About Muscular Dystrophy

Can Stem Cells Help?
Cell Transplantation
Combined stem cell and gene therapy
Looking to the Future
Canada
General Information

Muscular Dystrophy is a group of inherited neuromuscular disorders which are characterized by progressive muscle weakness and muscle wasting due to the malfunction of the nervous and musculoskeletal systems. The disease leads progressively to the inability to move, eat and breathe. While there are many types of muscular dystrophy, Duchenne's muscular dystrophy (DMD) is both the most common and most severe form of the disease. It afflicts one in every 3500 boys and is usually detected between the ages of 3 and 5 when the parent notices the child has difficulty walking. Duchenne's affects every muscle in the body and in the more severe forms results in progressive paralysis and even death.

Duchenne's muscular dystrophy is caused by a defective gene usually inherited from the mother. Children with this disease do not have the ability to make dystrophin, a protein vital to keeping muscle cells, also known as myofibers, intact following various stresses that may be inflicted on the muscle during a normal day.

Children have difficulty walking, breathing, talking, holding themselves upright, and they often require physiotherapy, steroid drugs, braces and other kinds of aid to slow progression, strengthen their muscles and cope with the challenges of this illness. Boys with Duchenne's are usually confined to a wheelchair by age 12 and if untreated, rarely live past age 20. Nowadays, with steroids and other drugs patients live well into their 30's, but scientists are actively looking for a cure.

Other types of muscular dystrophy affect both males and females, arise at various ages, or are usually less disabling.

Can Stem Cells Help?

Understanding how muscle cells normally work and what goes wrong at the molecular level when they fail to regenerate after injury or exercise is key to finding a cure for muscular dystrophy.

Myofibers are the building blocks of muscle tissue and are one of a few cells in the body that have more than one nucleus. They are formed from the fusion of thousands of single cells, extending from the tendons which attach the muscle to the skeleton. It is this unique property of myofibers that make them so amenable to cellular therapy. Indeed, most research focuses on identifying the best cell to introduce into the muscle for fusion that will counteract the degenerative process.

Since Duchenne's muscular dystrophy is due to the deficiency of one gene product - the protein dystrophin - some scientists are trying to find a way to either replace the protein through genetic engineering or through cell transplantation or a combination of both.

Cell transplantation

Early studies in the US successfully transplanted stem cells from healthy newborn mice into muscles of mice bred to simulate muscular dystrophy. These appear to provide the missing protein and be fully incorporated into muscle tissue.

This basic research contributed in part to the establishment in 2004 in Canada of the first clinical trial for the treatment of DMD. The results of this trial demonstrated that when muscle precursor cells (called "myoblasts") from a healthy donor are transplanted into the affected muscle of a patient with Duchenne's muscular dystrophy they fuse with the host myofibers and donate their healthy genome, allowing the correct form of the gene to be expressed. It is now known that a small fraction of healthy nuclei can provide enough dystrophin to completely cure a myofiber and ensure its survival.

The success of this cell transplantation protocol depends on suppressing the immune system with a new drug called Tacrolimus, on the injection of a high number of cells and on a short distance between the injection sites. In other words, the effects are localized to the site of cell injection.
The limitation of this technique is that cells have to be injected into each muscle in turn, whereas muscular dystrophy affects the entire body.

Combined stem cell and gene therapy

To circumvent this problem, in mouse and dog models, scientists have had some success in switching various genes on or off to improve muscle function by using "mesangioblasts," which are stem cells found in the walls of blood vessels that are known to restore muscle structure and function to a significant extent when injected into the bloodstream. Mesangioblasts can cross from the blood stream into muscle tissue, diffusing throughout the body and integrating into existing muscle. This makes them ideal candidates to deliver material into the muscle.

Scientists inserted a healthy version of the dystrophin-producing gene into the stem cell, multiplying these cells in the laboratory and then injecting them into the bloodstream of mice with muscular dystrophy. The cells migrated to the muscles and began producing healthy muscle tissue. The prospect of using this technique with a patient's own stem cells ("autologous transplant") would possibly prevent the immune reaction that is common with transplantation from a donor.

This technique was attempted recently with dogs, who suffer from a form of dystrophy very similar to the human form of Duchenne's. Italian scientists showed that mesangioblasts from healthy dogs injected into the sick dogs fused with existing myofibers and created enough dystrophin to restore functioning to a significant degree.

Viruses (adenoviral vectors) have also been used to "transport" genetically-modified cells throughout the body, but the effect is more diffuse.

Looking to the future

Because it is an inherited disorder, the research on DMD, especially the success of gene replacement therapy, will be applicable to many other genetic diseases.

One of the results of research into muscular dystrophy is that scientists have come to understand the "plasticity" of adult stem cells and demonstrated the production of muscle cells from different sources. Stem cells taken from fat tissue can become smooth muscle cells found in many organs, including the intestines, bladder and arteries. Bone marrow stem cells can create muscle cells. Blood vessel cells can differentiate into muscle, bone and cartilage cells. Controlling the mechanisms of stem cell differentiation in vitro is critical to using stem cells for therapy.

Studying muscle stem cells has brought some surprises as well. For example, Canadian scientists discovered that some muscle stem cells can become neural cells under the right laboratory conditions, a possible source of cells for neural transplants for degenerative diseases. US scientists discovered that female stem cells are better than males stem cells at regenerating muscle.

US and Canadian scientists have recently discovered a unique population of adult stem cells adjacent to the walls of blood vessels in muscles, known as "myoendothelial cells" that are likely related to mesangioblasts. These cells that appear to efficiently contribute to muscle repair. Interestingly, myoendothelial cells showed no sign of forming tumors - a concern with other stem cell therapies.

Just as promising is research related to satellite cells. Muscle satellite cells are the normal muscle precursor population closely associated with myofibers that are responsible for virtually all of the growth and repair of skeletal muscle. Researchers in Canada recently defined a subpopulation of satellite cells that function as stem cells. These “satellite stem cells” appear to maintain the overall satellite cell population and are capable of repopulating the satellite cell niche following transplantation.

This discovery may lead to being able to create a reservoir of muscle stem cells that can be mobilized in vivo to replace cells being damaged by MD. This represents tremendous progress over just injecting cells to repair the tissue. Researchers in Colorado are following up, experimenting with injecting a self-renewing population of stem cells to renew the satellite cell pool in an affected muscle.

Researchers in Canada, the US, the UK, Italy and Germany are working to translate these and other discoveries into strategies that can tested in clinical trials in the near future. Among other hurdles, since Duchenne’s is seen primarily in young boys, the question of conducting stem cell or gene therapy research with children requires ethical and legal guidance.

Canada

Canadian scientists are at the forefront of muscle stem cell research and Duchenne’s muscular dystrophy, particularly Dr Michael Rudnicki (Ottawa Hospital Research Institute), Dr Fabio Rossi (University of British Columbia) and Dr Jacques Tremblay (Laval University). Through their work it is known that a small fraction of healthy nuclei can provide enough functional dystrophin to completely cure a myofiber and ensure its survival. However, two interlinked problems have hindered the translation of cell delivery approaches into a routine therapeutic strategy.

While the number of cells that it is possible to obtain from healthy donors is considerable, it is still much below what would be required to treat enough muscle tissue to significantly improve its function. Furthermore, a strong immune reaction to transplanted cells is common, and even cells from the best-matched healthy donor will be rejected unless the patient is kept under constant immunosuppression. This in itself leads to a drastic reduction in life expectancy.

To overcome these limits, efforts are underway to establish a way to obtain transplantable muscle progenitors from embryonic stem cells. Embryonic stem cell cultures can be scaled up to very large quantities. These have the potential to generate a theoretically unlimited number of muscle progenitor cells which when injected will create muscle. Blood stem cells derived from the same embryonic stem cells that produced the muscle cells could be used to induce immune tolerance to that particular cell line in transplant recipients.

The research continues in order to develop a successful therapeutic strategy that can be made available in the clinic.