Novel strategies for enzyme replacement with gene therapy in models of Parkinson’s disease

Abstract: Parkinson’s disease is a brain disorder characterized by loss of dopaminergic neurons in the midbrain, resulting in the characteristic motor symptoms: resting tremor, rigidity, akinesia and postural instability. Symptomatic treatment is based on reconstituting the loss of dopamine in the brain, primarily by oral administration of its precursor L-DOPA. Although this drug is effective in the early years of treatment, patients experience troublesome side effects over time that are believed to develop at least partly due to route of administration in addition to the progressive worsening of the disease. An alternative to oral L-DOPA administration is continuous DOPA synthesis with gene therapy. This can be achieved by expressing enzymes needed for DOPA synthesis in neurons in the striatum, where the loss of dopamine is most profound in Parkinson’s disease. The addition of two enzymes is necessary for efficient synthesis of DOPA in striatal neurons, namely tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1), and these can be delivered with an adeno-associated virus (AAV) vector. In the first two studies in this thesis, a new AAV vector design including TH and GCH1 was developed and validated in a rat model of Parkinson’s disease. Expression of this vector in parkinsonian rats resulted in complete recovery in different tests of motor function and was dose dependent. The therapeutic efficacy in this model was associated with increased levels of DOPA and dopamine in the striatum and histological analysis showed widespread expression of both enzymes. The vector was also tested in a non-human primate model of Parkinson’s disease that showed safety but not efficacy. The aim of the third study in this thesis was to implement a regulation mechanism for the continuous DOPA synthesis approach. This is a long-standing goal for gene therapy and the strategy used here was based on destabilizing domains (DD) and the ligand trimethoprim (TMP). Combining DD regulation with the continuous DOPA synthesis approach resulted robust efficacy in parkinsonian rats when given TMP. In addition, the basal expression in absence of the TMP ligand was minimal. Efficacy of this approach should now be validated in nonhuman primates before clinical translation can begin.