BBA - Molecular Basis of Disease (v.1772, #2)

Duchenne muscular dystrophy is the most prevalent and severe form of human muscular dystrophy. Investigations into the molecular basis for Duchenne muscular dystrophy were greatly facilitated by seminal studies in the 1980s that identified the defective gene and its major protein product, dystrophin. Biochemical studies revealed its tight association with a multi-subunit complex, the so-named dystrophin–glycoprotein complex. Since its description, the dystrophin–glycoprotein complex has emerged as an important structural unit of muscle and also as a critical nexus for understanding a diverse array of muscular dystrophies arising from defects in several distinct genes. The dystrophin homologue utrophin can compensate at the cell/tissue level for dystrophin deficiency, but functions through distinct molecular mechanisms of protein–protein interaction.
Keywords: Dystrophin; Utrophin; Actin; Dystroglycan; Sarcoglycan; Syntrophin; Dystrobrevin; Costamere; Muscular dystrophy;

Nuclear envelope defects in muscular dystrophy by Kyle J. Roux; Brian Burke (118-127).
Muscular dystrophies are a heterogeneous group of disorders linked to defects in 20–30 different genes. Mutations in the genes encoding a pair of nuclear envelope proteins, emerin and lamin A/C, have been shown to cause the X-linked and autosomal forms respectively of Emery–Dreifuss muscular dystrophy. A third form of muscular dystrophy, limb girdle muscular dystrophy 1b, has also been linked to mutations in the lamin A/C gene. Given that these two genes are ubiquitously expressed, a major goal is to determine how they can be associated with tissue specific diseases. Recent results suggest that lamin A/C and emerin contribute to the maintenance of nuclear envelope structure and at the same time may modulate the expression patterns of certain mechanosensitive and stress induced genes. Both emerin and lamin A/C may play an important role in the response of cells to mechanical stress and in this way may help to maintain muscle cell integrity.
Keywords: Muscular dystrophy; Nuclear envelope; Nuclear lamia; Laminopathy; Emerin;

Molecular and cellular basis of calpainopathy (limb girdle muscular dystrophy type 2A) by Irina Kramerova; Jacques S. Beckmann; Melissa J. Spencer (128-144).
Limb girdle muscular dystrophy type 2A results from mutations in the gene encoding the calpain 3 protease. Mutations in this disease are inherited in an autosomal recessive fashion and result in progressive proximal skeletal muscle wasting but no cardiac abnormalities. Calpain 3 has been shown to proteolytically cleave a wide variety of cytoskeletal and myofibrillar proteins and to act upstream of the ubiquitin-proteasome pathway. In this review, we summarize the known biochemical and physiological features of calpain 3 and hypothesize why mutations result in disease.
Keywords: Calpain; Dystrophy; Muscle; Dysferlin; Titin;

During the last 10 years several muscular dystrophies within the group of distal myopathies have been clarified as to the molecular genetic cause of the disease. Currently, the next steps are carried out to identify the molecular pathogenesis downstream of the gene defects. Some early ideas on what is going on in the muscle cells based on the defect proteins are emerging. However, in no single distal muscular dystrophy these efforts have yet reached the point where direct trials for therapy would have been launched, and in many distal dystrophies the causative gene is still lacking. When comparing the gene defects in the distal dystrophies with the more common proximal muscular dystrophies such as dystrophinopathies or limb-girdle muscular dystrophies, there is a striking difference: the genes for distal dystrophies encode sarcomere proteins whereas the genes for proximal dystrophies more often encode sarcolemmal proteins.
Keywords: Muscular dystrophy; Distal dystrophy; Distal myopathy; Molecular genetics; Molecular pathogenesis; Molecular biology;

The congenital muscular dystrophies comprise a genetically and clinically heterogeneous group of disorders characterized by early onset of progressive muscle weakness and often involvement of other organ systems such as the brain and eyes. During the last decade, significant progress has been made to further characterize various forms of congenital muscular dystrophies based on their specific genetic and clinical appearance. This review represents an overview of the recent accomplishments as they relate to clinical, diagnostic, pathogenetic and therapeutic aspects of congenital muscular dystrophies.
Keywords: Muscular dystrophy; Laminin-α2; Dystroglycan; Selenoprotein N; Integrins;

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disorder characterized by progressive eyelid drooping, swallowing difficulties and proximal limb weakness. OPMD is caused by a small expansion of a short polyalanine tract in the poly (A) binding protein nuclear 1 protein (PABPN1). The mechanism by which the polyalanine expansion mutation in PABPN1 causes disease is unclear. PABPN1 is a nuclear multi-functional protein which is involved in pre-mRNA polyadenylation, transcription regulation, and mRNA nucleocytoplasmic transport. The distinct pathological hallmark of OPMD is the presence of filamentous intranuclear inclusions (INIs) in patient's skeletal muscle cells. The exact relationship between mutant PABPN1 intranuclear aggregates and pathology is not clear. OPMD is a unique disease sharing common pathogenic features with other polyalanine disorders, as well as with polyglutamine and dystrophic disorders. This chapter aims to review the rapidly growing body of knowledge concerning OPMD. First, we outline the background of OPMD. Second, we compare OPMD with other trinucleotide repeat disorders. Third, we discuss the recent advances in the understanding of the molecular mechanisms underlying OPMD pathogenesis. Finally, we review recent therapeutic strategies for OPMD.
Keywords: Polyalanine; Oculopharyngeal muscular dystrophy; Intranuclear inclusions; Poly(A) binding protein nuclear 1;

Facioscapulohumeral muscular dystrophy by Silvère M. van der Maarel; Rune R. Frants; George W. Padberg (186-194).
Facioscapulohumeral muscular dystrophy (FSHD) is caused by a cascade of epigenetic events following contraction of the polymorphic macrosatellite repeat D4Z4 in the subtelomere of chromosome 4q. Currently, the central issue is whether immediate downstream effects are local (i.e., at chromosome 4q) or global (genome-wide) and there is evidence for both scenarios. Currently, there is no therapy for FSHD, mostly because of our lack of understanding of the primary pathogenic process in FSHD muscle. Clinical trials based on suppression of inflammatory reactions or increasing muscle mass by drugs or training have been disappointing. A recent, probably the first evidence-based pilot trial to revert epigenetic changes did also not provide grounds for a larger clinical study. Clearly, better disease models need to be developed to identify and test novel intervention strategies to eventually improve the quality of life for patients with FSHD.
Keywords: FSHD; Muscular dystrophy; Review; D4Z4; Therapy; Epigenetic;

Myotonic dystrophy: Emerging mechanisms for DM1 and DM2 by Diane H. Cho; Stephen J. Tapscott (195-204).
Myotonic dystrophy (DM) is a complex multisystemic disorder linked to two different genetic loci. Myotonic dystrophy type 1 (DM1) is caused by an expansion of a CTG repeat located in the 3′ untranslated region (UTR) of DMPK (myotonic dystrophy protein kinase) on chromosome 19q13.3. Myotonic dystrophy type 2 (DM2) is caused by an unstable CCTG repeat in intron 1 of ZNF9 (zinc finger protein 9) on chromosome 3q21. Therefore, both DM1 and DM2 are caused by a repeat expansion in a region transcribed into RNA but not translated into protein. The discovery that these two distinct mutations cause largely similar clinical syndromes put emphasis on the molecular properties they have in common, namely, RNA transcripts containing expanded, non-translated repeats. The mutant RNA transcripts of DM1 and DM2 aberrantly affect the splicing of the same target RNAs, such as chloride channel 1 (ClC-1) and insulin receptor (INSR), resulting in their shared myotonia and insulin resistance. Whether the entire disease pathology of DM1 and DM2 is caused by interference in RNA processing remains to be seen. This review focuses on the molecular significance of the similarities and differences between DM1 and DM2 in understanding the disease pathology of myotonic dystrophy.
Keywords: Myotonic dystrophy; Repeat expansion; DMPK; ZNF9; MBNL; CUG-BP;

Modeling human muscle disease in zebrafish by Jeffrey R. Guyon; Leta S. Steffen; Melanie H. Howell; Timothy J. Pusack; Christian Lawrence; Louis M. Kunkel (205-215).
Zebrafish reproduce in large quantities, grow rapidly, and are transparent early in development. For these reasons, zebrafish have been used extensively to model vertebrate development and disease. Like mammals, zebrafish express dystrophin and many of its associated proteins early in development and these proteins have been shown to be vital for zebrafish muscle stability. In dystrophin-null zebrafish, muscle degeneration becomes apparent as early as 3 days post-fertilization (dpf) making the zebrafish an excellent organism for large-scale screens to identify other genes involved in the disease process or drugs capable of correcting the disease phenotype. Being transparent, developing zebrafish are also an ideal experimental model for monitoring the fate of labeled transplanted cells. Although zebrafish dystrophy models are not meant to replace existing mammalian models of disease, experiments requiring large numbers of animals may be best performed in zebrafish. Results garnered from using this model could lead to a better understanding of the pathogenesis of the muscular dystrophies and the development of future therapies.
Keywords: Zebrafish; Muscular dystrophy; Dystrophin; Animal models; Therapy;

Genetic modifiers of muscular dystrophy: Implications for therapy by Ahlke Heydemann; Katherine R. Doherty; Elizabeth M. McNally (216-228).
The genetic understanding of the muscular dystrophies has advanced considerably in the last two decades. Over 25 different individual genes are now known to produce muscular dystrophy, and many different “private” mutations have been described for each individual muscular dystrophy gene. For the more common forms of muscular dystrophy, phenotypic variability can be explained by precise mutations. However, for many genetic mutations, the presence of the identical mutation is associated with marked phenotypic range that affects muscle function as well as cardiac function. The explanation for phenotype variability in the muscular dystrophies is only now being explored. The availability of genetically engineered animal models has allowed the generation of single mutations on the background of highly inbred strain. Phenotypic variation that is altered by genetic background argues for the presence of genetic modifier loci that can ameliorate or enhance aspects of the dystrophic phenotype. A number of individual genes have been implicated as modifiers of muscular dystrophy by studies in genetically engineered mouse models of muscular dystrophy. The value of these genes and products is that the pathways identified through these experiments may be exploited for therapy.
Keywords: Dystrophin; Sarcoglycan; Modifier; Muscular dystrophy; Dysferlin;

Current treatment of adult Duchenne muscular dystrophy by Kathryn R. Wagner; Noah Lechtzin; Daniel P. Judge (229-237).
Patients with Duchenne muscular dystrophy (DMD) are living longer into adulthood due to a variety of improvements in health care practices. This growing patient population presents new therapeutic challenges. In this article, we review the literature on current treatment of adult DMD as well as our own experience as a multidisciplinary team actively caring for 23 men ages 19–38 years of age. Approximately one quarter of our adult DMD patients have remained on moderate dose corticosteroids. Daily stretching exercises are recommended, particularly of the distal upper extremities. Cardiomyopathy is anticipated, detected, and treated early with afterload reduction. Oxygen saturation monitoring, noninvasive positive pressure ventilation and cough assist devices are routinely used. Other medical issues such as osteoporosis, gastrointestinal and urinary symptoms are addressed. Current and future therapies directed at prolonging the lifespan of those with DMD will result in further increases in this adult population with special needs and concerns. These needs are best addressed in a multidisciplinary clinic.
Keywords: Duchenne muscular dystrophy; Cardiomyopathy; Multidisciplinary clinic;

The limb-girdle muscular dystrophies—Diagnostic strategies by Kate Bushby; Fiona Norwood; Volker Straub (238-242).
The limb-girdle muscular dystrophies are a group of disorders where our understanding of their underlying molecular basis has made huge strides over the past years, revealing great heterogeneity at the clinical and molecular level. The availability of direct protein and/ or gene based approaches to diagnosis means that these disorders can now be precisely defined, and such definition of a precise diagnosis is increasingly allowing directed management for these diseases by the ability to predict specific complications such as those of the cardiac or respiratory systems. An algorithm combining clinical, biochemical and molecular testing is described which will aid precision of diagnosis and direct specific testing towards the cases most likely to benefit. This brings advantages for the patients of today in recognising the specific risks of their disorders, and in the future will be the starting point for specific gene and protein based therapies.
Keywords: Limb girdle muscular dystrophies; Muscular dystrophy differential diagnosis; Muscular dystrophy management;

Viral-mediated gene therapy for the muscular dystrophies: Successes, limitations and recent advances by Guy L. Odom; Paul Gregorevic; Jeffrey S. Chamberlain (243-262).
Much progress has been made over the past decade elucidating the molecular basis for a variety of muscular dystrophies (MDs). Accordingly, there are examples of mouse models of MD whose disease progression has been halted in large part with the use of viral vector technology. Even so, we must acknowledge significant limitations of present vector systems that must be overcome prior to successful treatment of humans with such approaches. This review will present a variety of viral-mediated therapeutic strategies aimed at counteracting the muscle-wasting symptoms associated with muscular dystrophy. We include viral vector systems used for muscle gene transfer, with a particular emphasis on adeno-associated virus. Findings of several encouraging studies focusing on repair of the mutant dystrophin gene are also included. Lastly, we present a discussion of muscle compensatory therapeutics being considered that include pathways involved in the up-regulation of utrophin, promotion of cellular adhesion, enhancement of muscle mass, and antagonism of the inflammatory response. Considering the complexity of the muscular dystrophies, it appears likely that a multilayered approach tailored to a patient sub-group may be warranted in order to effectively contest the progression of this devastating disease.
Keywords: Dystrophin; Utrophin; Adeno-associated virus; Exon skipping; Antisense oligonucleotide;

Duchenne muscular dystrophy (DMD) is one of the most common lethal, hereditary diseases of childhood. Since the identification of the genetic basis of this disorder, there has been the hope that a cure would be developed in the form of gene therapy. This has yet to be realized, but many different gene therapy approaches have seen dramatic advances in recent years. Although viral-mediated gene therapy has been at the forefront of the field, several non-viral gene therapy approaches have been applied to animal and cellular models of DMD. These include plasmid-mediated gene delivery, antisense-mediated exon skipping, and oligonucleotide-mediated gene editing. In the past several years, non-viral gene therapy has moved from the laboratory to the clinic. Advances in vector design, formulation, and delivery are likely to lead to even more rapid advances in the coming decade. Given the relative simplicity, safety, and cost-effectiveness of these methodologies, non-viral gene therapy continues to have great promise for future gene therapy approaches to the treatment of DMD.
Keywords: Muscular dystrophy; Dystrophin; Antisense oligonucleotide; Plasmid gene therapy; Gene editing;

Stem cell based therapies to treat muscular dystrophy by F.D. Price; K. Kuroda; M.A. Rudnicki (272-283).
Muscular dystrophies comprise a heterogeneous group of neuromuscular disorders, characterized by progressive muscle wasting, for which no satisfactory treatment exists. Multiple stem cell populations, both of adult or embryonic origin, display myogenic potential and have been assayed for their ability to correct the dystrophic phenotype. To date, many of these described methods have failed, underlying the need to identify the mechanisms controlling myogenic potential, homing of donor populations to the musculature, and avoidance of the immune response. Recent results focus on the fresh isolation of satellite cells and the use of multiple growth factors to promote mesangioblast migration, both of which promote muscle regeneration. Throughout this chapter, various stem cell based therapies will be introduced and evaluated based on their potential to treat muscular dystrophy in an effective and efficient manner.
Keywords: Stem cell; Muscular dystrophy; Skeletal muscle; Regenerative medicine; Cell therapy;