Another Point for CRISPR: Targeted Treatment Using RNA Editing

Gene Editing Using Scissors

For more than a decade, the CRISPR-Cas technique, dubbed "molecular scissors," has been used for DNA-level gene modifications across various fields. Simply put, CRISPR is the immune system of prokaryotic organisms, allowing them to recognize genomic sequences of bacteriophages and subsequently target them with enzymes (proteins from the Cas family) that split them.

It has been shown that this system can be modified and the cleaving enzymes targeted at practically any nucleotide sequence. DNA can then be further modified. The discovery of the CRISPR-Cas technique earned the Nobel Prize in 2020.

To date, the CRISPR-Cas9 system has been used in several pilot gene therapy studies for hereditary diseases, such as transthyretin amyloidosis, sickle cell anemia, β-thalassemia, and Leber's congenital amaurosis, a progressive retinal degeneration caused by a mutation in the gene for centrosomal protein 290. However, gene therapy always carries the risk of introducing unwanted mutations into the genomic DNA.

RNA-Level Modification

One way to avoid the risk of unwanted genomic modification is to edit at the level of mRNA. This can be accomplished using the CRISPR-Cas13 system, which can specifically target a given sequence of mammalian mRNA. Compared to DNA modifications, RNA editing should provide better safety and more precise control over the extent of the effect or the ability to reverse the therapy's effects. Additionally, RNA editing does not require the presence of DNA repair enzymes through homologous recombination, so it can be used in non-dividing cells.

Variants of the Cas13 enzyme can correct point mutations present in mRNA or induce the degradation of the entire molecule (known as knockdown). This allows the technique to be used for treating diseases where the problem is the level of protein, not its faulty sequence.

Treatment of Macular Degeneration

The pilot study aims to reduce the level of protein in patients with age-related macular degeneration. This disease affects the central part of the retinal eye, beneath which newly formed vessels grow. These vessels are very permeable and fragile, leading to the exudation of fluid or bleeding under the retina, the formation of connective tissue, and progressive scarring, resulting in the loss of visual acuity, which can progress to functional blindness.

Currently, patients are most commonly given intraocular injections of a vascular endothelial growth factor (VEGF) inhibitor. However, frequent injections are associated with risks of complications such as submacular bleeding, intraocular hypertension, inflammation, or retinal detachment. Up to one-third of patients with aggressive forms of the disease do not respond to treatment.

RNA-Targeted Gene Therapy

In an open Phase I study approved by the U.S. Food and Drug Administration (FDA), approximately 15 patients are expected to receive experimental therapy with HG202, a sequence for CRISPR-Cas13 targeted with high efficiency at mRNA for VEGF, carried by an adenoviral vector.

The primary goal is to evaluate the safety and optimize dosing. Secondarily, the efficacy of the therapy will be assessed by measuring the best-corrected visual acuity (BCVA), central retinal thickness, and the need for rescue anti-VEGF therapy.

Preparing New Studies

Although targeted therapy using RNA editing is just beginning, it appears to be a promising option for the treatment of genetically conditioned diseases. Only a few months have passed since the approval of the very first study, but it is clear that more will soon follow.

Editorial Team, Medscope.pro

Sources:

1. Hui F., Liu G. S. RNA editing is the next frontier in gene therapy – here’s what you need to know. The Conversation 2024 Nov 21. Available at: www.theconversation.com/rna-editing-is-the-next-frontier-in-gene-therapy-heres-what-you-need-to-know-243938
2. HuidaGene Therapeutics receives the first-ever FDA clearance of CRISPR/Cas13 RNA-editing HG202 for macular degeneration. HuidaGene Therapeutics 2024 Nov 4. Available at: www.huidagene.com/new/news/70
3. CRISPR based RNA editors. HuidaGene Therapeutics 2025. Available at: www.huidagene.com/crispr/cas13
4. Open-label dose-escalation study for CRISPR/​cas13 RNA targeting therapy for the treatment of neovascular age-related macular degeneration in phase I trial (BRIGHT). NCT 06623279. ClinicalTrials.gov 2024 Oct 02. Available at: www.clinicaltrials.gov/study/NCT06623279