Molecular genetics and single-gene dichromats

    More information on this topic can be found elsewhere (Nathans et al., 1992; Sharpe et al., 1998; Sharpe et al., 1999; Stockman et al., 2000). Here, we provide a brief summary of those areas that are relevant to cone spectral sensitivity measurements made in dichromats in order to determine the "normal" M- and L-cone spectral sensitivities.

    The M- and L-cone photopigment genes lie in a head to tail tandem array on the q-arm of the X-chromosome. Each gene consists of six coding regions, called exons, which are transcribed to produce the opsin. Because the M- and L-cone photopigment genes are highly homologous and adjacent to one another, intragenic recombination between them is common and can lead to the production of hybrid or fusion genes, some of which code for anomalous pigments. Each hybrid gene can be identified by the site, usually between exons, at which the fusion occurs. For example, L3M4 indicates a hybrid gene in which exons 1 to 3 derive from an L-cone pigment gene and exons 4 to 6 from an M-cone pigment gene. Because exons 1 and 6 in the L- and M-cone pigment genes are identical, a L1M2 hybrid pigment gene encodes a de facto M-cone photopigment.

    The classification of hybrid genes, and genes in general, is complicated by polymorphisms in the normal population, the most common of which is the frequent substitution of alanine by serine at codon 180 in exon 3. Of 304 genotyped Caucasian males, we estimate that 56% have the serine variant [identified as L(S180)] and 44% the alanine variant [identified as L(A180)] for their L-cone gene (see Table 1, which summarizes data from Winderickx et al., 1993; Neitz & Neitz, 1998; Sharpe et al., 1998; and from Schmidt, Sharpe, Knau & Wissinger, personal communication).

Table 1

Summary of sources and data concerning the fraction of male Caucasian subjects with the L(S180) polymorphism. The mean fraction of 0.56 L(S180) to 0.44 L(A180) is the one used in the analysis.

    The L-cone polymorphism, and its distribution in the normal population, must be considered when estimating the "normal" L-cone spectral sensitivities. In contrast, the M(A180) versus M(S180) polymorphism for the M-cone pigment is much less frequent: 94% (Winderickx et al., 1993) or 93% (Neitz & Neitz, 1998) of males have the M(A180) variant.

    The spectral sensitivity of the photopigment that is encoded by the L2M3(A180) hybrid gene is practically indistinguishable from the photopigment encoded by the normal M(A180) [(or L1M2(A180)] cone pigment gene, its λ_max being only 0.2 nm (Merbs & Nathans, 1992) or 0.0 nm (Asenjo, Rim & Oprian, 1994) or insignificantly different (Sharpe et al., 1998) from that of the M(180) cone pigment. Thus, spectral sensitivities from protanopes carrying either L1M2(A180) or L2M3(A180) genes in their opsin gene array can be reasonably combined to estimate the normal M-cone spectral sensitivities, as was done by Stockman and Sharpe (2000).

    Dichromats with single photopigment genes in the M- and L-cone pigment gene array [e.g., L(A180), L(S180), L1M2(A180) or L2M3(A180)] are especially useful for measuring normal cone spectral sensitivities, since they should possess only a single longer wavelength photopigment. Dichromats with multiple photopigment genes are less useful, unless the multiple genes produce photopigments with the same or nearly the same spectral sensitivities: for example, L1M2(A180)+M(A180) or L2M3(A180)+M(A180).

References

Asenjo, A. B., Rim, J., & Oprian, D. D. (1994). Molecular determinants of human red/green color discrimination. Neuron, 12, 1131-1138.

Merbs, S. L., & Nathans, J. (1992). Absorption spectra of the hybrid pigments responsible for anomalous color vision. Science, 258, 464-466.

Nathans, J., Merbs, S. L., Sung, C.-H., Weitz, C. J., & Wang, Y. (1992). Molecular genetics of human visual pigments. Annual Review of Genetics, 26, 401-422.

Neitz, M., & Neitz, J. (1998). Molecular genetics and the biological basis of color vision. In W. G. K. Backhaus, R. Kliegl, & J. S. Werner (Eds.), Color vision: perspectives from different disciplines (pp. 101-119). Berlin: Walter de Gruyter.

Sharpe, L. T., Stockman, A., Jägle, H., Knau, H., Klausen, G., Reitner, A., & Nathans, J. (1998). Red, green, and red-green hybrid photopigments in the human retina: correlations between deduced protein sequences and psychophysically-measured spectral sensitivities. Journal of Neuroscience, 18, 10053-10069.

Sharpe, L. T., Stockman, A., Jägle, H., & Nathans, J. (1999). Opsin genes, cone photopigments, color vision and colorblindness. In K. Gegenfurtner & L. T. Sharpe (Eds.), Color vision: from genes to perception (pp. 3-50). Cambridge: Cambridge University Press.

Stockman, A., & Sharpe, L. T. (2000). Spectral sensitivities of the middle- and long-wavelength sensitive cones derived from measurements in observers of known genotype. Vision Research, 40, 1711-1737.

Stockman, A., Sharpe, L. T., Merbs, S., & Nathans, J. (2000). Spectral sensitivities of human cone visual pigments determined in vivo and in vitro. Vertebrate phototransduction and the visual cycle, Part B.  Methods in Enzymology, Vol. 316, pp. 626-650.  New York: Academic.

Winderickx, J., Battisti, L., Hibiya, Y., Motulsky, A. G., & Deeb, S. S. (1993). Haplotype diversity in the human red and green opsin genes: evidence for frequent sequence exchange in exon 3. Human Molecular Genetics, 2, 1413-1421.