Parkinson’s Disease (PD) is a genetic and neurodegenerative disorder characterized by loss of dopamine (DA) neurons in the brain, muscle rigidity, α-synuclein misfolding, Lewy bodies, and neuroinflammation. To define a few terms: α-synuclein is a 140 amino acid protein expressed in neurons that helps regulate the DA lifecycle, and when misfolded, can lead to the formation of Lewy bodies, which are protein accumulates (Calabresi 2023).

I. Underlying Genetic Mutations

In current PD genetics nomenclature, important chromosomal loci are termed PARK and numbered in chronological order. One of the best studied genes is the SNCA gene (PARK1-4), which codes for α-synuclein. Mutations in the SNCA are rare, but patients who do develop them usually have early onset PD. The first identified, p.A53T, a missense mutation, is the most frequent SNCA mutation, with high penetrance, as high as 85% (Klein 2012). Missense SNCA mutations cause PD through toxic gain of function—they impair the amino-terminal domain, and three-point mutants tend to form stable β sheets that exacerbate formation of Lewy bodies, which is the brain’s attempt to purge the cell of damaged α-synuclein (Klein 2012).

A second important gene in PD research is LRRK2 (PARK8), which is the most frequent known cause of late-onset autosomal-dominant PD. The neuropathology is inconsistent, developing both with and without Lewy bodies (Klein 2012). At least sixteen of the mutations appear to be pathogenic, including six recurrent mutations: p.R114C, p.R1441G, p.R1441H, p.Y1699C, p.G2019S, and p.I2020T. These pathogenic changes are clustered in ten exons that encode the carboxy-terminal region of the protein.

A third important gene in PD is Parkin (PARK2), the second largest gene in the human genome, coding for a 465 amino acid E3 ubiquitin ligase which conjugates ubiquitin proteins to lysine residues of target proteins, determining their cellular fate. The ubiquitin-like domain of Parkin shares 62% homology with ubiquitin and is important in stabilizing the structure and controlling its expression levels (Klein 2012). Parkin is an autosomal recessive mutation that is present in 50% of early PD cases and 15% of late PD cases (Moreno 2023).

II. Current Treatments

Currently, most pharmacological PD treatments target one of two areas: α-synuclein abnormal folding that forms Lewy bodies, or the DA neuron malfunction in the substantia nigra due to cytotoxic damage as a result of α-synuclein misfolding. The most common treatment option is Levodopa treatment. Levodopa, or L-DOPA, is a DA precursor whose administration has limitations due to adverse reactions like dyskinesia. Current Levodopa formulations contain decarboxylase inhibitors to prevent peripheral DA metabolism and allow for greater drug bioavailability (Moreno 2023). Levodopa is generally administered in conjunction with other drugs—monoamine oxidase or catechol-O-methyltransferase inhibitors—to stabilize DA levels and improve motor complications.

Other options for PD treatment are more invasive and consequently less popular. Deep Brain Stimulation (DBS) involves invasive surgery and is advised to be used during the final stages of the disease due to risks involved, including possible seizures. Patients with unwanted side effects of hypomania or psychotic symptoms have found their symptoms greatly improved by DBS (Moreno 2023). Another invasive procedure, Levodopa-carbidopa enteral suspension, is a surgical procedure that allows for safe delivery of high Levodopa doses via the implantation of a permanent tube through a percutaneous endoscopic gastronomy connected to a portable external pump. It prevents Levodopa fluctuations, leading to lower risk of dyskinesia (Moreno 2023).

III. New Therapies and Ethical Concerns

A potential new avenue of therapy would use CRISPR technology to upregulate the activity of tyrosine hydroxylase (TH), the rate limiting enzyme in DA synthesis. To ensure no DNA is cleaved, we would use dCas9, a dead Cas9 that would bind DNA but lack the endonuclease activity to cleave DNA. The Cas9 complex would be delivered with an adeno-associated virus (AAV) vector engineered with cell-specific promoters, such that it would only bind to the TH promoter on DA neurons. AAV serotype 9 (AAV9) could be used for its high tropism for neurons and ability to cross the blood-brain barrier. We would pick a gRNA that binds to the promoter region of the TH gene, and the entire complex would be fused with TH transcription factors. Once bound, the dCas9 activator complex would recruit transcriptional machinery to itself on the promoter and initiate TH transcription, thus increasing the amount in the cell, and subsequently the amount of dopamine produced. 

Unlike the current levodopa treatment, which brings in external L-DOPA, this would enhance the patient’s ability to create more of their own DA, which would hopefully be more long-lasting and reduce the need for constant treatment. However, it is uncertain how much off-target activation would occur, though this possibility would be reduced by having the drug by stereotactically injected into the brain.

A key ethical concern is off-target effects, where the CRISPR system unintentionally activates other genes, and while this possibility can be mitigated, it cannot be 100% eliminated. Another ethical concern is equitable access. Gene therapies are very costly, and it could limit access to only the wealthy. Further, individuals with advanced PD might not be able to consent to such procedures, and for this treatment to truly be a possibility, it would have to be run by comprehensive ethical review boards, and clinical trials run very carefully.

Works Cited

Calabresi P, Mechelli A, Natale G, Volpicelli-Daley L, Di Lazzaro G, Ghiglieri V. 2023. Alpha-synuclein in Parkinson’s disease and other synucleinopathies: from overt neurodegeneration back to early synaptic dysfunction. Cell Death & Disease. 14(3):1–16. doi:https://doi.org/10.1038/s41419-023-05672-9. https://www.nature.com/articles/s41419-023-05672-9.

Klein C, Westenberger A. 2012. Genetics of Parkinson’s Disease. Cold Spring Harbor Perspectives in Medicine. 2(1):a008888–a008888. doi:https://doi.org/10.1101/cshperspect.a008888. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3253033/.

Pardo-Moreno T, García-Morales V, Suleiman-Martos S, Rivas-Domínguez A, Mohamed-Mohamed H, Ramos-Rodríguez JJ, Melguizo-Rodríguez L, González-Acedo A. 2023. Current Treatments and New, Tentative Therapies for Parkinson’s Disease. Pharmaceutics. 15(3):770. doi:https://doi.org/10.3390/pharmaceutics15030770.