Suzanne de Bruijn

58 Chapter 1.2 data obtained from patient-derived biopsies might be more informative. However, for both HL and RD, samples derived from the tissues of interest are usually not available. For these purposes, animal models could provide a valuable alternative. Over the years, several studies have proven the suitability of studying ear- or eye-related disease in non-human primates and mouse models. 33,182 The International Mouse Phenotyping Consortium (IMPC) aims to generate mouse knockout models for all known genes in the mouse genome. 183 Furthermore, the zebrafish has proven its suitability as an animal model. In this model, retinal and inner ear function can already be studied five days post fertilization. 182,184 Limitations of the usage of animal models include ethical, time and financial considerations, in addition to the level of gene conservation. Stem cell technology and the development of differentiation protocols over the past decades have enabled the in vitro generation of patient-derived cells resembling retinal photoreceptors or inner ear hair cells. 185,186 These models can provide an alternative to method of studying the tissue of interest. Research has shown that differentiated cells can resemble the patient’s retina or inner ear. Several 2D- and 3D-differentiation protocols have been successfully applied to study both HL and RD. Differentiation approaches are rapidly being optimized, as the involved processes are still very time consuming and expensive. 185,186 More so, variability and cell heterogeneity are important hurdles, and these should be overcome in order to fully replace animal model studies. FUTURE DEVELOPMENTS Development of new technologies Chromosomal abnormalities and SVs are among the main causes of genetic diseases, which are being addressed in clinical application using routine cytogenetics methods, such as karyotyping and fluorescent in situ hybridization (FISH), comparative genomic hybridization (CGH), and SNP-microarrays. 187,188 However, these methods manifest significant limitations in the identification of SVs. For example, karyotyping allows the identification of different chromosomal abnormalities with a 5-10 Mb resolution. Although microarrays and CGH-arrays are able to identify gain and loss of chromosomal material as small as 10 kb, balanced rearrangements cannot be detected by these methods nor the exact location of the structural variation. 189,190 It is estimated that only 15-20% of chromosomal abnormalities can be detected by the application of these techniques, which indicates the great need for new technologies in the field of cytogenetics. 191