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R. Metlapally, A. Bulusu, A. E. Shay, C. C. Kroner, S. Zuchner, Y.-J. Li, T. L. Young; X-Linked High Myopia: Evaluation of CXorf2 as a Candidate Gene. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1322.
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© ARVO (1962-2015); The Authors (2016-present)
X-linked high myopia with mild cone dysfunction and protanopia has been mapped to chromosome Xq28. CXorf2 is a nested gene within the red and green opsin cone pigment gene tandem array on chromosome Xq28. We investigated whether the X-linked myopia phenotype is a consequence of CXorf2 gene alterations by screening for genomic DNA sequence mutations and copy number variations in a large pedigree (5 affected and 12 unaffected).
Primers were designed to amplify the 5 exons of the CXorf2 gene including intron /exon boundaries. Amplified PCR products were sequenced using standard techniques. To examine the copy number variation, quantitative real-time (ABI7900HT Fast Real-time PCR System) gene expression assays (Assays-by-DesignTM) consisting of unlabeled PCR primers and TaqMan® MGB probe (FAMTM dye-labeled) were performed using patient pedigree sample genomic DNA. A similar combined red-green opsin gene assay was designed to validate the sensitivity of the experiment, as the opsin copy numbers were previously determined in the pedigree. GAPDH served as an endogenous reference. Data were analyzed using the Comparative CT method, and results were plotted as the copy number present in each individual with respect to a calibrator.
A 5’ untranslated region (UTR) single nucleotide polymorphism (SNP) in CXorf2 was observed in all five affected males in the pedigree, but not in unaffected males. Of the 183 external controls screened, 6.5% possessed this SNP. Logistic regression analysis, which takes into account family strata, revealed significant association (p=0.0004) between this SNP and the disease status. The opsin gene copy number observed in affected males, control males, and a carrier female (4, 3 and 7, respectively) confirmed previous testing. Interestingly, while only 3 copies of CXorf2 have been reported within the opsin array, the CXorf2 gene targeted assay on affected male individuals in this pedigree revealed 5 copies.
The 5’UTR is thought to influence mRNA stability and translation efficiency, and 5’ UTR SNPs have been associated with disease. Copy number variations play a role in disease inheritance and susceptibility as they affect gene dosage. These CXorf2 gene alterations may be responsible for this phenotype.
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