Progress in Treatment and Understanding of Muscle Diseases

SAN FRANCISCO—Muscle diseases are a heterogeneous group of disorders, and the understanding of their causes—and development of treatments—is equally varied. From an FDA-approved treatment for Pompe’s disease to experimental therapeutics for muscular dystrophies to a unified theory of the mechanism of myotonic dystrophy, progress in muscle disease was the focus of a symposium at the 59th Annual Meeting of the American Academy of Neurology.

TREATMENT APPROVED FOR POMPE'S DISEASE

Pompe’s disease is due to a deficiency in the lysosomal enzyme acid a-glucosidase (GAA). The disease is also known as acid maltase deficiency and as glycogen storage disease type 2. The clinical spectrum of Pompe’s disease is determined by the underlying mutation in the gene. This determines the residual enzyme activity, which determines the degree of glycogen accumulation and the amount of muscle tissue damage, according to Priya Kishnani, MD, Division Chief of Medical Genetics and Professor of Pediatrics at Duke Medical Center in Durham, North Carolina.

The infantile form presents in the first few months of life with hypotonia and hypertrophic cardiomyopathy, and without treatment, death occurs within the first year of life. “There is a very short window of time for therapeutic intervention,” said Dr. Kishnani, who recently led a study of GAA replacement therapy in infants. Beginning at age 6 months, patients received IV infusions of recombinant human GAA every other week. All 18 patients enrolled in the trial survived to age 18 months, and more than three-fourths remained free of invasive ventilation. The mean cardiac mass was reduced almost to normal, and most patients made significant motor and functional gains as well. “We can clearly change the natural history of this disease,” Dr. Kishnani said.

Based on these results, the FDA approved GAA in 2006 as a treatment for Pompe’s disease in both infantile and late-onset forms. As of early 2007, there were nearly 600 patients receiving treatment in the United States.

The late-onset form may develop any time from early childhood to adulthood. Symptoms include limb girdle weakness and either an absence of or less severe cardiac involvement. The spectrum of disability in this form is wide, but as the disease progresses, respiratory insufficiency often leads to the need for a ventilator, and a wheelchair is often needed as a result of muscle weakness.

 “The range of symptoms in the late-onset form means that we may often miss the diagnosis if we are not thinking of it,” Dr. Kishnani said. “Pompe’s disease should be considered in patients with an undiagnosed limb girdle muscular dystrophy.”

Double-blind studies of GAA in late-onset Pompe’s disease are ongoing. In an open-label study of 18 patients ages 9 to 54, all of whom were treated for eight months to six years, three-fourths showed improved muscle strength, and half reduced the number of hours per day they spent on the ventilator. Several of Dr. Kishnani’s adult patients have been able to resume professional and community activities as a result of the treatment.

“We have to recognize that enzyme replacement therapy is one part of overall care,” Dr. Kishnani stressed. “Treatment needs to be provided within a multidisciplinary team. Outcome can really be positively impacted by early and comprehensive care. Early diagnosis is the key to optimal patient management.”

NEW ATTEMPTS AT THERAPY FOR DUCHENNE'S MUSCULAR DYSTROPHY

Despite enormous progress in understanding the genetic and pathophysiologic underpinnings of Duchenne’s muscular dystrophy (DMD), clinically proven therapeutics for the disease have not advanced since the introduction of steroid treatment more than two decades ago. Two recent developments may change that.

Following up a recent proof-of-principle study in mice, Richard Finkel, MD, presented preliminary clinical results for a drug that overrides the stop signal due to a “premature termination codon.” Such genetic errors account for about one in six cases of DMD. Dr. Finkel is Director of the Neuromuscular Program at Children’s Hospital in Philadelphia.

The drug, called PTC-124, “is genotype specific, not disease specific,” Dr. Finkel said, and it has the potential to treat many cases of cystic fibrosis, hemophilia, and other genetic disorders. PTC-124 causes the ribosome to “read through” a premature termination codon, allowing synthesis of the full protein.

Recently published studies have shown that PTC-124 is able to correct the muscle defects in a DMD mouse model and increase muscle force.

In a preliminary clinical trial in which 26 patients with DMD were given two oral doses of PTC-124 three times per day for 28 days, posttreatment muscle biopsies showed improvement in dystrophin expression and localization to the sarcolemmic membrane. However, the effect varied among patients. “Overall, about half of the patients at both the low and high dose did show some visible expression at the end of 28 days,” Dr. Finkel said. There was also a drop in the serum level of creatine kinase, an indicator of muscle damage, but no change in muscle strength.

He said that a trial in a third cohort, in which participants will receive a higher dose, is currently in progress. Tolerability and safety are “very good,” he said, and discussions are under way with the FDA, pending the outcome of further trials. PTC-124 was identified by a high-throughput screen of almost one million compounds, and other candidates identified by the screen are also in development.

Whether or not PTC-124 itself pans out, Dr. Finkel suggested this kind of treatment is likely to become more prominent in the future, and patients with genetic diseases may benefit if their mutation matches the mechanism of the treatment. “There will be emerging drug treatments that will depend on the specific type of mutation. I would urge you to get your patients genotyped, and get them enrolled in a registry,” he said.

Myostatin blockade is another strategy for muscular dystrophies of all types, one that doesn’t fix the underlying genetic problem, but attempts to compensate for its effects. Myostatin is an endogenous inhibitor of muscle growth that increases muscle size and strength in both animals and people.

Increasing strength by blocking myostatin is the goal of Kathryn Wagner, MD, PhD, Associate Professor of Neurology at the Johns Hopkins School of Medicine in Baltimore. “There is a linear decline in muscular dystrophy over time,” she said. “If we could increase strength, we could change the slope of that line and give the patient more disability-free years.”

Myostatin inhibits satellite cells, which produce new muscle fibers, and it stimulates fibroblasts, which increase fibrosis in damaged muscle. Dr. Wagner has shown that blocking myostatin reduces fibrosis, stimulates regeneration, and increases strength in animal models of chronic injury. Since myostatin is conserved across species, and at least one family of world-class athletes is known to have a null mutation in myostatin, “this gives us some hope that what we are observing in animals can be translated into humans,” Dr. Wagner said.

Clinical development of myostatin inhibitors has centered on monoclonal antibodies that neutralize the protein—in particular, one called myo-29. Dr. Wagner has just completed a double-blind trial of myo-29 in adults with a variety of types of muscular dystrophy, including Becker, limb girdle, and facioscapulohumeral forms. Patients received infusions every two weeks for 24 weeks and were then followed for an additional 12 weeks. Results are expected to be announced later this year.

NEW UNDERSTANDING OF THE MECHANISM OF MYOTONIC DYSTROPHY

There are no treatments yet, but recent developments in myotonic dystrophy (DM) have finally begun to shed light on the mechanism underlying this disease. “Myotonic dystrophy might be particularly treatable,” according to Charles Thornton, MD, Associate Professor of Neurology at the University of Rochester Medical Center in New York.

“The disease process in myotonic dystrophy is entirely different from most genetic diseases, because it is the RNA that is the problem,” he said, not the protein that the defective gene encodes. “The major effect is at the RNA level, not the protein level.”

Several lines of evidence point to this conclusion. In myotonic dystrophy type 1, the most common form, the mutation—an expanded CTG repeat—is in the DM protein kinase gene, but it comes after a stop codon. The repeat is transcribed but never translated. The protein is only modestly reduced in myotonic dystrophy cells, and mice completely lacking the protein appear to have no ill effects. Furthermore, the repeat is the only mutation that causes this form of myotonic dystrophy—after 15 years of genetic screening, no DM case has been identified in which the gene carries inactivating mutations within the coding region. “If the effect of this mutation were to inactivate this gene, certainly we would have seen other mutations that would cause the same effect,” Dr. Thornton said. “But the only mutation that causes the disease is an expansion of this repeat. So it’s hard to accept that the disease is caused by an effect on the protein.”

In contrast, the evidence is strong that RNA accumulation causes the symptoms of the disease. The transcripts form nuclear inclusions, which are found in each tissue affected by the disease, and RNA expansion reproduces features of myotonic dys­trophy in mice. Furthermore, RNA expansion and nuclear inclusions are also seen in myotonic dystrophy type 2, in a gene with no functional connection to DM protein kinase. In that disease, the expansion is in an intron and has no effect on expression of the encoded protein.

What is it about these particular RNAs that exerts a toxic effect? “We believe it results from the interaction of the expanded repeat with particular proteins in the nucleus,” Dr. Thornton said—specifically, a group of proteins known as muscleblinds. In mammals, these are the major nuclear proteins that bind to CUG-containing RNA (each of the disease-causing repeats includes CUG). They regulate RNA alternative splicing, and a single gene transcript can give rise to several different related proteins.

Alternative splicing occurs in the majority of genes and is tissue specific and developmentally regulated. Thus, the potential exists to affect a wide range of tissues, as seen in DM. Strong support for the role of aberrant splicing came in a new study showing that the myotonia in a mouse model of the disease could be mitigated by correcting the splicing of a chloride channel gene.

Muscleblind proteins are usually diffused in the nucleus but become highly localized when bound to the expanded repeat, which presumably hinders its ability to regulate splicing. “As a rough guess, you could estimate that dozens to a few hundred genes could be affected by this,” Dr. Thornton said.

Evidence for the central role of muscleblind sequestration comes from studies showing that the tissue effects of RNA expansion are mimicked closely by knocking out muscleblind expression and are rescued by overexpression. In summary, he said, “the current evidence would support the idea that interaction between a mutant RNA and a normal cellular protein is a critical aspect of myotonic dystrophy.”

“This makes me very hopeful about prospects for therapy,” Dr. Thornton said. This mutation does not directly eliminate any essential protein in muscle cells. The protein is not missing; it is only mislocalized and unable to do its normal job.”

Furthermore, he said, “based on the phenomenology of myotonic dys­trophy, you would predict the symptoms are likely to be reversible, up to a point,” since the major feature is muscle fiber atrophy, with very little fibrosis.

Strategies for treatment are under investigation, including accelerating the degradation of RNA, or blocking the interaction between the RNA and muscleblind.  Up-regulation of muscleblind is another possible avenue being pursued.           

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