## Vectors used in gene therapy

One of the priorities was and is the development of new, safer vectors. In addition to retroviruses and adenoviruses, adeno-associated viruses (AAV), which are very well tolerated by human immunity, are currently gaining attention. Non-viral vectors, e.g. fat particles – liposomes, gold particles and polymeric micelles, also seem to be promising. Each of the vectors mentioned has its advantages and disadvantages. Some viruses incorporate their DNA into the genome, while others leave their DNA in the form of episomes. Episomes are located in the nucleus, replicated and inherited along with the nuclear chromosomes without inserting themselves into the host genome. In this case, there is no risk that an important gene will be affected by the insertion. In dividing cells, on the other hand, there is a high probability that the episome will be lost over time and the patient will have to undergo repeated gene therapy.

When selecting a vector, it is also important to know what types of cells the virus can infect. This is referred to as viral tropism. Some viruses only infect dividing cells or are tissue specific, meaning they only infect cells of a certain type. All vectors have a limited coding capacity, which means that only a limited size of nucleic acid can be incorporated into the virion. Thus, if a long gene is to be repaired by gene therapy, this must be taken into account when selecting the vector. Here, non-viral vectors, for example liposomes, prove to be very suitable. They have a much greater capacity and also do not trigger such a strong immune response. On the other hand, they are less specific. Despite the advent of new vectors, it is still necessary to use relatively high doses, which contributes to a strong immune response. For this reason, gene therapy is often combined with the administration of immunosuppressants.