Retinal abnormalities associated with the G90D mutation in opsin

Several mutations in the opsin gene have been associated with congenital stationary night blindness, considered to be a relatively nonprogressive disorder. In the present study, we examined the structural and functional changes induced by one of these mutations, i.e., substitution of aspartic acid for glycine at position 90 (G90D). Transgenic mice were created in which the ratio of transgenic opsin transcript to endogenous was 0.5:1, 1.7:1, or 2.5:1 and were studied via light and electron microscopy, immunocytochemistry, electroretinography (ERG), and spectrophotometry. Retinas with transgenic opsin levels equivalent to one endogenous allele (G_0.5) appeared normal for a period of about 3-4 months, but at later ages there were disorganized, shortened rod outer segments (ROS), and a loss of photoreceptor nuclei. Higher levels of G90D opsin expression produced earlier signs of retinal degeneration and more severe disruption of photoreceptor morphology. Despite these adverse effects, the mutation had a positive effect on the retinas of rhodopsin knockout (R^-/-) mice, whose visual cells fail to form ROS and rapidly degenerate. Incorporation of the transgene in the null background (G ^+/-/R^-/- or G^-/-/R^-/-) led to the development of ROS containing G90D opsin and prolonged survival of photoreceptors. Absorbance spectra measured both in vitro and in situ showed a significant reduction of more than 90% in the amount of light-sensitive pigment in the retinas of G^+/+/R^-/- mice, and ERG recordings revealed a >1 log unit loss in sensitivity. However, the histological appearances of the retinas of these mice show no significant loss of photoreceptors and little change in the lengths of their outer segments. These findings suggest that much of the ERG sensitivity loss derives from the reduced quantal absorption that results from a failure of G90D opsin to bind to its chromophore and form a normal complement of light-sensitive visual pigment.