Deletion of M-Opsin Prevents M Cone Degeneration in a Mouse Model of Leber Congenital Amaurosis

Mutations in retinoid isomerase (RPE65) or lecithin-retinol acyltransferase (LRAT) disrupt 11-cis-retinal synthesis and cause Leber congenital amaurosis (LCA). Despite the success of recent RPE65 gene therapy, follow-up studies show that patients continue to experience photoreceptor degeneration and lose vision benefit over time. In Lrat(-/-) mouse model, mislocalized medium (M)-wavelength opsin was degraded, whereas mislocalized short (S)-wavelength opsin accumulated before the onset of cone degeneration. The mechanism for the foveal M/long-wavelength cone photoreceptor degeneration in LCA is unknown. By crossing Lrat(-/-) mice with a proteasome reporter mouse strain, this study showed that M-opsin-enriched dorsal cones in Lrat(-/-) mice exhibit proteasome stress because of the degradation of large amounts of M-opsin. Deletion of M-opsin relieves the proteasome stress and completely prevents M cone degeneration in Lrat(-/-)Opn1sw(-/-) mice (a pure M cone LCA model, Opn1sw encoding S-opsin) for at least 12 months. These results suggest that M-opsin degradation-associated proteasome stress plays a major role in M cone degeneration in Lrat(-/-) model. This finding may represent a general mechanism for M cone degeneration in multiple forms of cone degeneration because of M-opsin mislocalization and degradation. These results have important implications for the current gene therapy strategy for LCA that emphasizes the need for combinatorial therapies to both improve vision and slow photoreceptor degeneration.