Experimental and clinical application of plasmid DNA in the field of central nervous diseases

Novel therapeutic strategies utilizing plasmid DNA (pDNA) have been sought for non-treatable neurological disorders, such as ischemic stroke, Parkinson disease (PD), Alzheimer disease (AD), and multiple sclerosis (MS). One strategy is to induce overexpression of growth factors, such as vascular endothelial growth factor (VEGF), glial cell-line derived neurotrophic factor (GDNF), and hepatocyte growth factor (HGF), in the brain. Since ischemic stroke, PD, and AD show damage of neurons, the transfer of pDNA encoding these genes has been examined and shown to protect neurons from damage, associated with a better behavioral outcome. These growth factors have also been shown to accelerate angiogenesis, neurite outgrowth, and neurogenesis in the brain, and overexpression of these factors showed therapeutic effects in cerebral ischemia in rodents. Another application of pDNA is as a "DNA vaccine" to induce immunity against amyloid Aβ in AD, which requires a predominantly Th2 response to avoid autoimmune encephalomyelitis evoked by a Th1 response. Since the combination of pDNA and special devices and/or modification of pDNA could induce a predominantly Th2 response to a targeted antigen, a pDNA-based vaccine would be ideal for AD. Interestingly, pDNA could also induce immune tolerance, and pDNA-based vaccines to induce immune tolerance to autoimmune antibodies have been extensively examined in an animal model of MS. Based on the results, a pDNA vaccine has already been tried in MS patients and reported to be safe and partly effective in phase I/II clinical studies. In this review, we discuss the potential and problems of pDNA-mediated medicine in neurological disorders.