Dr. István Balogh, Dr. János Kappelmayer, Dr. József Tőzsér (2011)
University of Debrecen
The previously described point mutations (missense, nonsense, splicing or regulatory) represent the majority of the pathogenic mutations. There are other types of mutations, where the originally present nucleotide number is changed. The size of these changes range from only one to a scale that can be detected by light microscope after the adequate staining of the chromosomes.
When one or more nucleotides are incorporated into a DNA sequence, the result is insertion. The effect of insertion largely depends on the number of the inserted nucleotides. If the number of the inserted nucleotides that are introduced into the codin region of a gene cannot be divided by three, the open reading frame is shifted. In this case, downstream of the mutation site the sequence of the encoded protein will be completely different and the resulting protein will be almost certainly unable to fulfill the function of the original. Insertions that cause frameshift usually result in the incorporation of the premature stop codon. In Figure 3.1., an adenine insertion can be seen, which causes frameshift, whereby the originally present histidines will be replaced by threonine and the following stretch of serine amino acids.
Alterations that affect the genetic material are extremely diverse, ranging in their size from only one nucleotide (see above) to many thousands of nucleotides. In some cases, even chromosome regions can be mutated, as it can be seen in Figure 3.2. The size of this mutation might reach the detection limit of light microscope or fluorescence in situ hybridization (FISH). The effects of this large-scale mutations, which might affect hundreds of genes, can be numerous. Depending on the affected region and/or the break points, they might have a role in the development of tumour, but the increased amount of the expressed proteins from the duplicated chromosome region might also be harmful.
The opposite of the above mentioned insertion and duplication types of mutations is deletion. When one or more nucleotides are removed from a DNA sequence, the result is deletion. The effect of deletion is similar to the effect of insertion, in that it largely depends on the number of the inserted nucleotides. If the number of the deleted nucleotides introduced into the coding region of a gene cannot be divided by three, the open reading frame is shifted the same way as it is seen in the case of insertion. In this case, the downstream of the mutation site the sequence of the encoded protein will be completely different and the resulted protein will be unable to fulfill the function of the original. Deletions that cause frameshift usually result in the incorporation of the premature stop codon. In Figure 3.3, an adenine deletion can be seen, which causes frameshift and completely replaces the originally present histidine motif.
The open reading frame of the translation starts with the first ATG codon that encodes methionine. Insertion or deletion mutations, when the number of the affected nucleotides cannot be divided by three, will cause frameshift. A frameshift mutation is almost always pathogenic and results in premature stop codon. Such an mRNA molecule is usually degraded. It is not only small scale mutations that can cause a frameshift. In some cases, when entire exons are deleted or duplicated, the result can be a frameshift as well. A good example of that effect is one of the most prevalent monogenic diseases, the X-linked Duchenne/Becker type muscular dystrophy. In this case, the most prevalent cause of the disease are large deletions. The status of the open reading frame is an important prognostic factor, as mutations not affecting the open reading frame will result in Becker and mutations causing frameshift will result in Duchenne type of dystrophinopathy. Figure 3.4. shows the effect of a frameshift mutation, it does not show, however, whether the frameshift is caused by deletion or insertion.