Toms, Maria; (2018) Investigating the pathophysiology of KCNJ13 and USH2A retinopathies using zebrafish models. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Inherited retinal diseases (IRDs) encompass a large group of clinically and genetically heterogeneous diseases that affect approximately 1 in 3000 people, representing an important cause of severe visual loss in the human population. In the past two decades, significant contributions towards our understanding of these disorders has included the identification of over 250 causative genes. In this thesis, the pathophysiology of IRDs associated with KCNJ13 and USH2A was investigated using relevant zebrafish models. Mutations in KCNJ13 are associated with Leber congenital amaurosis, the most severe IRD subtype that causes blindness in childhood. KCNJ13 encodes the Kir7.1 channel, a potassium channel expressed on the apical retinal pigment epithelium (RPE). Longitudinal assessment of the kcnj13 mutant zebrafish (obelixtd15) revealed a late onset retinal degeneration at 12 months with retinovascular abnormalities, and a corresponding decline of visual function. Ultrastructural examination of the obelixtd15 RPE uncovered changes in phagosome clearance and mitochondrial growth prior to notable degeneration which indicate that the disease may be a primary phagosome failure with a secondary failure in mitochondrial physiology. In addition, gene and protein expression changes consistent with altered mitochondrial activity and retinal stress were observed. The alterations in the RPE are reminiscent of those seen in age-related macular degeneration and highlight potential therapeutic targets for KCNJ13 retinopathy. USH2A mutations are the most common cause of Usher Syndrome, characterised by combined retinitis pigmentosa and sensorineural deafness. USH2A encodes the large transmembrane protein, usherin, expressed in the photoreceptors and cochlear hair cells. CRISPR/Cas9 gene targeting was used to create an ush2a mutant zebrafish line, which showed a slowly progressive photoreceptor degeneration from 6 months with mislocalisation of rhodopsin. Developmental abnormalities in the neuromast and inner ear hair cells were also identified in ush2a larvae. The findings suggest that ush2a is involved in rhodopsin trafficking and rod photoreceptor maintenance in the zebrafish retina, as well as playing a role in mechanosensory system development. Overall, the ush2a zebrafish phenotype showed consistency with the clinical phenotype in USH2A patients, indicating the value of this model for gaining further insight into the disease pathophysiology.

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