Abstract
Abstract: :
Purpose:In the adult human outer retina, each of the 4 photoreceptor types is characterized by the exclusive expression of a different photopigment. The effect of targeted disruption of the rhodopsin gene, which is exclusively expressed in rods, has been studied in mice. Rod outer segments failed to form in homozygous mice lacking both rhodopsin alleles, and within months of birth, rods degenerated completely. Learning the effects of disrupting cone pigment expression is essential to understand the role of the opsin in the development of human cones. Methods:The Rochester Adaptive Optics Ophthalmoscope was used to image the cone mosaic of males who had a single X–linked opsin gene and one female who was heterozygous for a 52 kb deletion on one X–chromosome. This deletion disrupts all cone opsin gene expression in cones in which that X–chromosome is active. These retinas were compared to those from individuals with normal opsin gene arrays. Results:Males who had a single opsin gene on the X–chromosome had cone densities similar to that of individuals with normal opsin gene arrays. However the female heterozygote, who could not produce any cone opsin in about one–half of her cones (assuming random X–inactivation), had half the normal number of cones. Her remaining cones were expanded in size to about 1.4 times the normal diameter. Conclusions:Normal cone density observed here in single–gene dichromats is consistent with a theory in which the L and M cones represent a single cell type. Within each L/M cell the opsin genes compete for exclusive expression such that each mature cone expresses only L or only M. The only effect of reducing the number of X–linked genes to one is to eliminate this competition, so that all L/M cones express the one remaining opsin gene. However, eliminating opsin expression in a cone causes it to degenerate. Differences in the mosaic when no opsin is expressed in some cones, compared to a previously reported case in which a mutant cone opsin was expressed, suggest that the absence of opsin causes the cones to degenerate at a stage of development before the fovea is fully formed.
Keywords: color pigments and opsins • imaging/image analysis: non-clinical • photoreceptors