Scientific background of Duchenne Muscular Dystrophy
Duchenne Muscular Dystrophy (DMD) is caused by the absence of functional dystrophin protein. This protein fulfils an important linker function and connects the proteins of the skeleton of muscle fibers to the connective tissue that surrounds muscle fibers. Dystrophin acts as a shock absorber and protects muscle fibers against damage during muscle movement (contraction and relaxation of fibers).
You can envisage dystrophin as the rope between an anchor and a boat (Figure 1). The anchor can only function when the rope (dystrophin) is connected to both the anchor and the boat.
Figure 1: Dystrophin has an anchor function.
The blueprint of the dystrophin gene is embedded in the DMD gene. The parts containing the actual genetic information the cell needs to generate proteins are called “exons”. The DMD gene has 79 exons (Figure 2). These exons fit together like pieces of a jigsaw puzzle and form the genetic code for the dystrophin protein.
Figure 2. The exons of the DMD gene.
Duchenne patients have mistakes (called mutation)) in their DMD gene. The most common mistake is that one or more exons are missing from the gene: called a “deletion”. In the example in Figure 3 exon 48, 49 and 50 (48-50) are missing (deleted).
Figure 3. A deletion of exon 48-50.
When we zoom in to the part that flanks the mutation, we see that exon 47 does not fit to exon 51 (Figure 4).
Figure 4. Exon 47 does not fit to exon 51.
As exon 47 and exon 51 do not fit, the genetic code is broken. The consequence is that the blueprint becomes unreadable after exon 47 and the translation into dystrophin is stopped prematurely in exon 51. As dystrophin has to connect two different things, this dystrophin is completely non functional (Figure 5).
Figure 5. Due to the mistake in the DMD gene, only the beginning of the dystrophin protein can be made. The connection is completely lost: “the part of the rope bound to the boat is not present, so the boat sails off”.
Due to the lack of functional dystrophin muscle fibers from Duchenne patients are very sensitive to muscle damage. You are most likely familiar with the consequences.
It is possible that the mutation does not disrupt the genetic code of the DMD gene, as can be seen in an example in Figure 6 and Figure 7.
Figure 6. A deletion of exon 48-51.
Figure 7. Exon 47 fits to exon 52.
Due to the mutation exons 48, 49, 50 and 51 are missing. However, exon 52 fits to exon 47, so the genetic code is maintained and despite the mutation the blueprint remains readable and the protein synthesis can be continued after the mutation. As the beginning and the end of dystrophin are now present, the connection is not lost, although it is a bit shorter (the part that was described in exon 48-51 is missing). This dystrophin is thus largely functional (Figure 8).
Mistakes that do not disrupt the genetic code are found in Becker muscular dystrophy patients. As their dystrophins are partly functional, the muscle fibers are less sensitive to damage than those of Duchenne patients. Generally, the progression of the disease is less severe.
Scientific background of the exon skip therapy
The aim of the exon skip therapy is to recover the genetic code for Duchenne patients, resulting in the synthesis of a partly functional dystrophin protein instead of a non-functional protein. Because of this, hopefully the progression of the disease will be delayed or even be halted.
Exons can be skipped through so called antisense oligonucleotides (AONs). These are small, synthetic pieces of modified DNA that can cover a specific exon (Figure 9). As a result this exon will be skipped when the genetic code is assembled.
In the example of Figure 9 skipping of exon 51 restores the genetic code.
Figure 9. Exon skipping for a patient with a deletion of exon 48-50.
Due to the deletion exon 47 is joined to exon 51 when the genetic code is assembled. These exons do not fit and the genetic code is broken (and protein translation is stopped prematurely). Using an AON specific for exon 51 this exon is hidden and skipped during the assembly of the genetic code. Now exon 47 is joined to exon 52, which does fit: the genetic code is restored and protein synthesis can be completed.