DMD mutations: why aren't the Duchenne boys (and girls) able to produce dystrophin ?
The dystrophin gene has 79 exons
The blueprint for dystrophin is embedded in the DMD gene. The parts containing the genetic information the cell needs to generate proteins are called "exons". The DMD gene has 79 exons.
These exons fit together like pieces of a jigsaw puzzle and form the genetic code for the dystrophin protein, a protein that helps keep muscle cells intact.
When there is a mutation...
Duchenne patients have mistakes (mutations) in their DMD gene. The most common mistake is that one or more exons are missing from the gene (= a deletion).
As a result the genetic code is broken and the consequence is that the blueprint becomes unreadable after the missing (or mutated/duplicated) exon(s) and the translation into dystrophin is stopped prematurely.
Oscar's mutation is on exon 24 and it's a microdeletion. It means that a small part of that exon is missing. Unfortunately it still prevents him from producing dystrophin. It's not a common mutation, which isn't good news since laboratories are more interested in common mutations. Fortunately some of the promising research projects do not depend on the location of the mutation (see below).
Promising research strategies
Gene therapy approaches offer the possibility of delivering a functional copy of the dystrophin gene to muscle cells where it could restore production of the dystrophin protein. The most promising approach is based on the use of a harmless virus called Adeno-associated virus (AAV) which has been shown to effectively deliver genes to a range of different types of cells and tissues including muscle. One of the challenges is that the dystrophin gene is too big for the AAV vector. Researchers have made micro-dystrophin genes that have successfully been tested in animal models of Duchenne muscular dystrophy. A shortened but functional dystrophin is produced using this method. Different trials are planned and should be initiated soon.
CRISP/CAS9 > CRISPR/Cas 9 is an exciting genetic engineering technique. It has two key components: Cas9 which is an enzyme that can cut DNA at a precise point and CRISPR, a short strand of RNA, a chemical messenger. Three research groups, working independently of one another, recently reported that they had used the Crispr-Cas9 technique to treat mice with a defective dystrophin gene. Each group loaded the DNA-cutting system onto a virus that infected the mice’s muscle cells, and ‘cut out’ an exon from the gene. Without the defective exon, the muscle cells made a shortened but functional dystrophin protein, giving all of the mice more strength. There are high hopes for the application of the CRISPR/Cas9 technology for Duchenne however, for now, the state of the science and the corporate interest is classified as early stage.
Coaxing muscle cells into producing dystrophin protein without recoding dystrophin's basic genetic code is another strategy that scientists have also developed potential strategies for. These proposed cell therapies attempt to at least partially offset the muscle damage caused by the flawed genetic code. Scientists have begun to develop cell therapy techniques that use stem cells derived from muscle. These are essentially immature muscle cells with the potential to develop into a variety of types of tissues, including skeletal muscle.
Pharmacological approaches to formulating treatments for Duchenne do not seek to repair or replace the missing genetic information in a muscle cell, or to otherwise devise mechanisms to cause the muscle cell to produce normal dystrophin. Instead, pharmacological approaches seek to treat the symptoms of Duchenne without necessarily addressing the root causes. While pharmacological therapy may seem less dramatic than some of the newer methods being developed, pharmacological strategies also sidestep some of the most daunting obstacles associated with gene and cell therapies, most notably difficulties in achieving systemic delivery and overcoming immune response.
Potential alternatives to steroids
Studies show that, overall, children with Duchenne who are treated with steroids, stay walking for longer than those who are not treated with steroids. However, there can be important side effects. For this reason, different new drugs which have the potential to better balance the benefit and side effects than the traditional steroids are being developped :
- Vamorolone (VBP15) – Phase 2
- Edasalonexent (CAT1004) – Phase 2 (not a steroid)
In 1989, scientists discovered that a protein called utrophin exists in muscle cells, principally at the junction where the nerve meets the muscle cell. Since that time, scientists have observed that utrophin could potentially operate as a substitute for dystrophin (and protect the muscle cell membrane), if muscle cells could be coaxed into producing utrophin at locations other than the neuro-muscular junction. This strategy could perhaps lead to an effective treatment for Duchenne, using a biological process substantially simpler than those involved in gene and cell therapies.
- Ezutromid (SMT C1100) – phase 2
- Clinical trials with Biglycan are expected to start in 2017.
Scientists have long theorized that the body normally contains compounds that limit muscle growth. For example, certain breeds of cattle develop substantially more muscle than ordinary cattle. Researchers have isolated the cause of this disparity to a mutation in the gene that codes for the production of a hormone called myostatin, which tends to limit muscle growth. Scientists searching for a treatment theorize that inhibiting myostatin in people with Duchenne will cause them to develop more muscle mass initially. Ideally, this surplus will offset the muscle loss associated with Duchenne, allowing the patients to retain their ability to function for a longer period of time.
- PF-06252616 (Pfizer, phase 2)
- Adnectin (BMS 986089) (phase 2)
Scientists are not attempting to replace the genetic code; instead, they want the muscle cell to ignore the defective part of the dystrophin gene and make a smaller version of dystrophin. Scientists believe that this therapy could change the reading frame of a deletion in the dystrophin gene, so that an out-of-frame deletion in the dystrophin gene could be transformed into an in-frame deletion. Their hope is that this change would cause the muscle cell to produce a form of dystrophin that is at least partially functional, which could result in a significant improvement in the quality of life. Eteplirsen is the first exon-skipping drug to have received approval from the FDA (for exon 51). Unfortunately, Oscar's mutation is very rare no one is working on his mutation.
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