A clinical research team, based at the Nuffield Laboratory of Ophthalmology, John Radcliffe Hospital, Oxford, has published a report on the treatment of RNAi effectors aimed at treating ADRP (autosomal dominant retinitis pigmentosa). The study is aiming to inhibit toxic protein expression of dominant rhodopsin (RHO) alleles to treat disease. The atypical RNA interference effectors – known as “mirtrons” – are spliced from transcripts as short introns and the research team are proposing a gene therapy strategy using a mutation-independent treatment of RHO-related ADRP. Given the significant genetic and clinical heterogeneity within the rhodopsin gene, the development of multiple treatments may be extremely challenging. There are over 100 different mutations in the rhodopsin gene that may cause retinitis pigmentosa, potentially requiring 100 different therapies. The Oxford group aims to overcome this difficulty through “knockdown / replacement” approach allowing a single treatment to treat all 100 different mutations (or patients).
In their study, the research designed and validated artificial mirtrons directed against rhodopsin for their knockdown/replacement gene therapy application. The approach targets gene knockdown without compromising the transgene expression and can drive RNA-replacement by AAV to obtain rescue effect in a relevant model of rhodopsin related ADRP. Knockdown/replacement gene therapy has been demonstrated in transgenic models of ADRP usingrhodopsin-targeting shRNAs (short hairpin RNAs) under the control of Pol-III promoters, together with codon-modified rhodopsin supplementation under rod-specific Pol-II promoter control. Such studies have either employed separate suppression and supplementation vectors, or have combined the two therapeutic arms driven by separate promoters within a single AAV. A similar approach, envisioned in the 1990s using a similar “suppression and replacement” strategy applying antisense, ribozyme, RNAi, was also focused on ADRP, and other dominant genetic disorders. A significant advantage of the mechanism is that it is entirely independent of an individual’s mutation. Rather than focusing on each specific mutation, researchers can use a set of molecular “scissors” (the atypical RNAi) to remove all copies of a particular mRNA transcript associated with the disease. At the same time, as the mutated and normal mRNA transcripts are removed, a new functioning gene is provided, altered at one or more bases, such that its transcript escapes excision by the molecular scissors. This approach circumvents the necessity to address individual mutations and such approach may represent a core of the advantage: suppression and replacement.
Commenting on their study in their paper in Nature Communications (https://doi.org/10.1038/s41467-021-25204-3), the researchers stated that, “in summary, this study represents the first in vivo demonstration of RNA replacement gene therapy using artificial mirtrons. This strategy has several advantages over existing techniques and may be broadly applied for the treatment of any dominantly-inherited disease”. While the current work was based on both cell and animal models, clinical studies may be hopeful to drive forward clinical research in more comprehensive trials with Phase I/II studies.