Gene therapy typically involves the use of a noninfectious virus (i.e., a viral vector such as the adeno-associated virus) to shuttle genetic material into a part of the brain. Approaches have involved the expression of growth factors to prevent damage (Neurturin – a GDNF-family growth factor), and enzymes such as glutamic acid decarboxylase (GAD – the enzyme that produces GABA), tyrosine hydroxylase (the enzyme that produces L-DOPA) and catechol-O-methyl transferase (COMT – the enzyme that converts L-DOPA to dopamine). No safety concerns have been reported but the approaches have largely failed in phase two clinical trials. The delivery of GAD showed promise in phase two trials in 2011, but while effective at improving motor function, was inferior to DBS. Follow-up studies in the same cohort have suggested persistent improvement.
