Muscular dystrophy is a disorder caused by a mutation in the dystrophin gene (as you are aware). This gene links the muscle filaments (which do the pulling) to the muscle membrane (which links the muscle eventually to the bone). Dystrophin basically transduces the force. In MD, a faulty dystrophin is unable to transduce the force of muscle contraction.
The answer is obvious. As Gaspode said, replace the faulty gene. This is called “gene therapy.” There are a number of problems with this, however.
First, you have to clone a correct copy of the gene. This is already done. The problem is that dystrophin is a massive gene, so you need to find which part in a given individual is broken and construct your gene therapy around the individual’s mutation. This is a lot harder.
Next, you have to get the correct gene into the correct cell. Even a bigger problem. Most vector systems (the systems used to transport the gene) are based on viruses. You have to basically construct a virus to infect muscle. You need to strip this virus down so it is non-immunogenic. You need to suppress cell mediated immunity. You need to have a good delivery system for both the virus and for the DNA into the cell.
Third, you have to ensure a replacement. This is a huge problem. Out of 3,000,000 base pairs of genome, you need to find your appropriate gene site and replace all or part of the faulty gene. Not only do you have to inject the DNA into the cell (usually with a virus), but you have to hope that some of it gets to the nucleus. Muscle cells are quite large and form syncytiums with their nuclei peripherally. It makes a hard target to hit.
Fourth, you are now replacing a mutant protein (and one with which you are born) with a foreign protein. There can be all types of immunologic issues with this.
Gene therapy is in trials for several diseases right now. Some of these are cystic fibrosis, citrullemia, and several other single gene disorders. It is very hard. Most of the problems so far center around the virus vector (gutted adenovirus) that they are using. It is somewhat immunogenic, and actually caused a fatal cardiomyopathy in a clinical trial last year. That, and so far most have been unable to ensure a complete replacement.
This is a definite region into which medicine is heading. We are not there now. We probably won’t be there for at least a decade. It is incredibly difficult because it acts as a nexus to so many systems – immunology, molecular and human genetics, virology, biochemistry, cell biology, and pathology. We make progress every year, but this is not something in which there will be a big breakthrough anytime soon, unfortunately.