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Author(s): G. D. Smith [1, 2]; M. J. Gunthorpe [1, 2]; R. E. Kelsell [3]; P. D. Hayes [3]; P. Reilly [1]; P. Facer [4]; J. E. Wright [1]; J. [...]

Objective: To describe the phenotype in 4 families with dominantly inherited cone-rod dystrophy, 1 with an R838C mutation and 1 with an R838H mutation in the guanylate cyclase 2D (GUCY2D) gene encoding retinal guanylate cyclase-1., Methods: Psychophysical and electrophysiological evaluation and confocal laser scanning ophthalmoscopic imaging was performed on 10 affected members of 4 British families., Results: Although subjects had lifelong poor vision in bright light, a major reduction in visual acuity did not occur in most of them until after their late teens. Fundus abnormalities were confined to the central macula, and increasing central atrophy was noted with age. Increased background autofluorescence was observed surrounding the central atrophic area. Electrophysiological testing revealed a marked loss of cone function with only minimal rod involvement, even in older subjects. Photopic and scotopic static perimetry demonstrated central and peripheral cone-mediated threshold elevations with midperipheral sparing., Conclusion: The phenotype associated with autosomal dominant cone-rod dystrophy with either an R838C or R838H mutation in GUCY2D is distinctive, with predominantly cone system involvement. There is some variation in severity within the 3 families with the R838C mutation., Clinical Relevance: Families with the R838C or R838H mutation have a much milder phenotype than the family previously described that had 2 sequence changes, E837D and R838S, in GUCY2D.

Potassium channels are amongst the most heterogeneous class of ion channels known and are responsible for mediating a diverse range of biological functions. The most recently described family of K+ channels, the 'two pore-domain family', contain four membrane spanning domains and two pore-forming domains, suggesting that two channel subunits associate to form a functional K+ pore. Several sub-families of the two pore domain potassium channel family have been described, including the weakly inward rectifying K+ channel (TWIK), the acid-sensitive K+ channel (TASK), the TWIK-related K+ channel (TREK) and the TWIK-related arachidonic acid stimulated K+ channel (TRAAK). However, comparison of the mRNA expression of these channels has been difficult due to the differences in methods used and the species studied. In the present study, we used a single technique, TaqMan semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), to investigate the mRNA distribution of all currently known two pore potassium channels in human central nervous system (CNS) and peripheral tissues. TWIK-1 and the TWIK-1-like channel KCNK7 were predominantly expressed in the CNS, in contrast to TWIK-2 which was preferentially expressed in peripheral tissues such as pancreas, stomach, spleen and uterus. TASK-1 was expressed in the CNS and some peripheral tissues, whereas TASK-2 was exclusively expressed in the periphery except for mRNA expression observed in dorsal root ganglion and spinal cord. In addition, mRNA expression of the recently identified TASK-3, was almost completely exclusive to cerebellum with little or no mRNA detected in any other tissues. TREK-1 and TRAAK mRNA expression was predominantly CNS specific in contrast to the closely related TREK-2, which was expressed in both CNS and peripheral tissues. Studying the mRNA expression profiles of known two pore domain K+ channels will aid in the understanding of the biological roles of these channels. Furthermore, identification of common areas of expression may help identify which channels, if any, associate to form heteromeric K+ channel complexes.

We have isolated, by degenerate PCR, a complementary DNA encoding a novel two pore domain potassium channel. This is the 7th functional member of the human tandem pore domain potassium channel family to be reported. It has an open reading frame of 1.125 kb and encodes a 374 amino acid protein which shows 62% identity to the human TASK-1 gene: identity to other human members of the family is 31-35% at the amino acid level. We believe this gene to be human TASK-3, the ortholog of the recently reported rat TASK-3 gene: amino acid identity between the two is 74%. 'Taqman' mRNA analysis demonstrated a very specific tissue distribution pattern, showing human TASK-3 mRNA to be localised largely in the cerebellum, in contrast rat TASK-3 was reported to be widely distributed. We have shown by radiation hybrid mapping that human TASK-3 can be assigned to chromosome 8q24.3. Human TASK-3 was demonstrated to endow Xenopus oocytes with a negative resting membrane potential through the presence of a large K(+) selective conductance. TASK-3 is inhibited by extracellular acidosis with a mid-point of inhibition around pH 6. 5, supporting the predictions from the sequence data that this is a third human TASK (TWIK-related acid sensitive K(+) channel) gene.

Objective: To describe the clinical features of autosomal dominant cone-rod retinal dystrophy (CRD) in a British family mapping to chromosome 17p12-p13 (CORD6), with a heterozygous mutation (Glu837Asp/ Arg838Ser) of GUCY2D., Design: A prospective, clinical family survey., Patients: Ten affected members of a family with autosomal dominant CRD., Methods: Full clinical examinations were undertaken. Selected affected family members underwent electrophysiologic evaluation, scotopic static perimetry, dark adaptometry, and color vision assessment., Main Outcome Measures: Clinical appearance and electroretinographic responses., Results: Typical clinical and electroretinographic features of childhood-onset CRD were recorded. In addition, moderate myopia and pendular nystagmus were seen in affected individuals. Color vision assessment in the youngest affected individual showed no color discrimination on a tritan axis, but retention of significant red-green discrimination. Electronegative electroretinogram responses were seen on electrophysiology in the only young family member examined., Conclusions: The phenotype associated with GUCY2D CRD is clinically distinct from that associated with other dominant CRD loci. Unusual electroretinographic responses may indicate that this mutation of GUCY2D is associated with early defects in photoreceptor synaptic transmission to second-order neurons.

Mutations in the photoreceptor membrane guanylyl cyclase RetGC-1 have been linked to autosomal dominant cone-rod dystrophy. Three mutations were identified that alter strictly conserved residues within the RetGC-1 dimerization domain, a region predicted to form an amphipathic alpha-helical coil. Here we report on a biochemical characterization of one of the mutations, a substitution of cysteine for arginine at residue 838. We generated this mutation in vitro and measured its catalytic activity and sensitivity to guanylyl cyclase activating protein 1 (GCAP-1) and GCAP-2. The R838C substitution has several effects. It reduces the overall catalytic ability of RetGC-1 and dramatically reduces stimulation by GCAP-2, although GCAP-2 still appears to interact with the protein. The R838C substitution also increases the apparent affinity of RetGC-1 for GCAP-1 and alters the Ca(2+) sensitivity of the GCAP-1 response, allowing the mutant to be stimulated by GCAP-1 at higher Ca(2+) concentrations than wild type. The diminished response to GCAP-2, which we propose is not likely the cause of cone-rod degeneration in these patients, is interesting mechanistically because it separates the ability to bind a specific GCAP from the ability to be stimulated by it, and it also discriminates between the mechanisms of activation of GCAP-1 vs. GCAP-2. We suggest that the gain-of-function effects of R838C on RetGC-1 stimulated by GCAP-1, which are dominant in vitro and may cause an abnormal increase in cGMP synthesis in dark-adapted photoreceptors, may be the cause of the cone-rod degeneration.

We have performed genetic linkage analysis on a four generation British family with cone-rod dystrophy. Significant linkage to the disease gene was obtained with eight marker loci situated on chromosome 17p12-p13. A maximum two-point lod score of 5.93 with no recombination was obtained with marker locus D17S1844. Critical recombinants identified with flanking marker loci placed the disease gene between D17S796/D17S938 and D17S954, an interval estimated to be 8 cM in size. This new localisation for autosomal dominant cone-rod dystrophy (CORD6) overlaps with regions attributed previously to Leber's congenital amaurosis, central areolar choroidal dystrophy and dominant cone dystrophy. Given their differences in phenotype, the most plausible explanation would be that these different retinal disorders are caused by mutations in different genes mapping close together within the genome.

Purpose: The gene for progressive bifocal chorioretinal atrophy (PBCRA) has been linked to chromosome 6q, near the genomic assignment for North Carolina macular dystrophy. A study was undertaken to define the clinical features of a large PBCRA pedigree and to determine whether PBCRA and North Carolina macular dystrophy are phenotypically distinct entities., Methods: Fifteen affected individuals from 1 large family were examined clinically, which included angiography and electrophysiologic studies., Results: The PBCRA is an autosomal dominant chorioretinal dystrophy of early onset characterized by large atrophic macular and nasal retinal lesions, nystagmus, myopia, poor vision, and slow progression. A large atrophic macular lesion and nasal subretinal deposits are evident soon after birth. An atrophic area nasal to the optic nerve head appears in the second decade, which enlarges progressively. Electro-oculographic and electroretinographic studies indicated marked, diffuse abnormalities of rod and cone function. Fluorescein and indocyanine green angiography showed a large circumscribed area of macular choroidal atrophy with staining of deposits in the peripheral retina. In addition to previously documented features, nasal retinal abnormalities from a few weeks of age, marked photopsia in a number of patients, and retinal detachments in three eyes are reported as new features of the disease., Conclusions: An extended description of PBCRA is presented highlighting that the phenotype is distinct from North Carolina macular dystrophy, although some phenotypic similarities exist between the two conditions. These disorders may be the result of different mutations on the same gene or nearby genes.

Progressive bifocal chorioretinal atrophy (PBCRA) is a rare, autosomal dominant congenital chorioretinal dystrophy. We have performed genetic linkage analysis on a five-generation British pedigree. Two-point linkage analysis showed significant linkage with nine microsatellite marker loci mapping to chromosome 6q. Multipoint analysis gave a maximum lod score of 11.8 (theta = 0.05) between D6S249 and D6S283. This region overlaps with that to which the gene for North Carolina macular dystrophy (MCDR1) has been assigned. However, given the range of differences in phenotype between these two retinal disorders, it is likely that different mutation mechanisms are responsible for each disease.