RNA editing aims to treat genetic disease through altering gene expression at the transcript level. Pairing site-directed RNA-targeting mechanisms with engineered deaminase enzymes allows for the programmable correction of G>A and T>C mutations in RNA. This offers a promising therapeutic approach for a range of genetic diseases. For inherited retinal degenerations caused by point mutations in large genes not amenable to single-adeno-associated viral (AAV) gene therapy such as USH2A and ABCA4, correcting RNA offers an alternative to gene replacement. Genome editing of RNA rather than DNA may offer an improved safety profile, due to the transient and potentially reversible nature of edits made to RNA. This review considers the current site-directing RNA editing systems, and the potential to translate these to the clinic for the treatment of inherited retinal degeneration.

A questionnaire on substance abuse was distributed electronically to the heads of 185 Australian and New Zealand College of Anaesthetists accredited anaesthesia departments in Australia and New Zealand. The response rate was 57%. From January 2004 to December 2013, 61 cases of substance abuse were identified, giving an estimated incidence of 1.2 cases per 1000 anaesthetist years. Of 44 detailed reports completed, the majority were aged between 30 and 49 years, were male and of specialist grade. However, when corrected for gender and grade, the estimated overall incidence was higher in females and twice as high for trainees compared with specialists. When compared with prior surveys, the pattern of substance abuse in Australia and New Zealand appears to have changed significantly, with a notable increase in propofol and alcohol abuse and a decrease in reported cases of opioid abuse. Common presenting features of abuse included intoxication and witnessed abuse. Seventy percent of cases had more than one comorbid condition, most frequently either mental health or family problems. Only 32% of abusers had made a long-term recovery within the specialty. Death was the eventual outcome in 18% overall, with a particularly high mortality associated with propofol abuse (45%). Trainee suicide from all causes was reported at three times the rate of specialists. The findings indicate that substance abuse remains a significant problem in Australia and New Zealand and is associated with a significant mortality rate.

The treatment of dominantly inherited retinal diseases requires silencing of the pathogenic allele. RNA interference to suppress gene expression suffers from wide-spread off-target effects, while CRISPR-mediated gene disruption creates permanent changes in the genome. CRISPR interference uses a catalytically inactive ‘dead’ Cas9 directed by a guide RNA to block transcription of chosen genes without disrupting the DNA. It is highly specific and potentially reversible, increasing its safety profile as a therapy. Pre-clinical studies have demonstrated the versatility of CRISPR interference for gene silencing both in vivo and in ex vivo modification of iPSCs for transplantation. Applying CRISPR interference techniques for the treatment of autosomal dominant inherited retinal diseases is promising but there are few in vivo studies to date. This review details how CRISPR interference might be used to treat retinal diseases and addresses potential challenges for clinical translation.

CRISPR-Cas13 systems have therapeutic promise for the precise correction of point mutations in RNA. Using adenosine deaminase acting on RNA (ADAR) effectors, A-I base conversions can be targeted using guide RNAs (gRNAs). We compare the Cas13 effectors PspCas13b and Cas13bt3 for the repair of the gene USH2A, a common cause of inherited retinal disease and Usher syndrome. In cultured cells, we demonstrate up to 80% efficiency for the repair of the common c.11864 G > A and its murine equivalent c.11840 G > A, across different gRNAs and promoters. We develop and characterize a mouse model of Usher syndrome carrying the c.11840 G > A mutation designed for the evaluation of base editors for inherited retinal disease. Finally, we compare Cas13 effectors delivered via AAV for the repair of Ush2a in photoreceptors. Mean RNA editing rates in photoreceptors across different constructs ranged from 0.32% to 2.04%, with greater efficiency in those injected with PspCas13b compared to Cas13bt3 constructs. In mice injected with PspCas13b constructs, usherin protein was successfully restored and correctly localized to the connecting cilium following RNA editing. These results support the development of transcriptome targeting gene editing therapies for retinal disease.