Genome editing
With the CRISPR-Cas9 system, it has become possible to modify the genome of any organism – for example, a bacterium, an animal, a plant or that of a human being – in a targeted and precise manner. In this context, one also speaks of genome editing and genome surgery. The method was first described in 2012 and is currently being intensively researched and used worldwide (Jinek et al., 2012). The CRISPR-Cas9 system was developed starting from a defense mechanism of the adaptive immune system of bacteria. It enables prokaryotes to remove unwanted and infectious DNA, for example, from bacteriophages or plasmids.
- CRISPR stands for . It is repetitive DNA sequences in the genome of bacteria.
- Cas9 stands for .
How it works
How does the CRISPR-Cas9 system work? The Cas9 protein is a so-called endonuclease, i.e. an enzyme that cuts nucleic acids and, in this case, the DNA double strand. Cas9 is bound to an RNA that contains, in addition to a constant portion, a variable portion that recognizes a specific target DNA sequence. This RNA portion is called sgRNA (small guide RNA). The sgRNA interacts with the DNA to be cut by the nuclease. The resulting double-strand break can be completed by the cell’s own repair mechanisms. This can lead to mutations that are of interest, for example, in plant breeding. An RNA template can also be used to insert a new gene segment, for example to restore the function of a defective gene. Finally, it can also be used to inactivate disease-causing genes. For the CRISPR-Cas system to enter cells, a delivery method is also required. For example, viral vectors, liposomes or physical methods can be used for this purpose. The system differs from other methods in its relative simplicity, universality, speed, and in price. CRISPR-Cas can in principle be applied directly to humans or to cells that are removed from a patient, modified, propagated and subsequently reintroduced. This is referred to as ex vivo treatment or autologous transplantation.
Indications
CRISPR-Cas9 is already playing an important role in research and drug production today, just a few years after the method was described. In the future, additional gene therapy drugs will be developed. The potential medical applications are very numerous. These include, for example, severe genetic diseases such as Duchenne muscular dystrophy, cystic fibrosis, infectious diseases such as HIV and hepatitis, hemophilias, and cancers. CRISPR-Cas9 and similar genome editing methods are opening up the possibility for the first time to not only treat diseases symptomatically, but to cure them at the level of genetic information.
Adverse effects and risks
One potential problem is the selectivity of the method. It is possible that CRISPR could cause genetic changes at other undesirable sites in the genome. These are referred to as “off-target effects.” These can be dangerous and lead to side effects. In addition, the efficiency of the system still needs to be increased. In humans, in addition to somatic cells and stem cells, germ cells can also be manipulated, leading to inheritance of the altered genes. For example, it has already been shown that CRISPR-Cas9 can be applied to embryos, raising fears of “designer babies” and potentially having far-reaching consequences for the future development of humanity. Germline intervention is highly controversial and prohibited in most countries.
