Mechanisms of cone dysfunction in inherited retinal degenerations

This thesis examines the mechanisms of cone dysfunction in two different inherited retinal diseases: Achromatopsia and Retinitis Pigmentosa (RP). The first, minor, component of the thesis explored and characterised the loss of cone function in a novel mouse model of achromatopsia (cpfl10). The second, major, component of the thesis explored the loss of cone function in a mouse model of RP. I hypothesised that the cone function lost in RP could be restored using AAV vectors to over-express down regulated cone genes. This thesis subsequently developed and tested two AAV vectors delivering CRX and OPN1LW genes to photoreceptors in RP mice. Achromatopsia is a congenital condition characterised by absent or diminished cone function due to mutations of a single gene, typically those involved in the cone phototransduction cascade. This thesis provides a detailed phenotype of a novel cpfl10 mouse model of Achromatopsia that arose spontaneously in a local mouse colony. The causative (NM_013927)c.692G>A; p.(R231H) missense mutation in Cngb3 was also identified and explored. Retinitis pigmentosa (RP) is an inherited retinal degeneration in which secondary loss of cones follows the initial death of genetically abnormal rod photoreceptors. Strategies to prolong cone photosensitivity or survival would offer an important treatment. Such universal interventions could be effective regardless of which genetic mutation is underlying primary rod death. The molecular mechanisms by which cone photoreceptors lose photosensitivity in RP are poorly understood. I examined the gene expression profile of the cone phototransduction cascade in the Rho-/- mouse model of RP and correlated these changes to declining retinal function. The cone opsins were substantially down-regulated, as was the photoreceptor transcription factor Crx. I hypothesised that the cone function lost in RP could be restored using AAV vectors to over-express these down regulated cone genes. Using an AAV gene therapy vectors, I delivered the human homologs CRX and OPN1LW to the retinas of the same Rho-/- mouse model. Both vectors successfully transduced photoreceptors and RPE cells across three different doses. Transduced retinas did not show any improvement in cone function or survival. Whilst the observed down-regulation of cone genes correlates with cone function loss, it appears that both down-regulation and dysfunction arise from an unknown common cause. Ongoing research into mechanisms of metabolic starvation, oxidative stress and trophic factors in cone cell death offer promise for a common intervention to rescue cone photoreceptors.