Gene therapy by enzyme replacement for Parkinson’s disease Optimization of continuous DOPA delivery and development of a candidate vector for clinical application

Sammanfattning: The introduction of L-DOPA pharmacotherapy revolutionized the treatment of Parkinson’s disease close to fifty years ago. Before the discovery of dopamine as a neurotransmitter and its involvement in the disease, these patients had very little aid from medication and were bound to a life without movement. The dopamine neurons in the midbrain are a key player in the circuitry that is responsible for the initiation and control of movement. When the loss of dopamine neurons and especially their projection into the forebrain, becomes severe, the patients start to experience rigidity, postural instability and a gradual loss of the ability to move. L-DOPA is converted in the brain into dopamine and is very efficient at providing symptomatic relief. Unfortunately, within five to ten years, most patients start to experience side-effects, mainly the emergence of involuntary movements, so called dyskinesias. With time, the dose of L-DOPA needed to induce dyskinesias decreases to such an extent that the therapeutic window could be completely lost. These side-effects are thought to develop due to the intermittent, pulsatile delivery method of the drug. In this thesis, I have worked on a new method of providing a continuous DOPA production in the brain using adeno-associated viral vector mediated gene therapy. By providing the genes tyrosine hydroxylase and GTP cyclohydrolase 1 which code for the two enzymes required for DOPA production, we have been able to prevent the emergence of dyskinesias but also to provide symptomatic relief far more efficiently than the peripheral L-DOPA administration in research animals. A second goal of the thesis has been to pave the way to a clinical trial of these viral vectors for application in Parkinson’s disease. Towards this goal, I have investigated the role of the serotonergic system for the efficacy of the therapy and explored the usefulness of this treatment in advanced stage disease. Moreover, I have designed a novel viral vector construct that can mediate the expression of both genes from a single vector, and studied its effect in both rodents and monkeys. Based on the experimental studies presented in this thesis, I conclude that DOPA replacement by gene therapy has a true potential as a treatment for Parkinson’s disease patients at all stages of the disease and that the next step towards a clinical trial should be initiated.