Suzanne de Bruijn

286 Chapter 6 Genetic therapies Recently, the first retinal genetic therapy called Luxturna TM was approved for Leber congenital amaurosis or early-onset RP caused by pathogenic RPE65 variants and can now be prescribed to eligible individuals. A viral vector (adeno-associated virus (AAV)) that contains a wildtype copy of RPE65 cDNA is delivered to the retina by subretinal injection, which halts disease progression and, in some cases, even leads to improved visual function in treated individuals. 17-19 Additionally, hundreds of clinical trials are currently ongoing (https://www.clinicaltrials.gov/ ) to assess the safety and efficacy of genetic therapies to treat other forms of inherited RD and HL. Many scientists and clinicians anticipate that gene- or variant-specific therapeutic strategies hold the future for inherited sensory disorders. Genetic therapies areoftenpersonalized therapeutic strategies that combine knowledge of the underlying pathogenic variant and the molecular mechanisms involved. In case of loss-of-function, such as for the abovementioned pathogenic RPE65 variants, timely supplementation of a wildtype copy of the affected gene (gene augmentation) suffices. In principle, this approach may also be used in haploinsufficiency (dominant) cases, but it may then be a challenge to correct the defect with the right dose as too little protein may be ineffective and too much may also be detrimental. In case of toxic gain-of-function or dominant-negative mechanisms, a different approach is required. In chapter 3.2 , the design of an allele-specific approach to treat DFNA21 is described. AONs were designed that specifically bind the mRNA derived from the mutant allele implicated in DFNA21 and target the transcript for RNase H1-dependent degradation. A lead AON molecule was designed and validated in vitro . In the near future, the ability of the molecule to specifically knockdown mutant allele expression will be assessed in a mouse model of DFNA21. Over the years, a wide range of strategies to optimally design a genetic therapy have been explored, each holding its own pros and cons (extensively reviewed in references (20) and (21)). DNA therapies include gene augmentation (e.g. Luxturna TM ) and genome editing strategies. A gene augmentation approach based on AAVs, holds the advantage that there is no risk for gene integration in the genome. However, important challenges include delivery, immunogenicity and longevity. Despite these challenges, AAV vectors are considered the most safe and efficacious therapeutic vectors for retinal and inner ear delivery. Genome editing strategies have also been widely explored, especially the CRISPR/Cas9 system has received great attention in the past decade. This system that allows, for example, the correction of disease-causing variants or disruption of genes harboring