Regional localization of the murine Duchenne muscular dystrophy gene on the mouse X chromosome

The Ste20-like kinase - a Jack of all trades?

Over the past 20 years, the Ste20-like kinase (SLK; also known as STK2) has emerged as a central regulator of cytoskeletal dynamics. Reorganization of the cytoskeleton is necessary for a plethora of biological processes including apoptosis, proliferation, migration, tissue repair and signaling. Several studies have also uncovered a role for SLK in disease progression and cancer. Here, we review the recent findings in the SLK field and summarize the various roles of SLK in different animal models and discuss the biochemical mechanisms regulating SLK activity. Together, these studies have revealed multiple roles for SLK in coupling cytoskeletal dynamics to cell growth, in muscle repair and in negative-feedback loops critical for cancer progression. Furthermore, the ability of SLK to regulate some systems appears to be kinase activity independent, suggesting that it may be an important scaffold for signal transduction pathways. These various findings reveal highly complex functions and regulation patterns of SLK in development and disease, making it a potential therapeutic target.

Gene Therapy for Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a relatively common inherited disorder caused by defective expression of the protein dystrophin. The most direct approach to treating this disease would be to restore dystrophin production in muscle. Recent progress has greatly increased the prospects for successful gene therapy of DMD, and here we summarize the most promising developments.

AREAS COVERED

Gene transfer using vectors derived from adeno-associated virus (AAV) has emerged as a promising method to restore dystrophin production in muscles bodywide, and represents a treatment option applicable to all DMD patients. Using information gleaned from PubMed searches of the literature, attendance at scientific conferences and results from our own lab, we provide an overview of the potential for gene therapy of DMD using AAV vectors including a summary of promising developments and issues that need to be resolved prior to large-scale therapeutic implementation.

EXPERT OPINION

Of the many approaches being pursued to treat DMD and BMD, gene therapy based on AAV-mediated delivery of microdystrophin is the most direct and promising method to treat the cause of the disorder. The major challenges to this approach are ensuring that microdystrophin can be delivered safely and efficiently without eliciting an immune response.

Treatment with human immunoglobulin G improves the early disease course in a mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a severe hereditary myopathy. Standard treatment by glucocorticosteroids is limited because of numerous side effects. The aim of this study was to test immunomodulation by human immunoglobulin G (IgG) as treatment in the experimental mouse model (mdx) of DMD. 2 g/kg human IgG compared to human albumin was injected intraperitoneally in mdx mice at the age of 3 and 7 weeks. Advanced voluntary wheel running parameters were recorded continuously. At the age of 11 weeks, animals were killed so that blood, diaphragm, and lower limb muscles could be removed for quantitative PCR, histological analysis and ex vivo muscle contraction tests. IgG compared to albumin significantly improved the voluntary running performance and reduced muscle fatigability in an ex vivo muscle contraction test. Upon IgG treatment, serum creatine kinase values were diminished and mRNA expression levels of relevant inflammatory markers were reduced in the diaphragm and limb muscles. Macrophage infiltration and myopathic damage were significantly ameliorated in the quadriceps muscle. Collectively, this study demonstrates that, in the early disease course of mdx mice, human IgG improves the running performance and diminishes myopathic damage and inflammation in the muscle. Therefore, IgG may be a promising approach for treatment of DMD. Two monthly intraperitoneal injections of human immunoglobulin G (IgG) improved the early 11-week disease phase of mdx mice. Voluntary running was improved and serum levels of creatine kinase were diminished. In the skeletal muscle, myopathic damage was ameliorated and key inflammatory markers such as mRNA expression of SPP1 and infiltration by macrophages were reduced. The study suggests that IgG could be explored as a potential treatment option for Duchenne muscular dystrophy and that pre-clinical long-term studies should be helpful.

TNF-α-Induced microRNAs Control Dystrophin Expression in Becker Muscular Dystrophy

The amount and distribution of dystrophin protein in myofibers and muscle is highly variable in Becker muscular dystrophy and in exon-skipping trials for Duchenne muscular dystrophy. Here, we investigate a molecular basis for this variability. In muscle from Becker patients sharing the same exon 45-47 in-frame deletion, dystrophin levels negatively correlate with microRNAs predicted to target dystrophin. Seven microRNAs inhibit dystrophin expression in vitro, and three are validated in vivo (miR-146b/miR-374a/miR-31). microRNAs are expressed in dystrophic myofibers and increase with age and disease severity. In exon-skipping-treated mdx mice, microRNAs are significantly higher in muscles with low dystrophin rescue. TNF-α increases microRNA levels in vitro whereas NFκB inhibition blocks this in vitro and in vivo. Collectively, these data show that microRNAs contribute to variable dystrophin levels in muscular dystrophy. Our findings suggest a model where chronic inflammation in distinct microenvironments induces pathological microRNAs, initiating a self-sustaining feedback loop that exacerbates disease progression.

Skipping multiple exons of dystrophin transcripts using cocktail antisense oligonucleotides

Duchenne muscular dystrophy (DMD) is one of the most common and lethal genetic disorders, with 20,000 children per year born with DMD globally. DMD is caused by mutations in the dystrophin (DMD) gene. Antisense-mediated exon skipping therapy is a promising therapeutic approach that uses short DNA-like molecules called antisense oligonucleotides (AOs) to skip over/splice out the mutated part of the gene to produce a shortened but functional dystrophin protein. One major challenge has been its limited applicability. Multiple exon skipping has recently emerged as a potential solution. Indeed, many DMD patients need exon skipping of multiple exons in order to restore the reading frame, depending on how many base pairs the mutated exon(s) and adjacent exons have. Theoretically, multiple exon skipping could be used to treat approximately 90%, 80%, and 98% of DMD patients with deletion, duplication, and nonsense mutations, respectively. In addition, multiple exon skipping could be used to select deletions that optimize the functionality of the truncated dystrophin protein. The proof of concept of systemic multiple exon skipping using a cocktail of AOs has been demonstrated in dystrophic dog and mouse models. Remaining challenges include the insufficient efficacy of systemic treatment, especially for therapies that target the heart, and limited long-term safety data. Here we review recent preclinical developments in AO-mediated multiple exon skipping and discuss the remaining challenges.

High-resolution recombinational map of mouse chromosome 16

Five intersubspecific backcrosses and an intercross were used to establish a sex-averaged recombinational map spanning 56 cM across most of mouse Chromosome 16 (Chr 16). A total of 123 markers were ordered using an interval mapping approach to identify 425 recombination sites in a collection of 1154 meioses from 1155 progeny generated in the six crosses. The markers include the 10 "classic" Chr 16 reference markers, 26 additional genes or transcripts including two phenotypic markers (Pit1dw and Kcnj6wv), and 87 simple sequence length polymorphisms (SSLPs). One set of monozygotic twins was detected among the 304 meioses mapped to highest resolution. The reference markers and SSLPs allow the map to be well integrated with existing maps of Chr 16. The average distance between crossover sites is less than 500 kb for most chromosomes, making this collection of recombinant chromosomes useful as a binning and ordering resource for YAC-based physical map assembly on Chr 16.

Time course study of the isometric contractile properties of mdx mouse striated muscles

The isometric twitch and tetanic contractions of three hindlimb muscles (soleus, plantaris, extensor digitorum longus) were recorded in situ in groups of mdx and C57BL/10 control mice at young, adult and old ages (3, 4, 6, 8, 13, 26, 39 and 52 weeks). Based on a two-way analysis of variance (age/phenotype) the mdx phenotype did not modify the absolute tension but was associated with a significant decrease in the tetanic tension normalized to muscle weight in all the muscles which became heavier. These results suggest that the contractile material in mdx is not so powerful as in controls. Moreover, significantly faster time to peak and half-relaxation time were observed in mdx soleus and plantaris. Comparison between these contraction characteristics and those of other experimental models suggests that the high percentage of regenerated fibres in mdx muscles could play a role in modifying contractile properties.

A linkage map of mouse chromosome 1 using an interspecific cross segregating for the gld autoimmunity mutation

An interspecific backcross was used to define a high resolution linkage map of mouse Chromosome (Chr) 1 and to analyze the segregation of the generalized lymphoproliferative disease (gld) mutation. Mice homozygous for gld have multiple features of autoimmune disease. Analysis of up to 428 progeny from the backcross [(C3H/HeJ-gld x Mus spretus)F1 x C3H/HeJ-gld] established a map that spans 77.6 cM and includes 56 markers distributed over 34 ordered genetic loci. The gld mutation was mapped to a less than 1 cM segment on distal mouse Chr 1 using 357 gld phenotype-positive backcross mice. A second backcross, between the laboratory strains C57BL/6J and SWR/J, was examined to compare recombination frequency between selected markers on mouse Chr 1. Significant differences in crossover frequency were demonstrated between the interspecific backcross and the inbred laboratory cross for the entire interval studied. Sex difference in meiotic crossover frequency was also significant in the laboratory mouse cross. Two linkage groups known to be conserved between segments of mouse Chr 1 and the long arm of human Chrs 1 and 2 where further defined and a new conserved linkage group was identified that includes markers of distal mouse Chr 1 and human Chr 1, bands q32 to q42.

Age-related changes and tissue distribution of parvalbumin in normal and dystrophic mice of strain 129 ReJ

In murine muscular dystrophy, hindlimb muscle contains a functionally defective thiol protease inhibitor (TPI) which has been implicated in the onset and progression of the disease in mice. More recently, this protease inhibitor has been identified as parvalbumin, a calcium binding protein. In this study, a polyclonal antibody against mouse muscle parvalbumin was used to study the concentration and distribution of this protein in normal and dystrophic male mice at various ages. Immunodetection assays were used to screen extracts of hindlimb, forelimb, brain, heart, lung, liver, and kidney in 60-day-old normal and dystrophic male mice for parvalbumin content. Parvalbumin was detected in relatively high amounts in both hindlimb and forelimb muscle extracts, while much lower concentrations were detected in brains of normal and dystrophic animals. No parvalbumin was detected in the lung, liver, heart, or kidney extracts using the immunoassay. With aging, the parvalbumin concentration in hindlimb muscle of normal mice remained fairly constant for 90 days, whereupon the level increased at 120 days. In contrast, the parvalbumin concentration in hindlimb muscle of dystrophic mice decreased steadily with age to about 22%% of normal animals at 120 days. The parvalbumin content was also reduced in dystrophic brain.

Dystrophic changes in mdx muscle regenerating from denervation and devascularization

The regenerative capacity of young mdx muscle after a denervating and devascularizing injury (DD) was examined in extensor digitorum longus (EDL) and compared with that of age-matched control mouse EDL. DD of the right EDL was produced at the age approximating the onset of dystrophy in the mdx model, and mice recovered for 2 weeks. Contralateral unoperated EDLs from mdx and control mice served as internal controls for histopathology, myofiber cross-sectional area (CSA), and ultrastructure of fiber regeneration in DD-EDL. Mdx DD-EDL were composed of small, uniformly mature myofibers with mostly peripheral nuclei. This contrasted with control DD-EDL in which fibers were centrally nucleated. In addition, the unoperated mdx EDL exhibited the central nucleation of spontaneous recovery from dystrophy. The CSA distribution of mdx DD-EDL myofibers was significantly shifted toward smaller CSA compared with unoperated mdx EDL, although mean CSA did not differ between the two mdx muscle groups. The CSA distribution of control DD-EDL was significantly different and shifted toward smaller CSA from both unoperated control EDL and from mdx DD-EDL distributions. Ultrastructural features of dystrophy were present in both mdx DD-EDL and in the unoperated mdx EDL, although they appeared more prevalent in the latter. These results suggest that short-term plasticity of mdx muscle recovery from imposed injury may be greater than that of normal muscle in establishing a regenerating fiber population.

Sequence analysis of two exons from the murine dystrophin locus

We have isolated two genomic clones from the murine dystrophin locus, containing single exons encoding protein sequence from the putative actin-binding domain of the amino-terminus and the terminal portion of the triple helical domain. Using interspecific backcross progeny mice, both clones were shown to be X-linked. Sequence analysis indicated that the amino-terminal clone contains a 173 bp exon exhibiting 90% nucleotide sequence identity to human dystrophin exon 6, whilst the C-terminal clone contains a 61 bp exon with 93% nucleotide sequence identity to the human cDNA sequence.

Comparison of interspecific to intersubspecific backcrosses demonstrates species and sex differences in recombination frequency on mouse chromosome 16

One hundred fourteen progeny from an interspecific backcross between laboratory mice and M. spretus were typed for six markers spanning most of mouse Chromosome (Chr) 16. Additional maps of 9-10 markers of this chromosome were derived from analysis of over 500 progeny from four backcrosses between inbred laboratory strains and members of the Mus musculus group, M.m. musculus and M.m. molossinus (subspecies). The results of these analyses confirmed the gene order: (CEN)-Prm-1/Prm-2-Igl-1-Smst-Mtv-6-Gap43-Pit-1(dw)- D21S16h-App-Sod-1-Ets-2-Mx. Maps produced from these five crosses were of similar lengths, but recombination in several regions was affected by sex of the F1 parent or by the combination of strains used in the cross. As reported previously, recombination frequencies were elevated significantly at the distal end of the chromosome in a cross using F1 males. The male map showed significant compression in the interval Smst to Gap43. Both male and female intersubspecific maps were expanded near the proximal and distal ends of the chromosome relative to the interspecific cross. The spretus cross was compressed in the proximal interval, Prm-1-Igl-1-Smst, and was slightly expanded in the Smst-Gap43 interval, relative to intersubspecific crosses using F1 females. Female intersubspecific maps were expanded about 50% near the distal end of the chromosome when compared to the interspecific cross. The expansion or compression of maps using different strain or sex combinations has implications for the efficient production of high resolution recombinational maps of the mouse genome.

Linkage of a gene for neural cell adhesion molecule, L1 (CamL1) to the Rsvp region of the mouse X chromosome

L1 is a glycoprotein with an apparent molecular weight of 200 kDa in the developing fetus and adult central nervous system. In the peripheral nervous system, it has a molecular weight of 230 kDa. The L1 protein appears to be encoded by a single gene that has been located on the human X chromosome by in situ hybridization. In this paper we describe restriction variation in genomic DNA Southern analysis between Mus species for the K13 cDNA probe for the L1 neural cell adhesion molecule. We have designated the locus described by this variation as cell adhesion molecule L1, CamL1. The X chromosome linkage and the relative position on the X chromosome coincident with the genes Rsvp/G6pd/Cf-8 were defined in backcross matings involving M. spretus and M. musculus.

Efficient linkage of 10 loci in the proximal region of the mouse X chromosome

Interspecific Mus species crosses were used to construct a multilocus genetic map of the mouse X chromosome that extends for more than 50 cM. In these studies, we established the segregation of eight loci in more than 200 backcross progeny from crosses of M. musculus and M. spretus with a common inbred strain (C57BL/6JRos). Genetic divergence at the level of the nucleotide sequences makes these crosses a useful cumulative genetic resource for mapping additional genes defined by genomic or cDNA probes in a highly efficient manner. We have therefore devised a mapping strategy that uses a subset of these backcrosses that are recombinant between successive anchor loci to both localize and order an additional set of six genes without necessarily resorting to an analysis of the entire backcross series. Using this approach, we have defined the linkage of cytochrome b245 beta-chain (Cybb), synapsin (Syn-1), and two members of the X-linked lymphocyte-regulated gene family (Xlr-1, Xlr-2), as well as DXSmh141 and DXSmh172, two loci defined by random genomic probes. All six loci have been localized to the proximal portion of the mouse X chromosome and their order has been defined as Cybb, Otc, Syn-1/Timp, DXSmh141/Xlr-1, DXSmh172, Hprt, Xlr-2, Cf-9. Gene order was established by minimizing multiple recombination events across the region spanning an estimated 20 cM of the proximal X chromosome. The possible significance of the Xlr loci is discussed with respect to other X-chromosome loci that regulate the immune response.

Dystrophin: a clinical perspective

Dystrophin, the protein product of the gene related to Duchenne and Becker muscular dystrophies, is a large cytoskeletal protein associated with the muscle fiber membrane. Recently identified dystrophin-related myopathies affecting animals can serve as experimental models for human disease. Immunologic detection of dystrophin in clinical muscle biopsies provides a direct biochemical test for both Duchenne and Becker muscular dystrophies. Applications of dystrophin testing include improved diagnostic accuracy, carrier detection, fetal diagnosis, and evaluation of asymptomatic male infants identified as a result of neonatal screening for increased serum creatine kinase levels. Identification of dystrophin has brought us to the point of addressing rational therapies.

Aniridia, Wilms' tumor and human chromosome 11

Aniridia-a developmental abnormality of the eye in which the iris is apparently absent-has been shown to be genetically associated with Wilms' tumor (an embryonic nephroblastoma) in the WAGR syndrome. Genetic and cytogenetic evidence points to band p13 of human chromosome 11 as the localization of the genes responsible for these defects. Deleted chromosomes 11 from WAGR patients and clinically associated translocations involving 11p13 have been used to map and order genes and anonymous DNA markers around the WAGR locus refining the localization of the aniridia and Wilms' tumor genes to within about 1 million base pairs of DNA.

Dystrophin-related muscular dystrophies

The gene for the locus involved in Duchenne and Becker muscular dystrophies has been cloned and subject to intense analysis. The protein product of the locus is called dystrophin, and it has been shown to be associated with the muscle fiber membrane. The new knowledge of the molecular genetics of these disorders is being applied rapidly in clinical practice. Carrier detection and prenatal diagnosis have been revolutionized by the use of probes for the gene. These probes are also being employed to clarify cases where conventional clinical examination results in equivocal diagnoses. It is suggested that the disorders characterized by dystrophin abnormalities should be called dystrophin-related muscular dystrophies (DRMD). There are mouse and dog models for DRMD and these are being used to explore therapeutic strategies for treating DRMD patients.

Autonomous death of amphibian (Xenopus laevis) cranial myotomes

The death of cranial myotomes during Xenopus laevis embryogenesis is employed as a model system to study programmed cell death. The first primary myotomes to differentiate and functionally develop are in the occipital region of the embryonic head. Between stages 27 (tailbud) and 48 (feeding tadpole), they degenerate and disappear in a craniocaudal sequence. Descriptive and experimental studies were undertaken to establish whether this apparent cell (myotome) death program is autonomous or whether it depends on interactions with surrounding tissues (e.g., otic vesicle). Removal of the adjacent otic vesicle did not affect cranial myotome death. Likewise, grafting the otic vesicle to a novel location along the somite file did not induce local myotome degeneration (death). Cranial myotome primordia grafted into the trunk region degenerated on schedule. Trunk myotome primordia grafted to the cranial myotome location did not degenerate. It is therefore concluded that the cranial myotome death program has become autonomous by the time the cranial myotomes reach the developmental stage of segmentation.

Localization of dystrophin in cultures of human muscle

Dystrophin is a protein present in normal human muscle but absent in patients with Duchenne muscular dystrophy (DMD). Using a specific antibody, we have investigated the expression of dystrophin in human muscle which had regenerated in culture in the presence of nerve cells. Dystrophin was present and was correctly localized in the cultures of normal muscle but was absent from cultures of muscle from patients with DMD. Its control and function can now be studied.

Chromosome maps of man and mouse. IV

Current knowledge of man-mouse genetic homology is presented in the form of chromosomal displays, tables and a grid, which show locations of the 322 loci now assigned to chromosomes in both species, as well as 12 DNA segments not yet associated with gene loci. At least 50 conserved autosomal segments with two or more loci have been identified, twelve of which are over 20 cM long in the mouse, as well as five conserved segments on the X chromosome. All human and mouse chromosomes now have conserved regions; human 17 still shows the least evidence of rearrangement, with a single long conserved segment which apparently spans the centromere. The loci include 102 which are known to be associated with human hereditary disease; these are listed separately. Human parental effects which may well be the result of genomic imprinting are reviewed and the location of the factors concerned displayed in relation to mouse chromosomal regions which have been implicated in imprinting phenomena.

Recovery of induced mutations for X chromosome-linked muscular dystrophy in mice

We have used elevated levels of plasma creatine phosphokinase activity to identify muscular dystrophy phenotypes in mice and to screen the progeny of chemical mutagen-treated male mice for X chromosome-linked muscular dystrophy mutations. We were not successful in identifying heterozygous carriers of these induced muscular dystrophy mutations in greater than 8000 progeny. However, we were highly successful in identifying three additional alleles of the characterized mdx locus. These alleles of mdx were recovered from various mutagen-treated males and they occur on an X chromosome that carries flanking markers that allow us to follow the mutations in genetic crosses and in the development of corresponding mutant stocks. These alleles have been designated as mdx2Cv, mdx3Cv, and mdx4Cv. Preliminary data show that mice with mdx2Cv and mdx3Cv mutations have muscular dystrophic phenotypes that do not grossly differ from the characterized mdx mutation. These additional mdx mutations expand the value of mouse models of X chromosome-linked muscular dystrophy and potentially define additional sites of mutation that impair dystrophin expression.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

The localization of G6pd, glucose-6-phosphate dehydrogenase, and mdx, muscular dystrophy in the mouse X chromosome

A low activity mutant of glucose-6-phosphate dehydrogenase,G6pda-m1Neuhas been used to positionG6pdin the mouseXchromosome. Linkage tests withtabby,Taandharlequin,Hq, indicate a likely gene order ofHq–G6pd–Ta. Muscular dystrophy,mdx, has been located by two-and three-point crosses usingHprt,Pgk-1andMobloand suggest a gene order ofHprt–mdx–Pgk-1–Moblo. Together with existing linkage data a tentative order for the seven loci isHq–Hprt–G6pd–mdx–Ta–Pgk-1–Moblo. The relative positions ofG6pdandmdxhave not been directly tested andG6pdis assigned provisionally proximal tomdx. In the three point test usingHq,G6pdandTathe recombination frequency found betweenHqandTawas 9·9 ± 2·6%, substantially less than the value of 20·5 ± 2·1% reported by Isaacsonet al.(1974).

Multilocus molecular mapping of the mouse X chromosome

Using restriction fragment length polymorphisms (RFLPs) and enzymatic variants between distantly related mouse species, we have assigned three genes to the mouse X chromosome and concurrently mapped a total of eight genes spanning an estimated 50 cM of the chromosome. Segregation of RFLPs in over 200 male progeny from interspecies backcrosses between the inbred strain C57BL/6JRos and either wild-derived Mus musculus or Mus spretus was followed for the murine genes Timp (tissue inhibitor of metalloproteinases), Cf-8 (coagulation factor VIII), and Rsvp (red-sensitive visual pigment) and the known X-linked markers Otc, Hprt, Cf-9, G6pd, and Ags. From the centromere, the gene order was defined as Otc, Timp, Hprt, Cf-9, (Cf-8/Rsvp/G6pd), Ags, by minimizing the number of multiple recombinational events. No significant differences in map order or frequency of recombination were observed between the two backcross series studied. The use of Southern analysis has allowed us to add new genes to the map in a cumulative manner, and as probes become available, additional markers can be mapped, using the same set of mice, by utilizing existing blots or resampling the DNAs. The use of probes for functional genes has allowed us to directly compare the X chromosomes of mouse and man and has provided insight into chromosomal rearrangements which have occurred during the evolutionary divergence of these species, as well as to define the extent of linkage homologies.

Molecular genetics in muscular dystrophy research: revolutionary progress

The contribution of "reverse genetic" strategies to neuromuscular disease research is evident in the progression of breakthroughs that have recently culminated in the cloning of the Duchenne muscular dystrophy (DMD) cDNA. The resultant improvements in our understanding of the genetic basis of Becker muscular dystrophy (BMD) and DMD serve as models for similar investigation of other heritable disorders. These genetic advances have outpaced concurrent work on the molecular pathogenesis of the dystrophic process, with the curious result that inferences about the DMD protein's amino acid sequence have preceded any information about its function or intracellular localization. In recognition that this foundation sets the stage for the rapid elucidation of the disease's pathogenesis, we review the experimental basis of such advances, with reference to relevant progress in basic myology, pathology, and molecular biology. We conclude with a view towards the ultimate clinical implications of these experimental breakthroughs.

Expression of the murine Duchenne muscular dystrophy gene in muscle and brain

Complementary DNA clones were isolated that represent the 5' terminal 2.5 kilobases of the murine Duchenne muscular dystrophy (Dmd) messenger RNA (mRNA). Mouse Dmd mRNA was detectable in skeletal and cardiac muscle and at a level approximately 90 percent lower in brain. Dmd mRNA is also present, but at much lower than normal levels, in both the muscle and brain of three different strains of dystrophic mdx mice. The identification of Dmd mRNA in brain raises the possibility of a relation between human Duchenne muscular dystrophy (DMD) gene expression and the mental retardation found in some DMD males. These results also provide evidence that the mdx mutations are allelic variants of mouse Dmd gene mutations.

Dystrophin: the protein product of the Duchenne muscular dystrophy locus

The protein product of the human Duchenne muscular dystrophy locus (DMD) and its mouse homolog (mDMD) have been identified by using polyclonal antibodies directed against fusion proteins containing two distinct regions of the mDMD cDNA. The DMD protein is shown to be approximately 400 kd and to represent approximately 0.002% of total striated muscle protein. This protein is also detected in smooth muscle (stomach). Muscle tissue isolated from both DMD-affected boys and mdx mice contained no detectable DMD protein, suggesting that these genetic disorders are homologous. Since mdx mice present no obvious clinical abnormalities, the identification of the mdx mouse as an animal model for DMD has important implications with regard to the etiology of the lethal DMD phenotype. We have named the protein dystrophin because of its identification via the isolation of the Duchenne muscular dystrophy locus.