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Cone and Cone-Rod Dystrophies

  • ABCA4 (ATP-binding cassette, sub-family A (ABC1), member 4)
  • PRPH2/RDS (peripherin 2)
  • GUCY2D (Guanylate cyclase 2D, membrane)
  • AIPL1 (Aryl hydrocarbon receptor interacting protein-like 1)
  • CRX (Cone-rod homeobox)
  • GUCA1A (guanylate cyclase activator 1A)
  • CNGB3 (Cyclic nucleotide-gated channel, beta 3)
  • CNGA3 (Cyclic nucleotide-gated channel, alpha 3)
  • CERKL (Ceramide kinase-like)
Cone and cone-rod dystrophies (CRD) have an estimated prevalence of 1 in 40,000 individuals. CRD presents first as a macular disease or as a diffuse retinopathy with predominance of the macular involvement. The clinical signs of CRD reflect the predominant involvement of cones, leading to decreased visual acuity in the first decade of life. However, in some cases, diffuse retinopathy affects simultaneously cones and rods, resulting in both night blindness and loss of visual acuity. The visual field testing shows central scotomas, while the periphery is spared. Fundus examination shows pigment deposits and retinal atrophy in the macular region. At a later stage, patients are legally blind, even though large parts of the peripheral visual field remain preserved. The electroretinogram is distinguished by a more distinctive reduction of the photopic cone b-wave amplitude than the scotopic rod b-wave amplitude, compared to rod degeneration.

Using genomic DNA obtained from the submitted biological material, DNA sequence is obtained and analyzed for exon 13 of the GUCY2D gene, for the coding sequence and splice site junctions of the RDS (exons 1-3), CRX (exons 2-4), AIPL1 (exons 1-6), GUCA1A (exons 1-4), CNGA3 (exons 2-8), and CERKL (exons 1-13) genes. CNGB3 gene testing is offered in two tiers. Tier 1 includes sequence analysis of exon 10 for the common c.1148delC mutation. Tier 2 includes sequence analysis of the remaining 17 exons comprising the coding region and splice junctions of the CNGB3 gene. GeneDx also offers a panel including the sequencing of all coding exons of the RDS gene as well as the ABCA4 gene and their splice junctions that are sequenced using a solid-state sequencing-by-synthesis process.

Each test can be ordered as a reflex test or separately. Mutations found in the first person of a family to be tested are confirmed by repeat analysis using sequencing, restriction fragment analysis, or another appropriate method.

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