Therapeutic Approaches

Duchenne is a complex, multi-system disorder, caused by a mutation (or flaw), in one of the largest genes in the human body. It is thought that not any one treatment will end Duchenne, but rather successfully treating Duchenne will require a multi-faceted approach. This is why PPMD supports the idea of combination therapies. While we continue our long history of investing in single therapies as the foundation for combinations, we are also looking ahead at how best to study combinations, which therapies biologically make the most sense to combine, and the most efficient way to evaluate these combinations.

About Dystrophin

Since 1986, when the gene that produces dystrophin was discovered, the race to find treatments to end this devastating disease has been ongoing. Duchenne is a “dystophinopathy,” and is one of two diagnoses (Duchenne and Becker muscular dystrophy) that is caused by a mutation in the gene that makes a protein called dystrophin. Without dystrophin, muscles degenerate much more quickly and are not able to function or repair themselves properly.

Dystrophin is a shock absorber. Without this shock absorber, the muscle and muscle membrane becomes fragile and small breaks occur in the membrane. These breaks allow calcium and other molecules to enter into the muscle cells and destroy them. When muscle cells die, the body’s response is to activate inflammatory cells to ‘clean’ up or remove the damaged cells. Since there are many muscle fibers and they are breaking and repairing at different times, an individual with Duchenne lives in a constant state of inflammation.

Treating Duchenne will require a two-prong approach:

• Restoring or replacing dystrophin, the underlying cause of Duchenne; and
• Treating Duchenne symptoms that arise from the absence of dystrophin (i.e., reducing fibrosis and inflammation)

Restoring or Replacing Dystrophin

Dystrophin restoration or replacement aims to treat the underlying cause of the disease which is the lack of dystrophin, the protein that provides stability to the muscles. Gene therapy, exon skipping, and nonsense mutation readthroughs are all ways that dystrophin restoration/replacement is being explored.

What is the role of dystrophin?

Duchenne muscular dystrophy is caused by a change in the dystrophin gene. Genes are small pieces of DNA that contain the instructions for how to make a protein. The dystrophin gene is basically a recipe for how to make the dystrophin protein.

The dystrophin protein is critical for maintaining muscle cell structure and function. Changes to the gene, called mutations, can lead to differences in the amount or size of the dystrophin protein. Duchenne occurs because there is not enough dystrophin protein in the muscle cells or the dystrophin protein present does not work correctly. Some types of mutations in the dystrophin gene cause Duchenne, and other types of mutations cause Becker.

The dystrophin gene is one of the largest known human genes, so it frequently acquires mutations. Thousands of different mutations have been reported in the dystrophin gene. It is important to keep in mind that no one causes gene mutations, and they cannot be prevented. Each of us carries mutations in some of our genes, though we usually do not know it. Dystrophin restoration or replacement aims to treat the underlying cause of the disease which is the lack of dystrophin, the protein that provides stability to the muscles.

Muscle is highly organized into bundles of contractile proteins.  These are the proteins that produce force when muscles contract. Because muscle is a load bearing tissue, it is very important that each time it contracts, it doesn’t fall apart from the force of the contraction. There is a system that helps to protect the muscle from this issue. Encircling each of these muscle fibers is a membrane that is peppered with proteins. These proteins serve to anchor the muscle cell in the surrounding matrix and also help maintain membrane integrity. Dystrophin is like a rope that tethers this group of proteins to  the cell membrane. Without dystrophin, this complex system disappears. To learn more about Duchenne, click here. To learn more about the types of mutation in Duchenne, click here.

Given the importance of dystrophin, these first strategies all try to coax the body into making  dystrophin, or a protein that could substitute for dystrophin. There are four different ways that dystrophin restoration/replacement is being explored.

Gene Therapy

Gene therapy for Duchenne is centered on the goal of successfully introducing into a muscle cell the correct genetic code, or recipe, necessary to make the dystrophin protein. The most damaging effects of Duchenne are due to missing the protein dystrophin. By giving muscle cells the genetic code to produce a form of dystrophin, hopefully some of those effects will be stabilized, such as the amount of muscle strength and the development of fibrosis.

Gene therapy also refers to providing muscle cells with the genetic code to make other proteins that help the muscle stabilize and become stronger. Gene therapy will only work, however, if scientists can find a means of transporting the correct genetic code for the dystrophin or other proteins into each muscle cell in the body. Researchers today are using an adeno-associated virus, often referred to as AAV, as viruses have evolved over time to deposit their own genetic code into cells. Viral delivery harnesses the virus’s natural ability to deposit genetic material right to the muscle cell nucleus. The result of this viral “infection” would be the successful recoding of each muscle cell in the patient’s body.

In early 2017, PPMD launched our Gene Therapy Initiative, a long-term concept that seeks to accelerate the potential of gene therapy as a therapeutic for Duchenne. Our early strategy was to bring attention to and fund key questions that must be answered in order for the technology to progress towards approvals. Learn more about PPMD’s Gene Therapy Initiative.

Within gene therapy there are four strategies that are being pursued:

Micro-dystrophin / Mini-dystrophin

This strategy is the primary gene therapy strategy as it seeks to provide the genetic code for the missing protein that causes Duchenne. Unfortunately, because the dystrophin protein is so large, it won’t all fit inside the virus. Researchers have shortened the dystrophin genetic code, keeping the essential parts to provide function while eliminating parts that are not necessary. PPMD hosted a webinar on micro-dystrophin as part of our educational series on gene therapy. Click here to learn more.

For a comprehensive list of clinical trials in this, and other approaches, click here.

GalGT2

GalGT2 is a compound that encourages over-production of a protein that in turn produces other proteins that are important to stabilize the muscle and ultimately improve muscle function. This genetic code is also delivered via AAV. PPMD hosted a webinar on GalGT2 as part of our educational series on gene therapy. Click here to learn more.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Dup2

Dup2 focuses on developing exon-skipping therapies for rare exon mutations, specifically exon duplication mutations, accounting for around 6% of all mutations. Rare exon mutations are those that fall outside of the “hotspot,” the area where most mutations occur, generally considered to be from exon 45 throgh exon 55. By skipping over the duplication in a certain way, researchers believe almost normal dystrophin can be produced. Duplication in exon 2 is currently being studied. PPMD hosted a webinar on Dup2 as part of our educational series on gene therapy. Click here to learn more.

For a comprehensive list of clinical trials in this, and other approaches, click here.

CRISPR/Cas9

CRISPR/Cas9 is a technology that utilizes molecular “scissors” to guide an enzyme to a specific spot on the DNA, and then cut out one or more pieces of genetic material. This results in a permanent change to the DNA of the cell. In Duchenne, CRISPR/Cas9 is being used to cut out a letter in the genetic code, and much like exon skipping, our natural repair mechanism restores the flaw, allowing the translation of the gene to continue, and a functional dystrophin to be produced. At this early stage in development, CRISPR/Cas9 is being studied in mice and dogs, and, like gene therapy, is delivered using an adeno-associated virus (AAV). PPMD hosted a webinar on CRISPR/Cas9 as part of our educational series on gene therapy. Click here to learn more.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Exon Skipping  

One of the most common types of mutations in the dystrophin gene occurs when a piece of the code in the middle of the gene is missing or deleted. If the edges of this deleted piece of code are neat, the muscle cell can usually stitch the genetic message back together to make a shorter, but still functional dystrophin protein. This type of tidy-edged deletion, also known as an in-frame deletion, often results in Becker muscular dystrophy. However, when the edges of the deletion are ragged, or an out-of-frame deletion, the cell can’t use the mismatched edges to make any kind of dystrophin, resulting in Duchenne.

Researchers have been able to use short pieces of stabilized DNA called antisense oligonucleotides (“AONs”) to encourage the muscle cells to trim up those untidy edges in out-of-frame deletions. By leaving out or skipping additional segments of the dystrophin code called exons in the muscle cell’s working copy of the gene, the ragged edges of the deletion can be pulled back together to make a smaller, but still functional, dystrophin protein.

Deletions in different areas of the dystrophin gene will require different sets of these AONs to trim up the ragged edges. There are four approved exon skipping therapies with additional therapies in trials. There are also ongoing studies to test second generation AONs. Because of the many different mutations in the dystrophin gene, estimates suggest that it will take 20 different sets of AONs to effectively treat all the possible types of deletion mutations that are amenable to this technique. The time required to develop each of these AONs as a separate drug is time our community cannot spare and PPMD is working with companies and regulators on pathways to speed this process.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Nonsense mutation read–through

In Duchenne, sometimes the deletion occurs because a signal is given to stop reading the genetic code, or the “recipe.” This results in no full-length functional dystrophin at all.  Strategies involving small molecules that enable the production of full-length dystrophin by “reading through” or reading over this signal to stop are being developed.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Treating the Symptoms of Duchenne

When there is no dystrophin in the body, the muscle is damaged and the body naturally tries to repair this damage through muscle regeneration. This continual state of muscle degeneration and regeneration produces other effects such as fibrosis, inflammation, calcium imbalance, muscle wasting, cellular energy depletion, and cardiac dysfunction. These are called downstream effects because they happen due to the lack of dystrophin. There are therapeutic strategies aimed at addressing these downstream effects.  

Combating Fibrosis

Fibrosis, defined as the thickening and scarring of connective tissue, is a downstream symptom of the lack of dystrophin. Fibrosis occurs in Duchenne when chronic inflammation inhibits muscle repair. Reducing fibrosis may help decrease the breakdown of mature muscle cells and increase muscle strength.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Reducing Inflammation

Inflammation is a critical characteristic of Duchenne disease progression. Due to muscle degeneration and the resulting immune cells brought in to help regenerate the muscle, a whole host of inflammatory substances are released. The muscles of individuals with Duchenne are constantly in a state of inflammation. Corticosteroids are currently the standard of care to treat inflammation but have a number of side effects associated with long-term use. There are a number of experimental therapies in development that are aimed at reducing inflammation.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Regulating Calcium Balance

In Duchenne, because of the instability of the muscle membrane due to the lack of dystrophin, leaks in the muscle cell membrane can develop. These leaks can let too much calcium flow in and out of the muscle cell, disrupting cellular functions which further exacerbate cellular repair. Companies are developing compounds that aim to help regulate the calcium flow.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Improving Muscle Growth and Protection

Several therapeutic options intend to encourage muscle growth and discourage muscle breakdown. There are many strategies that can be explored in this domain, including approaches to enhance repair capabilities of the muscle, protect the muscle from breakdown, reduce inflammation and fibrosis, or induce muscle development.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Restoring the Cells Energy

Mitochondria are specialized structures within cells that supply chemical energy to power the activities of the cells, such as repairing muscle cells. Individuals with Duchenne lack efficient mitochondria in their cells. It is thought that by increasing or enhancing the mitochondria in muscle cells, the cellular functions of the muscle could be improved.

For a comprehensive list of clinical trials in this, and other approaches, click here.

Improving Heart Function

Cardiac function is a concern in Duchenne, as the heart is a muscle and is affected by the lack of dystrophin. There are a number of therapies in development that may impact both the skeletal muscle and cardiac muscle. However, some strategies are aimed primarily at the heart. Use of known cardiac interventions – Angiotensin-converting-enzyme (ACE) inhibitors, Angiotensin receptor blockers (ARBs), Eplerenone, or Betablockers – are part of care management of the heart based on physician recommendation.

For a comprehensive list of clinical trials in this, and other approaches, click here.