Gene therapy for Childhood Parkinsonism: Dopamine transporter deficiency syndrome

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Dopamine transporter deficiency syndrome (DTDS) is a devastating childhood neurotransmitter disorder. It clinically presents with progressive infantile parkinsonism-dystonia and is frequently misdiagnosed as cerebral palsy. The children are severely disabled with inability to control movement experiencing distressing hyperkinesia and dystonic postures. They progressively worsen with severe slowing of movements and muscle rigidity, termed parkinsonism. This results in severe neurodisability and is untreatable with medication or surgery, with a third dying by mid-adolescence. DTDS results from loss of function mutations the SLC6A3 gene coding for the Dopamine transporter (DAT). DAT functions to uptake dopamine to terminate dopamine neurotransmission and uptake dopamine for recycling and is physiologically expressed in specific brain regions (midbrain and striatum).There is medical need to develop impactful treatment for this untreatable childhood neurological disorders. We aim to develop a potential cure for DTDS by delivering Adeno associated virus (AAV) mediated hDAT gene therapy. This will result in expression of normally functioning DAT to restore dopamine homeostasis to improve motor function, health and neurodisability of DTDS children. Gene therapy for childhood neurological disorders is showing great promise. Adeno associated virus serotype 9 (AAV9) delivered intravenously rescued the Spinal muscular atrophy (SMA) mouse in 2010 and progressed to clinical trial within 4 years (NCT02122952). SMA type 1 is severe motor neuron disorder whereby affected babies do not achieve sitting and most die by 2years. Fifteen treated children have shown striking motor improvements, with 12 able to sit and some walking independently. Gene therapy is used to treat another childhood neurotransmitter disorder called Aromatic L-amino acid decarboxylase (AADC) deficiency, that is clinically similar to DTDS. Initially 4 children were treated by direct (stereotactic) injection of AAV2-AADC to a specific dopaminergic brain region called the putamen. This established clinical safety and all 4 children showed motor improvement. A subsequent phase II clinical trial has treated 18 children to date. Furthermore over 100 patients with Parkinson's disease, have safely received AAV2 gene therapy vectors delivered by stereotactic injection to the striatum or midbrain over the last decade. AAV2 gene therapies for Parkinson's disease have delivered dopamine synthesis genes to increase dopamine synthesis and provide good clinical evidence for the safety of AAV2 vectors and stereotactic injection. We previously undertook proof of principle gene therapy study for DTDS using the DAT-KO mouse model. This DAT-KO mouse recapitulates DTDS symptoms with early hyperlocomotion, 59% entering a Parkinsonian phase with bradykinesia, tremor, weight loss and reduced survival of 41%, by 5 weeks old. We delivered AAV9 gene therapy to neonatal DAT-KO mice by injection to the intracerebroventrical. This rescued the DAT-KO motor phenotype and survival. However with this approach, widespread intracranial expression of DAT was achieved, with significant off-target expression. We identified that off-target expression was associated with neuronal loss and reactive astrogliosis in the cortex. We therefore concluded that for clinical application, it is paramount to restrict hDAT expression to dopaminergic neurons. We aim to improve safety levels by changing AAV serotype to AAV2 that has a restricted expression profile compared to AAV9. We will also inject the vector into the dopaminergic brain regions affected in DTDS (midbrain and striatum). This is a logical step towards clinical translation whereby AAV2 and stereotactic injection methods have established clinical safety. We will evaluate expression, efficacy, and safety of stereotactically delivered AAV2.hDAT in adult DAT-KO mice as proof of concept preclinical development of gene therapy for DTDS.