|
|
| Aim: |
To deliver a healthy gene to Duchenne muscles, to allow normal dystrophin production. |
| Background: |
Genes consist of DNA and are located on chromosomes, which are present in the nuclei of all cells. The dystrophin gene contains the genetic code for dystrophin, which can be read by the cell and translated into the dystrophin protein. |
| |
| Challenge 1: |
Muscle consists of billions and billions of cells and the healthy gene has to be delivered to at least 20% of the billions of muscle cell nuclei. |
| Solution: |
Fortunately, there is an organism that is quite good at injecting genes into cells: the virus. Thus, the gene therapy field has developed viral vectors, where the viral genes are removed, so there is room for the new gene and the modified viruses are no longer pathogenic. |
| |
| Challenge 2: |
Most viruses like to infect dividing cells. Muscle tissue hardly divides and thus is a poor target. In addition, muscle fibers are enveloped by layers of connective tissue, which trap viral particles, so the virus cannot reach the muscle fiber to inject its dystrophin gene. |
| Solution: |
There is a virus that is relatively good at infecting muscle cells, the so called AAV virus. This virus can infect human cells but does is not pathogenic (it does not cause a disease). |
| |
| Challenge 3: |
Unfortunately, AAV is so small that the dystrophin genetic code does not fit (the entire gene is ~500 times to big, the genetic code ~4 times too big). |
| Solution: |
Scientists have attempted to create the smallest possible dystrophin, containing only the bare essential domains (micro-dystrophin). The genetic code of this micro-dystrophin is small enough to fit into the AAV vector. |
| |
In the Duchenne mouse model (mdx mouse) treatment with microdystrophin containing AAV viruses resulted in an improved muscle quality and function. |
| |
| Challenge 4: |
When a dog model (golden retriever muscular dystrophy or GRMD) was treated with AAV-microdystrophin, this resulted in an immune response. Consequently, cells infected with the AAV (thus containing microdystrophin) were destroyed by the immune system. From clinical trials in humans with other genes (e.g. to treat hemophilia) we know that AAV also induces an immune response in humans. The immune response will attack all foreign intruders (viruses, bacteria, parasites) and has no way of knowing this time the virus carries a good gene. |
| Solution: |
Ways to reduce the immune response are currently under investigations. This can be done by suppressing the immune response or by the identification of AAV subtypes that are less immunogenic (there are many AAV subtypes known). |
| |
| Clinical trials: |
A first clinical trial where patients received local AAV-microdystrophin injections in the arm muscle was performed in the USA (Mendell, Xiao Xiao and Samulski). Initial results of this trial have been presented recently at a scientific meeting. It has become clear that AAV-microdystrophin injections can lead to an anti-dystrophin immune response. |
| Future: |
Preparations are made for a larger trial where one or multiple muscles in the arm or leg are treated with AAV-microdystrophin. |
| |
|
Cell Therapy > |
