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For a model of a human disease to be valid and useful, it is important that key genotypic and phenotypic traits are shared between model system and human. The work in this thesis has been focused on generating new and characterizing spontaneous models of three genetic disorders affecting the retina: retinoblastoma, a childhood cancer with its origin in the fetal retina, Stargardt disease, a juvenile form of macular degeneration, and Bardet-Biedl syndrome, a pleiotropic ciliopathy featuring retinal degeneration. In Paper I, we generated two novel models of MYCN-driven retinoblastoma, in developing chicken retinas and in human retinal organoids, in order to identify its cell of origin and to determine whether MYCN is sufficient in driving tumorigenesis. We found that MYCN was indeed sufficient and that the expression was uniquely tolerated by the cone and horizontal cell progenitor which survived and proliferated anachronistically. The results from our models are in alignment with observations that MYCN-driven retinoblastoma results in a more anaplastic and aggressive form of the cancer and we propose that the fate-restricted progenitor for cones and horizontal cells is its cell of origin. In Paper II, we analyzed the effects of a novel, spontaneous model of ABCA4-mediated Stargardt disease in Labrador retrievers and found that dogs homozygous for the mutation exhibited visual impairment and photoreceptor degeneration. Cone photoreceptors were scarce and the retinal pigment epithelium autofluorescent, indicative of lipofuscin accumulation, a hallmark of Stargardt disease. The phenotype and its etiology in this model are akin to human Stargardt disease. In Paper III, we investigated whether a spontaneous mutation in the TTC8 gene resulted in syndromic Bardet-Biedl syndrome in golden retrievers. We found that, in addition to progressive photoreceptor degeneration and visual impairment, the TTC8 mutation resulted in pleiotropic clinical signs of Bardet-Biedl syndrome with great inter-individual variation, congruent with Bardet-Biedl syndrome in humans. We find that the models described in this thesis are representative of the corresponding genetic disorders in human and that they could be of value for hypothesis generation (in chicken), experimental gene therapy (in dog), and as a human system to test pharmacological interventions (retinal organoids).

In golden retriever dogs, a 1 bp deletion in the canine TTC8 gene has been shown to cause progressive retinal atrophy (PRA), the canine equivalent of retinitis pigmentosa. In humans, TTC8 is also implicated in Bardet&ndash, Biedl syndrome (BBS). To investigate if the affected dogs only exhibit a non-syndromic PRA or develop a syndromic ciliopathy similar to human BBS, we recruited 10 affected dogs to the study. The progression of PRA for two of the dogs was followed for 2 years, and a rigorous clinical characterization allowed a careful comparison with primary and secondary characteristics of human BBS. In addition to PRA, the dogs showed a spectrum of clinical and morphological signs similar to primary and secondary characteristics of human BBS patients, such as obesity, renal anomalies, sperm defects, and anosmia. We used Oxford Nanopore long-read cDNA sequencing to characterize retinal full-length TTC8 transcripts in affected and non-affected dogs, the results of which suggest that three isoforms are transcribed in the retina, and the 1 bp deletion is a loss-of-function mutation, resulting in a canine form of Bardet&ndash, Biedl syndrome with heterogeneous clinical signs.

In golden retriever dogs, a 1 bp deletion in the canine TTC8 gene has been shown to cause progressive retinal atrophy (PRA), the canine equivalent of retinitis pigmentosa. In humans, TTC8 is also implicated in Bardet–Biedl syndrome (BBS). To investigate if the affected dogs only exhibit a non-syndromic PRA or develop a syndromic ciliopathy similar to human BBS, we recruited 10 affected dogs to the study. The progression of PRA for two of the dogs was followed for 2 years, and a rigorous clinical characterization allowed a careful comparison with primary and secondary characteristics of human BBS. In addition to PRA, the dogs showed a spectrum of clinical and morphological signs similar to primary and secondary characteristics of human BBS patients, such as obesity, renal anomalies, sperm defects, and anosmia. We used Oxford Nanopore long-read cDNA sequencing to characterize retinal full-length TTC8 transcripts in affected and non-affected dogs, the results of which suggest that three isoforms are transcribed in the retina, and the 1 bp deletion is a loss-of-function mutation, resulting in a canine form of Bardet–Biedl syndrome with heterogeneous clinical signs.