Abstract
Purpose: :
To study the molecular basis and prevalence of different mutations in the red/green opsin gene cluster in a large series of patients with blue cone monochromacy (BCM).
Methods: :
The structure of the red/green opsin gene cluster was analyzed in 40 independent BCM patients by means of PCR amplification, PCR/RFLP, DNA sequencing and Southern Blot hybridizations. X chromosome inactivation assays were done for the methylation sensitive markers ARC and DXS6673E using DNA isolated from venous blood samples.
Results: :
In 12 patients we found deletions of various sizes that encompass either the locus control region (LCR) or the LCR and parts of the opsin genes itself. Single red or red/green hybrid opsin genes that carry the known Cys203Arg mutations were present in 18 patients. However we observed considerable opsin gene sequence variability in this group which excludes a common founder mutation event. In addition we identified the Cys203Arg in 9 subjects with multiple opsin genes where all copies harbour this mutation. Finally we identified one patient with a single red opsin gene that carries a new nonsense mutation (W90X). In 4 carrier females with reduced visual function we were able to show preferential inactivation of the non–mutant X chromosome.
Conclusions: :
The presence of the Cys203Arg mutation was the most prevalent cause of BCM that accounted for more than 2/3 of all cases in our sample. Yet there is considerable genetic heterogeneity in such cases with respect to opsin gene copy number and the identity of the mutated gene copy/copies. We therefore propose that unequal homologous recombination is the driving force for the multiple independent occurrence of this genotype. We also provide evidence that skewed X inactivation may result in reduced visual function in BCM carrier females.
Keywords: retinal degenerations: hereditary • color pigments and opsins • photoreceptors