S S Deeb

Analysis of red/green color discrimination in subjects with a single X-linked photopigment gene.

Many subjects despite having only a single X-linked pigment gene (single-L/M-gene subjects) are able to make chromatic discriminations by Rayleigh matching, especially when large fields are used. We used a combination of psychophysics (Rayleigh match), electroretinograms (ERG), and molecular genetic techniques to rule out several possible explanations of this phenomenon. Use of rods for chromatic discrimination was unlikely since strong adapting fields were employed and the large-field match results were not consistent with rod participation. A putative mid- to long-wavelength photopigment that escapes detection by current molecular genetic analysis was ruled out by finding only a single L/M photopigment in flicker ERGs from 16 single-L/M-gene subjects. Large-field match results were not consistent with participation of S cones. Amino acid sequence polymorphisms in the S-pigment gene that might have shifted the S cone spectrum towards longer wavelengths were not found on sequencing. The mechanism of chromatic discrimination in the presence of a single photopigment therefore remains unknown. Further possible explanations such as variations in cone pigment density and retinal inhomogeneities are discussed.

Severity of color vision defects: electroretinographic (ERG), molecular and behavioral studies.

Earlier research on phenotype/genotype relationships in color vision has shown imperfect predictability of color matching from the photopigment spectral sensitivities inferred from molecular genetic analysis. We previously observed that not all of the genes of the X-chromosome linked photopigment gene locus are expressed in the retina. Since sequence analysis of DNA does not necessarily reveal which of the genes are expressed into photopigments, we used ERG spectral sensitivities and adaptation measurements to assess expressed photopigment complement. Many deuteranomalous subjects had L, M, and L-M hybrid genes. The ERG results showed that M pigment is not present in measurable quantities in deutan subjects. Using these results to determine gene expression improved the correlations between inferred pigment separation and color matching. Furthermore, we found a subject who had normal L and M genes and normal proximal promoter sequences, yet he had a single photopigment (M) by ERG and tested as a protanope. These results demonstrate the utility of ERG measurements in studies of molecular genetics of color vision deficiencies, and further support the conclusion that not all genes are expressed in color deficient subjects. In particular, deuteranomaly requires a presently unknown mechanism of selective expression which excludes normal M genes and allows expression of L-M hybrid genes in one cone type, and the normal L in another.

Levels of expression of the red, green and green-red hybrid pigment genes in the human retina

Correlations between the genotype at the X-chromosome-linked red/green pigment genes and the colour vision phenotype will be aided by considering the relative levels of expression of these genes into mRNA and protein in the retina. The red and green pigment genes exist in head-to-tail arrays: one red pigment gene is found 5′ upstream of one or more green pigment genes. In addition to normal red and green pigment genes, 5′green-red3′ hybrid genes, typically associated with deuteranomaly, may also be found in subjects with normal colour vision. A locus control region (LCR), located 5′ upstream of the red pigment gene, is shown to be required for expression of the pigment genes in cone photoreceptors. We hypothesize that the expression of any pigment gene in an array is inversely proportional to its distance from this LCR. Green-red hybrid genes that occupy proximal positions in the array are more likely to be expressed and cause deuteranomaly, whereas those that occupy distal positions are less likely to be expressed, and do not result in defective colour vision.