What is gene therapy?
For over fifteen years, genes have been used to produce, with biotechnological techniques, pure proteins that are used as biopharmaceuticals (insulin, growth hormone, blood coagulation factors, erythropoietin, etc.). Gene therapy is a medical technology in which DNA is directly used as a pharmaceutical substance. With this technique, the genes or fragments of these are inserted into the human body to prevent, treat, or treating a disease. Carrier’s non-viral or viral origin achieves the transfer of the gene in the human body. The greatest difficulty encountered in transferring the gene is achieving a sufficient level of efficiency. Gene therapy can potentially cure many diseases, both genetic and acquired. The experimental clinical gene therapy began in 1990. To date it around 200 involving nearly 3,000 patients were made. Some of these trials have been conducted or are currently underway in Switzerland. However, the results obtained so far are strictly experimental.
What does 'somatic' gene therapy mean?
The term "somatic" is opposed to the term "germinal." If a gene is transferred to a germ cell line, the modification that is made to the genome will be transmitted to subsequent generations through the same germ cells (sperm and eggs). Currently, it is impossible to completely control the genetic alterations that are brought about by a gene transfer. Consequently, there is the risk of provoking physiological anomalies that would be transmitted to subsequent generations; furthermore, the alteration of heritable genetic material raises a series of ethical and moral questions. For these reasons, the genetic modification of germ cell lines is not considered an adaptable strategy, and its clinical trial is prohibited in Switzerland as in almost all other countries in the world.
With gene transfer on somatic cell lines, the genome of tissues (muscles, lungs, brain, bones, kidneys, heart, etc.) is changed, without this modification being passed on to the next generation; the genetic alteration concerns only the patient on whom it was made. This does not mean that such therapy is risk-free. We are studying the complications of random insertion of a foreign gene into the genome; according to the carrier that is used, the risks can be more or less high, thus determining the indications and contra-indications of a given treatment. Lungs, brain, bones, kidneys, heart, etc.), without this modification is being passed on to the next generation; the genetic alteration concerns only the patient on whom it was made.
What diseases can be treated with gene therapy?
Theoretically, all diseases can be treated through gene expression interventions. Commonly, gene therapy is thought to help heal hereditary diseases such as:
- muscular dystrophy
- cystic fibrosis
- hemophilia
- type I diabetes
- metabolic diseases (phenylketonuria)
- Physiological anomalies (mucopolysaccharidosis, Gaucher syndrome, etc.)
In reality, this therapy can also treat diseases that affect those genetically predisposed, but which depend heavily on environmental factors, such as:
- cancer
- Neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, etc.)
Finally, gene therapy can also treat acquired diseases such as:
- Various types of trauma (bone fractures, wounds, burns)
- Ischemias (tissue necrosis caused by an interruption of blood supply)
- Infections
Which gene transfer vectors are used today?
What is "in vivo" and "ex vivo?
The expression “in vivo treatment” refers to a gene transfer that occurs both locally (with an intramuscular or intratumor injection, by inhalation or by permeability, etc.), and systemically (intravenous injection) directly in the body. While “ex vivo " indicates a gene transfer that occurs in cells or tissues that have been first explanted, then grown in the laboratory, and subsequently re-implanted in the patient.
Physical methods
- hydrodynamic pressure
- biolistic method
- transfer mediated through an electric field
- direct injection into the tissues
In vivo, physical methods have limited applicability to the transfer of genes to surface tissues, while they are more conveniently used for ex vivo transfer. Direct injection into the tissues is effective only when therapy does not require a large amount of expression of the transferred gene (for example, when patients are treated with expression vectors for VEGF, growth factor of vascular endothelium).
Chemical methods
- cationic liposomes
- polycations such as DEAE dextran, polyethyleneimine
- polylysine complexes
The advantage of chemical methods is the ability to build the desired vector by combining the different chemical compounds, which are easily obtainable; however, the efficiency is 100 to 1,000 times lower than that of biological vectors. Future improvements of this method, such as the creation of compounds formed by chemical and biological substances, could lead to the formation of "virosomes" or "artificial viruses." These "hybrid vectors" are expected to become the most efficient vectors for in vivo systemic transfer. In fact, these vectors can be designed to accumulate in the desired body compartments, due to surface molecules that are specific ligands.
Biochemical methods
- polysyses associated with ligands
- "Transfer reinfection."
In biochemical methods, DNA is conjugated with proteins that can enter cells by endocytosis, for example, through internalization mediated by a receptor. These methods offer the advantage of specifically targeting certain cells, and such specificity is not possible with chemical methods.
Biological methods
- recombinant adenovirus
- recombinant adeno-associated viruses (AAV)
- recombinant mouse retrovirus
- recombinant herpes virus
- recombinant vaccinia virus
- recombinant measles virus
- recombinant lentiviruses (HIV, SIV, EIV, FIV)
Most of these viruses are constructed in such a way that they can only replicate in genetically manipulated cells. The major advantage these carriers offer is that they can transport genes with very high efficiency. Some vectors (for example, lentivirus) allow the integration of the introduced gene into the host cell genome. In this way, the expression of the introduced gene is permanent. The main risk we face is that the virus gives rise to particles capable of reproducing in an uncontrolled way. However, the use of modern protocols appears to have significantly reduced this risk. Some vectors (for example, recombinant adenoviruses) transport toxic components (such as capsid proteins) into the cell; this may preclude their use for non-serious diseases.
Finally, vectors (such as AAV) do not allow the packaging of genes with a long DNA sequence, causing a severe limitation for the treatment of diseases such as muscular dystrophy and cystic fibrosis, where the gene of interest is larger than the space available in vector. Many studies and experiments are conducted to make recombinant viruses capable of attaching to specific surfaces within the body to allow a systemic tissue-specific gene transfer. They are causing a serious limitation for the treatment of diseases such as muscular dystrophy and cystic fibrosis, where the gene of interest is larger than the space available in the vector. Currently, many studies and experiments are conducted with the aim of making recombinant viruses capable of attaching to specific surfaces within the body, so as to allow a systemic tissue-specific gene transfer. They are causing a serious limitation for the treatment of diseases such as muscular dystrophy and cystic fibrosis, where the gene of interest is larger than the space available in the vector. Currently, many studies and experiments are conducted with the aim of making recombinant viruses capable of attaching to specific surfaces within the body
, to allow a systemic tissue-specific gene transfer.
Author: Vicki Lezama