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    Home> Publications > QUEST Vol.15 No.5 September/October 2008
 
 
Research Updates

Viagra improved heart function in DMD mice

A recent study has found treatment with sildenafil (Viagra) significantly improved heart function in mice missing the protein dystrophin and showing a disease resembling Duchenne muscular dystrophy (DMD). Heart-muscle deterioration (cardiomyopathy) is a leading cause of death in boys and men with DMD and the closely related Becker muscular dystrophy (BMD).

A research team coordinated by Christine Des Rosiers at the University of Montreal, which included MDA grantee Basil Petrof at McGill University in Montreal, announced its findings May 13 in Proceedings of the National Academy of Sciences.

The idea of treating dystrophindeficient mice with sildenafil was based on previous studies suggesting that the hearts of these mice are more susceptible than normal hearts to stress-induced cell death because of a deficiency of a compound called cyclic guanosine monophosphate, or cGMP.

Black Mouse
Christine Des Rosiers and colleagues found the hearts of DMD mice treated with Viagra showed much less damage than those of untreated mice, while Summit Corp. researchers found the utrophin-stimulating compound C1100 increased strength in DMD mice.

Sildenafil and related medications, which have U.S. Food and Drug Administration approval to treat erectile dysfunction, as well as pulmonary hypertension, increase cGMP by blocking the enzyme that normally breaks it down. Increased levels of cGMP result in dilation of blood vessels, including the coronary arteries.

DMD-affected mice treated with sildenafil had 44 percent less damage to heart-muscle cells than did untreated mice when their hearts were stressed with a drug known to increase cardiac workload.

The researchers say their findings demonstrate that enhancing signaling by cGMP in dystrophin-deficient hearts improves cardiac contraction and energy production and helps the membranes surrounding cardiac muscle cells resist tearing under stress.

They note that treatment with sildenafil and similar medications, which already exist, “constitutes a potential clinical avenue for treatment of the dystrophin-related cardiomyopathies.” However, currently there is no clinical data to support the use of these drugs in people.

Utrophin-stimulating compound improves strength, endurance in mice

An experimental compound that stimulates production of the muscle protein utrophin increased strength in dystrophin-deficient mice with a disease resembling Duchenne muscular dystrophy (DMD).

The compound, called SMT C1100, is made by Summit Corp. of Oxford, United Kingdom. When combined with a corticosteroid (prednisone-like medication), it reduced muscle fatigue during exercise.

The findings were first presented at the New Directions in Muscle Biology and Diseases conference held in New Orleans April 27-30. The company says it plans to begin phase 1 clinical trials in 2009.

Mice

SMT C1100 is a small molecule designed to increase production of utrophin, a protein that at least partially compensates for the lack of functional dystrophin that characterizes DMD and Becker muscular dystrophy (BMD).

The major advantages of the utrophin-stimulating drug over some of the other strategies being developed for DMD and BMD are that the drug is oral; it has the potential to be effective in a wide range of patients; and, since people with DMD and BMD already make normal utrophin, it’s highly unlikely to cause an unwanted immune response.

Myotilin protein may be a viable therapeutic target in LGMD1A

Findings in a study published in the May issue of Muscle & Nerve show overproduction of the protein myotilin worsens the symptoms of type 1A limb-girdle muscular dystrophy (LGMD1A) in mice.

The myotilin gene, located on chromosome 5, carries instructions for the large myotilin protein found in the Z disc — the structure that anchors the thick and thin filaments that slide together to cause muscle to contract. Defects (mutations) in the myotilin gene lead to the production of abnormal (mutated) myotilin protein, which causes disorganization in the muscle fibers and is the underlying cause of LGMD1A.

MDA grantee Michael Hauser at the Center for Human Genetics at Duke University in Durham, N.C., and colleagues, examined the severity of muscle degeneration and conducted microscopic analysis of muscle fibers in mice bred to produce mutated myotilin alone or mutated myotilin and higher-than-average levels of normal myotilin.

They found the mice making mutant myotilin had signs of LGMD1A, but those making increased levels of normal myotilin in addition to the mutated protein had increased severity of symptoms. These included earlier onset and a greater involvement of muscles less affected in mice that produced only mutant myotilin.

The researchers indicated that a “knockdown” approach, in which the amounts of both abnormal and normal myotilin are reduced, might be an effective therapeutic strategy for LGMD1A, although Hauser suggests more work may be needed in developing highly efficient knockdown methods, and “it may turn out that effective therapy will require a combination of several different methods at once.”

Calling the mouse experiment “a big step forward in understanding the mechanism in LGMD1A,” Hauser noted, “This improved understanding of mechanism is an important part of designing effective therapies for any muscular dystrophy.”

Mitochondrial disease meeting highlights new directions

The mitochondria, complex bodies inside cells that have their own DNA, are the centers for cellular energy production. When they malfunction, either because of flaws in their own DNA or flaws in DNA in the cell’s nucleus that affect them, the energy deficit can be devastating, particularly for the functioning of the muscles and nervous system.

At a mitochondrial “summit” meeting jointly sponsored by MDA and the Friedreich’s Ataxia Research Alliance, and held in Phoenix May 20-21, some tantalizing new findings to treat mitochondrial myopathies were presented.

Friedreich’s ataxia (FA), although not often classified as a mitochondrial disease, is in fact a disease of the peripheral nervous system and heart for which the underlying cause is a deficiency of a mitochondrial protein.

TAT delivers proteins to mitochondria

Mark Payne, from Indiana University School of Medicine in Indianapolis, described his experiments with the TAT (“transactivator of transcription”) protein, which can carry molecular cargo across biological membranes, including those surrounding each mitochondrion. He said his group is the first to make this strategy work in an animal.

They applied the new technology to the restoration of the frataxin protein into the mitochondria. (Frataxin is deficient in FA.)

Frataxin, like many other mitochondrial proteins, is made from a gene in a cell’s nucleus. The cell “knows” it goes to the mitochondria because of a molecular tag that indicates its destination. “It’s sort of a zip code for the protein,” Payne said.

When frataxin, with its tag, was attached to TAT and injected into the abdomens of mice born without this protein, the compound crossed various membranes, including those around the mitochondria, where it was retained and increased the life span of the mice from an average of 27 days to an average of 39 days.

The strategy has promise for the delivery of many other proteins that have to cross mitochondrial and other membranes.

Blood stem cells provide needed enzyme in MNGIE

Also of note at the meeting was a talk by MDA grantee Michio Hirano, from Columbia University in New York, who described a promising experimental treatment for the mitochondrial disease known as MNGIE (mitochondrial neurogastrointestinal encephalomyopathy), which is due to a defect in an enzyme made from a gene in the cell’s nucleus.

The enzyme, thymidine phosphorylase, normally breaks down thymidine. Without it, thymidine accumulates in the blood, which has an indirect but highly toxic effect on mitochondria.

Hirano and colleagues have been experimenting with blood stem-cell infusions from healthy donors into patients with MNGIE, speculating that the new cells would restore the needed enzyme, which would eliminate the toxic metabolic product.

Hirano described a 30-year-old woman with MNGIE with severe gastrointestinal dysfunction causing weight loss, weakness, peripheral nerve abnormalities, lack of energy and abdominal pain. To supplement her meager food intake, she received 2,400 calories daily by intravenous infusions. Two-and-a-half years after receiving blood stem cells from a donor, he reported, she has shown a remarkable correction of the biochemical abnormalities in her blood, has gained weight, has less abdominal pain, can eat normally, does not require intravenous feeding and can run and jump.

Diagram
Normally (left), the thymidine phosphorylase enzyme breaks down thymidine, keeping mitochondria healthy. In MNGIE (right), a mutation in the gene for this enzyme keeps it from being produced, allowing toxic levels of thymidine to build up and damage mitochondria.

ID of protein that prevents myelin formation could have implications for CMT treatment

Scientists in the United Kingdom and Italy have found that a protein called c-jun keeps cells associated with nerve fibers from maturing and producing myelin, a fatty sheath that insulates the fibers and speeds transmission of signals to and from nerve cells.

Kristjan Jessen at University College London and colleagues, who reported their findings May 19 in the Journal of Cell Biology, say it’s likely the normal role of c-jun is to push myelin-making cells, known as Schwann cells, back to a more primitive state after nerves are injured. Schwann cells normally return to this earlier stage of development after injury, they note, as part of the process of nerve-fiber repair and regeneration.

However, in Charcot-Marie-Tooth disease (CMT), abnormal loss of myelination occurs, slowing nerve signals and leading to disability. Several forms of CMT, including the relatively common CMT1A and CMT1B, as well as other diseases, are characterized by abnormalities in myelination of nerve fibers.

The investigators say it will be important to determine whether c-jun is involved in causing these abnormalities. If so, they say, targeting c-jun might open new avenues for treatment.

Clinical Trials and Studies

Risk of sudden death 3 to 5 times above average in type 1 myotonic dystrophy

An electrocardiogram can detect abnormalities in the heart’s conduction system.
An electrocardiogram can detect abnormalities in the heart’s conduction system.

A significantly higher-than-average risk of sudden death exists for adults with type 1 myotonic dystrophy (MMD1, also known as DM1) and either a severely abnormal electrocardiogram (ECG) or a diagnosis of atrial tachyarrhythmia (an abnormally fast, irregular heart rate originating in the upper chamber, or atrium, of the heart), according to a study in the June 19 issue of the New England Journal of Medicine. The study was supported by MDA, the National Institutes of Health and the medical electronics manufacturer Medtronic.

A severely abnormal ECG correlated with a risk of sudden death that was 3.3 times above average. A clinical diagnosis of atrial tachyarrhythmia resulted in a fivefold increase in the risk of sudden death compared to the average risk.

The investigators defined a severe ECG abnormality as either a heart rhythm not originating from the normal place in the heart; a significant lengthening of the normal time it takes for signals to travel through the heart; or a partial blockage of signals from the upper to the lower part of the heart.

All these abnormalities reflect malfunctions of the heart’s conduction system, which regulates heart rate and rhythm via electricity-like signals. Conduction-system abnormalities, which are common in MMD1, can cause sudden death if they progress to complete blockage of cardiac signals and stoppage of the heartbeat; or if they lead to a fast, nonfunctional rhythm pattern in the lower heart chambers (ventricular tachyarrhythmia) that prevents adequate pumping action.

Cardiologist William Groh, who received MDA support at Indiana University’s Krannert Institute of Cardiology in Indianapolis, and colleagues studied 406 adults with MMD1 seen at 23 MDA clinics beginning in April 1997.

During an average follow-up period of 5.7 years, there were 81 deaths among study participants, 27 of which were classified as sudden. In addition, there were 32 deaths from respiratory failure due to respiratory muscle weakness, five nonsudden cardiac deaths and 17 deaths from other causes.

The investigators note that 10 percent of study participants had cardiac pacemakers by the last follow-up visit but that these did not appear to decrease the rate of sudden death or death from any cause.

A small number of participants had implantable cardioverter-defibrillators, which can bring an abnormally slow heartbeat up to a normal rate and also slow a dangerously fast heart rhythm, by delivering an electric shock. Sudden death occurred in two patients with these devices.

The investigators say their findings suggest cardioverter-defibrillators may be helpful in preventing sudden death in MMD1 but that their data didn’t allow firm conclusions to be drawn.

“In conclusion,” the authors write, “our study shows that adult patients with myotonic dystrophy type 1 are at high risk for sudden death.” They noted that the presence of a severe ECG abnormality or a clinical diagnosis of atrial tachyarrhythmia increased the risk of sudden death, whether or not the other condition existed.

Specific cardiac problem seen in MMD2

A type of cardiac abnormality known as “left ventricular noncompaction,” a condition in which the muscular wall of the heart’s main (lower left) pumping chamber is spongy and meshlike, rather than tightly compacted, might be specifically associated with type 2 myotonic dystrophy (MMD2, also known as DM2), says a group of researchers in France who published their findings in the April issue of Neuromuscular Disorders.

An echocardiogram assesses the heart’s pumping ability.
An echocardiogram assesses the heart’s pumping ability.

Karim Wahbi, at René Descartes University in Paris, and colleagues, describe a 61-year-old man with MMD2 with this abnormality, which can (but doesn’t always) lead to blood clots and interference with the heart’s pumping capacity. It was detected with an echocardiogram, which uses sound waves to produce images of the heart.

The researchers note that most cases of left ventricular noncompaction have been seen in association with a neuromuscular disorder, including MMD1. They recommend careful followup of MMD2 patients for cardiac complications.

Brain MRI abnormalities in MMD1 similar within families

The brains of family members with myotonic dystrophy may have more in common than those of people with the same size genetic defect or other shared disease characteristics.
The brains of family members with myotonic dystrophy may have more in common than those of people with the same size genetic defect or other shared disease characteristics.

A new study says brain MRI abnormalities known as “white matter lesions,” known to be common in type 1 myotonic dystophy (MMD1, or DM1), are more similar among family members affected by the disease than they are among people who share other disease characteristics, such as the size of the DNA abnormality that causes their disease.

Alfonso Di Costanzo at the University of Molise in Campobasso, Italy, and colleagues, who published their findings in the April issue of Neuromuscular Disorders, studied 60 people (30 of each gender) with the noncongenital form of MMD1. Each of the study participants, who came from 22 families, had at least two close relatives who also were affected.

When they analyzed the extent and location of the brain abnormalities in the study participants, the researchers found these were not correlated with the person’s gender or age at MMD1 onset, or with the gender of the parent from whom the disease was inherited; nor did they correlate with the size of the MMD1-causing DNA expansion, measured in blood cells, even though larger expansions are generally correlated with more severe disease symptoms and earlier onset.

The authors suggest that additional, as yet undefined, genetic factors, and/or environmental factors shared by families could be influencing the development of the MRI abnormalities. They also note, however, that MMD1-associated DNA expansions measured in blood cells don’t always match those measured in brain or other cells.

Study of 25 found SBMA has small impact on function, survival

A study of the survival and functional status of 39 people with spinal-bulbar muscular atrophy (SBMA, or Kennedy’s disease) conducted at the Mayo Clinic in Rochester, Minn., showed patients’ survival was only slightly altered compared to healthy subjects’ survival and that most patients had only mild neurologic impairment many years after diagnosis.

Eric Sorenson and colleagues, who published their findings in the May 20 issue of Neurology, reviewed the medical records of 39 people with a confirmed SBMA diagnosis who were seen at the Mayo Clinic between 1990 and 2005. They compared them to people unaffected by SBMA but otherwise similar.

Of the 39 SBMA-affected study participants, 33 (85 percent) were alive at the study’s start. In the six who had died, the average age of death was 76.

Eight could not be contacted, but the other 25 agreed to take part in the functional study, which used a standard rating scale to assess their abilities. The average score on this scale was 37, out of a possible 48.

The most common impairment was difficulty climbing stairs, although only five people said they were unable to do so.

Twenty reported some impairment in walking, but only two required a wheelchair.

Minor difficulty with swallowing was reported by 20 subjects, but all maintained normal oral intake and none used a feeding tube. No one required a speech augmentation device, and only one person needed respiratory support, using noninvasive positive pressure ventilation during sleep.

The results of the study suggest that “in addition to a good long-term survival, patients with SBMA continue to be independent in most of their activities of daily living,” the authors write. They say they don’t want to “diminish the significant impairment” SBMA causes in some patients; but that their findings suggest that “in the majority, SBMA follows a relatively benign course with comparatively good functional status years after the diagnosis.”

Researchers ID protein that drastically lessens severity of SMA in females

Researchers at several German and U.S. institutions recently announced they’ve identified a protein that can almost completely compensate for the genetic defect that causes chromosome-5-linked spinal muscular atrophy (SMA), which is by far the most common type of the disease. They say their findings may open new treatment pathways, as well as provide better understanding of the biology of SMA and possibly other, related diseases.

A research team coordinated by Brunhilde Wirth at the University of Cologne in Germany identified the activity level of the gene for the plastin 3 protein as a modifying factor in SMA in females.

Brunhilde Wirth and colleagues found high plastin 3 levels seem to reduce the severity of SMA in females. Elsewhere, Eugenio Mercuri’s research team saw motor function improve with albuterol in children with SMA2.
Brunhilde Wirth and colleagues found high plastin 3 levels seem to reduce the severity of SMA in females. Elsewhere, Eugenio Mercuri’s research team saw motor function improve with albuterol in children with SMA2.

In the process, they found that defects in the growth and maintenance of nerve fibers, or “axons,” which carry chemicals from cell to cell, are the primary problem in chromosome-5 SMA and are a potential point of intervention in its treatment. They published their findings in the April 25 issue of Science.

Chromosome-5 SMA, a genetic disease in which spinal nerve cells that control voluntary muscles (motor neurons) are lost, is caused by mutations in the SMN1 gene that result in varying degrees of deficiency of the nerve-cell protein SMN.

People with moderate SMN loss have weakness and limited mobility but generally live into adulthood, while babies born with almost no SMN have severely compromised breathing and swallowing functions and often don’t survive more than a few years.

Wirth’s team identified six families in which some of the females with specific SMN gene mutations and protein levels had the expected SMA symptoms, while others with the same SMN mutations and protein levels had very mild disease. Suspecting the presence of modifying genetic factors, the researchers measured activity levels of the siblings’ other genes.

Only one gene, carrying the code for the protein plastin 3, showed significantly different levels of protein-producing activity in cells from mildly affected, compared to more severely affected, siblings.

In boys, the level of plastin 3 protein didn’t make a difference in the degree of severity of SMA; but in the girls, high plastin 3 protein levels resulted in far less severe SMA symptoms or no symptoms at all with the same degree of SMN deficiency. The investigators don’t know the reason for this gender difference.

Further experiments in human cells and in mice and zebrafish showed that plastin 3 is important for bundling filaments that make up nerve axons and that extra plastin 3 can almost completely compensate for the detrimental effects of reduced SMN protein levels on these cellular structures.

The researchers note that their findings “support the view that axon biology is crucial for SMA ... and that proteins stabilizing the axon can modify the disease.”

Function of children with type 2 SMA improved with albuterol

Eugenio Mercuri from Catholic University in Rome, with researchers from several Italian institutions and the Dubowitz Neuromuscular Centre in London, found daily treatment with oral salbutamol (called albuterol in the United States) yielded some encouraging results in a small trial involving young children with type 2 spinal muscular atrophy (SMA2).

The investigators, who published their findings online June 23 in Neuromuscular Disorders, gave albuterol at a dosage of 2 milligrams three times a day for a year to 23 children who were between 30 months and 6 years old at study entry.

They assessed motor function using a standardized rating scale six months before the treatment started, when the treatment started, six months after starting treatment and a year after starting treatment.

There were no significant changes in function when the children were not on treatment, but the functional scores after six months and a year of treatment with albuterol were significantly higher than those recorded when the children started albuterol. There were no major side effects.

The researchers say their results suggest albuterol may be beneficial in SMA2 but that a larger trial, in which albuterol is compared to a placebo (inactive substance), is needed to confirm these early findings.

They also note that the drug’s actions in SMA are not well understood. It may reduce muscle wasting and/or increase production of the SMN protein, which is insufficient in this disease.