Suzanne de Bruijn

287 General discussion and perspectives gain-of-function variants using in vivo or ex vivo approaches . Nowadays, CRISPR/Cas9 is commonly used for mammalian genomic editing and for the efficient generation of knockin or knockout animal models. Evidently, several ethical concerns are attached to human gene editing, including the risk to introduce permanent off-target effects. CRISPR/Cas9 has clearly revolutionized the field of genetic research but the direct application of the technique in human subjects will most likely still remain a topic of ongoing debate for quite some years. In light of limitations of DNA therapies, RNA-targeting therapies, such as the AON- strategy for DFNA21 ( chapter 3.2 ), can be considered a more attractive alternative. AONs can be designed to redirect splicing (so called splice-AONs) or to silence gene expression (e.g. RNase H1-dependent AONs). Especially the transient and reversible nature of treatments with these molecules is considered an important advantage over DNA editing strategies. Delivery, longevity and potential off-target effects on the other hand, remain important challenges. Nevertheless, the recent report of a single AON injection (seporfarsen) that leads to visual improvement with an effect lasting for over 15 months as a treatment for CEP290 -associated RD is very promising. 22 Drug delivery, and specifically repeated delivery, to the inner ear is still less advanced as compared to the retina. However, with many ongoing efforts this limitation can be expected to be overcome soon (reviewed in (21)). The currently available genetic therapies are extremely expensive. The treatment of a single injection with Luxturna costs $425,000 per eye. 23 Additionally, for each mutated gene, pathogenic variant, or subsets of pathogenic variants, a personalized therapy needs to be designed depending on the pathogenic variants involved. This entails a time- and money-consuming process of clinical testing, that on average takes more than 10 years. An exception to this might be specific “N=1” situations. Last year, an AON- based treatment (milasen) received FDA approval within one year after the first contact with the patient. 24 The molecule was designed to treat an inherited lethal form of Batten disease, an autosomal recessive disorder that progressively affects the retina and the central nervous system. In this case, WGS revealed biallelic pathogenic variants in MFDS8 , among which a retrotransposon insertion in intron 6. The chemistry of the AON was based on an existing FDA-approved molecule, and combined with the urgency of the patient’s clinical situation and the sequence-specific design of the drug, rapid development of the treatment was possible. Despite all these issues, it is evident that genetic therapies entail great promises for the future. Nevertheless, there is first a crucial initial step to be taken: the establishment of a genetic diagnosis for all inherited disorders. Considering the important benefits of