Significant research and several clinical trials have been done regarding the efficacy of gene therapy in Parkinson’s disease (PD). While many claims have been made involving numerous wonder cures for PD, gene therapy may hold the exciting possibility of not only delaying disease progression but also halting it completely.
Parkinson’s disease is progressive a neurodegenerative disorder where there is steady loss of brain cells of the substantia nigra involved in the production of the chemical neurotransmitter dopamine. Refer to Parkinson’s Disease Brain Chemistry for more information.
Medication with levodopa and other related drugs are the mainstay of treatment which can only offer temporary relief but no lasting cure. Research focused on developing other methods, such as gene therapy, which can help to treat or modify the disease process more effectively is continuously being done.
What is gene therapy?
Simply, gene therapy involves the introduction of healthy genes into a person with defective genes which may be the cause of a certain medical problems, such as Parkinson’s disease. The healthy genes are expected to work on the target cells and cause changes in them that will trigger them to resume producing dopamine and thus reduce symptoms or altogether halt the progress of PD. For this new therapy to be acceptable, the benefits should be long-term and without other complications or side effects.
Virus Carriers for Gene Therapy
During gene therapy, genetic material is transferred to the recipient using a vector virus which is considered not to be harmful to humans. Either recombinant adeno-associated virus type 2 (rAAV2) or lentivirus vectors have been used
in clinical trials in an attempt to trigger :
- Increased dopamine levels via increased neurotransmitter production.
- Modulation of the neuronal phenotype.
Increased dopamine production may be achieved by means of the first two methods, by direct delivery of genes involved in neurotransmitter production (amino acid decarboxylase, tyroxine hydroxylate and GTP (guanosine triphosphate) cyclohydrolase 1).
To bypass the degenerating nigrostriatal pathway, rAAV2 is used to transfer glutamic acid decarboxylase (GAD) to the subthalamic nucleus (STN). GAD acts as a catalyst in the production of a neurotransmitter called GABA, which acts as a direct inhibitor on the overactive cells in the STN.
Protection of the degenerating nigrostriatum (neuroprotection) is hoped to be achieved by striatal delivery of rAAV2 containing the neuroprotective gene neurturin.
Stem Cells and Gene Therapy for Parkinson’s Disease
Research continues so as to find newer and more effective means of delivering genes and sustaining their effects. Using stem cells as a means of introducing genes is being studied. It is claimed that hematopoietic stem cells from blood and bone marrow can be genetically transformed outside the body and then re-introduced into the patient so as to eliminate the necessity of repeated introduction of gene into the patient but results have not been too promising. Embryonic stem cells (because of their versatility) may be a better alternative to hematopoietic stem cells but ethical issues may have to be looked into.
Although studies done so far seem promising regarding gene therapy as a means of treating PD, longer follow-up still needs to be done to determine if improvements can be sustained over a prolonged period. It is to be hoped that gene therapy will be more effective and less invasive than the current medical or surgical treatments available, with less side effects.