Suzanne de Bruijn

288 Chapter 6 genetic diagnostics related to genetic therapy, but also patient counseling and family planning, it is evident that genetic research is of utmost importance and significantly contributes to the patient’s quality of life. Additionally, with the rising number of commercially available DNA tests (e.g. 23andme) and ongoing debate of the value and ethical concerns of genetic risk factor screening 25 , complete understanding of the genomic variation will become more and more important. A study performed by Hanany et al. estimated that about 36% of all individuals carry a pathogenic variant that can cause autosomal recessive RD. 26 Preconception screening (e.g. “clinical exome” analysis) in consanguineous 27-29 and non-consanguineous 30 couples has shown its value. Unfortunately, after 30 years of research, it is not possible yet to provide a genetic diagnosis to all individuals affected by a genetic disorder. Which crucial challenges have been overcome in the past and which challenges are remaining that should be conquered to solve all the unsolved? These questions will be addressed in the following sections. GENETIC DIAGNOSTICS: CHALLENGES AND LESSONS LEARNED FROM THE PAST Thirty years after the discovery of the first HL- and RD-associated genes, novel disease- associated genes are still reported. Based on today’s count (May 2021), 271 RD- associated genes 31 and 154 HL-associated genes 32 have been described ( Figure 1 ). The number of genes gradually increased over the years, with developing technologies, increasing knowledge and decreasing sequencing costs being considered as important contributing factors. Noteworthy high-impact events over the years are the completion of the Human Genome Project and the introduction of WES, WGS and long-read sequencing technology. It was anticipated that WES and WGS would rapidly revolutionize the field of genetic diagnostics, lead to an exponential increase in disease- associated genes, and resolve the missing heritability. Unfortunately, this was not the case. In fact, in recent years, gene identification curves seem to have reached a plateau phase, suggesting that the majority of disease-associated genes have been discovered and attention should be shifted to (re)investigation of known disease genes in more depth, with specific emphasis on the involvement of non-coding elements. First generation sequencing As reviewed in chapter 1.2 of this thesis, the first HL- and RD-associated genes were identified by linkage studies and positional cloning ( CHM (1990) 33 , POU3F4 (1995) 34 ). Large and small deletions on the X-chromosome were linked to the respective diseases and enabled pinpointing the candidate disease genes and subsequent cloning.