Suzanne de Bruijn

293 General discussion and perspectives complete understanding of the human genome, chromosome function, and potentially of the impact of genomic variation on disease. Will this finally resolve all missing heritability? While soon we will be able to detect all genetic variation, most likely still a significant part will be categorized as “variants of unknown significance”. Sequencing technology is no longer holding us back, but knowledge is. A complete understanding of all (non- coding) elements of the human genome and their functional interdependencies is lacking. What are the steps that are required to collect this missing fundamental knowledge, and which next developments can we expect? 1. The introduction of integrative multi-omics approaches The involvement of non-coding elements in disease such as enhancers, microRNAs (miRNAs) and long noncoding RNAs (lncRNAs) is generally accepted these days. 59,60 miRNAs are a class of small noncoding RNAs of about 22 nucleotides long with a tightly regulated expression pattern. They are considered the “fine tuners” of gene regulation, and mediate posttranscriptional gene silencing. 61 Various studies have indicated the essential role of miRNAs in normal eye and ear development. 62,63 Pathogenic variants that affect the seed region of MIR96 have been described to underlie HL type DFNA50. 64 Also for RD, pathogenic variants affecting miRNAs have been implicated in disease: the disruption of MIR204 results in a unique eye phenotype that includes retinal degeneration and coloboma. 65 The biological function of lncRNAs is more diverse as compared to miRNAs. LncRNAs are multi-exon transcripts of usually 1,000-10,000 nucleotides in length, and are subjected to alternative splicing and post-translational modification such as polyadenylation. 66 LncRNAs are coined “the regulators of the regulators” and are involved in regulation of gene transcription (by functioning as enhancers, enhancer regulators, or pseudogenes), in post-transcriptional regulation of gene expression (by modulating splicing or acting as miRNA decoys) and in epigenetic regulation (by altering chromatin histone modifications. 67 The involvement of lncRNAs in disease is less explored, although this topic is gradually receiving more attention. LncRNAs have a high degree of tissue specificity and thousands are shown to be expressed in human inner ear and retinal tissues. 68-70 Over 20 of these lncRNAs show potential to influence expression of genes already known to be important for inner ear function in humans and mice among which a lncRNA that controls expression of miR96 (DFNA50). 71 Also, the lncRNA MALAT1 has been suggested to have a protective function in the development of glaucoma 72 , whereas elevated MALAT1 expression levels have been reported in patients diagnosed