This article contains items on: Duchenne muscular dystrophy. Becker muscular dystrophy, myotonic muscular dystrophy, Friedreich's ataxia, rare myopathies and the neuromuscular junction
Extending a gene-repair technique known as exon skipping to an additional type of gene mutation that causes Duchenne muscular dystrophy (DMD) or Becker muscular dystrophy (BMD) is feasible, say researchers coordinated by Kevin Flanigan at the University of Utah.
The team, which included MDA-supported Stephen Wilton at the University of Australia in Perth, published its results in the January issue of Annals of Neurology.
Exon skipping in DMD and BMD involves using compounds called antisense oligonucleotides (AONs) to mask errors in the dystrophin gene and restore production of functional dystrophin protein molecules. The technique has shown promise in a small clinical trial and is undergoing further testing.
So far, the strategy has been applied to errors in the dystrophin gene known as deletions, point mutations or nonsense mutations.
This new report shows that the technique can also be applied to “pseudoexon” mutations, errors in which the cell erroneously includes material in the genetic message that should have been spliced out.
The AONs blocked the erroneously inserted material in cells taken from two boys with DMD and one with BMD, allowing for production of full-length dystrophin.
MDA grantee Thomas Cooper at Baylor College of Medicine, with colleagues there and in France, has added a new piece to the puzzle of type 1 myotonic dystrophy (MMD1) that may help explain some of the differences between it and type 2 myotonic dystrophy (MMD2) and could ultimately lead to treatment advances.
Cooper, a professor of pathology and of molecular and cellular biology, coordinated the research team, which published its findings online Feb. 11 in Proceedings of the National Academy of Sciences.
The investigators first bred mice with an expanded stretch of DNA in the so-called DMPK gene, the same defect in the same location as the one that causes human MMD1. They say these mice mimic the human disease better than any other “mouse model” of MMD1 created so far because, in addition to myotonia (inability to relax muscles) and characteristic molecular abnormalities, these mice exhibit severe muscle wasting (atrophy), as observed in the human disease.
Previously developed mouse models of MMD1 have added the expanded DNA (which consists of chains of repeated DNA sequences) to a gene other than DMPK; or have inserted high numbers of normal-length repeated DNA pieces instead of a long, repeated DNA expansion; or have mimicked a secondary effect of the DNA expansion, the depletion of a protein known as MBNL1.
Cooper and colleagues say these other models exhibit some of the features and molecular events seen in human MMD1, but not all. They say their new model is the only one to mimic the muscle wasting that patients have and to show elevated levels of a protein called CUGBP1 in muscle cells, another characteristic of human MMD1.
They note that people with MMD2, which involves an expanded stretch of repeated DNA sequences in a gene other than DMPK, don’t have high levels of CUGBP1 and typically have milder muscle wasting than people with MMD1.
The increase in CUGBP1 levels, which has deleterious effects on muscle tissue and correlates with the severe muscle atrophy, seem to occur only when the expanded DNA sections are in the DMPK gene and not when they’re in other genes.
The study challenges a view widely held until now that the location of the expanded DNA stretch isn’t important in either type of MMD and that its existence in any location would cause roughly the same problems.
“Muscle atrophy is the primary cause of disability and death in individuals with MMD1,” Cooper said. “Having an animal model that reproduces this aspect of the disease provides an important tool to understand the process and to test therapies. This model has already given us a reason to think the CUGBP1 protein is involved, and this is an important lead to follow. We’re now testing how important CUGBP1 is to muscle wasting. If it’s a key factor, it gives us another target for therapies.”
Mutations (flaws) in an X-chromosome gene for a muscle protein called FHL1 have been implicated in a range of rare myopathies (muscle disorders) affecting skeletal and cardiac muscles. Until now, these myopathies have lacked an identifiable genetic cause.
The FHL1 protein is found in skeletal and cardiac muscles and is thought to play a role in the sarcomere, the contracting part of a muscle cell, and in the sarcolemma, the membrane surrounding the cell.
In January and February, three separate research groups, two of which had MDA funding, announced they had identified mutations in the FHL1 gene as underlying a muscle disorder.
In the January issue of the American Journal of Human Genetics, a team coordinated by Michio Hirano at Columbia University Medical Center in New York that included MDA-supported Catarina Quinzii at that institution described how a mutation in the FHL1 gene is the cause of weakness in the shoulder and lower leg muscles in a large Italian-American family.
In the same issue of the journal, a group led by Christian Windpassinger at the University of Toronto and the Medical University of Graz in Austria identified two mutations in the FHL1 gene, different from the one identified by Hirano and colleagues and from each other, as causing weakness of the shoulder and lower leg muscles with cardiac involvement in an Austrian and a U.K. family.
And in February, MDA-supported Carsten Bonnemann at Children’s Hospital of Philadelphia and colleagues described four additional mutations in the FHL1 gene as being responsible for “reducing body” myopathy, a rare muscle disease characterized by progressive weakness and the presence of abnormal protein deposits called reducing bodies in the muscle cells.
Bonnemann’s group analyzed muscle samples from four families in the United States and the United Kingdom and published its findings online Feb. 14 in the Journal of Clinical Investigation.
The Hirano and Windpassinger groups used a method called linkage analysis, in which a region of DNA difference in affected versus unaffected family members is used to identify a disease-causing gene.
The Bonnemann group, however, used a new method to reach its conclusions. Instead of starting with DNA analysis, which is the usual approach, they analyzed the content of the abnormal protein deposits in muscle samples from people with reducing body myopathy and found that the FHL1 protein was the largest component. They then analyzed the FHL1 gene in the four families and found it was abnormal in the affected patients.
When they put the abnormal FHL1 genes into cells in a lab dish, they saw the formation of reducing bodies just as they had in the patients’ muscles.
They say this new “laser microdissection proteomics” approach may become important in identifying the cause of other rare diseases that have prominent cellular changes.
Researchers in the laboratory of MDA-supported Lin Mei at the Medical College of Georgia in Augusta have found that muscle fibers do more than passively receive signals from nerve fibers that tell them to contract or relax.
|Scientists have known for decades that nerve fibers (axons) send chemical signals that muscle fibers need. Now it appears muscle fibers also send signals in the other direction, which nerve fibers need.|
Instead, say Mei and colleagues, who published their findings online in Nature Neuroscience Feb. 17, “backwards” (retrograde) signals coming from muscle fibers to nerve fibers profoundly influence nerve-fiber location and function.
When the investigators bred mice lacking a protein called beta-catenin in their muscles, they saw that branches of the phrenic nerves, which go to the respiratory diaphragm, were mislocated in the diaphragm muscle and that signal transmission was reduced.
However, when beta-catenin was depleted only in nerve cells in the mice, they didn’t have this type of neurological problem.
“These observations demonstrate that muscle beta-catenin is a key ingredient for neuromuscular junction formation,” Mei said, referring to places where nerve and muscle fibers meet. The findings also showed that beta-catenin may control other proteins necessary to nerve-cell health, she added.
“Muscles are known to produce elusive nutritional factors for nerve-cell survival and development,” Mei said. “And these findings could provide leads to their identification.”
In an accompanying editorial, researchers from Hong Kong University of Science and Technology describe the findings’ significance as “several-fold.” They say these experiments will help scientists understand more about how neuromuscular junctions develop and may add to current understanding of neuromuscular disorders, including muscular dystrophies and amyotrophic lateral sclerosis.
With joint support from MDA and the Friedreich’s Ataxia Research Alliance, or FARA (www.curefa.org), David Lynch at Children’s Hospital of Philadelphia and colleagues will refine their previously developed standardized measurements of disease progression in Friedreich’s ataxia (FA) and create a network for conducting clinical trials and other studies in this disease.
On the drawing board are a nine-center study of the natural history of FA and a parallel study to identify biological markers that reflect disease severity.