Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor and cognitive disturbances affecting 6.1 millions of people. This number is constantly growing and PD is bound to affect much higher number of people in the future. Current treatments provide only symptomatic relief, produce serious side effects and lose their efficiency after a long-term use. Therefore, alternative approaches are necessary to improve the PD treatment, such as gene therapies. Here I review the state-of-the-art gene therapies for Parkinson’s disease.
Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor and cognitive disturbances. PD affects around 6.1 millions of people worldwide (0.08%) and the number is constantly growing[1]. PD occurs sporadically in the population and less than 10% of cases are inherited. Exact causes of PD are not defined, but underlying pathophysiology includes misfolding and aggregation of α-synuclein, disruption of the autophagy-lysosome system, mitochondrial dysfunction, endoplasmic reticulum (ER) stress, and dysregulation of calcium homeostasis. These factors promote the loss of dopaminergic neurons in the part of the brain called Substantia Nigra (SN). Reduction of dopaminergic signalling causes hyperactivity in another part of the brain called striatum and, as a consequence, triggers motor dysfunctions. Current treatments (such as dopamine substitute L-DOPA) provide only symptomatic relief, produce serious side effects and lose their efficiency after a long-term use[2]. Therefore, alternative approaches are necessary to improve the PD treatment, such as gene therapies.
Gene therapies using viral vectors is one of the promising approaches to treat or to slow down the PD. Viral vectors allow to implement somatic gene therapy by delivering DNA to target cells with significantly lower cell toxicity. The main strategies now to cure or reduce symptoms of PD are aiming at restoring dopamine synthesis, normalizing functioning of basal ganglia, enhancing neuronal survival and neuritogenesis or usage of DNA editing tools such as CRISPR to prevent PD[3,4]. Using viral vectors to promotes expression of targets listed above can last for long time and does not require additional re-application as in case of delivery of molecules using micropumps. Thus, it minimizes risk of infection, meanwhile, the desired effect of the treatment remains. Viral vectors have been used in clinical trials since 2005 to treat PD and showed high efficiency, feasibility and safety for using them in human patients, thus they can soon be entering the market.
AAVs encoding an overexpression of the enzyme converting L-DOPA to dopamine (aromatic acid decarboxylase (AADC)) showed an improvement in PD symptoms in rodents, monkeys and in human trials. They decreased the need for medication and did not trigger significant adverse effects. Expression of the three enzymes (TH, GCH and AADC) involved in dopamine synthesis in the striatum is also well tolerated and efficient at reducing the parkinsonian symptoms in animal PD models and in clinical trials. Adding vesicular monoamine transporter 2 (VMAT2) overexpression further stabilized the dopaminergic tonus enabling a more stable release of dopamine.
This type of viruses was used to reduce striatal hyperactivity by overexpressing glutamate decarboxylase (GAD) converting glutamate to inhibitory neurotransmitter - GABA. GAD overexpression using AAV-GAD injected into STN diminished parkinsonian symptoms in rats. Following clinical trials also showed improved UPDRS scores in patients.
Another group of viruses is dedicated to the stimulation of neuronal survival and neuritogenesis using neurotrophic factors (glial cell-line derived neurotrophic factor - GDNF, Neurturin - NRTN, Nurr1, Cerebral dopamine neurotrophic factor - CDNF, mesencephalic astrocyte-derived neural factor - MANF, Brain-derived neurotrophic factor - BDNF, Vascular endothelial growth factor – VEGF, Neuropeptide Y - NPY). Using viral constructs triggering overexpression of neurotrophic factors showed promising results in vitro and in vivo in animal models of PD. GDNF, Neurturin1, Nurr1, CDNF, MANF, BDNF, VEGF, NPY showed neuroprotective properties, improved survival of dopaminergic neurons in vitro. They also improved the motor behaviour in animal models of PD in most of the cases. However, in some studies GDNF overexpression caused some side effects, such as a weight loss. Nonetheless, clinical trials on AAV2-GDNF are approved, but the results have not yet been published. NRTN overexpression in the putamen or SN and putamen together had improved PD symptoms in phase 1 clinical trials, but failed to do so in phase 2. AAV-mediated overexpression of Nurr1 improved also preserved a large amount of striatal fibres in monkeys. Striatal expression of CDNF and MANF showed an improved locomotion in PD animal models in some studies, meanwhile nigral expression, in another study, was not so efficient. Two studies using AAV-BDNF in SN in showed a general improvement in locomotor activity with signs of neuroprotection only in one of these studies. However, chronic delivery of BDNF to the SN in healthy rats produces a hypodopaminergic phenotype. VEGF has neuroprotective effects in the CNS possibly, via increased angiogenesis, improvement of microcirculation, microglial proliferation and reduced apoptosis. However, VEGF is also implicated in signalling cascades involved in tumorous cell growth, thus, the dosage of VEGF should be carefully studied in animal models before advancing to human trials. NPY is elevated in basal ganglia of PD patients, possibly as a compensatory mechanism. NPY ligands showed promising results in other neurodegenerative diseases.
Mutations in several genes have been associated with both familial- and sporadic PD, including parkin, LRRK2, SNCA, PINK1, DJ-1, VPS35, DNAJC13, CHCHD2. Vectors encoding CRISPR-CAS9 constructs can be used to correct gene mutations responsible for focal disease activity. However, this approach has not yet been approved for trials.
Current targets for viral vectors are not able to tackle the source of the problem, sine the exact causes of PD are not known. Major players of cell loss (α-synuclein, autophagy-lysosome system, mitochondrial dysfunction, endoplasmic reticulum (ER) stress, calcium homeostasis) are not targeted directly by any of these strategies. The strategies restoring dopamine synthesis do not provide fine restoration of dopaminergic signalling that occurs locally in particular synapses. Those synapses vary significantly by their function but they get activated by unspecific increase of dopamine in the ambience. In striatum, the main cell type - medium sized spiny neurons (MSNs) divided into two major populations expressing predominantly only one type of dopaminergic receptors (D1 or D2). D1-positive MSNs project to direct pathway, meanwhile D2-positive MSNs project predominantly to indirect pathways. Imbalance of these pathways in PD is the major cause of parkinsonian symptoms[5]. However, precise control of either pathway is not possible with such viral vectors. Nonetheless, this approach reduced the quantity of L-DOPA in patients and improving PD symptoms. Similar problem is triggered by GAD-overexpressing viruses, where GABA-mediated inhibition of D1- and D2-positive MSNs is again unspecific.Viral vectors delivering various trophic factors aiming to neuroprotection and to restore neuretogenesis showed less consistent positive results in clinical trials, meanwhile they triggered more pronounced side effects. Although, this approach has a potential and currently used in ongoing clinical trials, this type of vectors should be considered with greater care since theirs effect is less direct and affects complex signalling cascades controlling various functions in the cells. This can explain observed side effects and it highlights the possibility that this type of viruses can trigger other long-term side effects in patients that cannot be easily diagnosed. CRISPR technology can be used to identify the genes involved in PD development and to correct it. However, the efficiency of CRISPR is highly variable (10-25% in neurons) thus it cannot be a complete cure.
The viral strategies restoring dopamine synthesis represent the most promising approach, but it is a temporary solution that has a potential to stay on the market until more robust solution will be found. Considering that the clinical trials started in 2005 they have the best chance to enter the market first as a symptomatic treatment. Overall, gene therapies for PD offer a lot of hope to people suffering from it.
Please, if you like this article, please share it with your friends, subscribe to our news and free networking events here and leave your comments below.
[1.] Ray Dorsey, E. et al. Global, regional, and national burden of Parkinson’s disease, 1990–2016: a systematic analysis for the Global Burden of Disease Study 2016. Lancet Neurol. 17, 939–953 (2018).
[2.] Massano, J. & Bhatia, K. P. Clinical approach to Parkinson’s disease: Features, diagnosis, and principles of management. Cold Spring Harb. Perspect. Med. 2, 1–15 (2012).
[3.] Axelsen, T. M. & Woldbye, D. P. D. Gene therapy for Parkinson’s disease, an update. J. Parkinsons. Dis. 8, 195–215 (2018).
[4.] Coune, P. G., Schneider, B. L. & Aebischer, P. Parkinson’s disease: Gene therapies. Cold Spring Harb. Perspect. Med. 2, 1–15 (2012).
[5.] Lindgren, H. S., Ohlin, K. E. & Cenci, M. A. Differential involvement of D1 and D2 dopamine receptors in L-DOPA-induced angiogenic activity in a Rat model of parkinson’s disease. Neuropsychopharmacology 34, 2477–2488 (2009).