What is Gene Therapy? Genetic Diseases Types of Gene Therapy The Science – How does Gene Therapy Work? Gene of the Moment: p53 Biotechnology and Gene Therapy Challenges to Gene Delivery Types of Vectors Bibliography

What is Gene Therapy?

Gene therapy is the treatment of a disease by introducing a new gene into a cell.

Gene therapy aims to treat disease by changing the expression of a person's genes, with the ultimate goal to cure or prevent genetic diseases. Despite the incredible amount of buzz surrounding gene therapy, it is still in the very early stages of development – more than a decade after the first gene therapy experiment.

The Concept

Genes are the blueprint for the development of the human body over a life time. If a person is born with a gene that is defective or mutated, he or she may develop a disease that is associated with that gene mutation. Instead of treating the symptoms that arise from such a genetic mutation, gene therapy proposes to fix the problem at the root: add the correct gene, or repair the existing one.

Gene Therapy Targets

Cells in the affected area are the targets of gene therapy. Although all cells in our bodies have the same set of genes – called a genome – different genes are active or expressed, depending on the kind of cell. For example in a lung cell, only the genes that contribute to the functioning of the lung are expressed. Therefore, a gene therapy procedure for cystic fibrosis – which is a disease of the lung – involves treating the genes of lung cells, rather than the cells of the entire body.

In the case of cancer, many researchers are looking into gene therapy which aims to eliminate cancer cells – i.e. tumours. Although cancer cells have the same set of genes as normal cells, some malfunction in cancer cells allows them to grow rapidly and indefinitely. Research is under way to deliver genes into these cancer cells that will bring about apoptosis – cell suicide.

Genetic Diseases

Not all diseases are caused by genetic mutations nor do all genetic mutations lead to disease. Environmental factors play a role in disease development. Take cancer for example. It is known that certain gene mutations lead to specific types of cancer. However, having these gene mutations does not mean the person will develop the disease. Certain known carcinogens, such as tar and radiation, are believed to trigger cancer development.

On the other hand, there are single gene mutations that will guarantee the development of disease, as in the case of cystic fibrosis and Huntington's disease. Such diseases are the prime candidates for gene therapy.

Learn more about Huntington's Disease and Respiratory Disorders, such as cystic fibrosis.

Types of Gene Therapy

Germ Cell Gene Therapy – Germ cells are the reproductive cells – egg and sperm cells. Therefore, changes made to germ cells will be passed on to the next generation. At present, germ cell gene therapy remains technically difficult and is fraught with ethical issues. More research is being focussed on somatic cell gene therapy.

Somatic Cell Gene Therapy – Somatic cells are all cells that aren't germ cells. When gene therapy is in the news, the discussion is most likely around this type of gene therapy.

The Science – How does Gene Therapy Work?

In the lab, a virus is stripped of its disease-causing genes, while the genes that enable it to infect cells are retained. A therapeutic gene is inserted into this virus. This allows the virus to "infect" cells with the therapeutic gene. This viral vector is injected into the specific diseased tissues. The virus attaches to the diseased cells and gets sucked into the cell by process called endocytosis. The virus breaks apart inside the cell and the genetic material from the virus enters the nucleus of the cell. If the procedure is successful, the cell begins to produce the proteins encoded by the newly delivered therapeutic gene.

Gene of the Moment: p53

Cancer is a major focus for gene therapy research at the moment. The most common gene therapy strategy for treating cancer is delivering therapeutic genes that can induce apoptosis (programmed cell death) in cancer cells. The gene that is delivered in the majority of these cases is a gene that produces p53 – an identified "tumour suppressor" that is mutated or deleted in many cases of human cancer.

The role of p53 is to keep a check on the cell cycle. If anything damages the DNA in a cell, p53 stops the cell cycle and tries to repair it, or if the damage is beyond repair, it induces apoptosis. If the gene for p53 has been mutated and no longer works, damaged DNA and cells can go unchecked. In the case of cancer, tumours can develop.

Many cancer gene therapies focus on repairing or adding the p53 gene into the cancer cells in order to induce apoptosis. A drug based on the p53 gene is currently in late stage clinical trials.

Biotechnology and Gene Therapy

Two key biotechnology advances have led to the current boom in gene therapy research:

• Recombinant DNA Technology – Also known as genetic engineering, this technology allows researchers to change the genetic structure of viruses to render them harmless for gene therapy.
  
  
• Human Genome Project – Research in genomics through the Human Genome Project has identified all the genes in the human genome. Research is now underway to identify the functions of these genes. Understanding the functions of the numerous genes in the human body will help researchers discover the relationship between disease.

Learn more about Genomics

Gene Therapy Vectors

Vectors are the vehicles that carry the gene or DNA strand into the target cells. Designing better vectors is the focus of gene therapy. Currently, genetically engineered viruses are the most commonly used vectors.

What's So Special About Viruses?

Viruses are favoured in gene therapy experiments because of the way they infect human cells. Viruses insert their own RNA or DNA into the host cell's genes and prompt the host cells to produce the proteins that are encoded by the viral genes. Essentially, they are extremely efficient at manipulating a host cell.

Using genetic engineering techniques, researchers cut out all the genes that enable them to cause disease and replicate. They only keep the genes that are responsible for the virus's infecting mechanism. They then add the therapeutic gene(s) into the virus so that when the virus infects a cell, the therapeutic gene is inserted, not the viral ones.

Challenges to Gene Delivery

There are several challenges that an ideal vector has to meet.

Getting to its destination is the vector's first challenge. The biggest obstacle to this is a person's immune system; it sees the vectors as foreign and often destroys them before they can get to the target cells.

The second challenge is inserting the foreign gene or DNA strand into the nucleus of the cell. If the genetic material does not go into the nucleus, the expression of the genes is brief and cannot be sustained. Retroviruses are often used because they can efficiently enter the nucleus.

The third challenge lies in the integration of genetic material upon its entry into the nucleus. The integration into a host cell's genome is significant because this means gene therapy treatments do not have to be administered in large amounts and as frequently. However, gene integration at present is very random and researchers are not able to control it. It can potentially interrupt another gene's functions by inserting itself anywhere.

Researchers are working to improve and stabilize gene delivery.

Types of Vectors

Examples of vectors area as follows:

Vector	Description	Advantage	Disadvantage
Retrovirus	RNA virus	Integrates into host cell, therefore, long term expression Not toxic to host cells Does not elicit strong immune responses	Only infects rapidly dividing cells (i.e. cancer cells) Degrades rapidly inside the body Integrates into host cell randomly, therefore could interrupt the expression of other host cell genes
Adenovirus	Larger than other viruses	Infects a wide range of cells Does not integrate into the host cell's genome, therefore, minimal risk of interrupting the expression of other host cell genes	Highly detectable by immune system. For this reason, it is often administered at high doses but this can lead to toxic immune responses, which may lead to the death of the individual Does not integrate into the host cell's genome, therefore the expression of genes is temporary and brief Integrates into host cell randomly, therefore could interrupt the expression of other host cell genes
Adeno-Associated Virus (AAV)	A small DNA virus that does not seem to be associated with any human diseases.	Infects a wide range of cells Does not seem to elicit an immune response Integrates into the host cell's genome, therefore long term expression	Carries small therapeutic payloads Does not seem to elicit an immune response Safety concerns over method of vector preparation in the lab: AAV needs another virus to replicate in culture. Issues of impurity Integrates into host cell randomly, therefore could interrupt the expression of other host cell genes
Cationic Lipopsomes	Non-viral. Liposomes are fatty droplets that mimic the human cell membrane structures	Generates minimal immune response	Not very efficient in transferring genes into the host cell
Naked DNA	Non-viral. Free DNA plasmids that are injected into an individual.	Generates minimal immune response	Low rate of gene transfer and weak long term expression.

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