Abstract
Light absorption by photopigment molecules expressed in the photoreceptors in the retina is the first step in seeing. Two types of photoreceptors in the human retina are responsible for image formation: rods, and cones. Except at very low light levels when rods are active, all vision is based on cones. Cones mediate high acuity vision and color vision. Furthermore, they are critically important in the visual feedback mechanism that regulates refractive development of the eye during childhood. The human retina contains a mosaic of three cone types, short-wavelength (S), long-wavelength (L), and middle-wavelength (M) sensitive; however, the vast majority (~94%) are L and M cones. The OPN1LW and OPN1MW genes, located on the X-chromosome at Xq28, encode the protein component of the light-sensitive photopigments expressed in the L and M cones. Diverse haplotypes of exon 3 of the OPN1LW and OPN1MW genes arose thru unequal recombination mechanisms that have intermixed the genes. A subset of the haplotypes causes exon 3- skipping during pre-messenger RNA splicing and are associated with vision disorders. Here, we review the mechanism by which splicing defects in these genes cause vision disorders.
Highlights
Exons comprise overlaid splicing and protein codes
Middle wavelength sensitive (M) opsin genes (OPN1LW and OPN1MW, respectively) of Old World nonhuman primates have stereotyped differences [8] that presumably were shaped by the dual evolutionary pressures of the superimposed exon splicing and protein codes [9,10]
The OPN1LW and OPN1MW genes are arranged in a head-to-tail tandem array on the X-chromosome [16,17]; they share greater than 96% nucleotide sequence identity over their entire ~40 kilobase pair length [18]
Summary
Exons comprise overlaid splicing and protein codes. Exonic splicing enhancers and silencers (ESEs and ESSs) were initially discovered in alternatively spliced exons but are known to control the splicing of constitutive exons [1,2,3,4,5,6,7]. LVAVA splicing-defective opsin gene haplotype is associated with both syndromic and non-syndromic myopia caused by excessive axial growth of the eye [22,30,32,33]. The LVAVA splicing-defective opsin gene haplotype is associated with both syndromic and non-syndromic myopia caused by excessive axial growth of the eye [22,30,32,33]. An inherited form of high myopia linked to Xq28 [59] occurs in males who express an Xq28 cone opsin gene with the LVAVA haplotype and second cone opsin gene with a haplotype exhibiting very little exon 3-skipping [30] These high myopes have two types of L/M cones that harbor dramatically different amounts of photopigment. The human data were consistent with a slow degeneration associated with LVAVA but not LIAVA, presumably due to the small amount of mutant photopigment made in cones expressing the LVAVA haplotype
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