Leber Congenital Amaurosis 1 (LCA1) and Cone-Rod dystrophy 6 (CORD6) via the GUCY2D Gene

Leber congenital amaurosis (LCA, OMIM 204000) is the most severe form of inherited retinal degeneration that is usually evident at birth or during the first months of life. LCA is clinically characterized by poor visual function often accompanied by nystagmus, abnormal pupillary responses, photophobia, high hyperopia, markedly diminished electroretinogram (ERG) and keratoconus condition due to oculo-digital signs of Franceschetti such as eye poking, pressing, and rubbing the eyes with a knuckle or finger (Weleber et al. GeneReviews, 2013; Perrault et al. Nat Genet 14(4):461-464, 1996). The estimated prevalence of LCA is 2-3 per 100,000 live births and accounts for 10-18% of congenital blindness (Fazzi et al. Eur J Paediatr Neurol 7(1):13-22, 2003). The less severe forms of inherited retinal degeneration are usually considered juvenile retinitis pigmentosa (RP) (Gu et al. Nature Genetics 17(2): 194 -197, 1997), which represents a group of hereditary retinal dystrophies with a worldwide prevalence of ~1 in 4000 ((Booij et al. J Med Genet 42(11): e67, 2005). LCA and juvenile RP overlap both phenotypically and genotypically, and the intermediate phenotypes are described as Cone rod dystrophies (CORDs/CRDs) or early onset retinal degeneration (Booij et al., 2005). CORDs are 10 times less common than RP (1/40,000). One of the forms, CORD6 (OMIM 601777), is characterized by dysfunction or degeneration of cone photoreceptors with relative preservation of rod function in the initial stages. The most common symptoms are photophobia, decreased visual acuity, and dyschromatopsia. Fundus changes may vary from mild pigment granularity to a distinct atrophic lesion in the central macula. As the disease progresses, degeneration of rod photoreceptors also occurs and leads to progressive night blindness and peripheral visual field loss (Hamel. Orphanet J Rare Dis 2:7, 2007).

LCA is a genetically heterogeneous disorder and is often inherited in an autosomal recessive manner. To date, approximately 19 genes have been implicated in the pathogenesis of different types of LCA (Weleber et al., 2013; Chen et al. Invest Ophthalmol Vis Sci 54(6):4351-4357, 2013). Together these genes account for 70% of the LCA cases. These genes encode proteins that have a wide range of retinal functions, such as photoreceptor morphogenesis, phototransduction, vitamin A cycling, guanine synthesis, and outer segment phagocytosis (den Hollander et al. Prog Retin Eye Res 27(4):391-419, 2008). Mutations in these genes cause not only LCA but also other retinal disorders (Weleber. Ophthalmic Genet 23(2):71-97, 2002).

GUCY2D (also known as RetGC-1), which encodes the membrane-bound retinal guanylyl cyclase, is a major causative gene for LCA, and is also known to be causative for CORD6. GUCY2D is expressed in both cone and rod photoreceptors, but primarily in the cone outer segments (Dizhoor et al. Neuron 12(6):1345-1352, 1994). GUCY2D catalyzes visual photransduction, which is a biochemical process responsible for light conversion into electrical signals in the rod and cone cells. In this process, the GUCY2D protein helps in restoring photoreceptor sensitivity by synthesizing cyclic guanosine monophosphate (cGMP), which is regulated by guanylate cyclase activator proteins (GCAPs). GCAPs are sensitive to changes in the cytoplasmic Ca++ concentrations. At lower cytoplasmic Ca++ concentrations, the GCAPs bind to the GUCY2D dimerization domain and stimulate it, and in consequence, the cGMP level is restored. Due to genetic or environmental factors, if the Ca++ homeostasis is disrupted in the photoreceptor cells it would lead to the apoptosis of the cells (Baehr and Palczewski. Subcell Biochem 45:71-91, 2007; Garcia-Hoyos et al. Mol Vis 17:1103-1109, 2011). Recessive mutations in GUCY2D cause LCA, whereas dominant mutations result in a less severe form, CORD6. The majority of the identified GUCY2D mutations in CORD6 patients localize to codon 838 on exon 13 (dimerization domain), indicating a mutation hotspot. LCA1-associated mutations were dispersed throughout the gene but none were identified at exon 13. These mutations have a more damaging effect on GUCY2D function than CORD6 mutations (Kitiratschky et al. Invest Ophthalmol Vis Sci 49(11):5015-5023, 2008; Hamel, 2007), which explains the severity of the LCA.

A study by Rozet et al. (2001) identified GUCY2D mutations in ~20% (25/130) of their LCA patients, whereas Kitiratschky et al. (2008) identified mutations in ~40% (11/27) of their CORD families. All CORD mutations affected codon 838 (Rozet et al. Invest Ophthalmol Vis Sci 42(6):1190-1192, 2001; Kitiratschky et al. Invest Ophthalmol Vis Sci 49(11):5015-5023, 2008). Another study identified GUCY2D mutations in 11% of the patients with juvenile RP (Booij et al. J Med Genet 42(11): e67, 2005). Ugur Iseri et al. (2010) identified GUCY2D mutations in 12% of all LCA cases and up to 40% of dominant CORD6 cases (Ugur Iseri et al. Eur J Hum Genet 18(10):1121-1126, 2010).

A study by Perrault et al. (2000) identified two gross deletions and one duplication in GUCY2D out of 118 patients affected with LCA (Perrault et al. Eur J Hum Genet 8(8):578-582, 2000).

This test provides full coverage of all coding exons of the GUCY2D gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.