UC San Diego

Ciliopathies comprise a highly heterogeneous group of genetic disorders attributed to dysfunction of the primary cilium. Joubert syndrome is a ciliopathy that primarily affects the central nervous system (CNS), and is specifically characterized by ataxia, hypotonia, and neonatal apnea associated with cerebellar hypoplasia and malformation of the midbrain-hindbrain junction. Absence of AHI1, which encodes a cilium-localized protein, has been shown to cause a form of Joubert syndrome that is also highly penetrant for retinal degeneration¹'²but the mechanisms underlying the pathogenesis of Joubert syndrome and related disorders are not well understood. To study the role of Ahi1 in a more tractable model, I have targeted the Ahi1 gene in the mouse to generate germline and conditional null alleles, and used tissue culture, mouse genetic analysis, histopathology and immunochemical techniques to study these mouse mutants. In contrast to human patients with the disease, brains from Ahi1-/- mice were nominally affected. Despite this, analysis of retina from Ahi1-/- mice revealed a phenotype consistent with the retinal involvement frequently observed in patients with deleterious AHI1 mutations. Specifically, Ahi1 knockout mice failed to form outer segments (OS), and showed abnormal distribution of opsin throughout photoreceptors. This was followed by rapid degeneration of the outer nuclear layer through apoptotic mechanisms attributed to the ectopic accumulation of opsin. Through analysis of double mutant mouse lines, I also found that the phenotype displayed dosage-sensitive genetic interactions with Nphp1, another ciliopathy gene. Although not a primary cause of retinal blindness in humans, I found that an allele of AHI1 modifies the relative risk of retinal degeneration greater than 7 fold within a cohort of nephronopthisis patients. These data support context-specific roles for AHI1 as a contributor to retinopathy and may explain a proportion of the variability of retinal phenotypes observed in nephronophthisis