Two therapies for Duchenne patients are currently being tested in clinical trials, which are applicable only to patients with specific mutations: PTC124 (treats only stop mutations) and exon skipping (restores the genetic code for certain deletions). Here I will explain why these approaches can not be applied to all patients. (An overview of therapeutic approaches currently being developed for DMD can be found on the TREAT-NMD website)
A you can read in the mutations section, a disruption of the genetic code leads to the use of aberrant protein subunits, whereas a point mutations cause a premature stop signal.
Point mutations are small changes in the DNA that alter a protein subunit code into a stop signal. These stop signals are normally only present at the end of an mRNA (figure 1).
Figure 1. Messenger RNA contains the genetic code for a protein. The translation of the mRNA into protein is performed by the protein factory. This factory needs a “start” signal to initiate protein translation (in the first exon) and a stop signal (at the end of the mRNA, i.e. exon 79 for dystrophin) to indicate that the protein is complete.
Due to stop mutations, in addition to the normal stop signal at the end of the genetic code, an extra stop signal is present in the middle of the genetic code (figure 2).
Figure 2. A stop mutation causes the premature stop of protein translation (and thus only half a protein is made).
Generally, there are additional signs that indicate to the protein factory that the translation into protein is almost complete (e.g. normal stop signals are located at the end of the mRNA and not in the middle). You can compare this with a stop sign in traffic: it makes sense at a very busy crossing, but not in the middle of a highway. There is thus a difference between normal and aberrant stop signals. Nevertheless, the protein factory obeys the aberrant stop signal and protein translation is stopped.
PTC124 is a drug that can make the protein factory ignore the stop signals that do not make much sense (in the middle of a highway), so that protein translation can continue and a complete protein can be generated (figure 3). The real stop signal (at the end of mRNA (or at a busy crossing)) are not ignored.
Figure 3. PTC hides the aberrant stop signals and protein translation can continue until the end of the mRNA (the real stop signal).
PTC does not affect mutations that disrupt the genetic code (deletions or duplications of one or more exons, small deletions or duplications within an exon, or small mutations that disrupt exon definition during splicing (splice site mutations)). For these mutations, there is not a single stop signal at an unusual place, but rather aberrant protein subunits are included into the protein. PTC will not change this (even with PTC the parts that make a model airplane, still cannot make a model car).
Applicability of exon 51 skipping
Skipping exon 51 is applicable to the largest group of Duchenne patients (13% of all patients). Figure 4 shows how exon 51 skipping can restore the genetic code. Figure 14 explains why exon 51 skipping does not work for mutations that require the skipping of other exons.
Figure 4. Exon 51 skipping for a patient with an exon 45 deletion. A deletion of exon 45 disrupts the genetic code (exon 44 and exon 46 do not fit). This deletion can NOT be restored by exon 51 skipping. When exon 51 AONs are used anyway this will result in a disruption of the genetic code at two positions: at the location of exon 44-46 (does not fit) and at the location of exon 50-52 (also does not fit). So exon 51 does not restore the genetic code for this mutation, but instead disrupts it at an additional position.