O17 Highly efficient base editing of homozygous and heterozygous COL7A1 variants in fibroblasts from patients with recessive dystrophic epidermolysis bullosa using adenine base editor 8e mRNA system delivered via novel lipid nanoparticle formulations

Abstract Introduction and aims Adenine base editors (ABEs) present an attractive approach to gene editing in the inherited blistering disease recessive dystrophic epidermolysis bullosa (RDEB), which Results from pathogenic variants in COL7A1. We previously showed that ABE8e, a new ABE variant, confers remarkable efficiency (94.6%) and a favourable safety profile in correcting variants and restoring collagen 7 (C7) expression when electroporated into RDEB fibroblasts. With a goal to develop topical base editing creams, we now explore use of lipid nanoparticle (LNP)-encapsulated ABE8e mRNA system to edit two distinct variants in RDEB fibroblasts. Methods Primary patient fibroblasts harbouring a homozygous G>A mutation (exon 42) or a heterozygous C>T mutation (exon 54) were gene edited using ABE8e mRNA, delivered via lipofectamine or six different LNPs. To assess the gene editing efficiency, Sanger sequencing and next-generation sequencing were employed. Cell viability was evaluated via a lactate dehydrogenase cytotoxicity assay, while functional restoration of C7 expression was assessed via Western blotting of cell lysate and supernatant. Finally, topical delivery was tested on normal human skin equivalents (HSEs). Results We identified an optimal ratio of 1 : 7 for sgRNA : ABE8e, at which editing efficiency increases with ABE8e dosage, with 400 ng being sufficient for 90% biallelic editing. Formulations containing DTDTMA (C14) and DHDTMA (C16) LNPs demonstrated approximately 90% and approximately 80% editing (with and without the peptide, respectively), in addition to reduced cytotoxicity compared with lipofectamine. Depending on the nature of the mutation, the edit corresponded to an increase in C7 expression in supernatant only or cell lysate as well, whereby C14 LNPs enabled twofold C7 restoration. Finally, our HSEs demonstrated viability of topical delivery of lipofectamine-encapsulated green fluorescent protein mRNA. Conclusions We demonstrated the potential value of base editing as a gene therapy for RDEB in addition to the potential of these LNPs for clinical translation of the therapy. Nonetheless, further safety and functionality tests still need to be conducted.