Parkinson’s disease (PD) is the second most prevalent neurological condition after dementia [1, 2]. According to an analysis by the Global Burden of Disease Study (2016), which was published in Lancet Neurology two years later, the probability of having PD increases with aging, with rates ranging from 41 to 100,000 in those in their 50s and up to 1,900 per 100,000 in those aged 80 and above [3]. Furthermore, it was reported that a prevalence percentage increase of 54.9% to 66.2% occurred from the year 1990 to 2021 [4]. Within this chronic, severe neurodegenerative condition, which is recognized by a rapid degeneration of dopamine (DA)-associated neurons in the pars compacta of the substantia nigra par compacta (SNpc), an intracellular protein called synuclein alpha (SNCA) is broadly dispersed [5, 6].

PD is a complicated condition that is triggered by genetic, environmental, or age variables. Motor manifestations develop whenever DA within the basal ganglia is insufficient [7]. People with PD often exhibit olfactory impairment, rapid eye movement (REM) sleep behavioral disorder, and autonomic nervous system (ANS) problems such as orthostatic hypotension, bladder malfunction, and constipation, coupled with the usual motor warning signs of rigidity, resting tremor, bradykinesia, and gait abnormalities, as well as lack of postural stability [8].

PD normally progresses until it leaves its patients completely debilitated. The rate of progression and its course vary among patients. The course is relatively benign in some patients with little disability after twenty years, and may be more aggressive among others who may be severely disabled after ten years. Untreated PD worsens over the years, and it may lead to the deterioration of all brain functions and premature death.

According to [9], the breakdown of DA pathways that connect the SNpc with the striatum is thought to be caused by an aberrant buildup of fibrillar SNCA. This leads to a lack of DA and poor motor activity.

The gold standard for treating PD motor symptoms is levodopa (L-DOPA) [10, 11]. In order to increase L-DOPA availability in the brain, carbidopa functions as a decarboxylase inhibitor [12]. PD drugs have a lot of adverse effects (AEs). The most frequent AEs of the combination of L-DOPA and carbidopa are nausea, compulsive gambling, depression, low blood pressure, hallucinations, and irregular sleep patterns.

L-DOPA treatment-induced off symptoms and dyskinesia are still unsolved; according to one study, off symptoms and dyskinesia affect 55.9% and 13.5% of the study group, respectively [13]. It’s unclear how long-term L-DOPA medication causes dyskinesia [14]. It underscores the essential need to investigate novel therapeutic options for the medical management of PD. By suppressing or delaying DA-producing cell death within the brain, disease alteration and neural protection are key therapy options to reduce PD symptoms related to movement [15]. L-DOPA is one of the only short-term PD drugs currently on the market [16].

This review aims to explore the advancing role of gene therapy in PD treatment, with an emphasis on its ability to modify PD progression, tackle underlying molecular and genetic mechanisms, and surmount the challenges of current symptomatic treatments. The objective of this review is to highlight recent advancements, current methods and strategies, and future perspectives, coupled with the evaluation of the limitations, ethical concerns, and clinical outcomes associated with its uses in the management of PD.

The etiology of PD constitutes a cellular buildup of the SNCA gene in insoluble particles, which damages cells [17]. Depletion of dopaminergic neurons leads to SNCA assembly, disrupting several pathways in the SNpc that rely on the dopaminergic tone. Because of this imbalance, the striatal projections in the neural circuits that contribute to the Parkinsonian phenotype diverge. PD is mostly brought on by a mix of both external and genetic variables. Subsequently, it is considered that PD is multidimensional rather than the outcome of a single, unambiguous cause.

Pharmacotherapy and functional neurosurgery are the two categories of treatment for PD. There are two forms of pharmacotherapy: dopaminergic and non-dopaminergic methods. Varieties of dopaminergic treatments for PD include monoamine oxidase (MAO) inhibitors, DA receptor agonists, and catecholamine-O-methyl transferase (COMT) inhibitors [11, 14] (Table 1). Patients’ motor fluctuations can be minimized by employing COMT and MAO inhibitors. The mainstay of modern therapy for PD consists of L-DOPA-based medications. L-DOPA-based medications work by replacing the DA that has been depleted in the striatum. Since L-DOPA, the precursor of DA, can cross the blood-brain barrier (BBB), it is employed for PD treatment. Following its passage through the BBB, 3,4-dihydroxyphenylalanine (DOPA) decarboxylase transforms L-DOPA into DA. Serious motor impairments could arise from peripheral, extra-central conversion by the enzyme. MAO-B inhibitors selectively raise DA within synaptic clefts, block catabolism of DA, increase DA signaling, and decrease MAO-B activity in the brain. These drugs have inverse correlations with striatal dopaminergic activity. Dopaminergic activity rises as the MAO-B inhibiting enzyme diminishes. This family of drugs can occasionally be appropriate to ease initial symptoms of PD.

In PD, anticholinergic medications are also given. They function via non-dopaminergic pathways. Acetylcholine activity is decreased by anticholinergic medications. The main purpose of these medications is to alleviate moderate motor symptoms, particularly muscle stiffness and tremors. Also, amantadine, a non-dopaminergic medication that blocks the N-methyl-D-aspartate (NMDA) receptor, has some therapeutic advantages [21]. It was first developed mainly to treat flu, but it has also been used to treat a number of PD conditions with symptoms that include resting tremors and stiffness in particular. Amantadine is titrated up from a lower beginning dose, just like L-DOPA [22]. This medication causes headaches, sweating, nightmares, and insomnia as side effects. Clinical trials and animal experiments are still being conducted on other non-dopaminergic drugs, such as antagonists of metabotropic glutamate receptors.

Two surgical options for treating PD are subthalamotomy and subthalamic nucleus deep brain stimulation (DBS), as well as pallidotomy and globus pallidus internus DBS [23]. Functional neurosurgery was utilized for DBS and lesions in severe PD [24]. While globus pallidus internus surgery directly reduces dyskinesia, subthalamic nucleus DBS has the benefit of reducing the dosage of dopaminergic drugs [25].

Numerous drugs or therapies have exhibited clear neuroprotection in DA-producing neurons when tested in vitro as well as in vivo; yet, a number of them did not pass clinical trials [26]. Novel therapies have been developed as a result of the previously outlined flaws of the drugs that are already available. Gene therapy has been utilized in clinical investigations for ameliorating a range of disorders in the human brain, and it might provide an option for PD treatment. It is a very innovative technique for treating PD. Gene therapy is highly versatile, and its numerous approaches are being tested for curing PD. A lot of research studies were either finished or are currently in progress.

In order to restore normal cellular function and provide therapeutic benefits, gene therapy attempts to replace or repair a damaged gene by injecting genetic material into specific cells [27]. By protecting and repairing dopaminergic neurons, gene therapy for PD seeks to slow or even reverse the course of the disease. The primary treatment for the motor symptoms of PD has been L-DOPA for about six decades. Despite being effective, this treatment may have a number of AEs, which may only be somewhat controlled by changing the drug’s formulation or taking it with other drugs that increase L-DOPA’s effectiveness and lessen its AEs.

DBS and continuous enteral or subcutaneous dopaminergic infusions are two alternate therapies that have been developed for the relief of symptoms of PD, although their efficacy is limited by their non-physiological mechanisms of action. Furthermore, when neuron loss worsens, these treatments become less effective since they don’t address the underlying neurodegeneration. To considerably enhance people’s well-being, a treatment that slows down the progression of the condition and/or gives a robust, long-lasting functional benefit is still required.

Adeno-associated viruses (AAVs) are small viruses with single-stranded DNA, and they are members of the family Parvoviridae [28]. Despite their small size, they are the most promising gene therapy vehicle due to their ability to generate long-term transgene expression and their clinical safety and effectiveness in transduction of quiescent and proliferative cells [28]. Almost all PD gene therapy clinical trials have employed AAV-based vectors. Effective AAV-based gene therapy requires tissue tropism, biodistribution, and sensitivity to neutralizing antibodies, all of which are largely determined by AAV serotypes [29]. AAV2 has also been used in a number of clinical trials. Due to its sustained expression in neurons and relative safety profile, it is presently considered a viable vector for gene treatment of neurodegenerative diseases like PD.

Targeting SNCA is considered one of the vital approaches in reducing the progression of PD. Furthermore, proteins in the DA synthesis pathway have been an obvious target for gene therapy. A gene therapy approach that enhances DA synthesis is promising for the treatment of PD. Also, targeting of trophic factor genes has been reported to result in the continuous production of trophic factors and increased DA turnover [30].

Gene therapy holds promise for a long-lasting impact that could not only enhance existing dopaminergic treatment but also change the progression of PD. Gene therapy’s main goal is to restore the synaptic function of the nigrostriatal dopaminergic pathway, which could help prevent long-term issues brought on by maladaptive brain remodeling and side effects from stimulation or medication. Figure 2 shows the different methods of gene delivery for PD.

AAVs have emerged as the most reliable vectors for central nervous system (CNS) gene therapy due to their ability to achieve long-term transgene expression, coupled with their established clinical safety and efficacy in transducing both quiescent and dividing cells. However, their use is constrained by the limited size of the therapeutic payload they can accommodate. Studies of PD gene therapy have primarily used AAV-based vectors.

Multiple genetic therapy techniques are currently being explored to restore the dopaminergic pathways [31]. AAV-L-amino acid decarboxylase (AADC), AAV-growth-derived neurotrophic factor (GDNF), AAV-glutamic acid decarboxylase (GAD), and striatal viral transduction have all been examined in studies in monkey models of PD [32, 33]. In non-human primates (NHPs), these studies demonstrated enhanced motor behavior, persistent gene transfer, robust expression, and regeneration of DA signaling in PD. AAV vector is the most commonly used vehicle in PD due to its biocompatibility, safety, delivery capability, and non-pathogenic characteristics. Although there are currently no active clinical studies or traditional medical treatments for PD, targeting microRNAs (miRNAs) seems to be a viable therapeutic strategy. One promising strategy appears to be the development of PD therapies using miRNAs that target and inhibit the production of SNCA. It has been demonstrated that in mice with sporadically produced SNCA, AAV-induced SN synthesis of short hairpin RNA (shRNA) targeting SNCA reduces neurological impairments [34].

Additionally, the SN of the adult male Lewis rats receives AAV-shRNA targeting SNCA, which lowers SNCA protein levels by roughly three-quarters and shields the animals from the toxicity of mitochondrial rotenone [35], as indicated in Table 2.

The efficacy of delivering the typical glucocerebrosidase 1 (GBA1) gene via an AAV vector to various animal experimental models of PD or PD-GBA has been shown by multiple independent researchers, as shown in Table 2. These studies include various genetic studies involving models with SNCA or GBA1 gene mutations [39, 40]. In the majority of these in vivo investigations, the AAV-GBA1 vector was injected into the mouse’s CNS. In these several PD models, it has been demonstrated that increasing GCas and its functionality via the administration of a GBA1 vector reduces irritation and aggregated SNCA formation. This research indicated improvements in motor behavior, secure transmission of genes, robust expression, and recovery of DA communication [43]. There aren’t any human trials for disease-modifying compounds that target DJ-1 at the moment. Nonetheless, some intriguing preclinical research could result in future clinical uses.

Several investigations using rat PD models have demonstrated the effectiveness of recombinant wild-type DJ-1 in preserving dopaminergic neurons [44, 45]. It was observed that lower 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity was related to better behavior and less dopaminergic dysfunction as revealed in a hemi-Parkinsonian mouse model [46]. Finding medications that prevent the excessive oxidation of a crucial cysteine (Cys) that is still present at position 106 of the DJ-1 protein (Cys106) is a second crucial tactic. There are presently no known investigations focusing on PRKN or PINK1 mutations or the associated metabolic pathways. Nonetheless, some promising preclinical research could result in future clinical uses. As was previously indicated, two essential hallmarks of PRKN, alongside PINK1-related PD, are mitophagy and mitochondrial dysfunction. Addressing the PINK1/Parkin route to restore normal mitophagy thus seems to be the most promising treatment approach [47]. In cell models, it has been shown that the matrix protein nipsnap homolog 1 (NIPSNAP1) affects this mitophagy pathway, indicating that it may restore PINK1-Parkin-dependent mitophagy in PINK1 phenotypes [48]. Reactive oxygen species (ROS) were shown to be elevated, and brain mitophagy decreased in zebrafish deficient in NIPSNAP1 [48].

Gene therapy has shown promising results in the preclinical models of PD in numerous studies, resulting in several clinical trials. Results from the clinical trials may at first seem somewhat disappointing, but they have shown that safe delivery of viral vectors into the brain is feasible.

AAV2-hAADC (human AADC) and lentiviral-GTP cyclohydrolase 1-tyrosine hydroxylase (LV-GCH1-TH)-AADC are important clinical trials that are presently being conducted to evaluate safety, dose escalation, and efficacy in PD patients.

As indicated in Table 3, there were several enhancements in the off-state Unified Parkinson’s Disease Rating Scale (UPDRS)-III using gene therapy strategies that focused on DA restoration (AAV2-AADC and LV-GCH1-TH-AADC) or basal ganglia network neuromodulation (AAV2-GAD). With the gene therapy approach based on increasing the synthesis of neurotrophic factors (AAV2-GDNF), a varying degree of off-state UPDRS-III score reduction or stability was also noted.

In the AAV2-AADC and AAV2-GDNF studies, an increase in AADC activity or DA storage terminal capacity was validated by the use of neuroimaging biomarkers, primarily fecal microbiota transplantation (FMT) and 18F-DOPA positron emission tomography (PET) [53–56]. Following treatment, metabolic alterations by fludeoxyglucose-18 (FDG) PET demonstrated a functional reconfiguration of the pathways connecting the SN with cortical motor areas with AAV2-GAD [49]. Given its relative improvement, neuroimaging may prove to be a crucial goal for gene therapy research; nevertheless, there has been inconsistency and challenge in the correlation between clinical change and neuroimaging.

A plethora of clinical trials have investigated the preliminary efficacy and safety of gene therapy for PD using viral vectors [37, 49, 53–57]. Four of these investigations [37, 53, 54, 56] have reported their outcomes from similar patient cohorts. In this long-term observational extension experiment, individuals who have previously undergone AAV2-hAADC therapy are asked to take part (NCT03733496). Also, one completed research study and one ongoing investigation have used ProSavin, which expresses GCH1, TH, and AADC (LV-GCH1-TH-AADC) [58, 59]. As indicated in Table 4, there is a significant unmet need to find disease-modifying therapeutics for PD, and some active clinical trials are also looking into alternative viral vectors (AAV2) and recombinant genes. A number of PD gene therapy studies have demonstrated relative safety in a variety of clinical trials intended to improve the production of trophic factors (NCT04167540) or restore DA synthesis [37, 49, 53–57].

Unfortunately, some of the clinical trials on gene therapy for PD have been discontinued due to the inability to achieve the set objectives and the inability to produce the desired patient outcomes. The efficacy of gene therapy for PD can be affected by various factors, with the route and mode of administration being one of the most important. Intraparenchymal administration has been used in many PD trials to deliver the gene vector directly to the target area of the brain [37, 49, 53–57]. This approach significantly lowers the vector dose and the probability of off-target diffusion into peripheral tissues, both of which lower the potential immunogenicity [61]. Intracerebroventricular, intrathecal, subpial, and intracisternal infusions can also be employed when a broader delivery inside the CNS is required. These methods are limited by a more heterogeneous distribution, higher dose requirements, and lower target tissue levels of vector transduction.

Few studies look at the long-term safety profile, and no firm conclusions can be made regarding the long-term therapeutic efficacy, even if the follow-up period in PD gene therapy is sufficient to show the short-term safety profile for all examined medicines. Given that neurotrophic factors are anticipated to exhibit their neuroprotective effect over an extended period of time, this is extremely important.

Investigating the “disease-modifying” effect of gene therapy in PD was hampered by the preferred inclusion of advanced PD patients and the lack of cerebrospinal fluid biomarkers to track the course of PD. Current research is expected to yield more information by including a population of early PD patients, even though the absence of broadly accepted biomarkers of disease progression remains a barrier to evaluating the long-term expected impact of gene therapy in PD. Lastly, the ability to do a meta-analysis and a direct comparison between the various therapy options is limited by the variety of study designs.

The most important developments in gene therapy are expected to come from advancements in molecular techniques for modifying gene expression and target specificity, as well as growing interest in a precision-medicine approach. These developments will likely be crucial for future clinical trials.

NBIb-1817, also known as VY-AADC, is an investigational recombinant AAV serotype 2 vector encoding the gene for hAADC, which is intended to help brain cells produce the AADC enzyme that converts L-DOPA to DA. In early 2019, Voyager Therapeutics and Neurocrine Biosciences entered into a strategic collaboration in an effort to combine Neurocrine Biosciences’ expertise in neuroscience, drug development, and commercialization with the creative gene therapy programs targeting severe neurological diseases established by Voyager [78]. Targeted delivery aided by magnetic resonance imaging (MRI) is used in conjunction with intraoperative monitoring.

The Food and Drug Administration (FDA) informed Neurocrine in December 2020 that it was putting a clinical hold on the phase 2 clinical trial of NBIb-1817, called RESTORE-1 (NCT03562494), after the data safety and monitoring board (DSMB) requested that dosing be stopped until it received information about MRI abnormalities seen in trial participants. The agency gave Neurocrine information so that it could respond fully to the FDA regarding the hold, including an assessment of any potential link between the agent and the negative findings, a plan to mitigate and manage those findings, and supporting documentation to demonstrate that there is a favorable benefit/risk profile. Before this, the medication seemed to be headed toward success in patients with advanced PD. According to 3-year data released in September 2020 from two cohorts, VY-AADC01 treatment led to stable or better motor function, quality of life, and a significant reduction in the demand for antiparkinsonian drugs. When compared to a second phase 1 open-label evaluation, the gene therapy and related administration method were generally well-tolerated within the time frame and seemed to support further clinical development in this population [79]. Three cohorts of fifteen patients received bilateral intraoperative MRI-guided putaminal infusions of the drug. A dose of ≤ 7.5 × 10¹¹ vg was given to Cohort 1, ≤ 1.5 × 10¹² vg to Cohort 2, and ≤ 4.7 × 10¹² vg to Cohort 3. There were no documented severe AEs associated with VY-AADC01. One person experienced atrial fibrillation and pulmonary embolism, while another experienced two small intestinal obstructions six days apart, 29 months after the treatment. These four nondrug-related significant adverse events were all eventually resolved. Headache (Cohort 1: n = 5; Cohort 2: n = 3; Cohort 3: n = 3) and hypoesthesia (Cohort 1: n = 3; Cohort 2: n = 1; Cohort 3: n = 3) were the most frequently reported adverse events.

Patients with PD can still benefit from gene therapy, which has the potential to significantly improve their quality of life. Despite the lack of approval, encouraging outcomes from completed and continuing clinical trials guide the development of gene therapy. For PD gene therapy trials to be successful, patient selection must take into account the mechanism of action of a specific vector design as well as its broad distribution in the targeted brain region. Longer observation periods and early participation are anticipated to be necessary to assess the growth factor-based gene therapy capacity to alter the course of the disease. Moreover, there is a need for combination strategies like the applications of both gene therapy and neuroprotective drugs, the development of efficient CNS-delivery vectors, better patient stratification (prodromal PD, genetic subtypes), and extensive testing to achieve adequate safety for the treatment of PD.