Visual function restoration for a pre-clinical in vivo IRD model using  a novel therapeutic “base editing”. 

Researchers at the Seoul National University College of Medicine, Republic of Korea, have demonstrated a proof-of-principle using “base editing” (BEs) to address Leber congenital amaurosis (LCA).  The pre-clinical in vivo study used adenine base editors (ABEs) with trans-splicing proteins (inteins) with subretinal injection of ABEs in retinal pigment epithelial cells of rd12 models, resulting in an A to G transition. The gene editing appeared to show sufficient recovery of the RPE65 protein and light-induced electrical responses from the retina. According to the researchers, “our results suggest that, considering the editing efficacy and functional recovery, ABEs could be a strong, reliable method for correction of pathogenic variants in the treatment of LCA”.

Gene editing has a long history of technical developments aimed to treat genetic disease on the basis of altering a DNA sequence.  Antisense oligonucleotides, ribozymes, aptamers, RNA interference (RNAi), zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats).  One of the recent versions of gene editing now uses base editing, as a gene therapy approach designed to more finely-tune “surgery”, changing one “letter” of the gene’s code at a time, such as changing a C to T or A to G, aiming to correct the error with a potential promise of more precision, efficiency and safety.  Prior to earlier technologies, CRISPR/Cas9 was a significant advance on making alterations on a given specific DNA sequence involving double-strand breaks with a homology-directed repair (HDR) strategy.  However, as commented in their report, the Korean researchers stated that “there are also critical concerns that CRISPR-mediated DSBs frequently induce unexpected large deletion, chromosomal depletion, and complex rearrangement”.  Alternatively, base editing or prime editing tools can result in the correction and recovery of disease phenotypes without the generation of acute DSBs. DNA base editors (BEs), including cytosine base editors (CBEs) and adenine base editors (ABEs), consist of a cytidine/adenosine deaminase fused to nuclease-deficient Cas9 nickase (nCas9), with the conversion of C to T and A to G, respectively”. This method avoids breakages on the genomic DNA strands, thereby avoiding random insertion and deletions (“indels”) potentially arise other pathologies.

In the current study, a pre-clinical model aims to address LCA type 2 or murine rd12 model with a nonsense mutation of C to T transition at the 130th position in exon 3 of the Rpe65 gene (c.130C>T, p.R44X).  To correct this mutation, the strategy used an optimized version of the adenine base editor (ABE) to correct the nonsense mutation of the Rpe65 gene in rd12 model.  The adenine base editing tool, with a dual-AAV vector system, were deigned to target the pathogenic variants in the RPE65 gene and then restore the functional RPE65 protein for the purpose of both treating gene therapy and genome editing.    Commenting in their study, researchers stated that, “[t]aken together, we suggest AAV-based adenine base editing as a permanent and direct therapeutic approach for the treatment of patients with LCA-associated substitutions. Base editing is a better method than HDR for correcting substitutions, due in part to the higher efficiency and lower indels. In terms of specificity and efficacy, the impact of base editing might be greater with the improved versions of BEs”.