[Front edge] gene therapy depends on the gene carrier

Gene therapy is a new treatment that can treat a variety of diseases, including cancer, hereditary, infectious, cardiovascular and autoimmune diseases. Gene therapy refers to the introduction of a therapeutic gene into a target cell, and the therapeutic gene is expressed in a cell as a specific protein, thereby treating a disease caused by a genetic abnormality or defect. The gene therapy process is divided into three parts: the selection of therapeutic genes, the introduction of therapeutic genes into host cells, and the expression of therapeutic genes. The most crucial of these is the second step - gene delivery. Many experiments have shown that free genes can be endocytosed into cells, but the progress of this method is too slow for effective biological effects. In addition, free DNA or RNA is very sensitive to the digestion of nucleases in serum and is susceptible to degradation. Because intracellular and extracellular disorders impede gene delivery, transcription, and expression, gene therapy requires a suitable delivery system. Therefore, the development of effective gene vectors is important for gene therapy.

Gene vectors are vehicles that introduce therapeutic foreign genes into targeted cells, a process that is the most critical step in gene therapy. Gene vectors can be divided into two broad categories: viral gene vectors and non-viral gene vectors. The virus itself has the ability to infect cells, integrate the gene of interest into the virus, and then carry it into the cell by the virus to complete the transfection process. The simplest gene vector is the introduction of plasmid DNA into a host cell by a prokaryote.

Figure 1 Schematic diagram of gene therapy

The advantage of the viral gene carrier is that the transfection efficiency is high and the biocompatibility is good. At present, the viral carrier is often used for clinical treatment. Non-viral gene vectors are mainly divided into two types, polymer gene vectors and liposome gene vectors. The polymer gene carrier utilizes a positive charge on the surface of the cationic polymer to complex with the DNA and is taken up by the cells, thereby releasing the therapeutic gene into the cytoplasm to complete the transfection process. Polymer gene carriers can be engineered to regulate their molecular weight and molecular structure to achieve different performance requirements. Liposomes are vesicular structures, and DNA can enter its internal hydrophilic core, which allows the DNA to enter the cell when it phagocytose the vesicle. Compared with viral vectors, non-viral vectors have lower transfection efficiency, but their advantages are also more obvious: they are safe, do not produce rejection, and are not passed on to the next generation through reproduction.

Figure 2 Schematic diagram of adenovirus

Viral gene vectors mainly include retrovirus vectors, adenoviruses, herpes viruses, and lentiviruses. The molecular sequence of the retroviral vector has been solved, so it was first used for gene therapy. Retroviruses are double-stranded. RNA viruses, such as the commonly used murine leukemia virus, are well suited for the transcription of foreign genes that require long-term expression. However, its shortcomings are also obvious. After entering the host cell, the retrovirus may have an indeterminate genetic integration with the host cell, thus creating a potentially life-threatening adverse effect. The ability of adenovirus to proliferate makes it very popular in the field of gene therapy. Most of the adenoviruses used in clinical trials are derived from adenovirus type 5, which is composed of the outer envelope and the inner double-stranded DNA. Although it can infect most types of cells, its disadvantages are obvious: it does not integrate with host cells, which leads to shorter gene expression times, and adenoviruses may cause a strong immune response.

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