Suzanne de Bruijn

41 The impact of modern technologies on molecular diagnostics the complete ABCA4 gene (coding and non-coding regions) in 1,054 individuals with Stargardt disease, who were previously screened for variants in the coding regions and remained genetically unexplained. This study proved that a smMIP-based approach is a cost-effective approach in case of a strong genotype-phenotype correlation. The method allowed deep-sequencing of the region of interest, and causal structural and deep-intronic variants were identified in 25% of the investigated cases, who were genetically undiagnosed after pre-screening methods. 41 Targeted NGS techniques have several advantages, such as less data storage, high sequencing accuracy due to high coverage, cost-, and time-effectivity. 42 However, this approach is unable to detect variants in novel (candidate) disease-associated genes. Furthermore, pathogenic variants residing in non-coding regions, and structural variants (SVs) can be missed if only exons are analyzed. Due to decreasing prices of both WES and WGS, these approaches have become rapidly preferred to overcome the disadvantages of targeted NGS. Whole exome sequencing versus whole genome sequencing Protein coding regions comprise 1-2% of the human genome. However, it is estimated that they harbor approximately 85% of disease-causing variants. 43-45 Therefore, the enrichment of coding regions utilized in WES quickly became an accurate and efficient method to investigate the coding regions of the genome for potential pathogenic variants, and this is now widely applied in genetic diagnostics. 46 One of the striking features of WES is in the success rates of genetic diagnostics of diseases with extensive locus heterogeneity, such as inherited HL and RD. 4,47 Currently, the diagnostic yield for RD using WES is estimated to be between 50% and 80%, dependent on the phenotype studied. 4,48-50 According to a study performed by Haer-Wigman et al., the highest yields were obtained for retinitis pigmentosa (63%) and lowest yields are obtained for macular dystrophy (28%) and rare unspecified types of RD (25%). 4 For HL, the genetic diagnostic rate is also highly dependent on phenotype (e.g. syndromic or non- syndromic phenotype, mode of inheritance). The highest rates are observed in patients with a positive family history or with a congenital or symmetric type of HL. 47 The overall estimated detection rate for HL, when employing WES, varies between 30-40% based on different large cohort studies and largely depends on phenotypic diversity. 3,51 The diagnostic yield for HL is importantly influenced by the involvement of environmental factors (e.g. noise, ototoxic drugs and trauma), which likely explains the difference in yield compared to RD. Genetic causes have been estimated to underlie approximately two-thirds of the cases of congenital and early childhood HL; the remaining cases can be explained by acquired causes. 52 This genetic contribution decreases with the