De novo mutations in the CRX homeobox gene associated with Leber congenital amaurosis

Leber congenital amaurosis (LCA) is a clinically heterogeneous group of childhood retinal degenerations, generally reported to be inherited in an autosomal recessive manner. Affected infants have little or no retinal photoreceptor function as tested by electroretinography. Mutations in the photoreceptor guanylate cyclase gene, RETGC as well as in the retinal pigment epithelial cell gene, RPE65 have been identified in some individuals with autosomal recessive LCA. Additional LCA candidate genes include those involved in phototransduction processes or RPE function. Alternatively, mutations in genes required for photoreceptor development or maintenance may also lead to an LCA phenotype. We reported previously that mutations in a novel photoreceptor-specific homeobox gene, CRX, cause autosomal dominant cone-rod dystrophy (adCRD) a disorder in which visual acuity and colour vision losses begin in the first decade of life or later. This disease progresses to cause marked central visual field defects and pigmentary retinopathy. The CRX homeodomain transcription factor regulates the expression of photoreceptor outer segment proteins such as rhodopsin, IRBP, beta -PDE and arrestin. Because CRX is essential for photoreceptor maintenance, and because expression of a dominant-negative CRX allele in developing retina prevented outer segment biogenesis, we tested the hypothesis that CRX mutations are responsible for the LCA phenotype by examining the CRX gene of 74 LCA patients. Probands met the following diagnostic criteria: severe visual loss with nystagmus noted in the first few months of life, a markedly reduced or non-detectable electroretinogram, absence of other ocular causes for the visual disturbance, and no accompanying systemic diseases. Informed consent was obtained from all subjects after the nature of the procedures was fully explained. Using SSCP analysis and direct sequencing of PCR-amplified exons as described previously, we identified two putative disease-causing de novo deletion mutations in CRX: E168 Delta 2bp and G217 Delta 1bp. These mutations were not present in 360 control CRX alleles.