Gene therapy in Parkinson’s disease is a specific neurotransmitter ( dopamine ), protect the neural system, or the modification of genes that are related to the disease. Then these cells are transplanted to a patient with the disease. There are different kinds of treatments that focus on reducing the symptoms of the disease but there is no cure.
Parkinson’s disease (PD) is a progressive neurological condition resulting from the death of the cell that contains and produces dopamine in substantia nigra . People with PD may develop disturbance in their motor activities. Some activities can be tremor or shaking, rigidity and slow movements ( bradykinesia ). Patients may eventually present certain psychiatric problems like depression and dementia .  Current pharmacological intervention consists of the administration of L-dopa, a dopamine precursor. The L-dopa dopamine increases dopamine production of the remaining nigral neurons.  Other therapy is the deep brain electrical stimulation to modulate the overactivity of thesubthalamic nucleus to the loss of dopamine signaling in the stratum.  However, with this treatment, the number of substantia nigra neurons decreases so it becomes less efficient. 
These treatments try to reduce the symptoms of dopamine but they do not cure the disease. The new treatments for PD are in clinical trials and most of them are centered on gene therapy . With this, researchers expect to compensate for the loss of dopamine or to protect the dopamine neurons from degeneration.  The pharmacological and surgical therapies for PD focus on compensating the ganglia dysfunction caused by the degeneration of the dopaminergic neuron from substantia nigra. 
Gene therapy background
There are many new PD treatments in which the dopamine neurons from degeneration can be compensated for. There are some important reasons for focusing on gene therapy as a treatment for PD. First of all, there is no cure for this disease. Secondly, some genes have been identified that can modulate the neuron phenotype or act as neuroprotective agents. Also, the environment of the brain can not afford repeated injections into the region where the substance meets the striatum, the nigrostriatum. Therefore, gene therapy could be a single treatment appealing, viral vectors used in the therapy are diffusible and able to do transduction of the striatum. 
Gene therapy bases
The main idea of the gene therapy is to create new generations of cells that produce neurotransmitter (dopamine) and then transplant these cells to patients with PD.  This is because the neurons can not proliferate nor be renewed; and replace lost neurons it is a process that is currently going under investigation. Also, the use of embryonic dopaminergic cells can not be used because these cells are difficult to obtain  and can be modified for somatic cells , not germline.  With the modifications of the transplanted cell, there can be a change in the expression of the genes or normalize them. 
Types of gene therapy
There are several types of gene therapy. There are therapies for symptomatic approaches like the generation of ectopic L-dopa , the full ectopic dopamine synthesis, the ectopic L-dopa conversion or the use of glutamic aciddescarboxylase (GAD). Also there are disease modifying therapies like NTN or GNDF (glia cell line-derived neurothrophic factor), the regulation of the α-synuclein and Parkin gene expression. Currently, there are a number of vector-based tools that have been used.  In the different types of gene therapy, the investigations are encoding enzymes that are necessary for dopamine synthesis, such as tyrosine hydroxylase ,GTP cyclohydrolase 1 and AADC. 
A symptomatic approach is a treatment focused on the symptoms of patients. The first one, consists in the ectopic dopamine synthesis. Here, the production of ectopic L-dopa in the striatum is another alternative gene therapy. This therapy consists of transferring TH and GTP cyclohydrolase 1 genes into the MSNs because the endogenous AADC activity is able to convert L-dopa into dopamine.  In an experiment in 2005, using tyrosine hydroxylase (TH) and GCH1 altogether with vectors, they could provide normal levels of L-dopa to rats. The results of this experiment showed reduced dyskinesia by 85% and the reversal of abnormal projections in the strium using TH-GCH1 gene transfer. 
Dopamine synthesis can be fully ectopic. In this case, the AADC enzyme is in charge of converting the levodopa to dopamine. In Parkinson disease, the loss of neurons from the nigrostriatum leads to the inability to convert levodopa to dopamine. The goal of AAV2-hAADC is to restore normal levels of AADC in the striatum so it could be more conversion of levodopa, and thus reducing levodopa-induced dyskinesia.  Using the gene therapy, in 2012, an experiment was accomplished with primates testing tyrosine hydroxylase (TH) transgene in primate astrocytes. Gene therapy was made with TH full-length cDNA using rat TH. The results showed behavioral improvement in the monkeys that received the plasmid, unlike the monkey control. 
Another type is the ectopic L-dopa conversion in which they use a gene replacement enzyme that can be used to increase the efficacy of the pharmacological L-dopa therapy by using AAV vectors. This AAV vectors have been designed to AADC coding sequence to the MSN (medium spiny neurons) in the striatum to be able to convert L-dopa into dopamine. 
Other causes of gene therapy are the use of glutamic acid decarboxylase (GAD) expression in the subthalamic nucleus. This is a gene replacement therapy that can be used to increase the efficacy of L-dopa therapy by using AAV vectors. This AAV vectors have been designed to deliver the AADC coding sequence to the MSN in the striatum to be able to convert L-dopa into dopamine.  A phase 2 study, published in the journal Lancet Neurology Parkinson, says that a gene therapy called NLX-P101 dramatically reduces movement damage. In this study, they used glutamic acid decarboxylase (GAD). They introduced genetic material in the brain related to motor functions. The symptoms included tremor, stiffness and difficulty in movements; and were improved in half of the group in gene therapy, while in the control group, 14% improved them. 
There are therapies in development based on the modification of the disease. The first one is the neurotrophic factors gene delivery. In this therapy, GNDF or NTN are used to protect the system. GNDF is a factor of TGFβ superfamily, is secreted by astrocytes ( glia cells that are in charge of the survival of the dopaminergic neurons ) and is homologous to NTN, persephin and artemin. Preclinical studies of the nigrostriatal dopaminergic in relation to Parkinson’s disease system have shown that GNDF and NTN are very potent neuroprotective agents. Another type in the disease’s technical modification is the synuclein silencing. Some cases of PD are related to polymorphisms in the α-synuclein promoter and also in the multiplication of the locus that carries the α-synuclein gene. Therefore, trying to down-regulate the α-synuclein expression could impact the development of the disease. There have been several viral vector-based gene delivery systems that interfere with α-synuclein expression, and they depend on the interference of the RNA (destabilizing the α-synuclein RNAm) and / or the block protein translation (using short hairpin RNA gold micro RNA directed against the α-synuclein RNAm sequence). 
The discovery of the Parkin is another type of modification of PD. The Parkin gene is linked with mutations associated with autosomal recessive juvenile parkinsonism (previous state of Parkinson’s with the typical symptoms and pathology but with slow progression). The mutations in Parkin are responsible for the development of autosomal recessive juvenile parkinsonism. 
New projects and investigations
More gene therapy trials have been conducted for PD (with the adeno-associated virus 2 gene), the objectives and strategies used for the researches are clear, the research tries to translate the experience of the trials and try to improve the development of new technology for the gene therapy of PD. 
- Jump up^ “Parkinson’s disease: Diagnosis and management in primary and secondary care” . National Institute for Health and Care Excellence. June 2006. Archived from the original on 26 January 2013 . Retrieved 12 April 2013 .
- ^ Jump up to:a b c d Feng, RS; Maguire-Zeiss, KA (March 2010). “Gene therapy in Parkinson ‘s disease: rationale and current status” . CNS Drugs . 24 (3): 177-92. doi : 10.2165 / 11533740-000000000-00000 . PMC 2886503 . PMID 20155994 .
- ^ Jump up to:a b c d e f g h Coune Philippe G; Schneider, Bernard L .; Aebischer, Patrick (April 2012). “Parkinson’s Disease: Gene Therapies”. Cold Spring Harbor Perspectives in Medicine . 2 : a009431. doi : 10.1101 / cshperspect.a009431 .
- ^ Jump up to:a b Horellou, Philippe; Mallet, Jacques (October 1997). Ex Vivo Gene Therapy for Parkinson ‘s Disease: Intracerebral Transplantation of Genetically Modified Cells . Molecular Neurobiology . SpringerLink. 15 (2): 241-256. doi : 10.1007 / BF02740636 . Retrieved 12 April 2013 .
- Jump up^ Palfi, SP; Gurruchaga, JM; Ralph, GS; Lepetit, H .; Lavisse, S .; Buttery, PC; Watts, C .; Miskin, J .; Kelleher, M .; Deeley, S .; Iwamuro, H .; Lefaucheur, JP; Thiriez, C .; Fenelon, G .; Lucas, C .; Brugieres, P .; Gabriel, I .; Abhay, K .; Drouot, X .; Tani, N .; Kas, A .; Ghaleh, B .; Corvoisier, P .; Dolphin, P .; Breen, DP; Mason, S .; Guzman, NV; Mazarakis, ND; Radcliffe, PA; Harrop, R. (2014). “Long-term safety and tolerance of Pro Savin , a lentiviral vector-based gene therapy for Parkinson’s disease: A dose escalation, open-label, phase 1/2 trial”. The Lancet . 383 : 1138-1146. doi : 10.1016 / S0140-6736 (13) 61939-X . PMID 24412048.
- ^ Jump up to:a b c d Forsayeth, J .; Bankiewicz, KS; Aminoff, MJ (2010). “Gene therapy for Parkinson’s disease: Where are we going?” Expert Review of Neurotherapeutics . 10 (12): 1839-1845. doi : 10.1586 / er.10.161 .
- Jump up^ Carlsson T, C Winkler, C Burger, Muzyczka N, RJ Mandel, Cenci A, Björklund A, Kirik D (March 2005). “Reversal of dyskinesia in an animal model of Parkinson ‘s disease by continuous L – DOPA delivery using rAAV vectors”. Brain . 128 (3): 559-69. doi : 10.1093 / brain / awh374 . PMID 15659429 .
- Jump up^ Campos-Romo, Aurelio; Ojeda-Flores, Rafael; Moreno-Briseño, Pablo; et al. (2012). “Behavioral improvement in MPtP-treated nonhuman primates in the HALLWAY task after transfer of tH cdnA to host astrocytes” (PDF) . Acta Neurobiol. Exp. Polish Neuroscience Society – PTBUN, Nencki Institute of Experimental Biology. 72 : 166-176 . Retrieved 15 April 2013 .
- Jump up^ “Gene Reversing Therapy Symptoms of Parkinson’s Disease” . Science Daily . Retrieved 12 April 2013 .
- Jump up^ Kitada T, Asakawa S, Hattori N, Matsumine H, Yamamura Y, Minoshima S, Yokochi M, Mizuno Y, Shimizu N (April 1998). “Mutations in the parkin cause cause autosomal recessive juvenile parkinsonism”. Nature . 392 (6676): 605-8. doi : 10.1038 / 33416 . PMID 9560156 .