Somatic and Germline Gene Therapies are the types of gene therapy where a gene is introduced in a somatic cell or germ cell, respectively.
Gene Therapy Introduction
Gene therapy refers to the method wherein a genetic defect is treated by inserting a functional gene or section of DNA in cells. The functional gene inserted into an individual or an embryo compensates for the non-functional gene present.
The newly inserted DNA corrects the effect of the mutated gene. Gene therapy was developed in 1972. The first therapeutic gene therapy was given to a four year old girl in 1990 to treat adenosine deaminase (ADA) deficiency.
Gene Therapy Types
On the basis of types of cells involved in the gene transfer, gene therapy is of two types:
- Somatic gene therapy: Here the desired gene is transferred to a somatic cell. The gene is not transferred to the offspring.
- Germline gene therapy: Here the desired gene is introduced in the germ cell. The gene gets transferred from one generation to another generation.
Somatic Gene Therapy
The somatic gene therapy targets body tissues. These cells do not produce sperm or eggs. therefore the gene is not transferred to the next generation.
In the somatic cell gene therapy, therapeutic DNA is transferred to a somatic cell, which is any cell other than gametic cells, undifferentiated stem cell or gametocytes. The DNA is either inserted into the genome or present as an external genome like plasmid.
Genes can be inserted into a cell by using a viral vector or by liposomes. The recombinant DNA is introduced in the target cells, e.g. bone marrow, lungs, liver, muscles, etc.
There are two types of somatic gene therapy:
- Ex-vivo – Here the cells are taken out from the body and grown in the laboratory. These cells are then exposed to the virus containing the desired gene and then after recombination, the recombinant cells are returned to the patient.
- In-vivo – Here the genes are transferred to the cells present inside the patient’s body.
To treat ADA deficiency, lymphocytes from patients were cultured and the functional ADA cDNA is introduced into the cells through a retroviral vector. The genetically engineered lymphocytes were then transferred back to the patient. These cells need to be infused periodically as the cells die and need to be replaced.
It is considered a safer approach. The downside is that the effect is not passed to the future generation and is short-lived. As cells and tissues die and get replaced, periodic treatment needs to be given for maintaining the effect.
Somatic cell gene therapy is accepted to treat many disorders such as muscular dystrophy, cancer, cystic fibrosis, certain infectious diseases, etc.
A single gene disorder like haemophilia, thalassaemia, cystic fibrosis, etc. have good chances of being cured by somatic cell gene therapy. There is still research going on to find a complete cure.
Germline Gene Therapy
The germline gene therapy targets germinal or reproductive cells. These cells produce male and female gametes therefore the inserted gene passes to the future generations. The transfer can also be done during early embryonic development, e.g. during in-vitro fertilisation, then the desired gene can be inserted in all the cells of a developing embryo.
The germline gene therapy can be more effective and can be a permanent cure for genetic diseases that run in families. It has the potential to eliminate a disease from the population. But it is not yet legal in many countries due to ethical issues. Some people may use it for enhancements rather than treatments. There is also insufficient knowledge of risks posed to the future generations.
Gene Therapy Methods
There are various techniques to perform gene therapy:
- Gene augmentation therapy – Here a functional gene is introduced, which produces sufficient levels of proteins to compensate for non-functional genes. This is used to treat disease where there is a loss of function. E.g. cystic fibrosis, ADA deficiency, etc.
- Gene inhibition therapy – This is used when a gene activity is altered and needs to be suppressed, e.g. cancer, infectious diseases. Here a gene is introduced, which either inhibits the gene expression of another gene or interferes with the activity of another gene product. E.g. Gene inhibition therapy can be used to eliminate the activity of oncogenes and prevent further uncontrolled growth of cells.
- Killing Specific Cells – This is used to destroy a group of cells such as in cancer. Here a specific DNA called suicide DNA is inserted into diseased cells inorder to destroy it. Here the inserted DNA produces a product that helps the immune system to identify and attack those cells. It is important to target only diseased cells so that functional cells do not die.
The desired gene or a DNA fragment is inserted into the vector such as virus, bacteria or plasmid. The recombinant DNA is introduced into the nucleus of the host cell, e.g. by micro-injection. The gene is expressed inside the cell and produces a functional product that has a therapeutic effect.
There are multiple aspects which need to be taken care of for a successful gene therapy.
- The gene should be correctly inserted into the target cell and switched on for the activity.
- The immune response against the foreign gene needs to be avoided.
- The new gene should not affect the functionality of other normal genes.
- Gene therapy is still new and developing. It requires a very specific approach depending on an individual, so it may be expensive.
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