S-Cone Dystrophy in Tritan Color-Vision Deficiency Revealed by Adaptive-Optics Retinal Imaging

Abstract

Purpose:: Tritan color-vision deficiency is an autosomal dominant disorder associated with amino-acid substitutions in the S-cone-pigment gene that are predicted to perturb the structure or stability of the S-cone pigment. The aim of this work was to determine whether the loss of S-cone function is accompanied by physical disruption of the cone mosaic.

Methods:: An adaptive-optics ophthalmoscope was used to image the cone mosaic of a 56-year-old male and his 33-year-old daughter, each having a novel mutation (R283Q) in one of their S-cone pigment genes. The density and regularity of the cone mosaic of each subject was compared with that of normal controls. Subjects also performed standard color-vision tests and surface-color matching under different illuminants.

Results:: The father’s behavior on all color-vision tests was characteristic of a tritanope, whereas the daughter made only mild tritan errors. His surface-color judgments were also characteristic of a tritanope. Retinal imaging revealed different S-cone mosaics consistent with their discrepant phenotypes: no evidence for S cones was found in the retinal images from the father, whereas the daughter had normal S-cone density. Voronoi and nearest-neighbor analyses showed that the father’s mosaic was significantly more irregular than of normal controls.

Conclusions:: The absence of S-cones coupled with the abnormal packing arrangement of the remaining cones in the 56-year-old tritan suggests that heterozygosity for the R283Q mutation ultimately results in the death of S-cones. It is hypothesized that the phenotypic difference between the father and daughter with the same mutation is due to their being at different stages in a progression where dominant negative interactions compromise the function and viability of S-cones.

Keywords: color vision • retinal degenerations: hereditary • imaging/image analysis: clinical