Cell and gene therapies aim to introduce reparative cells or correct the effect of the disease-causing gene and thereby treat the ultimate underlying cause of the medical problem.
Gene therapy is the insertion of genes into an individual’s cells and tissues to treat disease, in particular, hereditary diseases. Gene therapy typically aims to supplement a defective gene with a functional one. Although the technology is still in its infancy, it has already been used with some success.
In most gene therapy studies, a normal gene is inserted into the genome to replace the function of an abnormal disease-causing gene. The gene therapy is delivered using a DNA containing specialised ‘vector’ containing genetic material to the patient’s target cells. Currently, the most common type of vectors are viruses that have been genetically modified to carry normal human DNA – Mother Nature’s own nanotechnology! Over millions of years, viruses have evolved to deliver their genes to human cells and scientists have tried to harness this ability by manipulating the viral genome to remove disease-causing genes and insert therapeutic ones.
Diseased cells such as the patient’s liver or lung cells are targeted by the vector. The vector then unloads the therapeutic human gene into the target cell. The generation of a functional protein product from the therapeutic gene restores the target cell to a normal state.
DIFFERENT TYPES OF GENE THERAPY
In theory, it is possible to insert genes into either:
- somatic cells (most cells of the body) including adult stem cells; and
- embryonic stem cells or cells of the germline (such as sperm cells and ova).
All human gene therapy has thus far been directed at somatic cells, whereas germline engineering in humans remains only a highly controversial prospect. For the introduced gene to be transmitted normally to offspring, it needs to be permanently inserted into the germ cells.
WHAT IS SOMATIC CELL GENE THERAPY?
Somatic cell gene therapy can be broadly split into two categories: ex vivo, which means ‘outside the body’ (where cells are removed, genetically modified and then transplanted back in again); and in vivo, which means ‘in the body’ (where genes are introduced directly into cells in the body). Attempts to correct the actual disease gene itself directly (recombination-based approaches) in vivo is are especially difficult, because for most cases successful targeting of the one gene from 30,000 has a very low probability or working.
USE OF VIRUSES IN GENE THERAPY
All viruses introduce their genetic material into the host cell as part of their life cycle. This genetic material contains basic ‘instructions’ of how to produce more copies of these viruses, hijacking the body’s normal production machinery to serve the needs of the virus. The cell will carry out these instructions and produce additional copies of the virus, leading to more and more cells becoming infected. Some types of viruses actually physically insert their genes into the cell’s DNA (it is the defining feature of retroviruses, the family of viruses that includes HIV). This incorporates the genes of that virus among the genes of the host cell for the life span of that cell.
Doctors and molecular biologists realised that viruses like this could be used as vehicles to carry ‘good’ genes into a human cell. First, the ‘molecular surgeons’ would remove the genes in the virus that cause disease. Then they would replace those genes with a genes encoding the desired effect (for instance, insulin production in the case of diabetics). This procedure must be done in such a way that the genes which allow the virus can still to insert itself efficiently genome iinto a patient’s cellsits host’s genome are left intact. This can be confusing and requires significant research and understanding of the virus’s genes to determine which to keep and which can be discarded, whilst maintaining utmost safety.
This explanation of viruses being used in genes is an over-simplication and numerous problems exist preventing gene therapy from using viral vectors on a large scale. Such problems include: complexities arising from preventing undesired effects, ensuring the virus will modify the correct target cell in the body and ensuring that the inserted gene doesn’t disrupt any existing vital genes already in the genome. However, this basic mode of gene introduction currently shows much promise and doctors and scientist continue to improve the platform technology. At stake is the future of human health!