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Candice Davidoff, Jay Neitz, Maureen Neitz; Frequency of atypical genotypes associated with normal and defective color vision. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3022.
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The majority of red-green color vision deficiencies result from gene rearrangements that cause the loss of normal L or M opsin expression. This can be due to deletion of an L or M gene or insertion of an extra L gene between the normal L and M genes, displacing the M gene to a non-expressed position. Thus, characterizing the number of L and M genes is a good predictor of the presence and type of congenital red-green color vision defects. However, two atypical types of mutations, missense mutations and extra L opsin genes downstream of the expressed positions, would result in misdiagnoses based on L and M gene stoichiometry. This study aims to estimate the frequency of these types of mutations in the population.
For 803 males unselected for color vision deficiencies, PCR was performed to selectively amplify L or M opsin genes. From these products exons 3 and 4 were sequenced. The number and type of opsin genes on the X-chromosome were determined by SNP analysis using Sequenom's MassARRAY system. For samples found to have extra L genes, long range PCR was performed to amplify the most downstream gene and verify whether it encoded an L or M opsin. For samples with missense mutations, parts of the last gene were sequenced to determine if the mutation was in a gene in an expressed position.
3 missense mutations were identified in L opsin genes: R151T, V171L and V232L. 8 mutations were found in M opsin genes: six C203R, one R163I and one synonymous mutation. No C203R mutation was observed in any L opsin gene. 74 samples were found to have extra L genes. Of these 25 were confirmed to have the extra L in an expressed position, 20 were confirmed to have L genes at the end of the array and thus are likely to have normal color vision, and 29 had arrays too long to determine the position of the extra L gene.
C203R mutations are unlikely to cause protan defects. Missense mutations that may cause protan defects were present in 0.37% of the sample and those that may cause deutan defects were detected in 1%. 9% of men in this sample have more than one L gene. Of those, at least a quarter had the genetic basis for normal color vision despite the presence of an extra L gene. 3.1% of men could be confirmed to have an L gene in the second position and are presumed to have deuteranomaly and another 3.6% may be deuteranomalous depending on the location of the extra L gene(s) in arrays with > 3 genes.
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