The following story includes items about: Becker muscular dystrophy, centronuclear myopathy, type 1A Charcot-Marie-Tooth disease, congenital muscular dystrophy, Duchenne muscular dystrophy, limb-girdle muscular dystrophy, Miyoshi myopathy (distal muscular dystrophy), type 1 myotonic muscular dystrophy, myotubular myopathy, nemaline myopathy, and spinal muscular atrophy
Researchers at the University of Rochester (N.Y.) Wellstone Muscular Dystrophy Cooperative Research Center have identified a compound that has the potential to be developed into a treatment for type 1 myotonic dystrophy (MMD1, or DM1).
|In type 1 MMD, extra-long pieces of RNA form hairpin-like structures, which stick to and trap MBNL1 protein molecules. Thornton and colleagues believe their CAG25 molecule stuck to the abnormal RNA, opening up the “hairpin” and releasing the trapped MBNL1 protein.|
The compound, dubbed CAG25, is an “antisense oligonucleotide,” a type of construct that’s used to block RNA, a close chemical relative of DNA.
In the current experiments, the antisense oligonucleotide was given to mice with a disease resembling human MMD1. The researchers wanted to see whether it would counteract any of the effects of MMD1 by sticking to abnormally long RNA and freeing a protein called MBNL1 that would otherwise become trapped in it.
Charles Thornton, who co-directs the MDA clinic at the University of Rochester Medical Center, coordinated the research team, which published its findings July 17, 2009, in the journal Science.
The Wellstone Center at the University of Rochester has had funding from MDA and the National Institutes of Health.
The hypothesis Thornton’s research group set out to test was whether releasing protein molecules stuck to long strands of RNA would allow the proteins to resume their normal activities and improve symptoms in mice with an MMD1-like disease.
These mice were given injections of CAG25 into one leg muscle and an inactive substance into the same muscle on the other leg. Researchers interpreting the results didn’t know which legs had received CAG25. The CAG25-treated legs showed improvements in myotonia, the inability to relax muscles, which is a hallmark of myotonic dystrophy.
“What we have now is proof of concept that this general approach for treating myotonic dystrophy is potentially effective,” Thornton said, noting that results of the new study should encourage researchers to improve and refine the strategy.
Scientists at three U.S. institutions have used a very small synthetic molecule to correct the genetic defect in cells taken from a person with spinal muscular atrophy (SMA).
Genetic information moves from its storage form as DNA to a set of instructions known as RNA, from which protein molecules are made. Most of the RNA instructions from SMN1 genes, which are missing in SMA, tell the cell to make full-length SMN protein. Most of the instructions from SMN2 genes, which are present in SMA, tell the cell to make short SMN protein. Antisense to mask some of the instructions in the SMN2 RNA can cause synthesis of full-length SMN from the SMN2 gene.
The multicenter research team, which published results in the July-August-September 2009 issue of RNA Biology, was coordinated by MDA grantee Ravindra Singh at Iowa State University in Ames. The team also included MDA-supported Laxman Gangwani at the Medical College of Georgia in Augusta.
The molecule the researchers developed is called an “antisense oligonucleotide,” a type of compound that can cause cells to skip over erroneous genetic instructions. The compound is being tried experimentally in a number of genetic diseases to block the effects of abnormal genetic material (see “Freeing MBNL1 protein” above).
In SMA, the compound is being used to mask genetic instructions that, when present, result in the synthesis of a short, nonfunctional SMN protein. A full-length SMN protein is needed to treat this disease.
Antisense oligonucleotides have been used this way previously in SMA, but the molecules have been larger. The new, smaller version has potential advantages for both safety and effectiveness, the researchers say.
Scientists in the United States and Japan have identified a three-protein cluster that reseals damaged muscle-fiber membranes. The findings, published June 5, 2009, in the Journal of Biological Chemistry, could have implications for development of treatments for muscular dystrophies.
|Defects in the muscle-fiber membrane underlie many muscular dystrophies. The newly identified “repair complex” consisting of mitsugumin 53, caveolin 3 and dysferlin provides a new therapeutic target.|
In experiments using mouse muscle fibers, the investigators determined that mitsugumin 53 (MG53), a protein they announced in January as contributing to muscle-fiber repair, works closely with two other proteins, dysferlin and caveolin 3.
Scientists have known for a few years that dysferlin is involved in muscle-fiber membrane repair and that mu-tations of the gene for dysferlin or caveolin 3 can cause limb-girdle muscular dystrophy (LGMD). They’ve also known that mutations of dysferlin can cause Miyoshi myopathy, a form of distal muscular dystrophy. Now it appears that these three proteins — dysferlin, caveolin 3 and MG53 — form a cluster (complex) that repairs damaged membranes. Targeting the molecular functions of this cluster provides a new and promising avenue for therapeutic research, the researchers say.
Such research is especially important for muscular dystrophies in which membrane damage plays a major role, such as Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), some types of limb-girdle muscular dystrophy (LGMD) and possibly some types of congenital muscular dystrophy (CMD).
In experiments in mice, Michael Rudnicki, an MDA grantee at the Sprott Center for Stem Cell Research at Ottawa Hospital Research Institute (OHRI), and colleagues, found the WNT7a protein stimulates muscle repair by causing proliferation (an increase in number) of “satellite stem cells.” They say the protein probably operates similarly in humans. The findings were published June 5, 2009, in the journal Cell Stem Cell.
|Satellite cells are located near muscle fibers in mice and humans and stay in a dormant state until called upon for repair work. The WNT7a protein causes them to proliferate.|
Satellite cells are located near mature muscle fibers in mice and humans and stay in a dormant state until called upon for repair work. In earlier experiments, Rudnicki found that some satellite cells function as stem cells and maintain overall numbers of satellite cells. He distinguished these from other satellite cells, which are in various stages on the road to becoming muscle tissue.
In muscular dystrophy, satellite cells are believed to become depleted quickly because tissue damage places great demands on them for repairs. Enhancing their numbers could slow the process of muscle degeneration, even in the face of disease.
“In muscle degenerative diseases, one of the big problems is thought to be that the muscles run out of repair cells,” says Paul Muhlrad, a research program coordinator at MDA. “Rudnicki’s laboratory has figured out the biochemical pathways the body uses to maintain the supply.”
When the OHRI researchers injected genes for the WNT7a protein into muscle fibers in mice, they saw an increase in satellite stem cell numbers and enhanced muscle regeneration compared to what they saw in fibers that weren’t treated this way.
“The identification of satellite stem cells and the pathways that regulate their function is an important advance in our knowledge,” Rudnicki said. “We believe that this discovery points the way forward toward the development of new drugs that will stimulate muscle repair.”
A protein present in skeletal muscles during fetal development and in the heart after birth apparently can compensate for a similar protein that’s missing in a small percentage of patients with the muscle disease known as nemaline myopathy.
MDA research grantee Nigel Laing at the University of Western Australia in Perth was part of a multinational team of scientists who published their findings May 25, 2009, in the Journal of Cell Biology.
When the investigators bred mice missing the gene for the skeletal-muscle alpha-actin protein but with extra cardiac-muscle alpha-actin protein, they found the cardiac actin compensated well for the loss of the skeletal-muscle actin.
The findings open up the possibility of developing a treatment for some patients with human nemaline myopathy by increasing their own production of cardiac actin or giving them cardiac actin protein or genes.
Nemaline myopathy results from defects in a number of different muscle protein genes, all of which have to do with the muscle’s contractile filaments. These filaments slide over each other during muscle contraction in both cardiac and skeletal muscle tissue. Actin is a major filament component.
In its severest form, nemaline myopathy results in death in early infancy. In its less severe forms, affected children attain motor milestones slowly and may weaken further at puberty.
Previously, researchers had found mice bred not to produce any skeletal-muscle actin died by 9 days of age. In contrast, in this latest study, mice bred to produce extra cardiac-muscle actin but no skeletal-muscle actin survived into old age and had virtually normal muscle function.
These mice had grip strength and motor activity equal to that of healthy mice, and their muscles displayed a normal appearance, even under an electron microscope.
The researchers say their results show cardiac actin can effectively replace skeletal-muscle actin in muscles after birth, at least in mice and possibly in humans.
Previously, Laing said, it’s been shown that higher cardiac actin levels in patients without skeletal-muscle actin correlate with higher levels of function. The present results might indicate that increasing the level of cardiac actin even more in these patients would improve their motor abilities, he said.
The researchers caution that these experiments only showed compensation for a deficiency of the skeletal-muscle actin protein, not an abnormality in the protein. Patients with abnormalities in the skeletal-muscle alpha-actin gene that result in abnormalities of the actin protein, rather than deficiency, might not be helped by extra cardiac actin.
“Our results show that cardiac actin can work remarkably well in skeletal muscle,” Laing said. “This means that cardiac actin is a valid target for developing therapies for skeletal-muscle actin disease. However, we have a long way to go to be able to apply this to human patients. We have to find ways to increase cardiac actin in the muscles of human patients. That could take a long time, although we remain hopeful.”
A protein called osteopontin has been implicated as a cause of some of the detrimental inflammation and scarring (“fibrosis”) of muscle tissue that takes place in Duchenne muscular dystrophy (DMD).
Eliminating osteopontin was beneficial to mice with a DMD-like disease, and the researchers concluded that reducing osteopontin should be investigated as a possible therapy for DMD.
Sylvia Vetrone at the University of California-Los Angeles (UCLA) and colleagues published their findings online May 18, 2009, in the Journal of Clinical Investigation.
The eight-person study team was coordinated by Melissa Spencer at UCLA and included Carrie Miceli, a UCLA immunologist whose contribution Spencer called “hugely significant.” Also on the team was Eric Hoffman, who has MDA support for related work at Children’s National Medical Center in Washington.
Osteopontin plays a role in promoting tissue damage in autoimmune diseases, disorders in which the immune system mistakenly attacks the body’s own tissues, the investigators note.
Although DMD is a genetic disease whose underlying cause is the loss of the muscle protein dystrophin, it shares some features with autoimmune disorders, such as inflammatory tissue changes. The inflammatory changes are believed to be secondary to the loss of dystrophin.
The investigators in this study found elevated osteopontin levels in muscle biopsy samples from people with DMD and in the blood and muscles of dystrophin-deficient mice with a disease resembling human DMD. Elevation of osteopontin correlated with progression of the disease process in the mice.
To see the effects of eliminating osteopontin, the researchers bred mice lacking both osteopontin and dystrophin. In these dystrophin-deficient, osteopontin-deficient mice, they saw fewer immune-system cells and more regulatory cells known to dampen the immune response than they saw in the dystrophin-deficient mice. The mice missing both dystrophin and osteopontin also showed lower levels of a protein known to cause fibrosis than the mice missing only dystrophin.
Dystrophin-deficient, osteopontin-deficient mice were stronger than dystrophin-deficient mice when they were tested at 4 and 8 weeks of age.
Although they didn’t maintain this strength advantage at 6 months, their diaphragm and heart muscles did show less scarring than those of the dystrophin-deficient mice at the age of 6 months. (Spencer said studies are under way to test diaphragm and heart function in these mice.)
The researchers interpreted these early findings to mean that osteopontin promotes inflammation and contributes to the deposition of scar tissue in dystrophic muscles.
They say their studies suggest that blocking osteopontin “may be a promising therapeutic target for reducing inflammation and fibrosis in individuals with DMD.” They note that further studies should be designed to find ways of reducing osteopontin in muscle tissue and to better understand the relationship among osteopontin, regulatory cells and the dystrophic process.
Zarife Sahenk at Nationwide Children’s Hospital and Ohio State University in Columbus, and colleagues, has found mice with a disease resembling type 1A Charcot-Marie-Tooth disease (CMT1A) benefited from a transfer of genes for the neurotrophin 3 protein. CMT1A is caused by a duplication of the PMP22 gene.
Jerry Mendell, who has received many MDA research grants and co-directs the MDA clinic at Nationwide Children’s, was part of the study team, as was Brian Kaspar, who has received MDA support at Nationwide.
The researchers injected the leg muscles of CMT1A mice with either neurotrophin 3 genes inside shells made from adeno-associated viruses, or with a sham injection. The legs injected with the genes showed better grip strength and had more normal-looking nerve fibers.
The investigators concluded that neutrophin 3 gene therapy is a promising avenue for treatment development in human CMT1A.
These findings were reported at the 2009 American Academy of Neurology meeting, which was held recently in Seattle.
|Molecular geneticist Alan Beggs has had MDA support to study centronuclear and other myopathies at Children’s Hospital in Boston.|
Families affected by muscle diseases known as centronuclear myopathies, including myotubular myopathy, gathered for a conference in Houston July 24-26, 2009. The organizers have made the conference proceedings available as a Webcast at www.mtm-cnm.com.
Among the speakers are Alan Beggs, a molecular geneticist at Children’s Hospital in Boston, who has had MDA support to study centronuclear and other congenital myopathies; and Susan Iannaccone, a pediatric neurologist at Children’s Medical Center in Dallas who has received MDA support for neuromuscular disease research and directs the MDA clinic at her institution.
Recent advances have shown that the course of disease in centronuclear myopathies is highly variable, with myotubular myopathy being the most severe form. (See “Taking a Closer Look at Myoubular Myopathy,” Quest, September-October 2007.)