Investigating Disease Presentation and Mechanism in RP1L1-Associated Photoreceptor Degeneration

Abstract

Photoreceptor disease results in irreparable vision loss and blindness, which has a dramatic impact on quality of life. Retinitis pigmentosa 1-like 1 (RP1L1) is a component of the photoreceptor axoneme, the backbone structure of the photoreceptor’s light-sensing outer segment. Pathogenic variants in RP1L1 lead to a cone disease called occult macular dystrophy (OMD) and a rod disease known as retinitis pigmentosa (RP). This indicates that RP1L1 has important, but distinct, roles in rod and cone biology, though its exact function is unknown. In this thesis, I summarize reported RP1L1-associated photoreceptor conditions and report novel associations between RP1L1 and additional photoreceptor conditions outside of OMD and RP. I found that OMD can progress to an RP1L1 maculopathy with visible fundus findings in patients with the most common OMD-causing RP1L1 mutation. Further, I report RP1L1 maculopathies with novel RP1L1 variants which may have initially started as OMD and progressed to more severe macular degenerations. In addition, RP1L1 was associated with a rod-cone dystrophy. This is the first report of simultaneous rod and cone disease due to RP1L1 mutations. Zebrafish are an instrumental system for the generation of photoreceptor degeneration models, which can be utilized to determine underlying causes of photoreceptor dysfunction and death. To investigate the potential function of RP1L1 in photoreceptors and model RP1L1-associated disease, I generated an rp1l1 zebrafish mutant using CRISPR/Cas9 genome editing. The introduced rp1l1 mutation is predicted to severely truncate the Rp1l1 protein and abolish all functional domains. Homozygous rp1l1 mutant zebrafish had progressive photoreceptor functional defects that began with the rod photoreceptors, as determined by electroretinographic assessment. Live imaging via optical coherence tomography revealed that rp1l1 homozygotes had gaps in the photoreceptor layer, disrupted photoreceptor mosaics, and thin retinas, indicative of outer retinal degeneration. Mutant photoreceptor outer segments were disorganized and lacked uniformity, with wavy discs, and in some cases appeared extremely abnormal with outer segment vesicles or atypical morphology. Intriguingly, the rp1l1 mutants also had lipid-rich subretinal deposits between the photoreceptors and retinal pigment epithelium. The rp1l1 mutant is a novel model of RP1L1-associated photoreceptor disease and the first zebrafish model of photoreceptor degeneration with reported subretinal drusenoid-like deposits, a feature of age-related macular degeneration. This model could be further utilized to determine the mechanism underlying photoreceptor degeneration in RP1L1-associated RP. Additionally, as a unique zebrafish model of photoreceptor disease with subretinal lipid deposits, these rp1l1 mutants could be further utilized to inform etiology of ocular deposits observed in other retinal degenerative conditions.