Five Strategies for Treating Neuromuscular Disease

Quest takes a look at gene therapy, 'antisense' and other cutting-edge scientific approaches and how they're being applied to diseases in MDA's program

Article Highlights:
  • This article describes five leading therapeutic strategies currently being tried in neuromuscular disease research: antisense oligonucleotides, stem cells, small molecules, protein therapy and gene therapy.
  • These strategies are relevant in ALS, Becker MD, centronuclear myopathies/myotubular myopathy, Charcot-Marie-Tooth disease, congenital MD, Dejerine-Sottas disease, distal MD (Miyoshi myopathy), Duchenne MD, facioscapulohumeral MD, Friedreich’s ataxia, limb-girdle MD, myotonic MD and spinal muscular atrophy.
  • The strategies were the focus of an MDA scientific conference in March 2011 that was designed to help advance these strategies from “microscope to marketplace.”
by Amy Madsen on July 1, 2011 - 4:15pm

QUEST Vol. 18, No. 3

Antisense oligonucleotides block flawed genetic instructions

Antisense oligonucleotides — also called antisense, oligos, or simply AONs — are pieces of genetic code that keep other genetic code from being processed. Designed to pair up with a particular sequence of DNA or RNA, AONs can change, block or destroy targeted genetic instructions in a variety of ways.

In facioscapulohumeral muscular dystrophy (FSHD), the apparently toxic DUX4 protein instructions potentially can be blocked by AONs. In type 1 myotonic dystrophy (MMD1 or DM1), genetic instructions for the DMPK protein could be blocked or destroyed by AONs. Laboratory experiments in these diseases have had promising early results.

In type 2 myotonic dystrophy (MMD2 or DM2), AONs potentially could interfere with the genetic instructions for the ZNF9 protein. These experiments are not yet under way.

In the SOD1-related form of familial ALS (amyotrophic lateral sclerosis), AONs could block the genetic instructions for toxic SOD1 protein. A clinical trial to test this approach is under way in participants with SOD1-associated familial ALS.

In Duchenne muscular dystrophy (DMD), AONs can be used to mask certain segments of genetic instructions for the dystrophin gene, so that a functional dystrophin protein can be made. This strategy — known as exon skipping because it causes the cell to skip over certain regions (exons) of the genetic instructions for the dystrophin gene — currently is being tested in clinical trials.

In spinal muscular atrophy (SMA), the goal is to use AONs to change the way the cell reads genetic instructions for the SMN protein that’s lacking in this disease, so that a functional SMN protein can be produced.

Experiments are under way to cause inclusion of a section of the SMN gene called exon 7. In this case, AONs would cause exon inclusion rather than exon skipping.

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Stem cells can display 'disease in a dish'

Stem cells are cells in the very early stages of development. They may be destined to turn into a specific cell type (such as muscle or nerve cells) or they may still retain pluripotency — the ability to develop into any of a number of different cell types.

Scientists now are working with induced pluripotent stem cells or iPSCs, which are mature cells — sometimes taken directly from the patients themselves — that have been reprogrammed into a stemlike state. From there they can be coaxed to become whichever type of cell is needed.

Patient-derived iPSCs are being used in FSHD to study how the disease develops and to test potential therapies in a “disease in a dish.”

In ALS, disease-affected stem cells serve as models for studying the disease process and screening potential therapeutic strategies.

They also are in development as cell-transplantation therapies. In an ongoing ALS clinical trial, neural precursor cells derived from a fetal spinal cord are being injected into participants’ spinal cords.

In a different ALS clinical trial, transplantation into the spinal cord of mesenchymal stem cells, derived from bone marrow, has been found to be both feasible and safe.

In DMD, various types of muscle stem cells may replace or repair degenerating muscle fibers. A clinical trial is under way in Italy to test the safety and possible benefits of a type of stem cell known as the mesoangioblast, which is associated with blood vessels but which can develop into muscle tissue.

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Small-molecule drugs effectively target tissues and pathways

Synthesized small-molecule drugs are considerably smaller than proteins and many times smaller than cells, making them generally easier to deliver to targeted tissues and less likely to elicit an unwanted immune response.

A small molecule belonging to a chemical class of compounds called histone deacetylase (HDAC) inhibitors is being developed as a potential treatment for Friedreich’s ataxia (FA).

Also being studied in FA are small molecules known as antioxidants, which can counteract a type of cell damage known as oxidative stress.

In SMA, a small molecule under development is designed to change the way cells interpret the genetic instructions for SMN, coaxing production of this needed protein.

In DMD and the related Becker muscular dystrophy (BMD), clinical trials are under way of a small molecule known as sildenafil (brand name Viagra) and a similar small molecule, tadalafil (brand name Cialis). These drugs appear to increase blood flow to exercising muscle and also have cardiac effects that may be beneficial in DMD and BMD.

Another emerging use of small molecules in DMD, and possibly in BMD, is to combat inflammation.

Inflammation-reducing corticosteroids like prednisone and deflazacort are widely used in DMD, but they have undesirable side effects, such as weight gain and bone loss, if given for long periods at relatively high doses.

Efforts are under way to develop anti-inflammatories without the side effects of corticosteroids. A small molecule compound known as VBP-15 appears promising, and is likely to be carried forward into clinical trials.

In ALS, small molecules are being used to target the cell-death pathway in motor neurons. A small molecule called necrostatin 1 appears to be a potent inhibitor of the BNIP3 cell death pathway.

A similar strategy is being investigated for SMA, in which a degenerative pathway known as JNK is being targeted by a small molecule designed to inhibit JNK signaling.

In centronuclear myopathies (CNMs), including myotubular myopathy (MTM), evidence points to a defect at the neuromuscular junction, the place where nerve and muscle fibers meet.

Some with CNM/MTM have responded to a small-molecule drug called pyridostigmine (brand name Mestinon), which has been used for many years to treat neuromuscular transmission defects such as those that occur in myasthenia gravis (MG) and other forms of myasthenia.

Pyridostigmine appears to modestly increase function and may improve quality of life for some with these conditions.

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Protein therapies are difficult to develop, with some notable exceptions

Proteins carry out the majority of cellular functions in the human body. Many forms of neuromuscular disease are caused by missing, deficient or toxic proteins.

But development of protein-based therapeutic agents generally poses more challenges than development of other types of therapies. This is due to several factors.

Protein molecules’ large size and other properties can make it difficult to get them into cells. Delivering the molecules to the right cells is another challenge.

Once inside cells, proteins may be modified, recycled, or degraded and destroyed by cellular “housekeeping” machinery.

Unwanted immune responses not only can neutralize protein therapy but can cause harmful reactions in patients.

When produced in large quantities (for mass marketing), inherent characteristics of protein molecules change, such as stability or the risk of misfolding and/or clumping.

Nonetheless, there have been remarkable success stories in this field.

A shining example is the approval by the U.S. Food and Drug Administration (and regulatory agencies in other countries) of Myozyme and Lumizyme to treat Pompe disease (acid maltase deficiency or AMD). The drugs replace the missing or deficient acid maltase protein.

In DMD, the protein biglycan appears to attract the beneficial protein utrophin to the muscle-fiber membrane in animal experiments. Utrophin can partially compensate for the dystrophin protein missing in this disease.

Administering the protein laminin 111 has shown benefit in animal models of congenital muscular dystrophy (CMD) that is caused by the loss of the laminin alpha 2 protein.

A protein called netrin 1 appears to protect nerve fibers in laboratory experiments and has potential in Charcot-Marie-Tooth disease (CMT) and Dejerine-Sottas disease.

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Gene therapy could yield long-term treatments, once challenges are addressed

Gene therapy, or gene transfer, refers to the delivery of genes as therapeutic agents. Since genes carry the instructions for protein synthesis, they can lead to production of proteins that are directly or indirectly therapeutic in neuromuscular diseases.

Because transferred genes potentially can continue to produce protein for some time, gene therapy may offer a more permanent fix than other therapies.

But gene therapy faces many technical challenges, as well as a high bar set by regulatory agencies like the FDA.

The key challenges are:

  • delivering the genes to the targeted tissue while avoiding off-target tissues; and
  • avoiding unwanted immune response to the proteins made from the new genes, or to the delivery vehicles in which the new genes are delivered.

Gene therapy for DMD has been in development for several years. A recent clinical trial involving injection of a miniaturized dystrophin gene uncovered unexpected types of immune responses that are not yet completely understood and are being explored.

Although the therapy was found to be safe, at least at the low doses used, the immune responses appear to have interfered with prolonged production of the dystrophin protein. Immunosuppressant drug treatment may be necessary in conjunction with dystrophin gene transfer in humans.

Another type of gene therapy being considered in DMD involves transferring genes for the protein claudin 5 to treat cardiac aspects of the disease. This approach has shown great promise in treating heart-muscle damage in a mouse model of the disease.

Blocking the myostatin protein via a protein called follistatin is a strategy that has potential for treating DMD and likely many other neuromuscular diseases. Mice with a DMD-like disease that received genes for the follistatin protein showed an overall increase in body mass and weight of individual muscles. Monkeys that received follistatin gene transfer had stronger, larger muscles.

In FSHD, genes for antisense oligonucleotides are in development that block toxic proteins in a mouse model of the disease. This strategy could provide long-term production of protein-blocking agents.

Also in development is a gene therapy strategy that involves transfer of the dysferlin gene, which is mutated in the type 2B form of limb-girdle muscular dystrophy (LGMD2B) and in distal muscular dystrophy (Miyoshi myopathy). Results in dysferlin-deficient mice are encouraging.

Delivery of genes to the central nervous system has been complicated by the blood-brain and blood-spinal cord barriers. A new delivery vehicle, based on the shell of a type 9 adeno-associated virus (AAV9), has overcome this barrier in laboratory models of SMA.

Mice with an SMA-like disease have shown remarkable benefit from the delivery of SMN genes in AAV9 shells and the strategy is being considered for human clinical trials. A caveat is that SMN may need to be given very early in life for maximum benefit.

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Multiple strategies increase the chances of success

Although they hold promise, the five strategies described here still must undergo further refinement and rigorous testing before they can receive FDA approval for use in humans.

But, the more potential therapies under development, the better the odds one or more will work.

 

Some 300 researchers, clinicians, representatives from the biotech and pharmaceutical industries, and students specializing in neuromuscular disease attended the 2011 MDA National Scientific Conference, March 13-16 in Las Vegas.

The conference, which focused on five key strategies under development for neuromuscular diseases, marked the first in a planned series of MDA-sponsored meetings that will emphasize new research and current medical care.

The interchange of information at the conference was valuable in further developing these strategies.

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