Mutations

Introduction
A mutation in the gene containing the genetic code for the dystrophin protein is found in nearly all Duchenne patients. There are different types of deletions, but the result of each mutation is the same: no functional dystrophin can be produced. Nevertheless the type of mutation matters for some therapeutic approaches (exon skipping and PTC124), as these only apply to some specific mutations.

The most common mutations found in the dystrophin gene is described below.

From gene to protein
Genes contain the blueprint (genetic code) for proteins. Genes consist of DNA and are dispersed over 23 pairs of chromosomes, that are located in the nucleus of every cell in our body. The protein factory (the system that translates the genetic code into protein) is located outside the nucleus in the so called cytoplasm. As the DNA cannot leave the nucleus, genes and the protein factory are physically separated. Therefore, when a protein needs to be generated, a temporary copy is made from a gene. You can compare this with a recipe (gene) you need from a book (DNA) that cannot leave the library (nucleus). In order to prepare the recipe at home (cytoplasm) you make a copy of the recipe that you take with you. Temporary copies of a gene consist of RNA and are transported from the nucleus to the protein factory where the copy is translated into protein.

The genetic code within a gene is not continuous, but is dispersed over so called exons. In between exons reside introns, that do not contain protein information. Before the RNA can be transported to the protein factory to be translated into protein, these introns have to be removed and the exons joined together (Figure 1). This process, called splicing, takes place in the nucleus. To compare again: the recipe contains a lot of advertisements which can be cut out of the text. This will make the recipe very accessible, without losing the value of the recipe.
The resulting product only contains the genetic code (exons). This “messenger” RNA (mRNA) is subsequently transported into the cytoplasm and translated into protein.

splicing_nederlands
Figure 1. The genetic code is dispersed over exons, which are interrupted by introns. First, an RNA copy is generated of a gene, then the introns are removed and exons are joined during the splicing process.

When RNA is translated into protein, 3 RNA units code for a single protein unit (Figure 2). The translation starts with a start code for the initiation protein subunit, which is the same for all proteins. After this first subunit, different subunits (amino acids) follow. In this simplified example only two types of subunits are used (blue and yellow), but in reality there are over 20 different protein subunits. Proteins consist of tens, hundreds and sometimes thousands of subunits. For instance, the dystrophin protein contains 3685 protein subunits.
A stop code is present at the end of the mRNA to indicate that protein translation is finished. After this stop signal, the completed protein is transported to its site of action (e.g. close to the muscle membrane for dystrophin). Due to the different properties of the protein subunits (big or small, soluble in water or in lipids, having an electric charge or not etc), the properties of a protein are determined by the amount of subunits and their individual properties.


Figuur 2. The translation of messenger RNA (mRNA) to protein.

Mutations
As written above the genes contain the blueprint for proteins. If a gene contains a mistake (mutation) this will affect the protein product. Duchenne patients can not produce functional dystrophin protein due to mutations in their dystrophin gene. There are three types of mutations commonly found in Duchenne patients: deletion (removing information), duplication (multiplying information) and “point mutations” (small errors in the gene).