Medical Professional Publications

Muscular Dystrophy: Advances in Research Lead to Improved Quality of Life

(From Pediatric Directions, Issue 38)

Duchenne muscular dystrophy (DMD) is the most common severe childhood form of muscular dystrophy, affecting 1 in 3,500 males. The disease is transmitted through X-linked recessive inheritance. Signs of DMD are progressive loss of muscle strength. Children demonstrate decreased ability to run or jump between ages 3 and 5 years, and then by age 12 often lose the ability to walk.

Twenty-five years ago, MDA-funded investigators identified the dystrophin gene that, when mutated, causes Duchenne muscular dystrophy, as well as the somewhat milder Becker muscular dystrophy (BMD). The team of investigators at the Research Institute at Nationwide Children’s Hospital is continuing to gain new ground each day in the quest to understand the origin of DMD.

Walking for patients with DMD can be prolonged by one to two years using corticosteroids. Patients, who historically died from respiratory complications, are able to have their life prolonged through use of antibiotics, vaccines and other ancillary methods, which protect the respiratory system and prolong life. This unmasks an evolving need for more intensive cardiac intervention.

Isometric strength is tested with DMD patients using a force transducer, which is attached to the extremity being tested by a cuff. The force generated is recorded on a computer.

Becker muscular dystrophy (BMD) is a milder variant of the disease. Some BMD patients lose the ability to walk as early as the late teenage years, while others ambulate until late middle life. BMD patients typically survive into their 40s or 50s. Dystrophinopathy can cause predominant cardiac manifestations with relative sparing of skeletal muscle (X-linked cardiomyopathy). Non-progressive cognitive impairment is common in DMD and BMD.

Pictured Right: Isometric strength is tested with DMD patients using a force
transducer, which is attached to the extremity being tested by
a cuff. The force generated is recorded on a computer.

DMD Research
Twenty-five years ago, Muscular-Dystrophy-Association-funded investigators identified the dystrophin gene (DMD gene). This subsequently led to the identification of the 427-kD cytoskeletal dystrophin protein, without which the muscle fiber is predisposed to repeated cycles of necrosis and regeneration. As a result, the regenerative capacity of the muscle is diminished as satellite cells are depleted, resulting in muscle fiber loss with replacement by fat and connective tissue.

Based on years of research, three major treatment strategies have evolved to correct or repair the DMD gene defect, and each depends on the specifics of the DMD gene mutation.

  • DMD Gene Suppression of Stop Codon Mutations
  • Exon Skipping
  • Gene Replacement

Extensive research continues to help refine treatment delivery and improve patient outcomes. A brief overview of the research that has resulted in these recommended DMD treatments is provided.

The following information contains excerpts from the Current Rheumatology Report (2011) 13:199–207

Mutation Suppression
Mutation suppression, or “stop codon read-through,” is a treatment strategy applicable to boys with DMD with premature termination codons (PTCs). These are estimated to occur in approximately 13 percent to 15 percent of the population. Using an antibiotic to bind to the decoding site of ribosomal RNA, stop codon read-through allows the ribosome to ignore the PTC and insert an amino acid that ensures continued translation to produce full-length protein.

A series of clinical trials was initiated in DMD patients, as a follow-up to lab studies in mdx mice where the aminoglycoside antibiotic, gentamicin, was successfully used to suppress stop codon mutations. Although initial trials in four DMD patients reported dystrophin expression in three of four patients with stop codons, clinically meaningful outcomes, such as timed walking and stair climbing, were not achieved.

A third clinical trial performed by a group at The Research Institute at Nationwide Children’s consisted of a 14-day course of gentamicin, which reduced Creatine Kinase (CK) by 50 percent in boys with DMD with stop codons. This was followed by a long-term, six-month course of gentamicin. Six of 12 DMD patients showed increased dystrophin levels. In three patients, dystrophin increases reached a potentially therapeutic range, with levels between 13 percent and 15 percent of wild-type protein. These dystrophin levels were in a similar range to those observed in the mdx murine gentamicin study (10% – 20%). Although there was increased dystrophin expression, clinical efficacy was modest. Serum CK was reduced and muscle strength showed a reduced rate of decline supported by a slight increase in forced vital capacity, although significance was not reached.

The data presented in this clinical trial supports the therapeutic concept of mutation suppression by aminoglycoside treatment of DMD stop codon mutations. Of particular interest is the concept that pretreatment dystrophin expression on muscle biopsies could potentially serve as a marker for patients most likely to respond to stop codon read-through therapy. Clinical efficacy improving functional outcomes will require greater levels of dystrophin expression. More practical read-through agents that can be taken orally are in clinical testing, and if effective, have the potential to greatly expand stop codon read-through as a treatment strategy for boys with DMD. Initial studies show no significant difference in the six-minute walk test, but further studies are pending.

Exon Skipping
Exon skipping represents a second molecular treatment strategy for DMD. Antisense oligonucleotides (AONs), single strands of DNA or RNA that are complementary to a chosen sequence, can be used to target dystrophin exons. This allows one or more exons to be excluded and therefore correct the reading frame to yield a truncated, yet functional, dystrophin protein. It has been predicted that through preplanned skipping of targeted exons, as many as 60 percent to 80 percent of DMD mutations can be corrected.

AVI BioPharma completed a 19-patient clinical trial in the United Kingdom confirming the potential of eteplirsen to be a safe and effective disease-modifying drug for DMD, capable of skipping exon 51 of the dystrophin gene. (The Lancet, July 25, 2011).

The research team at Nationwide Children’s has initiated the first trial of eteplirsen in the USA using higher doses compared to the studies conducted in Europe. This randomized, double-blind, placebo-controlled, 12-patient trial is needed to further test safety, efficacy and optimal dosing of eteplirsen. On August 15, 2011, the first three boys participating in the trial received the first of 24 weekly doses of eteplirsen or a placebo by intravenous (IV) infusion. Since then, all boys have been enrolled and the study is well underway.

By administering eteplirsen by IV for 24 weeks, our goal is to find the best dosage to trick the body into skipping over genetic disruptions present in some cases of DMD, to produce dystrophin levels that could improve muscle function.

Gene Replacement
The size of the dystrophin gene is a formidable obstacle for DMD gene therapy because of the limited packaging capacity (<5 kb) of adeno-associated virus (AAV). Fortunately, dystrophin can retain significant function, even when missing large portions of its coding sequence.

DMD genes packaged into viral vectors strengthen muscles in mouse models of muscular dystrophy. Using this information, we translated basic research from the animal model to patients with DMD. Six boys with DMD gene deletions were treated by injecting a viral vector containing a corrected DMD gene into the biceps muscle of one arm. However, when the patients were evaluated three months later, long-term production of dystrophin protein from the corrected DMD gene was not detected.

To understand why this therapy failed, researchers at Nationwide Children’s measured immune responses against dystrophin, concerned about immunity caused by the T lymphocyte. The natural role of T cells is to protect individuals from infection and cancer by destroying cells that are recognized as different or foreign. Parts of the corrected dystrophin protein are clearly foreign because of the patient’s DMD gene deletion, and so unwanted T cell immunity targeting the repaired muscle cells was a possibility.

Researchers at Nationwide Children’s did detect T cell immunity against foreign segments of the corrected dystrophin protein in one patient with a large DMD gene deletion. However, stronger and faster T cell immunity was detected in a second patient with a much smaller DMD gene deletion. The strong, rapid immunity in the second patient with a very small DMD gene deletion was a surprise. The amount of corrected dystrophin protein that is foreign should also be small, and possibly ignored altogether by the T cells. In addition, the T cell immunity to dystrophin was found to have been present in this patient even before treatment. Careful examination of the muscle revealed that the T cells present before gene therapy recognized dystrophin that is produced in a very small percentage of muscle cells that naturally self-correct the defective DMD gene. Delivery of the gene therapy vector to bicep muscles boosted and accelerated this pre-existing immune response.

This study is significant because it documents immunity against a dystrophin protein designed to treat the disease. That may be broadly important to the entire field of gene therapy. This gene therapy study has led to the new basic discovery that even small amounts of dystrophin naturally produced from self-correcting DMD genes can trigger destructive T cells, and they may target muscle cells in a process that resembles autoimmunity. Once researchers understand the scope and significance of the T cell response against muscle in DMD, it may be possible to harness the same approaches to shut them off. This will move researchers closer to the goal of slowing muscle loss in DMD and ultimately to prevent immune responses against therapeutic dystrophin protein.

Early Detection May Be Key to Effective Treatment
Nationwide Children’s is one of just two sites in the world that are conducting newborn screenings for DMD, the other is Antwerp Belgium. Since 2006 Nationwide Children’s has been working in coordination with the Ohio Department of Health to implement a newborn screen for DMD. Research indicates that early intervention can make a significant difference in treatment outcome for patients. To date, approximately 40,000 newborn males among four major birthing hospitals have been screened. This work has been supported by the Center for Disease Control and will introduce the nation to the potential for newborn screening in the USA.
Next Steps - NIH Funding and Wellstone
The Center for Gene Therapy has received the National Institutes of Health funding to become a Paul D. Wellstone Muscular Dystrophy Cooperative Research Center (MDCRC), one of three national award recipients in 2010. This is part of the effort established through the MD-Care Act. The MDCRCs are composed of a multidisciplinary mixture of basic, translational and clinical studies that are tightly integrated to foster the development of new therapies for the muscular dystrophies. With each new step that is taken toward advancing research and treatment for DMD, the result is improved outcomes and quality of life for those affected by this devastating disease.

For additional information regarding DMD research at Nationwide Children’s, visit the Paul D. Wellstone Muscular Dystrophy Cooperative Research Center (MDCRC) or see an interview with Dr. Mendell.

Author: Jerry R. Mendell, MD, is an attending neurologist at Nationwide Children’s and director of the Center for Gene Therapy at The Research Institute of Nationwide Children’s, director of the Paul D. Wellstone Muscular Dystrophy Cooperative Research Center (MDCRC), as well as co-director of the Muscular Dystrophy Association clinic, and professor of Pediatrics, Neurology, Pathology, and Physiology and Cell Biology at The Ohio State University College of Medicine.

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