Homozygosity Mapping Reveals PDE6C Mutations in Patients with Early-Onset Cone Photoreceptor Disorders

Department of Ophthalmology, Erasmus Medical Centre, Rotterdam, The Netherlands.
The American Journal of Human Genetics (Impact Factor: 10.93). 08/2009; 85(2):240-7. DOI: 10.1016/j.ajhg.2009.06.016
Source: PubMed


Cone photoreceptor disorders form a clinical spectrum of diseases that include progressive cone dystrophy (CD) and complete and incomplete achromatopsia (ACHM). The underlying disease mechanisms of autosomal recessive (ar)CD are largely unknown. Our aim was to identify causative genes for these disorders by genome-wide homozygosity mapping. We investigated 75 ACHM, 97 arCD, and 20 early-onset arCD probands and excluded the involvement of known genes for ACHM and arCD. Subsequently, we performed high-resolution SNP analysis and identified large homozygous regions spanning the PDE6C gene in one sibling pair with early-onset arCD and one sibling pair with incomplete ACHM. The PDE6C gene encodes the cone alpha subunit of cyclic guanosine monophosphate (cGMP) phosphodiesterase, which converts cGMP to 5'-GMP, and thereby plays an essential role in cone phototransduction. Sequence analysis of the coding region of PDE6C revealed homozygous missense mutations (p.R29W, p.Y323N) in both sibling pairs. Sequence analysis of 104 probands with arCD and 10 probands with ACHM revealed compound heterozygous PDE6C mutations in three complete ACHM patients from two families. One patient had a frameshift mutation and a splice defect; the other two had a splice defect and a missense variant (p.M455V). Cross-sectional retinal imaging via optical coherence tomography revealed a more pronounced absence of cone photoreceptors in patients with ACHM compared to patients with early-onset arCD. Our findings identify PDE6C as a gene for cone photoreceptor disorders and show that arCD and ACHM constitute genetically and clinically overlapping phenotypes.

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Available from: Anneke den Hollander
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    • "The disease is characterized by decreased visual acuity, photophobia, nystagmus, and complete colorblindness [1]. Thus far, mutations in five genes have been identified to cause achromatopsia; they encode key components of the cone phototransduction cascade: the alpha (PDE6C) and gamma (PDE6H) subunits of cone cyclic guanosine monophosphate (cGMP) phosphodiesterase [2,3], the alpha subunit of cone transducin (GNAT2) [4,5], as well as the alpha (CNGA3) and beta (CNGB3) subunits of cone cyclic-nucleotide gated channel [6-8]. In the majority of patients, achromatopsia is a channelopathy and caused by mutations in either the CNGA3 or CNGB3 gene, with CNGB3 being affected most commonly [9-12]. "
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    ABSTRACT: Background Achromatopsia is an autosomal recessive disease characterized by the loss of cone photoreceptor function that results in day-blindness, total colorblindness, and decreased central visual acuity. The most common causes for the disease are mutations in the CNGB3 gene, coding for the beta subunit of the cyclic nucleotide-gated channels in cones. CNGB3-achromatopsia, or cone degeneration (cd), is also known to occur in two canine breeds, the Alaskan malamute (AM) and the German shorthaired pointer. Results Here we report an in-depth characterization of the achromatopsia phenotype in a new canine breed, the miniature Australian shepherd (MAS). Genotyping revealed that the dog was homozygous for a complete genomic deletion of the CNGB3 gene, as has been previously observed in the AM. Identical breakpoints on chromosome 29 were identified in both the affected AM and MAS with a resulting deletion of 404,820 bp. Pooled DNA samples of unrelated purebred Australian shepherd, MAS, Siberian husky, Samoyed and Alaskan sled dogs were screened for the presence of the affected allele; one Siberian husky and three Alaskan sled dogs were identified as carriers. The affected chromosomes from the AM, MAS, and Siberian husky were genotyped for 147 SNPs in a 3.93 Mb interval within the cd locus. An identical shared affected haplotype, 0.5 Mb long, was observed in all three breeds and defined the minimal linkage disequilibrium (LD) across breeds. This supports the idea that the mutated allele was identical by descent (IBD). Conclusion We report the occurrence of CNGB3-achromatopsia in a new canine breed, the MAS. The CNGB3-deletion allele previously described in the AM was also observed in a homozygous state in the affected MAS, as well as in a heterozygous carrier state in a Siberian husky and Alaskan sled dogs. All affected alleles were shown to be IBD, strongly suggesting an affected founder effect. Since the MAS is not known to be genetically related to the AM, other breeds may potentially carry the same cd-allele and be affected by achromatopsia.
    Full-text · Article · Apr 2013 · BMC Genetics
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    • "Cyclic GMP is essential for the ability of rods and cones to respond to the light stimuli and the control of its level is critical for proper functioning of photoreceptors. Mutations in genes involved in cGMP synthesis (RetGCs and GCAPs) or degradation (PDE6) (Chang et al., 2009; Dizhoor, 2000; Grau et al., 2011; Piri et al., 2005; Hunt et al., 2010) can lead to various forms of retinal dystrophies such as some types of retinitis pigmentosa (Bowes et al., 1990; McLaughlin et al., 1993), progressive cone dystrophy (Thiadens et al., 2009), dominant cone degeneration (Behnen et al., 2010; Jiang and Baehr, 2010), cone-rod dystrophy (Buch et al., 2011; Sokal et al., 2005; Tucker et al., 1999) and Leber congenital amaurosis (Perrault et al., 2000, 1996). Retinitis Pigmentosa (RP) is a common form of rod-cone dystrophy , constituting the largest Mendelian genetic cause of blindness in the developed world. "
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    ABSTRACT: Inherited retinal degenerations affecting both rod and cone photoreceptors constitute one of the causes of incurable blindness in the developed world. Cyclic guanosine monophosphate (cGMP) is crucial in the phototransduction and, mutations in genes related to its metabolism are responsible for different retinal dystrophies. cGMP-degrading phosphodiesterase 6 (PDE6) mutations cause around 4-5 % of the retinitis pigmentosa, a rare form of retinal degeneration. The aim of this study was to evaluate whether pharmacological PDE6 inhibition induced retinal degeneration in cone-enriched cultures of porcine retina similar to that found in murine models. PDE6 inhibition was induced in cone-enriched retinal explants from pigs by Zaprinast. PDE6 inhibition induced cGMP accumulation and triggered retinal degeneration, as determined by TUNEL assay. Western blot analysis and immunostaining indicated that degeneration was accompanied by caspase-3, calpain-2 activation and poly (ADP-ribose) accumulation. Oxidative stress markers, total antioxidant capacity, thiobarbituric acid reactive substances (TBARS) and nitric oxide measurements revealed the presence of oxidative damage. Elevated TNF-alpha and IL-6, as determined by enzyme immunoassay, were also found in cone-enriched retinal explants treated with Zaprinast. Our study suggests that this ex vivo model of retinal degeneration in porcine retina could be an alternative model for therapeutic research into the mechanisms of photoreceptor death in cone-related diseases, thus replacing or reducing animal experiments.
    Full-text · Article · Apr 2013 · Experimental Eye Research
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    • "Mutations in the two subunits (CNGA3, CNGB3) of cone photoreceptor cyclic nucleotide-gated (CNG) channels account for approximately 75% of all cases of complete achromatopsia [1], [4]–[9]. Some of the remaining cases are caused by mutations in alpha subunit of cone transducin (GNAT2) [10], [11] and the catalytic alpha subunit of cone phosphodiesterase (PDE6C) [12], [13]. In cone photoreceptors, CNG ion channels are integral tetrameric plasma membrane proteins composed of two A3 and two B3 subunits. "
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    ABSTRACT: Achromatopsia is a rare autosomal recessive disorder which shows color blindness, severely impaired visual acuity, and extreme sensitivity to bright light. Mutations in the alpha subunits of the cone cyclic nucleotide-gated channels (CNGA3) are responsible for about 1/4 of achromatopsia in the U.S. and Europe. Here, we test whether gene replacement therapy using an AAV5 vector could restore cone-mediated function and arrest cone degeneration in the cpfl5 mouse, a naturally occurring mouse model of achromatopsia with a CNGA3 mutation. We show that gene therapy leads to significant rescue of cone-mediated ERGs, normal visual acuities and contrast sensitivities. Normal expression and outer segment localization of both M- and S-opsins were maintained in treated retinas. The therapeutic effect of treatment lasted for at least 5 months post-injection. This study is the first demonstration of substantial, relatively long-term restoration of cone-mediated light responsiveness and visual behavior in a naturally occurring mouse model of CNGA3 achromatopsia. The results provide the foundation for development of an AAV5-based gene therapy trial for human CNGA3 achromatopsia.
    Full-text · Article · Apr 2012 · PLoS ONE
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