Suzanne de Bruijn

37 The impact of modern technologies on molecular diagnostics INTRODUCTION In previous decades, different methods for disease gene identification have been established and successfully employed. All these technologies have significantly contributed to identifying the large number of genes that are associated with inherited forms of hearing loss (HL) (>150 genes) 1 and retinal dystrophy (RD) (>270 genes). 2 Especially after the introduction next generation sequencing (NGS) techniques, it was anticipated that soon all HL- and RD-associated genes would be identified. Nevertheless, the diagnostic yield suggests a significant portion of missing heritability, which can potentially be explained by unrecognized disease genes or missed variants. 3,4 To provide a genetic diagnosis for all inherited cases, it has become evident that there is no single technique that can serve as the gold standard. To be able to detect and interpret all genetic variation of the human genome, classical methods such as linkage analysis or homozygosity mapping should be combined with novel state-of-the-art techniques. 5,6 The observed high genetic heterogeneity is not unique for these inherited sensory disorders; they have also been described for other inherited disorders including intellectual disability, ciliopathies and inherited susceptibility for cancer. 7-9 Although in general, disease gene identification strategies applied in these fields rely on the same principles and have undergone a similar development, an optimal diagnostic strategy depends heavily on key factors such as evolutionary pressure and involvement of multifactorial versus monogenic causes. For example, for intellectual disability, de novo causes are more frequent due to a strong reduction of reproductive fitness; this impacts the optimal diagnostic strategy. This review focuses on the identification of monogenic causes of inherited HL and RD. In this review, we aim to provide an overview of the development of techniques that have enabled disease gene discovery throughout the years. Additionally, we evaluate and highlight the complexity and different aspects of candidate variant and candidate gene interpretation. Finally, we describe recent and upcoming improvements and innovations of existing technologies and the development of novel technologies in the field.