Suzanne de Bruijn

39 The impact of modern technologies on molecular diagnostics Homozygosity mapping Genome-wide homozygosity mapping has proven to be a powerful tool to identify disease-associatedgenes forautosomal recessivedisorders. Forboth inheritedHLandRD, a significant number of disease-associated genes were identified using this strategy. 13,17 In consanguineous families, a pathogenic variant is often present in a homozygous state as it is inherited from a recent common ancestor (grandparent or great grandparent). Homozygosity mapping can be used to determine regions that contain consecutive homozygous variants 16,18 , which is often performed using SNP-arrays. Although the average size of homozygous stretches is larger in consanguineous families (typically between 30 to 100 megabase (Mb)-sized regions) 19,20 , several studies have indicated that this method is also an effective tool for non-consanguineous families (1-30 Mb- sized homozygous regions 19-21 ). EYS is one of the most frequently mutated genes in RD and was identified using homozygosity mapping in a non-consanguineous family. 22,23 Other examples of disease gene identification using homozygosity mapping in a non- consanguineous family include PDE6C 24 which is associated with RD, and OTOG 25 and MYO15A 26 , which are implicated for HL. The size of a homozygous disease-associated haplotype decreases over subsequent generations due to meiotic recombination. Well- characterized families and detailed phenotypic information are prerequisites for the successful application of this technique. Next generation sequencing DNA studies have been revolutionized by the conventional Sanger sequencing technique, which was introduced in 1977. 27 It is known as an enzymatic sequencing or chain-termination method, which utilizes labeled di-deoxynucleotides acting as chain terminators. 27 The first human genome was sequenced based on the Sanger sequencing technology in 2001, which took almost 13 years to complete and cost $2.7 billion, and was part of a large collaborative and international publicly funded project. 28 In parallel, efforts to sequence the first human genome were also performed in a commercial setting by the company Celera Corporation, whose results were revealed in joint publications with the public human genome project. 29,30 The Celera project employed a whole genome shotgun sequencing approach and proceeded at a much faster pace and lower cost, although it benefited significantly from the data that were already generated by the public human genome project. 29,31 As a result of both efforts to sequence the human genome, it became clear that the scale, efficiency, and cost needed to be vastly optimized for routine use in clinical diagnostics. Therefore, shortly after the release of the human genome sequence, the aimwas re-established to achieve a $1,000 human genome within 10 years. 32