Unique point mutations in OPN1LW associated with protanopia.




Anthony W. Paratore

Introduction

The OPN1LW gene, located on the q-arm of the X-chromosome at location Xq28, is commonly mutated in individuals with protanopia, a moderately severe color vision defect that renders dichromatic vision. The product of OPN1LW translation is a normally functioning photosensitive pigment named opsin located exclusively in the cone cells of the retina. Point mutations, deletions, etc. to OPN1LW disrupt the cone cell’s spectral sensitivity to light, especially at wavelength 492 nm, dimming the brightness of red, orange, and yellows to a level where reds can be confused with black or dark gray, and red traffic lights may not appear be illuminated. Recently published studies indicate protanopia affects 7 percent of the US male population (approximately 10 million men). Primers were designed to anneal the six exons of OPN1LW, and subsequent PCR allowed for the comparison of sequences from an individual with protanopia (mutant), an individual without protanopia (wild type) and the published National Center for Biotechnology Information sequence for the OPN1LW gene.

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In this study, exons 1 – 6 of the OPN1LW gene were amplified and analyzed via PCR and sequencing technologies. The full sequence of the exon (protein coding portion) of the gene was aligned using multiple sequence aligning software (Geneious Pro 2.5.4). Aligned exonic regions were also translated into their protein sequence (Geneious Pro 2.5.4) to confirm similarity in eventual gene products and to assess the effect individual mutations had on amino acid selection. Similarity for both nucleotide sequence and protein product were analyzed. Mutant type sequence (DNA from Anthony Paratore) and wild type sequence (DNA from Berish Rubin) were compared to the published sequence (NCBI NM_000513) for OPN1LW gene. Alignment of mutant and wild type to the published sequence was performed to show the variability of sequence that can occur without generating protanopia. Analysis of exon 1, 2 and 4 demonstrated that the mutation causing protanopia is not located in these coding regions. Analysis of exon 3, 5, 6 was inconclusive because reverse sequencing data was not available to verify sequencing results. Further sequencing is required to rectify this problem. Ironically, exon 6, despite numerous mutations, did not exhibit the characteristic Gly->Glu substitution, well established to cause some forms of protanopia. This may indicate that protanopia is more likely caused by whole gene rearrangement rather than by individual point mutations. In theory, red-green dichromacy (protanopia) can arise from a single point mutation occurring in the coding sequence of the L- or M-cone pigment genes (i.e. OPN1LW) which vitiates and destabilizes the opsin protein or impairs its quantum efficiency (ability to capture wavelengths of differing value that correspond to different colors). Alternatively, protanopia may arise from major deletions/insertions in the coding sequences, such as those exhibited in exon 6 of the mutant sequence. Furthermore, 4 – 8% percent of Caucasian males with normal vision have a 5'M 3'L hybrid gene in addition to normal L- and M-cone pigment genes. However, it is evident that the expression level of this hybrid gene is insufficient to disrupt normal color vision in these individuals.

Full Paper

Acknowledgments

I would like to extend my gratitude to Dr. Berish Rubin and Dr. Sylvia Anderson for thoughtful comments and guidance during this project. Also, I am deeply indebted to Leleesha Samaraweera, Jinsong Qiu and Joseph A. Frezzo for their patience and endless supply of answers for my endless supply of questions. I would also like to thank the biology department of Fordham University for funding this project.


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