinous and neuromuscular junctions. When dystrophin is absent utrophin is abundant on the sarcolemma. This has raised the possibility that up-regulation of utrophin may be of therapeutic benefit. Two full-length transcripts of utrophin, A and B, have been identified, which are regulated by alternatively spliced 5' promoters. In dystrophic mouse muscle, the A isoform is present on the sarcolemma, whereas the B form is confined to blood vessels. We show here using immunohistochemistry and human isoform-specific antibodies that A- and B-utrophin localisation is the same in human muscle. The A isoform is present on the sarcolemma of foetal human muscle fibres, regenerating fibres, fibres deficient in dystrophin and on blood vessels and neuromuscular junctions. B-utrophin is only detected on blood vessels. We also show that muscle adjacent to some soft tissue tumours shows increased sarcolemmal utrophin-A, showing that utrophin and dystrophin can simultaneously localise to the sarcolemma and raising the possibility that factor(s) from the tumour cells or accompanying inflammatory cells may have a role in regulating utrophin.

The utrophin A 5'-untranslated region confers internal ribosome entry site-mediated translational control during regeneration of skeletal muscle fibers

Utrophin up-regulation in muscle fibers of Duchenne muscular dystrophy patients represents a potential therapeutic strategy. It is thus important to delineate the regulatory events presiding over utrophin in muscle in attempts to develop pharmacological interventions aimed at increasing utrophin expression. A number of studies have now shown that under several experimental conditions, the abundance of utrophin is increased without a corresponding elevation in its mRNA. Here, we examine whether utrophin expression is regulated at the translational level in regenerating muscle fibers. Treatment of mouse tibialis anterior muscles with cardiotoxin to induce muscle degeneration/regeneration led to a large (approximately 14-fold) increase in the levels of utrophin A with a modest change in expression of its transcript (40%). Isolation of the mouse utrophin A 5'-untranslated region (UTR) revealed that it is relatively long with a predicted high degree of secondary structure. In control muscles, the 5'-UTR of utrophin A caused an inhibition upon translation of a reporter protein. Strikingly, this inhibition was removed during regeneration, indicating that expression of utrophin A in regenerating muscles is translationally regulated via its 5'-UTR. Using bicistronic reporter vectors, we observed that this translational effect involves an internal ribosome entry site in the utrophin A 5'-UTR. Thus, internal ribosome entry site-mediated translation of utrophin A can, at least partially, account for the discordant expression of utrophin A protein and transcript in regenerating muscle. These findings provide a novel target for up-regulating levels of utrophin A in Duchenne muscular dystrophy muscle fibers via pharmacological interventions.

A 1.3 kb promoter fragment confers spatial and temporal expression of utrophin A mRNA in mouse skeletal muscle fibers

Upregulation of utrophin in muscle is currently being examined as a potential therapy for Duchenne muscular dystrophy patients. In this context, we generated transgenic mice harboring a 1.3 kb human utrophin A promoter fragment driving expression of the lacZ gene. Characterization of reporter expression during postnatal muscle development revealed that the levels and localization of beta-galactosidase parallel expression of utrophin A transcripts. Moreover, we noted that the utrophin A promoter is more active in slow soleus muscles. Additionally, expression of the reporter gene was regulated during muscle regeneration in a manner similar to utrophin A transcripts. Together, these results show that the utrophin A promoter-lacZ construct mirrors expression of utrophin A mRNAs indicating that this utrophin A promoter fragment confers temporal and spatial patterns of expression in skeletal muscle. This transgenic mouse will be valuable as an in vivo model for developing and testing molecules aimed at increasing utrophin A expression.

The utrophin promoter A drives high expression of the transgenic LacZ gene in liver, testis, colon, submandibular gland, and small intestine

BACKGROUND

Duchenne muscular dystrophy (DMD) is caused by the absence of the muscle cytoskeletal protein dystrophin. Utrophin is an autosomal homologue of dystrophin, and overexpression of the protein is expected to compensate for the defect of dystrophin. The utrophin gene has two promoters, A and B, and promoter A of the utrophin gene is a possible target of pharmacological interventions for DMD because A-utrophin is up-regulated in dystrophin-deficient mdx skeletal and cardiac muscles. To investigate the utrophin promoter A activity in vivo, we generated nuclear localization signal-tagged LacZ transgenic mice, where the LacZ gene was driven by the 5-kb flanking region of the A-utrophin gene.

METHODS

Four transgenic lines were established by mating four independent founders with C57BL/6J mice. The levels of mRNA for beta-galactosidase in several tissues were examined by RT-PCR. Cryosections from several tissues were stained with hematoxylin and eosin (H&E) and with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal).

RESULTS

The 5-kb upstream region of the A-utrophin gene showed high transcriptional activity in liver, testis, colon, submandibular gland, and small intestine, consistent with the endogenous expression of utrophin protein. Surprisingly, the levels of both beta-gal protein and mRNA for the transgene in cardiac and skeletal muscles were extremely low, even in nuclei near the neuromuscular junctions. These results indicate that the regulation of the utrophin gene in striated muscle is different from that in non-muscle tissues.

CONCLUSIONS

Our results clearly showed that the utrophin A promoter is not sufficient to drive expression in muscle, but other regulatory elements are required.

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.

Factors Associated with Induced Chronic Inflammation in mdx Skeletal Muscle Cause Posttranslational Stabilization and Augmentation of Extrasynaptic Sarcolemmal Utrophin

Chronic inflammation in tibialis anterior muscles of mdx mice was produced by a single injection of a recombinant adenovirus vector (AV) expressing an immunogenic beta-galactosidase (beta-gal). In regions of intense beta-gal staining, mononuclear infiltrates abounded, and muscle fibers showed strong extrasynaptic utrophin immunostaining, restoration of dystrophin-associated protein complex, and a marked reduction of the prevalence of centronucleation. Immunoblot analysis confirmed an increase of endogenous utrophin without an increase of the mRNA of the major muscle isoform utrA. Significantly better maximal tetanic force values were demonstrated in the inflammatory versus control mdx muscles. The resistance to lengthening contraction- induced damage was also significantly increased in the former. In muscles of mice lacking TNF-alpha gene, AV vector did not induce inflammation and extrajunctional utrophin increase did not occur. In the inflammatory mdx muscles, proteolytic activity of calcium-activated calpain was reduced, and in mdx myotubes in vitro, incubation with NO donors also reduced calpain-mediated utrophin proteolysis. Since utrophin was shown to be a natural substrate of calpain and known inhibitors of calpain in cultured mdx myotubes increased utrophin levels, the above results were consistent with the following conclusions: (1) extrasynaptic utrophin increase is mainly responsible for the antidystrophic effect; (2) extrasynaptic utrophin increase is a result of posttranscriptional mechanism(s) related to proinflammatory factors; and (3) reduction of endogenous muscle calpain activity by inflammatory cytokines has an important role in the stabilization and increase of the extrasynaptic utrophin.

Okadaic acid augments utrophin in myogenic cells

Duchenne muscular dystrophy is a fatal childhood disease caused by mutations that abolish the expression of dystrophin in muscle. Utrophin is a paralogue of dystrophin and can functionally replace it in skeletal muscle. A potential therapeutic approach is to increase utrophin levels in muscle. One way to achieve this aim is to increase the expression of the utrophin gene at a transcriptional level via promoter activation. In this study, we have shown that utrophin A mRNA levels can be induced by okadaic acid in murine myogenic C2C12 cells. We have found that a utrophin A promoter reporter can be induced by Sp1 in C2C12 myoblasts, but not in myotubes. This activation can be enhanced by okadaic acid treatment. Our data suggest that this induction is due to Sp1 phosphorylation during myogenesis and thus, utrophin expression in muscle could be regulated by treatment with phosphatase inhibitors. Control of utrophin promoter activation could then be used to increase the expression of utrophin, and thus ameliorate the symptoms of Duchenne muscular dystrophy.

A-utrophin up-regulation in mdx skeletal muscle is independent of regeneration

Duchenne muscular dystrophy is a fatal childhood disease caused by mutations that abolish the expression of dystrophin in muscle. Utrophin is a paralogue of dystrophin and can functionally replace it in skeletal muscle. A method to induce utrophin up-regulation in muscle should therefore be therapeutically useful in Duchenne muscular dystrophy. The search for such a method needs to be informed by an understanding of the mechanisms controlling utrophin expression in muscle. Two full length utrophin isoforms are expressed: A and B. A-utrophin is up-regulated in dystrophin deficient skeletal muscle and we sought to test the hypothesis that this up-regulation occurs as a consequence of ongoing regeneration. We measured utrophin expression by immunohistochemistry and immunoblotting in the oesophageal outer muscular layer and in gamma-irradiated limb muscle from mdx mice. Skeletal muscle in these tissues is dystrophin deficient but not regenerating; we found that A-utrophin up-regulation still occurred. We conclude that utrophin up-regulation in skeletal muscle does not depend on regeneration. An alternative hypothesis involving competition for binding sites between utrophin and dystrophin is discussed. These results have important implications for future studies aiming to effect therapeutic utrophin up-regulation in Duchenne muscular dystrophy patients.

Therapeutic Strategies for Duchenne and Becker Dystrophies

Duchenne muscular dystrophy (DMD), a severe X-linked genetic disease affecting one in 3500 boys, is the most common myopathy in children. DMD is due to a lack of dystrophin, a submembrane protein of the cytoskeleton, which leads to the progressive degeneration of skeletal, cardiac, and smooth muscle tissue. A milder form of the disease, Becker muscular dystrophy (BMD), is characterized by the presence of a semifunctional truncated dystrophin, or reduced levels of full-length dystrophin. DMD is the focus of three different supportive or therapeutic approaches: gene therapy, cell therapy, and drug therapy. Here we consider these approaches in terms of three potential goals: improvement of dystrophic phenotype, expression of dystrophin, and overexpression of utrophin. Utrophin exhibits 80% homology with dystrophin and is able to perform similar functions. Pharmacological strategies designed to overexpress utrophin appear promising and may circumvent many obstacles to gene and cell-based therapies.

Identification of transcripts from a subtraction library which might be responsible for the mild phenotype in an intrafamilially variable course of Duchenne muscular dystrophy

While frame-shift mutations are usually found in Duchenne muscular dystrophy (DMD), in-frame mutations are associated with the less severe phenotype of Becker's muscular dystrophy. Exceptions have been reported in both directions suggesting the existence of modifying genes, which might be helpful for innovation of new therapeutic strategies. We report on the very rare case of an intrafamilially different course of DMD, with the younger brother being far less affected than the older one when compared at the same age. In this context, we constructed a subtraction library enriched for transcripts over-expressed in the patient with the milder phenotype. Twelve random clones were sequenced, followed by database analysis. Six of them, casein kinase 1 alpha 1, RAP2B, dynactin 3 light chain, core binding factor beta, myosin light polypeptide 2 and one hypothetical gene, were further analysed by real-time RT-PCR. All these genes were over-expressed 3-20 times in the less affected patient compared with the more severely affected one. Casein kinase 1 and the hypothetical gene showed even a slightly higher expression than the control. Up-regulation of myosin light polypeptide 2, one of the most sensitive markers of muscle fibre regeneration, obviously reflects the milder phenotype. Casein kinase 1, dynactin and core binding factor are supposed to be involved in cell cycle pathways. RAP is a component of the signalling network which controls fundamental cellular processes such as proliferation and differentiation. All four might be interesting candidates for a therapeutic approach to diminish progression of dystrophy in DMD.

Advances in Duchenne muscular dystrophy gene therapy

Since the initial characterization of the genetic defect for Duchenne muscular dystrophy, much effort has been expended in attempts to develop a therapy for this devastating childhood disease. Gene therapy was the obvious answer but, initially, the dystrophin gene and its product seemed too large and complex for this approach. However, our increasing knowledge of the organization of the gene and the role of dystrophin in muscle function has indicated ways to manipulate them both. Gene therapy for Duchenne muscular dystrophy now seems to be in reach.