Researchers at Radboud University Medical Center, Nijmegen, the Netherlands, have reported a novel approach to use exon skipping that may “patch” part of a gene sequence and tolerate re-engineered pre-mRNA transcripts allowing to generate intact protein translation. In brief, the researchers suggest a ”therapeutic potential of skipping combinations of two exons that together encode exactly one single protein domain” may provide a viable approach. Researchers stated that “this approach may serve as a future paradigm to develop treatments aimed at slowing down or halting the progression of photoreceptor degeneration in USH2A-associated RP (retinitis pigmentosa)”. This alternative strategy skipping a combination of exons encoding a complete protein domain has demonstrated a similar approach in patient-derived cell models for Duchenne muscular dystrophy (DMD), providing a significant opportunity to address the debilitating disorder.
Usher syndrome is characterised by dual sensory disease that affects hearing, balance and vision and there may be at least 9 genes that can cause the different types of Usher syndrome including: type 1 genes for MY07A, USH1C, CDH23, PCHD15, and USH1G; type 2 genes include: USH2A, GPR98, and WHRN, and; type 3 genes include: CLRN1. Patients with USH2A may present a combination of RP and congenital hearing impairment with a delayed onset and slowly progressive pathology. USH2A gene translates a protein called usherin found in basement membranes in the inner ear and in the retina that plays an important role in cell development and maintenance. The USH2A gene spans an estimated 800kb of DNA on chromosome 1q41 and the usherin coding sequence comprises 15.6kb. This size of the coding sequence is challenging in the context of attempting to deliver a gene augmentation strategy with current viral gene capacity. In addition, there are over 600 different mutations identified in the transcript, encoding the large isoform of usherin which are distributed across the gene.
In the current study using an experimental functional model, an exon skipping process used a protein domain-oriented dual strategy creating a genomic deletion of the frequently mutated human USH2A exons number 30-31 or 39-40. Removing these in-frame combinations of exons allow restoration of usherin expression and rescue the photopigment mis-localization. According to the report, the researchers commented that “to translate these findings into a future treatment in humans, we employed in vitro assays to identify and validate antisense oligonucleotides (ASOs) with a high potency for sequence-specific dual exon skipping. Together, the in vitro and in vivo data demonstrate protein domain-oriented ASO-induced dual exon skipping to be a highly promising treatment option for RP caused by mutations in USH2A”.