Purpose: Recessive mutations in the human IQCB1/NPHP5 gene are associated with Senior-Løken syndrome (SLS), a ciliopathy presenting with nephronophthisis and Leber congenital amaurosis (LCA). Nphp5-knockout mice develop LCA without nephronophthisis. Mutant rods rapidly degenerate while mutant cones survive for months. The purpose of this study was to reinitiate cone ciliogenesis in a Nphp5 -/-; Nrl -/- mouse with viral expression of full-length NPHP5 and rescue function.
Methods: Nphp5 -/- mice were mated with Nrl -/- mice to generate Nphp5-/-; Nrl-/- double-knockouts. Nphp5-/-; Nrl-/- mice and Nphp5+/-; Nrl-/- controls were phenotyped with confocal microscopy from postnatal day 10 (P10) until 6 months of age. Nphp5-/-; Nrl-/- mice and Nphp5+/-; Nrl-/- controls were injected at P15 with self-complementary adenoassociated virus 8 (Y733F) (AAV8(Y733F)) expressing GRK1-FL-cNPHP5. Expression of mutant NPHP5 was verified with confocal microscopy and electroretinography (ERG).
Results: In the Nphp5 -/- and cone-only Nphp5 -/-; Nrl -/- mice, cone outer segments did not form, but mutant cones continued to express cone pigments in the inner segments without obvious signs of cone cell death. The mutant cone outer nuclear layer (ONL) and the inner segments were stable for more than 6 months in the cone-only Nphp5 -/-; Nrl -/- retinas. Viral expression of NPHP5 initiated after eye opening showed that connecting cilia and RP1-positive axonemes were formed. Furthermore, cone pigments and other cone outer segment proteins (cone transducin and cone PDE6) were present in the nascent mutant cone outer segments, and rescued mutant cones exhibited a significant photopic b-wave (30% of Nphp5 +/-; Nrl -/- controls).
Conclusions: Nphp5-/-; Nrl-/- cones persistently express cone pigments in the inner segments without obvious degeneration, providing an extended duration interval for viral gene expression. Viral expression of full-length NPHP5 initiates ciliogenesis between P15 and P60, and mutant cones are, in part, functional, encouraging future retina gene replacement therapy.