What are the researchers aiming to do?
Gene therapy for Duchenne muscular dystrophy aims to increase the amount of dystrophin protein by delivering a dystrophin gene to the muscle. Viruses, which have been modified to prevent them causing illness, can act as very efficient carriers for delivering micro-dystrophin into muscle cells. Prof Dickson has been investigating the adeno-associated virus (AAV) as a means to deliver dystrophin to the muscle.
Viral vectors work by taking the instructions for the missing protein and delivering it to the target cell, in this case muscle cells. AAV vectors are limited in the amount of information they can carry, so researchers have had to produce a shortened version of the dystrophin gene called micro-dystrophin that will fit inside the AAV-vector. Many different versions of micro-dystrophin have been produced and making a small change to the micro-dystrophin gene can make it more efficient at producing dystrophin protein in the muscle.
Prof Dickson and his team have produced a series of different micro-dystrophin genes which they will test for their ability to produce protein effectively and safely in the muscle cell.
The researchers will also be exploiting a natural process that occurs in cells to try and deliver a full-length dystrophin gene to the muscle. Using a full-length dystrophin gene could have advantages over micro-dystrophin as they can then be certain that all of the parts important for the correct functioning of dystrophin are present. This process, called trans-splicing, involves using three different AAV-vectors. Each vector carries a different, but overlapping piece of the dystrophin gene. When the muscle cell begins processing the information contained in the gene pieces, it is thought that the cell will assemble them together. This will create one single messenger molecule with all the instructions for a full-length dystrophin protein.
Prof Dickson will use a mouse model of Duchenne muscular dystrophy to test the different micro-dystrophin genes and trans-splicing AAV vectors. He will study how efficient they are at producing dystrophin in the muscles as well as determining which is the best at delivering dystrophin to all of the muscles of the body. An important aspect of this will be assessing how well they restore muscle function as well as whether they cause an immune response.
Prof Dickson is aiming to address the three main challenges that have prevented gene therapy such as this from being used to treat Duchenne muscular dystrophy. These are the size of the dystrophin gene, delivering enough of the gene to the muscle to alleviate symptoms, and the immune response that is often generated as a reaction to gene therapy.
The main problem, which researchers have been working to overcome, is the size of the dystrophin gene – it is very large. The AAV virus can only carry a limited amount of information and the full dystrophin gene is too much information for it to hold. Prof Dickson’s research is addressing this problem and looking at ways to either modify the micro-gene to get the most protein produced in the muscle or to deliver the full dystrophin gene by a different method.
In order to halt the progression of Duchenne muscular dystrophy the viral vector must be able to deliver large enough quantities of the dystrophin gene to all the affected muscles. Making a small change to the micro-dystrophin gene can increase its efficiency at producing dystrophin protein in the muscle. Combining this approach with testing different types of AAV vector, Prof Dickson hopes to optimize delivery of the dystrophin gene to the muscles.
Finally, it is known that gene therapy can elicit an immune response. This can either by triggered by the viral vector or the newly introduced dystrophin protein itself. The body will see proteins that it does not recognise as foreign. Since boys with Duchenne muscular dystrophy do not produce the dystrophin protein, it is thought that the body sees the newly produced dystrophin as a foreign protein and can mount an attack against it. In order to get around this problem Prof Dickson will be investigating ways to make the immune system ignore the dystrophin protein and AAV-vector so that when the muscle cells produce the dystrophin protein they will not be attacked by the immune system.
By addressing these problems, Prof Dickson may speed up the translation of this technology from the bench to the bedside.
How will the outcomes of the research benefit patients?
This is pre-clinical research, a crucial stage in getting a treatment into clinical trial. Animal models will be used to try to optimize the safety and efficiency of this particular gene therapy. If these researchers can show it is possible to deliver sufficient quantities of the dystrophin gene to muscle, while minimizing or preventing an immune reaction, these studies may provide a gene therapy agent that can move forward into clinical trial. Several of the therapies that are currently under investigation and in clinical trial, such as exon skipping and Ataluren (formerly PTC124) can only be applied to boys with specific types of mutation and so they would not be of benefit to all boys with Duchenne. This gene therapy approach is not dependent on the type of mutation and so could potentially be used to treat all boys with Duchenne.
Background information
The aim of gene therapy is to compensate for faulty genes by introducing a normal copy of the gene back into the cell. In the case of Duchenne muscular dystrophy, it is the gene that codes for the dystrophin protein that is faulty. The genetic fault causes an absence of the functional dystrophin protein, which eventually leads to the muscles weakening and wasting.
To get the new gene into the cell, a vector is required. Viruses are often used as vectors as they have evolved to deliver genetic material into cells when they infect them. Scientists can take advantage of this and instead of introducing viral genes they can modify them to introduce human genes for gene therapy. The viruses have been modified so that they can no longer cause infections in people. In the case of the AAV-vector, it carries the gene into the cell and injects it into the nucleus. It is in the nucleus that the messenger molecule, RNA, can “read” the DNA code, which is the first step in producing a protein.
AAV vectors are currently being used in several clinical trials including for Duchenne muscular dystrophy and limb girdle muscular dystrophy type 2D.
Project leader: Prof. George Dickson
Location: Royal Holloway, University of London
Condition: Duchenne muscular dystrophy
Duration of project: 2 years
Total Project Cost: £130,931
Official project title: Enhancing the Therapeutic Functionality of Adeno-associated Virus (AAV) Vectors Encoding Dystrophin for Duchenne Muscular Dystrophy Gene Therapy
Further information and links
Find out more about Duchenne muscular dystrophy.
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