Recent research funded by the Muscular Dystrophy Campaign has enhanced gene therapy technology, making it a more feasible treatment for Duchenne muscular dystrophy.
The researchers, led by Professor George Dickson at the Royal Holloway-University of London, sought to improve a gene therapy approach which uses a virus to introduce a healthy copy of the dystrophin gene into muscle cells. This new research modified the introduced dystrophin gene to increase the efficiency of dystrophin protein production in the cells. This was successful in a mouse model of Duchenne muscular dystrophy. The treated mice had high levels of dystrophin produced in their muscles, including the heart. Importantly, the treated mice had muscles that worked as well as healthy mice.
These results are encouraging because they overcome some of the previously encountered hurdles of gene therapy for Duchenne muscular dystrophy. Increasing the efficiency of the technology will reduce the dose of virus required and this will improve the safety and practicality of this gene therapy approach. However, it should be pointed out that the experiments were carried out in animals and it remains to be seen how well boys with Duchenne will repond to this treatment.
This gene therapy strategy is attractive because it could potentially be used to treat any mutation causing Duchenne muscular dystrophy. Once proven, the principles could also be applied to the development of gene therapy for other neuromuscular conditions.
- What are the challenges facing the development of gene therapy for Duchenne muscular dystrophy?
- How have the scientists improved the gene therapy?
- What does this mean for people with Duchenne muscular dystrophy?
- Statement from Professor George Dickson
- Further information
What are the challenges facing the development of gene therapy for Duchenne muscular dystrophy?
Viruses called adeno-associated virus (AAV) are particularly useful vehicles for delivering a healthy copy of a gene to cells affected by a genetic disorder. This is because they are able to infect human cells but do not cause any disease. Studies have also shown that the immune system doesn’t normally have a strong reaction to this type of virus.
Mutations in the dystrophin gene cause Duchenne muscular dystrophy. Dystrophin is an important structural component of muscle cells and if absent or faulty the cells are fragile and deteriorate. Unfortunately the dystrophin gene is too large to fit inside adeno-associated viruses so scientists have designed abbreviated versions of the gene. They have removed sections of the gene thought to not be essential for its function, resulting in a gene that is one third the original length. These so called micro-dystrophins, could transform Duchenne muscular dystrophy into a much slower progressing Becker form.
Previously tested versions of micro-dystrophin have been shown to give some improvement in muscle function in mouse models, but very large doses of virus were required in these studies. These large doses make the prospect of gene therapy extremely troublesome because of the practicalities of producing the large amounts of virus needed to treat humans. Large doses may also cause problems with the safety of the treatment.
How have the scientists improved the gene therapy?
The researchers in George Dickson’s laboratory improved the gene therapy by making the micro-dystrophin more efficient and stable. One way of explaining this is to consider the information in genes to be like instructions for making proteins that are written on a piece of paper. The researchers performed the equivalent of writing the instructions in large, bold, simple words and laminating the piece of paper. This made the instructions easier and more efficient for the muscle cells to read. The instructions were also more stable so they would last for longer in the cells.
The virus containing the optimised microdystrophin was injected into the blood stream of young adult (10 week old) mice. Remarkably, more than 100 times more dystrophin protein was seen in the hearts of these mice compared to the non-optimised micro-dystrophin. Large amounts of the micro-dystrophin protein was also seen in the leg and diaphragm muscles. The muscles were healthier and stronger and were protected from exercise induced damage. The mice treated with the optimised micro-dystrophin were almost indistinguishable from healthy mice.
What does this mean for people with Duchenne muscular dystrophy?
Gene therapy using a micro-dystrophin would not cure Duchenne muscular dystrophy but would have the potential to transform it into a much slower progressing Becker form. If successful, gene therapy could be used to treat all Duchenne muscular dystrophy patients with any type of mutation.
There is currently a phase 1 clinical trial of gene therapy for Duchenne muscular dystrophy being conducted in Ohio, USA. During this trial an adeno-associated virus containing micro-dystrophin was injected into the biceps (upper arm muscles) of 6 boys with Duchenne muscular dystrophy. The purpose of this small trial is to test whether the treatment is safe and try to prove the principle of this gene therapy approach. It is expected to finish at the end of this year and the results will be published as soon as all data are analysed. It has been reported that no serious side-effects were observed, suggesting that the therapy was well tolerated.
The clinical trial is using a different type of adeno-associated virus and a different version of micro-dystrophin to that discussed in this article. The results of the clinical trial will help to clarify if this gene therapy strategy is likely to be safe and effective. Should this be the case then the results discussed here should be considered in the planning of future clinical trials.
Statement from Professor George Dickson
“One major hurdle in developing human gene therapy for Duchenne muscular dystrophy is the high doses of dystrophin gene vectors currently required. These treatments are almost prohibitively difficult and expensive to produce at industrial scale, and carry significant risks of toxic side-effects in patients. We have worked to optimise and refine a new second generation dystrophin gene therapy vector which is some 30-times more effective. This type of enabling advance is hugely important since it makes feasible widespread Duchenne muscular dystrophy gene therapy at doses potentially 100-times lower than currently used. This second generation reagent is available now to the clinical community across the world for immediate development and testing in clinical trials. We hope it will become the therapeutic material of choice for gene therapy trials for Duchenne muscular dystrophy.”
Find out more information about clinical trials.
The original paper called is not freely available and is written in scientific language. The reference for the paper is:
Foster H, Sharp PS, Athanasopoulos T, Trollet C, Graham IR, Foster K, Wells DJ, Dickson G. Codon and mRNA Sequence Optimization of Microdystrophin Transgenes Improves Expression and Physiological Outcome in Dystrophic mdx Mice Following AAV2/8 Gene Transfer. Mol Ther. 2008 Sep 2
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