Gene Therapy for Parkinson’s Disease - Viral Vector Approaches in Neurobiology and Brain Diseases - page 175

to provide disease-modifying therapies. From all the potentially
therapeutic genes tested in PD preclinical models only glial cell line-
derived neurotrophic factor (GDNF) and associated trophic factor
family, such as neurturin, have been shown to provide concomitant
neuroprotection and regenerative axonal regrowth abilities using
gene therapy approaches, as well as enhance dopamine function.
Based on the potential observed in preclinical research, clinical trials
for GDNF and neurturin have been attempted. In the case of
GDNF, the clinical trials were ultimately unsuccessful [ 18 - 20 ]. In
addition to the clinical trials, a preclinical study in primates using a
prolonged GDNF infusion strategy to the striatum caused weight
loss and multifocal cerebellar Purkinje cell loss [ 21 ]. Consequently,
no further clinical trials using GDNF infusions or GDNF gene ther-
apy approaches were carried out. Neurturin, another potent neuro-
trophic factor, has been used to develop a disease-modifying gene
therapy for PD as an alternative to GDNF. Neurturin also showed
great promise in preclinical PD models [ 22 - 24 ]. However, the
gene therapy approach using adeno-associated vectors to deliver
neurturin was not successful. Although the approach was safe, it
was not effective in treating patients suffering from PD [ 25 ].
Subsequent analysis of patient brains showed that there was a very
limited expression of neurturin in the putamen and very little neur-
turin was found in the SNpc of the patients [ 26 ]. Taken together,
the clinical trials for PD using neurotrophic factors indicate that
gene therapy approaches need to be localized and regulated.
Unwanted side effects from gene therapy may occur if a range
of cell types or the wrong cell types are targeted. Although the cell
specifi city can be greatly increased by using an endogenous cell-
specifi c promoter such as the neuron-specifi c enolase (NSE) or glial
fi brillary acidic protein (GFAP) promoters [ 27 ], there is still a need
for promoters that are relevant in PD. Transcriptome data from
parkinsonian patients can be used to identify genes that are highly
expressed in PD and the promoters of these genes could potentially
be used in vectors for PD gene therapy. By using a promoter from
a gene showing a high expression in PD, transgene expression may
be directed to the cells that are affected by the disease.
This chapter provides a methodology on how PD relevant pro-
moters can be selected from patient microarray data, evaluated in
vivo and how specifi city can be further enhanced by adding
microRNA (miRNA) target sites. miRNA are 22 nucleotide
stretches that mediate posttranscriptional gene regulation by tar-
geting messenger RNA containing complementary sequences for
degradation [ 28 ]. Since miRNAs are expressed in a cell-specifi c
manner, Brown and colleagues observed miRNA could be used to
regulate viral tropism [ 29 ]. Briefl y, if the target sequence of a
miRNA is present in the gene or untranslated region (UTR) of a
messenger RNA transcribed from a viral vector, the messenger
RNA will be degraded in a cell that expresses the specifi c miRNA,