CRISPR-Cas9: A solution for Neurodegenerative Diseases

Agnibes Sen, Amity University Kolkata

Neurodegenerative diseases (NDs) like Alzheimer’s (AD), Huntington’s (HD), and Parkinson’s diseases (PD), are some of the most dreaded medical complexities, due to the lack of precise diagnosis and definite cure. Growing evidence suggests that genetic and cellular alterations giving rise to misfolded proteins, and uncontrolled accumulation of them lead to severe damages, overwhelming the protein-disposal mechanism of the neurons. This constitutes the common pathological feature of these NDs. If only the functionality or connectivity could be restored to its prior state, the alterations and accumulated damages could be improved upon.

Targeted nucleases enable researchers to virtually manipulate any genomic sequences, leading to the facile creation of isogenic cell lines and animal models for the study of human diseases. This technology promotes exciting new possibilities for human gene therapy. Development of gene therapy with nuclease-based editing tools by applying ingenious in vivo designs has achieved the first cases of clinical patient referrals.

What is CRISPR-Cas9?

In the field of genome editing, Clustered Regularly Interspaced Short Palindromic Repeats–associated nucleases (CRISPR-Cas9), has emerged as a novel tool for generating specific animal models. These are used for unravelling the underlying mechanisms and screening potentials of various drugs against NDs, as well as for manipulating sequence-specific genes to help individuals with NDs to regain functionality and connectivity of their neurons.

Background:

CRISPR was first discovered in 1987 within the Escherichia coli genome. CRISPR is found in nearly 90% of sequenced archaea and approximately 50% of sequenced bacterial genomes. The Cas (CRISPR-associated) protein shows helicase and nuclease motifs which recognizes and cuts foreign pathogenic DNA. This is allowing researchers to utilize this naturally available machinery as an artificial tool for genome editing.

Applications in Treatment:

Since the conventional medicinal approaches are proving to be insufficient in their viabilities in modern times, CRISPR/Cas9 based gene therapy is one of the viable options promising satisfactory outcomes. The main advantage of the CRISPR/Cas9 gene-editing system in mammalian cells is the high efficiency and precision that’s observed when knock-out models are made by the non-homologous end joining (NHEJ) repair mechanism and the union of non-homologous ends. Some of the most useful applications of this technology can be seen in the inhibition/activation of a target gene; in detecting epigenetic modifications; in Multiplexing; in carrying out genome-wide scans; for visualizing genomic loci using fluorophores and for purifying genomic regions.

Alzheimer’s disease

Alzheimer’s disease is one of the most widely prevalent ND which unfortunately also accounts for one of the most common causes of deaths amongst aged people. One of the main causes of the onset of AD is due to the occurrence of mutations is presenilins, subunits of γ-secretase, and in the amyloid precursor protein (APP). Experiments conducted in AD knock-in mouse models showed a satisfactory decrease in amyloid plaque build-up due to reduction in the secretion of amyloid β. This feat was accomplished by the action of nanoparticles that were coated with Cas9-sgRNA complex, which targeted the BACE1 gene (it codes for an enzyme that cleaves APP). Another study reported the correction of a mutation in PSEN2 of cholinergic neurons using CRISPR/Cas9, which lead to the restoration of electrophysiological activity of the brain. 

Parkinson’s disease:

This progressive neurodegenerative disease is caused due to the loss of dopaminergic neurons in substantia nigra. This loss is caused due to certain sudden as well as genetic mutations in important genes like SNCA (α-synuclein) and LRRK2 (leucine-rich repeat kinase 2). Using CRISPRi, the expression of SNCA was reduced, which ultimately lead to enhancement of mitochondrial ROS and hence overall cellular health.

Huntington’s disease:

Like the previous two, Huntington’s disease is also among the list of most devastating neurodegenerative diseases. It is caused from the overgeneration of CAG repeats of the Huntington (HTT) gene. To help treat this, CRISPR was used to delete an important portion of the genes’ sequence which contained the promoter and the transcription start site. This led to a series of events which ultimately resulted in decreased neurotoxic inclusions by two folds. It also led improved movement abilities, thereby increasing the lifespan.

Conclusion:

Although the alliance between genome engineering and regenerative medicine still resides in its juvenile stages, evaluating the full potential of these technologies in stem cells requires that their functional landscape be fully explored in these genetic backgrounds. Only then will genome editing technologies truly be able to reprogram the cell so that its fate and behaviour can be manipulated for the next generation of advances in synthetic biology and gene therapy.

Also read: Non-Invasive Prenatal Test (NIPT) towards breast cancer detection and monitoring!

Reference(s):

1. Genome editing: A perspective on the application of CRISPR/Cas9 to study human diseases (Review) – Diana Raquel Rodríguez-Rodríguez, Ramiro Ramírez-Solís, Mario Alberto Garza-Elizondo, María De Lourdes Garza-Rodríguez, and Hugo Alberto Barrera-Saldaña. [Int J Mol Med. 2019 Apr; 43(4): 1559–1574.] 

DOI: https://doi.org/10.3892/ijmm.2019.4112 

2. Development and Applications of CRISPR-Cas9 for Genome Engineering – Patrick D. Hsu, Eric S. Lander and Feng Zhang. [Cell. 2014 Jun 5; 157(6): 1262–1278.] 

DOI:  https://doi.org/10.1016/j.cell.2014.05.010 

3. Editing the Central Nervous System through CRISPR/Cas9 Systems – Agustin Cota-Coronado, Néstor Fabián Díaz-Martínez, Eduardo Padilla-Camberos and N. Emmanuel Díaz-Martínez (2019). [Front. Mol. Neurosci;] 

DOI: https://doi.org/10.3389/fnmol.2019.00110

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