Gene Therapy

In the 1990s there was great hope that this novel approach may provide an answer to many hitherto uncurable diseases. The basic idea is to correct defective genes responsible for disease development. This can be achieved in a number of ways:

• Homologous recombination can be used to swap a defective gene for a normal functioning one.
• By altering the regulation of the gene, to effect its function.
• By using selective reverse mutation to repair a defective gene.
• But the most common method is to insert a normal gene into a non-specific location to replace a defective gene.

When a normal gene is inserted into the genome a carrier molecule, a vector is used. This will deliver the new gene to the target cells. The most commonly used vectors are viruses. The most commonly used viruses are:

• Retroviruses
• Adenoviruses
• Adeno-associated viruses
• Herpes simplex viruses

These viruses are altered to carry normal human DNA. The patients target cells are infected with the vector, which deposits its genetic load including the gene to be replaced . The target cell is then able to produce a functioning protein. More recently success has been seen by combining a tumour specific adenovirus vector and several single therapy genes. Targeting Gene-virotherapy has killed tumour cells with minimal damage to normal cells in mice.^1,2
There are also non-viral insertion options. The simplest method is direct introduction of new DNA into the target tissues. This is limited by the type of tissue and the amount of DNA required. An artificial lipid sphere with an aqueous core is created, a liposome, which can both carry the therapeutic DNA and pass it through the target cells membrane.
The therapeutic DNA can also bind chemically to molecules that will attach to target cell receptor sites. These are then taken into the cells interior. This tends to be less effective than the other methods.

Human Gene Therapy is still largely in the experimental phase. There have been few big breakthroughs since the first trial started in 1990. There has also been at least one death attributed to therapy and two cases of leukaemia developing post-therapy.
There are also technical problems involved:

• Problems with viral vectors such as toxicity, inflammatory and immune responses.
• Disorders resulting from multiple gene defects such as heart disease, Alzheimers and diabetes are not good candidates for gene therapy. Single gene defects are the most responsive to therapy.
• Gene therapy tends to be of short duration. The functioning DNA that is introduced must remain functioning and stable to effect a cure. Many cells divide quite rapidly, so multiple treatments are required.
• The immune response to any foreign material may reduce the effectiveness of the treatment. It also makes multiple treatments unfeasible.

• A team from Moorfields Eye Hospital and University College in London have conducted the first human gene therapy trials to treat Leber's congenital amaurosis which is caused by a single abnormal gene: (the procedure has already been successful at restoring vision for dogs). This is the first trial to use gene therapy in an operation to treat blindness in humans.³
• A combination of two tumour suppressing genes delivered in lipid-based nanoparticles has reduced the number and size of human lung cancer tumours in mice during trials conducted by researchers from The University of Texas.⁴
• The National Cancer Institute (NCI), has successfully re-engineered lymphocytes, to target and attack cancer cells in patients with advanced metastatic melanoma.⁵ This is the first time that gene therapy is used to successfully treat cancer in humans.

In a bid to alleviate disease at the earliest possible stage, in-utero fetal gene therapy has also been tried.⁶ Pre-natal screening for severe genetic disease such as Crigler-Najjar, Pompe's and Haemophilia B has been tested in mouse models. There have been issues with the development of liver tumours, insufficient target cells are reached and the therapy is not toxic enough to target cells.
There are attempts underway to manufacture anti-tumour vaccines.In this technique Epstein-Barr virus vectors mediate gene transfer into human B lymphocytes.⁷In the hope of being able to vaccinate against cervical cancer, cell death has been induced in human papilloma virus type 18 positive cancer cells. The E6 oncogene is targeted by E6 siRNA.⁸
Other areas of research include:

• Arthritis⁹
• Cartilage defects¹⁰
• Pulmonary tissue injury¹¹
• Type 1 diabetes¹²
• Traumatic brain injury¹³

A recent trial, approved by the American Food and Drugs administration, is for the treatment of Parkinsons Disease. This is a phase 1 clinical trial with 11 patients already enrolled. They are aiming to produce the neuroprotective and restorative subthalamic glutamic decarboxylase. There have been no adverse events reported to date.¹⁴