Suzanne de Bruijn

283 General discussion and perspectives This year, 2021, we celebrate the 20 th anniversary of the human genome. It has been twenty years since the public Human Genome Project and Celera Corporation jointly released the first sequences of our genome. 1,2 As expected, this greatly impacted modern genetic diagnostics. Nowadays, a putative variant can be readily interrogated by extracting all relevant information such as genomic position and population allele frequencies. The costs to perform whole genome sequencing (WGS) have dropped from $2.7 billion for the first complete human genome sequence, to less than $1,000. 2,3 In combination with other technological advancements and functional analyses, the field of human genetics revolutionized. Finally, we are slowly reaching a complete understanding of the human genome and the impact of genetic variation. Despite all these advancements, interpretation of genetic variation can still be challenging (reviewed in chapter 1.2 ). Although we are able to detect the full spectrum of genetic variation, the knowledge that is required to interpret all these variants is still lagging behind, which prevents optimal translation of findings to clinical care. This knowledge gap is considered the most important contributor to the missing heritability that is described for both inherited retinal dystrophies (RD) and hearing loss (HL). About 20-50% 4-7 of RD cases and 60-70% 8,9 of HL cases still lack a genetic diagnosis when whole exome sequencing (WES) is performed. The focus of this thesis was to shed light on the missing heritability for the inherited sensory disorders HL and RD. To do so, a variety of genetic and functional strategies was employed. In chapter 2 , functional evidence was collected to validate the association of KIAA1549 with retinitis pigmentosa (RP). Novel pathogenic KIAA1549 variants were identified and the number of known patients affected by these variants was increased. To prove causality of KIAA1549 pathogenic variants in RP, in vitro expression studies and immunohistochemistry were performed. The studies described in the following chapters of this thesis were focused on previously identified loci for RD and HL, for which the genetic defects were still elusive. After decades of research, the defects underlying two of these loci were finally identified: DFNA21, that is associated with dominantly- inheritedHL ( chapter 3 ) and RP17, associatedwith dominantly-inherited RP ( chapter 4 ). As a continuation of this research, the first steps towards a genetic therapy for DFNA21 have been described in chapter 3.2 with the development of a gene silencing strategy using RNase H1-dependent antisense oligonucleotides (AONs). The findings described in chapters 3 and 4 do not only provide a genetic explanation for DFNA21 and RP17; the important insights that are gained can also be applied to other unsolved Mendelian diseases.