A gene is a piece of DNA, which carries unique genetic information that is often but not always used in our cells to make new proteins. We each have around 23,000 genes and these determine the physical and mental traits we inherit from our parents. Genes also play a part in how likely we are to develop a particular illness or condition. Sometimes the genes we inherit are defective – which can lead to a range of disorders – or we may inherit the correct genetic information but, as a natural part of the ageing process (or perhaps as a result of environmental factors) the proteins that are coded by the genes may deviate from normal, resulting in illness or disease.

Gene therapy is part of a rapidly growing field of medicine termed “biologicals” which refers to a new approach to treating or preventing certain conditions using genes instead of traditional drugs as we commonly use today. Gene therapy can take several forms, depending on the condition it aims to treat. Most commonly it involves inserting a healthy gene into the cells of a patient with the aim of treating their disease. This can work by the new gene instructing the body to build proteins that are missing but necessary for the normal functions of an organ. It may also repair or prevent cells from dying. Other forms of gene therapy involve inserting a copy of a gene to replace a defective one or deactivating a gene that is not functioning properly.

Scientists believe that gene therapy may be effective both for conditions caused by defective genes and for disorders that have an unknown genetic cause or are only partially caused by defective genes, as is the case with Parkinson’s. This, they hope, will either halt or even reverse the progression of the condition or bring about a cure.

Although it is widely agreed that Parkinson’s is not usually inherited through the genes our parents pass on, researchers believe that gene therapy will one day be able to prevent the death of dopamine-producing cells in the brain and help to revitalise dying cells in the early stages of the disease. This is important as current treatments tend to become less effective over time and only control symptoms – they do not slow or halt the progression of Parkinson’s. Gene therapy has the potential to alter the way neurons (nerve cells) in the brain work.

The aim of current research is to be able to deliver specific genes directly into neurons in the affected area of the brain. Three gene therapy approaches have already entered clinical testing: 

1. Proteins that increase levels of dopamine production in the brain - these trials are testing whether gene therapy will instruct brain cells not normally damaged in Parkinson’s patients to produce dopamine and help recover the lost function resulting from the loss of dopamine neurons. If successful, this approach can either replace medications such as levodopa or dopamine agonists, or make levodopa more effective. Scientists believe that achieving normal levels of dopamine without fluctuations many experience will result in better management of symptoms than is currently possible.
2. Restoring nerve cell activity in the brain - in Parkinson’s, nerve cells in the basal ganglia area of the brain become overactive. This part of the brain helps to control movement so any changes in this region will affect movement. For some time, scientists argued that this over-activity could be lessened by increasing the activity of enzymes used to produce GABA - a neurotransmitter normally produced in the brain. However, the results of early trials did not support the efficacy of the approach and have been discontinued. New ways to control the abnormal activity (by for example using light responsive genes/proteins) are under development and are likely to form the second generation approaches in this area.
3. Growth factors – growth factors can help protect neurons from further damage and also promote their rejuvenation and survival. Two related factors have been tested separately in a series of clinical trials using different methods of administration. Although there were signs of change in some of the patients, so far none of the trials have provided statistically significant effects. Improvements in the design of the constructs, mode and place of delivery, as well as a better assessment of the most suitable patients to benefit from this type of treatment, are expected to form the basis of new trials in the near future.

Gene therapy generally uses a carrier molecule, called a vector, to deliver the therapeutic gene to the target cells. At present, the most commonly used vectors are viruses that have been genetically modified to carry therapeutic genes, although other methods are under development. Before introducing the viruses, scientists ensure that these particles do not have the capacity to cause infection and travel around the body, making them safe tools to express genes only in the intended areas of the brain. Thus, each viral particle can only infect one cell and then disappear. The gene product, on the other hand, will be expressed for a very long time, probably for the lifetime of the patient.

In the case of Parkinson’s, a viral vector – often the adeno-associated virus or a lentivirus – is injected into the brain. The vector passes its genetic material to the cells so that the therapeutic gene can work locally in the brain where it is needed.

Researchers believe that gene therapy will be useful for a number of people with Parkinson’s, irrespective of whether their condition has been genetically caused. However, as with most treatments, it won’t be suitable for everyone. As part of the clinical trial process, scientists will establish who this treatment will suit and those for whom it is not recommended.

As with all treatments, there are some risks. It is thought that there may be side effects within the central nervous system that relate to long-term, high levels of exposure to therapeutic genes, or there may be an immune response to the treatment. Current research efforts are directed at implementing further measures to reduce the probability of these risks, even if they are unlikely.

Gene therapy is in clinical development and is therefore not yet widely available as a treatment option. Ongoing clinical trials involve a small number of patients and have strict adherence to specific protocols. These are run from only a few clinical centres. Further research and wider clinical trials are, of course, required to obtain approval from regulators before this type of treatment can be more widely used.

Key aspects researchers are addressing currently include:

• the possibility of regulating the amount of therapeutic material that is present at any time so that the best results can be achieved
• the possibility of targeting brain cells through less invasive delivery routes, e.g., by injections into the systemic circulation or the cerebrospinal fluid
• ensuring that the viruses introduced are safe and do not induce an immune reaction
• ensuring that the treatment itself does not induce unexpected detrimental effects due to the ability of inserted therapeutic genes to alter things that were not intended to be changed.

We would like to thank Professor Deniz Kirk (Lund University, Sweden) for his help in reviewing this information.

Articles from Parkinson's Life online magazine

• Parkinson’s gene therapy could improve motor symptoms