Retinal-specific guanylate cyclase gene mutations in Leber's congenital amaurosis

Characterization of retinal guanylate cyclase-activating protein 3 (GCAP3) from zebrafish to man

Calmodulin-like neuronal Ca2+-binding proteins (NCBPs) are expressed primarily in neurons and contain a combination of four functional and nonfunctional EF-hand Ca2+-binding motifs. The guanylate cyclase-activating proteins 1-3 (GCAP1-3), the best characterized subgroup of NCBPs, function in the regulation of transmembrane guanylate cyclases 1-2 (GC1-2). The pairing of GCAPs and GCs in vivo depends on cell expression. Therefore, we investigated the expression of these genes in retina using in situ hybridization and immunocytochemistry. Our results demonstrate that GCAP1, GCAP2, GC1 and GC2 are expressed in human rod and cone photoreceptors, while GCAP3 is expressed exclusively in cones. As a consequence of extensive modification, the GCAP3 gene is not expressed in mouse retina. However, this lack of evolutionary conservation appears to be restricted to only some species as we cloned all three GCAPs from teleost (zebrafish) retina and localized them to rod cells, short single cones (GCAP1-2), and all subtypes of cones (GCAP3). Furthermore, sequence comparisons and evolutionary trace analysis coupled with functional testing of the different GCAPs allowed us to identify the key conserved residues that are critical for GCAP structure and function, and to define class-specific residues for the NCBP subfamilies.

The genetics of keratoconus

Keratoconus is a relatively common, bilateral, non-inflammatory corneal ectasia. The aetiology of this condition is probably multifactorial, or it represents the final common pathway for a variety of different pathological processes. Although a familial history is present only in a minority of cases, one of the major aetiological factors is certainly genetic. This is evidenced by: the condition's familial inheritance; its discordance between monozygotic and dizygotic twins; and its association with other known genetic disorders such as Down's and Marfan's syndromes. In the keratoconic cornea, a possible genetic predisposition to increased sensitivity to apoptotic mediators by keratocytes has also been hypothesized. Differences in prevalence between ethnic groups have been identified. Recent advances in computerized topographic diagnostic techniques for keratoconus, including forme fruste keratoconus, enables higher accuracy in delineating abnormal from normal, and helps define study populations for genetic linkage studies. However, genetic heterogeneity and the phenotypic diversity of keratoconus means that genetic analysis continues to be a complex process. None the less, it is foreseeable that over the next decade, improved diagnostic techniques, in combination with molecular genetics, may reveal conclusive data on the precise nature of the genetic inheritance of keratoconus in specific populations. This review considers the evidence that suggests keratoconus is primarily an inherited condition, and examines research strategies aimed at unveiling the genetic predisposition, and the enigma of environmental influences on its phenotypic expression.

Molecular aspects of retinal degenerative diseases

Retinal degeneration, either acquired or inherited, is a major cause of visual impairment and blindness in humans. Inherited retinal degeneration comprises a large group of diseases that result in the loss of photoreceptor cells. To date, 131 retinal disease loci have been identified, and 76 of the genes at these loci have been isolated (RetNet Web site). Several of these genes were first considered candidates because of their chromosomal localization or homology to genes involved in retinal degeneration in other organisms. In this review, I will discuss recent advances in the identification of genes that cause retinal degeneration, and I will describe the mechanisms of photoreceptor death and potential treatments for retinal degenerative diseases.

Dominant Leber congenital amaurosis, cone-rod degeneration, and retinitis pigmentosa caused by mutant versions of the transcription factor CRX

We summarize 18 mutations in the human CRX gene that have been associated with Leber congenital amaurosis (congenital retinal blindness), cone-rod degeneration, or retinitis pigmentosa. Except for one obviously null allele not definitely associated with a phenotype (a frameshift in codon 9), all CRX mutations appear to be completely penetrant and cause disease in heterozygotes. These dominant alleles fall into two categories. In one group are missense mutations and short, in-frame deletions; in the second group are frameshift mutations, all of which are in the last exon. All of these dominant mutations are likely to produce stable mRNA that is translated. Mutations in the missense group preferentially affect the conserved homeobox (codons 39-98), and all frameshift mutations leave the homeodomain intact but alter the OTX motif encoded by codons 284-295 at the carboxy terminus. We could not uncover any correlation between type of disease (congenital amaurosis vs. cone-rod degeneration or retinitis pigmentosa) and the type of mutation (missense vs. frameshift). Four of the 18 mutations (approximately 20%) were de novo mutations, and all of these were found in isolate cases of Leber congenital amaurosis. Dominant CRX mutations have not been associated with mental retardation or developmental delay that has sometimes been found in Leber congenital amaurosis caused by other genes. Implications regarding potential future therapies are discussed.

Complete exon-intron structure of the RPGR-interacting protein (RPGRIP1) gene allows the identification of mutations underlying Leber congenital amaurosis

Leber congenital amaurosis (LCA) is a genetically heterogeneous autosomal recessive condition responsible for congenital blindness or greatly impaired vision since birth. So far, six LCA loci have been mapped but only 4 out of 6 genes have been identified. A genome-wide screen for homozygosity was conducted in seven consanguineous families unlinked to any of the six LCA loci. Evidence for homozygosity was found in two of these seven families at the 14q11 chromosomal region. Two retinal specific candidate genes were known to map to this region, namely the neural retina leucine zipper (NRL) and the retinitis pigmentosa GTPase regulator interacting protein (RPGRIP1). No mutation of the NRL gene was found in any of the two families. Thus, we determined the complete exon-intron structure of the RPGRIP1 gene. RPGRIP1 encompasses 24 coding exons, nine of which are first described here with their corresponding exon-intron boundaries. The screening of the gene in the two families consistent with linkage to chromosome 14q11 allowed the identification of a homozygous null mutation and a homozygous missense mutation, respectively. Further screening of LCA patients unlinked to any of the four already identified LCA genes (n=86) identified seven additional mutations in six of them. In total, eight distinct mutations (5 out of 8 truncating) in 8/93 patients were found. So far this gene accounts for eight out of 142 LCA cases in our series (5.6%).

Leber's congenital amaurosis with anterior keratoconus in Pakistani families is caused by the Trp278X mutation in the AIPL1 gene on 17p

BACKGROUND

Leber's congenital amaurosis (LCA) represents the earliest and severest form of retinal dystrophy leading to congenital blindness. A total of 20% of children attending blind schools have this disease. LCA has a multigenic basis and is proving central to our understanding of the development of the retina. We describe the clinical and molecular genetic features of four inbred pedigrees from neighbouring remote villages in northern Pakistan, in which some of the affected members have concurrent keratoconus.

METHODS

History-taking and physical and eye examinations were performed in the field. Venipuncture, DNA extraction, studies of linkage to known LCA genes, automated sequencing and polymorphism analyses for haplotype assessments were done.

RESULTS

We examined 12 affected and 15 unaffected family members. By history, there were an additional nine blind people in the four pedigrees. In each pedigree a consanguineous marriage was evident. We found a homozygous nonsense mutation in the AIPL1 gene, which replaces a tryptophan with a stop codon (Trp278X). The phenotype is severe and variable, despite the common molecular genetic etiology in each family. Affected patients had hand motion to no light perception vision and fundus findings ranging from maculopathy to diffuse pigmentary retinopathy. Three affected members had definite keratoconus, and two were suspects based on mild cone formation in the cornea of at least one eye.

INTERPRETATION

We have identified four Pakistani families with a severe form of LCA that is associated with severe keratoconus in some affected members. The molecular etiology in all four families is a homozygous nonsense mutation, Trp278X, in the photoreceptor-pineal gene AIPL1. To our knowledge, this is one of the first phenotype-genotype correlations of AIPL1-associated LCA.

Identification of functional regions of guanylate cyclase-activating protein 1 (GCAP1) using GCAP1/GCIP chimeras

Guanylate cyclase-activating protein 1 (GCAP1) and guanylate cyclase-inhibitory protein (GCIP) are calmodulin-related Ca2+-binding proteins expressed in vertebrate photoreceptor cells. GCAP1 activates photoreceptor guanylate cyclase 1 (GC1) at low free [Ca2+] (<50 nM, in the light), but inhibits it at physiological high [Ca2+] (1 microM, in the dark). GCIP, a Ca2+-binding protein from frog retina, inhibits GC1 at approximately 1 microM [Ca2+], but is unable to stimulate cyclase at low [Ca2+]. In this study, we probed the interaction between GCAP1 and GC1 by producing GCAP1/GCIP chimeras and tested their capability to stimulate GC1. We prepared eight pairs of constructs in which the N-terminal portions of GCIP and GCAP1 were successively replaced by corresponding domains of GCAP1, and GCIP, respectively. The expressed proteins were purified and tested for stimulation of GC1 at 50 nM [Ca2+], and their ability to competitively inhibit GC1 stimulation by a Ca2+-insensitive GCAP1 mutant, GCAP1-tm, at high [Ca2+]. While all GCAP1/GCIP chimeras competitively inhibited GC1 stimulation at high [Ca2+] by GCAP1-tm, several of the GCIP/GCAP1 chimeras had no effect. A chimera consisting of residues 1-20 of GCIP and 21-205 of GCAP1 had no effect on GC1 at low [Ca2+], suggesting that the N-terminal region MGNIMDGKSVEELSSTECHQ, which has no sequence similarity to GCIP, is among the key components necessary for GC1 stimulation. A GCAP1/GCIP chimera consisting of residues 1-43 (including nonfunctional EF1) of GCAP1 and residues 56-206 of GCIP stimulated GC1 at low [Ca2+] and inhibited GC1 at high [Ca2+], suggesting that the essential components required to transform an inhibitory to an activating protein are contained within the N-terminal region of GCAP1 (residues 1-43).

Pupil size following dark adaptation in patients with retinitis pigmentosa

According to the equivalent light hypothesis, molecular defects in the photoreceptor lead to a continuous activation of the photoreceptor cascade in a manner equivalent to real light. The consequences in diseases such as retinitis pigmentosa (RP) are as disruptive to the cells as real light. Two forms of the equivalent light hypothesis can be distinguished: strong - mutations in rhodopsin or other cascade proteins in some forms of RP continuously excite the visual phototransduction cascade; weak - disruption of outer segments in all patients with RP eliminates circulating dark current and blocks neurotransmitter release in a manner similar to real light. Both forms of the equivalent light hypothesis predict that pupils of patients with RP will be constricted like those of normal subjects in the light. The purpose of this study was to test the equivalent light hypothesis by determining whether steady-state pupil diameter following full dark adaptation is abnormally small in any of a sample of patients with RP. Thirty-five patients with RP and 15 normal subjects were tested. Direct steady-state pupillometric measures were obtained from one eye in a full-field dome after 45 min of dark adaptation by videotaping the pupil with an infrared camera. Mean pupil diameter in the dark was comparable (t = -0.15, P = 0.88) between patients with RP (6.85 +/- 0.58 mm) and normal subjects (6.82 +/- 0.76 mm). The results of the present study are clearly counter to the prediction of the second (weaker) form of the equivalent light hypothesis.

Interactions within the coiled-coil domain of RetGC-1 guanylyl cyclase are optimized for regulation rather than for high affinity

RetGC-1, a member of the membrane guanylyl cyclase family of proteins, is regulated in photoreceptor cells by a Ca(2+)-binding protein known as GCAP-1. Proper regulation of RetGC-1 is essential in photoreceptor cells for normal light adaptation and recovery to the dark state. In this study we show that cGMP synthesis by RetGC-1 requires dimerization, because critical functions in the catalytic site must be provided by each of the two polypeptide chains of the dimer. We also show that an intact alpha-helical coiled-coil structure is required to provide dimerization strength for the catalytic domain of RetGC-1. However, the dimerization strength of this domain must be precisely optimized for proper regulation by GCAP-1. We found that Arg(838) within the dimerization domain establishes the Ca(2+) sensitivity of RetGC-1 by determining the strength of the coiled-coil interaction. Arg(838) substitutions dominantly enhance cGMP synthesis even at the highest Ca(2+) concentrations that occur in normal dark-adapted photoreceptor cells. Molecular dynamics simulations suggest that Arg(838) substitutions disrupt a small network of salt bridges to allow an abnormal extension of coiled-coil structure. Substitutions at Arg(838) were first identified by linkage to the retinal degenerative disease, autosomal dominant cone rod dystrophy (adCORD). Consistent with the characteristics of this disease, the Arg(838)-substituted RetGC-1 mutants exhibit a dominant biochemical phenotype. We propose that accelerated cGMP synthesis in humans with adCORD is the primary cause of cone-rod degeneration.

Leber congenital amaurosis and retinitis pigmentosa with Coats-like exudative vasculopathy are associated with mutations in the crumbs homologue 1 (CRB1) gene

Mutations in the crumbs homologue 1 (CRB1) gene cause a specific form of retinitis pigmentosa (RP) that is designated "RP12" and is characterized by a preserved para-arteriolar retinal pigment epithelium (PPRPE) and by severe loss of vision at age <20 years. Because of the early onset of disease in patients who have RP with PPRPE, we considered CRB1 to be a good candidate gene for Leber congenital amaurosis (LCA). Mutations were detected in 7 (13%) of 52 patients with LCA from the Netherlands, Germany, and the United States. In addition, CRB1 mutations were detected in five of nine patients who had RP with Coats-like exudative vasculopathy, a relatively rare complication of RP that may progress to partial or total retinal detachment. Given that four of five patients had developed the complication in one eye and that not all siblings with RP have the complication, CRB1 mutations should be considered an important risk factor for the Coats-like reaction, although its development may require additional genetic or environmental factors. Although no clear-cut genotype-phenotype correlation could be established, patients with LCA, which is the most severe retinal dystrophy, carry null alleles more frequently than do patients with RP. Our findings suggest that CRB1 mutations are a frequent cause of LCA and are strongly associated with the development of Coats-like exudative vasculopathy in patients with RP.

Rod-cone interactions: developmental and clinical significance

During the last decade, numerous research reports have considerably improved our knowledge about the physiopathology of retinal degenerations. Three non-mutually exclusive general areas dealing with therapeutic approaches have been proposed; gene therapy, pharmacology and retinal transplantations. The first approach involving correction of the initial mutation, will need a great deal of time and further development before becoming a therapeutic tool in human clinical practice. The observation that cone photoreceptors, even those seemingly unaffected by any described anomaly, die secondarily to rod disappearance related to mutations expressed specifically in the latter, led us to study the interactions between these two photoreceptor populations to search for possible causal links between rod degeneration and cone death. These in vivo and in vitro studies suggest that paracrine interactions between both cell types exist and that rods are necessary for continued cone survival. Since the role of cones in visual perception is essential, pending the identification of the factors mediating these interactions underway, rod replacement by transplantation and/or neuroprotection by trophic factors or alternative pharmacological means appear as promising approaches for limiting secondary cone loss in currently untreatable blinding conditions.

Pathways to photoreceptor cell death in inherited retinal degenerations

The mutations that cause many forms of inherited retinal degenerations have been identified, yet the mechanisms by which these mutations lead to death of photoreceptor cells of the retina are not completely understood. Investigations of the pathways from mutation to retinal degeneration have focused on spontaneous and engineered animal models of disease. Based on the studies performed to date, four major categories of degeneration mechanism can be identified. These include disruption of photoreceptor outer segment morphogenesis, metabolic overload, dysfunction of retinal pigment epithelial cells, and chronic activation of phototransduction. Future investigations will likely identify additional mechanisms of photoreceptor damage. This review will summarize what has been learned from studying animal models of non-syndromic inherited retinal degenerations.

The guanylyl cyclase family at Y2K

During the 1980s the purification, cloning, and expression of various forms of guanylyl cyclase (GC) revealed that they served as receptors for extracellular signals. Seven membrane forms, which presumably exist as homodimers, and four subunits of apparent heterodimers (commonly referred to as the soluble forms) are known, but in animals such as nematodes, much larger numbers of GCs are expressed. The number of transmembrane segments (none, one, or multiple) divide the GC family into three groups. Those with no or one transmembrane segment bind nitric oxide/carbon monoxide (NO/CO) or peptides. There are no known ligands for the multiple transmembrane segment class of GCs. Mutational and structural analyses support a model where catalysis requires a shared substrate binding site between the subunits, whether homomeric or heteromeric in nature. Because some cyclases or cyclase ligand genes lack specific GC inhibitors, disruption of either has been used to define the functions of individual cyclases, as well as to define human genetic disease counterparts.

Regulation and role of adenylyl cyclase isoforms

At least nine closely related isoforms of adenylyl cyclases (ACs), the enzymes responsible for the synthesis of cyclic AMP (cAMP) from ATP, have been cloned and characterized in mammals. Depending on the properties and the relative levels of the isoforms expressed in a tissue or a cell type at a specific time, extracellular signals received through the G-protein-coupled receptors can be differentially integrated. The present review deals with various aspects of such regulations, emphasizing the role of calcium/calmodulin in activating AC1 and AC8 in the central nervous system, the potential inhibitory effect of calcium on AC5 and AC6, and the changes in the expression pattern of the isoforms during development. A particular emphasis is given to the role of cAMP during drug and ethanol dependency and to some experimental limitations (pitfalls in the interpretation of cellular transfection, scarcity of the invalidation models, existence of complex macromolecular structures, etc).

Screening for CRX gene mutations in Chinese patients with Leber congenital amaurosis and mutational phenotype

PURPOSE

To screen for possible disease-causing mutations in the CRX gene in Chinese patients with Leber congenital amaurosis (LCA) and to enrich the understanding of its mutational phenotype.

METHODS

Genomic DNA was collected from 27 patients with LCA. The coding sequences of the CRX gene were analyzed using the PCR-heteroduplex-SSCP method. Mutations were confirmed by DNA sequencing.

RESULTS

We identified two heterozygous variations in the CRX gene in two patients with LCA. One was a deletion (GCC-->-CC, A181D1bp) leading to a frameshift and protein truncation. This mutation was present in a patient with LCA, but not in his healthy parents. The ocular manifestations of this A181Delta1bp mutation are described. An intronic variation (IVS1-13G-->C) was found in a patient with LCA as well as in his healthy father.

CONCLUSION

A heterozygous A181D1bp mutation in the CRX gene caused an LCA phenotype in a Chinese patient.

Spiroplasma sp. 16S rDNA in Creutzfeldt-Jakob disease and scrapie as shown by PCR and DNA sequence analysis

The pathogenesis of the transmissible spongiform encephalopathies (TSE), which include Creutzfeldt-Jakob disease (CJD) in humans and scrapie in sheep, remains an enigma. In this paper we present evidence for the association of Spiroplasma sp., a wall-less prokaryote, with TSE. We have shown PCR amplification of Spiroplasma 16S rDNA in TSE-infected brain tissues (13 of 13 CJD cases and 5 of 9 scrapie cases) and not in control brains (0 of 50). Direct sequencing of the amplified PCR products has confirmed the presence of Spiroplasma-like DNA in all 5 of the TSE brains tested. Our evidence is not necessarily in conflict with involvement of a PrPres--a protease-resistant host-derived protein referred to as the prion--in the pathogenesis of TSE, since there is evidence that another factor is involved. We propose a bacterium, namely Spiroplasma, as this associated factor although the role of Spiroplasma in TSE cannot be determined from these experiments. The presence of the nucleic acid sequence of this microbe in all cases of TSE in our laboratory and not in controls provides direct evidence of the association of Spiroplasma sp. with TSE.

Null RPGRIP1 alleles in patients with Leber congenital amaurosis

We isolated and characterized the entire coding sequence of a human gene encoding a protein that interacts with RPGR, a protein that is absent or mutant in many cases of X-linked retinitis pigmentosa. The newly identified gene, called "RPGRIP1" for RPGR-interacting protein (MIM 605446), is located within 14q11, and it encodes a protein predicted to contain 1,259 amino acids. Previously published work showed that both proteins, RPGR and RPGRIP1, are present in the ciliary structure that connects the inner and outer segments of rod and cone photoreceptors. We surveyed 57 unrelated patients who had Leber congenital amaurosis for mutations in RPGRIP1 and found recessive mutations involving both RPGRIP1 alleles in 3 (6%) patients. The mutations all create premature termination codons and are likely to be null alleles. Patients with RPGRIP1 mutations have a degeneration of both rod and cone photoreceptors, and, early in life, they experience a severe loss of central acuity, which leads to nystagmus.

Guanylyl cyclases as a family of putative odorant receptors

Mammals can discriminate among a large number (> 10,000) of unique odorants. The most highly supported explanation for this ability is that olfactory neurons express a large number of seven transmembrane receptors that are not spatially organized at the level of the olfactory epithelium, but whose axonal projections form a distinct pattern within the olfactory bulb. The odor-induced signaling pathway in olfactory neurons includes a Gs-like protein (G(olf)) that activates a specific adenylyl cyclase (type III) isoform, resulting in elevations of cyclic AMP and subsequent activation of a cyclic nucleotide-gated channel. The channel also can be regulated by cyclic GMP. Recently, an olfactory neuron-specific guanylyl cyclase was discovered in rodents, and subsequently a large family of sensory neuronal guanylyl cyclases was identified in nematodes. These guanylyl cyclases are concentrated in the plasma membrane of the dendritic cilia and contain extracellular domains that retain many of the primary sequence characteristics of guanylyl cyclases known to be receptors for various peptides. Thus, the guanylyl cyclases appear to represent a second family of odorant/pheromone receptors.

Mutations in a new photoreceptor-pineal gene on 17p cause leber congenital amaurosis. Nat gen 2000;24:79-83

Leber congenital amaurosis (LCA, MIM 204000) accounts for at least 5% of all inherited retinal disease1 and is the most severe inherited retinopathy with the earliest age of onset2. Individuals affected with LCA are diagnosed at birth or in the first few months of life with severely impaired vision or blindness, nystagmus and an abnormal or flat electroretinogram (ERG). Mutations in GUCY2D (ref. 3), RPE65 (ref. 4) and CRX (ref. 5) are known to cause LCA, but one study identified disease-causing GUCY2D mutations in only 8 of 15 families whose LCA locus maps to 17p13.1 (ref. 3), suggesting another LCA locus might be located on 17p13.1. Confirming this prediction, the LCA in one Pakistani family mapped to 17p13.1, between D17S849 and D17S960—a region that excludes GUCY2D. The LCA in this family has been designated LCA4 (ref. 6). We describe here a new photoreceptor/pineal-expressed gene, AIPL1 (encoding arylhydrocarbon interacting protein-like 1), that maps within the LCA4 candidate region and whose protein contains three tetratricopeptide (TPR) motifs, consistent with nuclear transport or chaperone activity. A homozygous nonsense mutation at codon 278 is present in all affected members of the original LCA4 family. AIPL1 mutations may cause approximately 20% of recessive LCA, as disease-causing mutations were identified in 3 of 14 LCA families not tested previously for linkage.

Prevalence of AIPL1 mutations in inherited retinal degenerative disease

Leber congenital amaurosis (LCA) is the most severe form of inherited retinal dystrophy and the most frequent cause of inherited blindness in children. LCA is usually inherited in an autosomal recessive fashion, although rare dominant cases have been reported. One form of LCA, LCA4, maps to chromosome 17p13 and is genetically distinct from other forms of LCA. We recently identified the gene associated with LCA4, AIPL1 (aryl-hydrocarbon interacting protein-like 1) and identified three mutations that were the cause of blindness in five families with LCA. In this study, AIPL1 was screened for mutations in 512 unrelated probands with a range of retinal degenerative diseases to determine if AIPL1 mutations cause other forms of inherited retinal degeneration and to determine the relative contribution of AIPL1 mutations to inherited retinal disorders in populations worldwide. We identified 11 LCA families whose retinal disorder is caused by homozygous or compound heterozygous AIPL1 mutations. We also identified affected individuals in two apparently dominant families, diagnosed with juvenile retinitis pigmentosa or dominant cone-rod dystrophy, respectively, who are heterozygous for a 12-bp AIPL1 deletion. Our results suggest that AIPL1 mutations cause approximately 7% of LCA worldwide and may cause dominant retinopathy.

Recent advances in the molecular basis of inherited photoreceptor degeneration

To date, 118 loci have been associated with photoreceptor degenerative disease. In this review, we will discuss recent advances in the identification of genes that cause progressive photoreceptor cell death when mutated. We will focus on 12 genes isolated within the last two years that have been shown to be photoreceptor-specific, or that have provided insight into photoreceptor biology and the mechanisms of photoreceptor cell death. To aid in understanding the biologic basis for these diseases, we also briefly review photoreceptor biology. Finally, we report on recent advances towards the treatment of these disorders.

Ca(2+)-binding proteins in the retina: structure, function, and the etiology of human visual diseases

The complex sensation of vision begins with the relatively simple photoisomerization of the visual pigment chromophore 11-cis-retinal to its all-trans configuration. This event initiates a series of biochemical reactions that are collectively referred to as phototransduction, which ultimately lead to a change in the electrochemical signaling of the photoreceptor cell. To operate in a wide range of light intensities, however, the phototransduction pathway must allow for adjustments to background light. These take place through physiological adaptation processes that rely primarily on Ca(2+) ions. While Ca(2+) may modulate some activities directly, it is more often the case that Ca(2+)-binding proteins mediate between transient changes in the concentration of Ca(2+) and the adaptation processes that are associated with phototransduction. Recently, combined genetic, physiological, and biochemical analyses have yielded new insights about the properties and functions of many phototransduction-specific components, including some novel Ca(2+)-binding proteins. Understanding these Ca(2+)-binding proteins will provide a more complete picture of visual transduction, including the mechanisms associated with adaptation, and of related degenerative diseases.

Molecular genetics of human retinal disease

The past decade has witnessed extraordinary progress in retinal disease gene identification, the analysis of animal and tissue culture models of disease processes, and the integration of this information with clinical observations and with retinal biochemistry and physiology. During this period over twenty retinal disease genes were identified and for many of these genes there are now significant insights into their role in disease. This review presents an overview of the basic and clinical biology of the retina, summarizes recent progress in understanding the molecular mechanisms of inherited retinal diseases, and offers an assessment of the role that genetics will play in the next phase of research in this area.

Autosomal dominant cone-rod retinal dystrophy (CORD6) from heterozygous mutation of GUCY2D, which encodes retinal guanylate cyclase

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 a new photoreceptor-pineal gene on 17p cause Leber congenital amaurosis

Leber congenital amaurosis (LCA, MIM 204000) accounts for at least 5% of all inherited retinal disease and is the most severe inherited retinopathy with the earliest age of onset. Individuals affected with LCA are diagnosed at birth or in the first few months of life with severely impaired vision or blindness, nystagmus and an abnormal or flat electroretinogram (ERG). Mutations in GUCY2D (ref. 3), RPE65 (ref. 4) and CRX (ref. 5) are known to cause LCA, but one study identified disease-causing GUCY2D mutations in only 8 of 15 families whose LCA locus maps to 17p13.1 (ref. 3), suggesting another LCA locus might be located on 17p13.1. Confirming this prediction, the LCA in one Pakistani family mapped to 17p13.1, between D17S849 and D17S960-a region that excludes GUCY2D. The LCA in this family has been designated LCA4 (ref. 6). We describe here a new photoreceptor/pineal-expressed gene, AIPL1 (encoding aryl-hydrocarbon interacting protein-like 1), that maps within the LCA4 candidate region and whose protein contains three tetratricopeptide (TPR) motifs, consistent with nuclear transport or chaperone activity. A homozygous nonsense mutation at codon 278 is present in all affected members of the original LCA4 family. AIPL1 mutations may cause approximately 20% of recessive LCA, as disease-causing mutations were identified in 3 of 14 LCA families not tested previously for linkage.

Physical map of 17p13 and the genes adjacent to p53

Genetic lesions in the p53 tumor suppressor gene are the most frequently observed alterations in human cancers. Typically in tumors, one allele of the p53 gene is initially mutated, followed by deletion of the remaining wildtype allele. In human colon cancer, for example, approximately 70% of late stage tumors are hemizygous mutant p53. Since the precise gene environment surrounding the p53 gene is not known, the neighboring genes concomitantly lost with wildtype p53 deletion remain undetermined. A restriction enzyme map and clone array of 1.1 Mb surrounding the p53 gene were constructed using a combination of YAC, BAC, NotI linking, and NotI jumping clones. The resulting physical map and clone array include approximately 400 kb telomeric and 700 kb centromeric to the p53 gene. Sequence determination and analysis adjacent to NotI and AscI sites, indicative of CpG islands, allowed the rapid identification of numerous genes within the cloned region. Twenty-seven transcription units were identified, including 18 characterized genes. Limited analysis of primary human colon tumors, hemizygous for the p53 gene, indicates loss of the entire 1.1-Mb region upon deletion of wildtype p53.

Regulation of photoreceptor membrane guanylyl cyclases by guanylyl cyclase activator proteins

Guanylyl cyclase (GC) plays a central role in the responses of vertebrate rod and cone photoreceptors to light. cGMP is an internal messenger molecule of vertebrate phototransduction. Light stimulates hydrolysis of cGMP, causing the closure of cGMP-dependent cation channels in the plasma membranes of photoreceptor outer segments. Light also lowers the concentration of intracellular free Ca(2+) and by doing so it stimulates resynthesis of cGMP by guanylyl cyclase. The guanylyl cyclases that couple Ca(2+) to cGMP synthesis in photoreceptors are members of a family of transmembrane guanylyl cyclases that includes atrial natriuretic peptide receptors and the heat-stable enterotoxin receptor. The photoreceptor membrane guanylyl cyclases, RetGC-1 and RetGC-2 (also referred to as GC-E and GC-F), are regulated intracellularly by two Ca(2+)-binding proteins, GCAP-1 and GCAP-2. GCAPs bind Ca(2+) at three functional EF-hand structures. Several lines of biochemical evidence suggest that guanylyl cyclase activator proteins (GCAPs) bind constitutively to an intracellular domain of RetGCs. In the absence of Ca(2+) GCAP stimulates and in the presence of Ca(2+) it inhibits cyclase activity. Proper functioning of RetGC and GCAP is necessary not only for normal photoresponses but also for photoreceptor viability since mutations in RetGC and in GCAP cause photoreceptor degeneration.

Targeted gene disruption in the development of mouse models to elucidate the role of receptor guanylyl cyclase signaling pathways in physiological function

The physiological role of receptor guanylyl cyclases (GCs), which transduce a signal via the generation of intracellular cyclic GMP, has been somewhat speculative since there are few specific inhibitors that discriminate among various receptor isoforms. Although the natriuretic peptide receptors have been thought to regulate cardiovascular and renal function, the exact contribution of the receptor subtypes has not been clarified. The normal role of the heat-stable enterotoxin receptor guanylyl cyclase remains undefined, and several orphan members of the family await the identification of ligands as well as function. Targeted gene disruption, familiarly known as gene knockout, has emerged during the past decade as a powerful technique for probing the function of gene products, and has been used to develop animal models of inherited human diseases. We are just beginning to apply gene targeting technology to the guanylyl cyclase receptor family. Reviewed here is the information gained to date from the targeted disruption of several members of the guanylyl cyclase receptor family, their ligands, or effector molecules.

Regions in vertebrate photoreceptor guanylyl cyclase ROS-GC1 involved in Ca(2+)-dependent regulation by guanylyl cyclase-activating protein GCAP-1

The membrane bound guanylyl cyclase (GC) photoreceptor membrane GC1 (ROS-GCI) of photoreceptor cells synthesizes cGMP, the intracellular transmitter of vertebrate phototransduction. The activity of ROS-GCI is controlled by small Ca(2+)-binding proteins, named GC-activating proteins (GCAPs). We identified and characterized two short regulatory regions (M445-L456 and L503-1522) in the juxtamembrane domain (JMD) of ROS-GC1 by peptide competition and mutagenesis studies. Both regions are critical for the activation of ROS-GCI by GCAP-1.

Leber congenital amaurosis

Leber's congenital amaurosis (LCA) is the earliest and most severe form of all inherited retinal dystrophies responsible for congenital blindness. Genetic heterogeneity of LCA has been suspected since the report by Waardenburg of normal children born to affected parents. In 1995, we localized the first disease causing gene, LCA1, to chromosome 17p13 and confirmed the genetic heterogeneity. In 1996, we ascribed LCA1 to mutations in the photoreceptor-specific guanylate cyclase gene (retGC1). RetGC1 is an essential protein implicated in the phototransduction cascade, especially in the recovery of the dark state after the excitation process of photoreceptor cells by light stimulation. In 1997, mutations in a second gene were reported in LCA, the RPE65 gene, which is the first specific retinal pigment epithelium gene. The protein RPE65 is implicated in the metabolism of vitamin A, the precursor of the photoexcitable retinal pigment (rhodopsin). Finally, a third gene, CRX, implicated in photoreceptor development, has been suspected of causing a few cases of LCA. Taken together, these three genes account for only 27% of LCA cases in our series. The three genes encode proteins that are involved in completely different physiopathologic pathways. Based on these striking differences of physiopathologic processes, we reexamined all clinical physiopathological discrepancies and the results strongly suggested that retGC1 gene mutations are responsible for congenital stationary severe cone-rod dystrophy, while RPE65 gene mutations are responsible for congenital severe but progressive rod-cone dystrophy. It is of tremendous importance to confirm and to refine these genotype-phenotype correlations on a large scale in order to anticipate the final outcome in a blind infant, on the one hand, and to further guide genetic studies in older patients on the other hand.

Genetic analysis of the guanylate cyclase activator 1B (GUCA1B) gene in patients with autosomal dominant retinal dystrophies: Table 1

The guanylate cyclase activator proteins (GCAP1 and GCAP2) are calcium binding proteins which by activating Ret-GC1 play a key role in the recovery phase of phototransduction. Recently a mutation in theGUCA1A gene (coding for GCAP1) mapping to the 6p21.1 region was described as causing cone dystrophy in a British family. In addition mutations in Ret-GC1have been shown to cause Leber congenital amaurosis and cone-rod dystrophy. To determine whether GCAP2 is involved in dominant retinal degenerative diseases, the GCAP2 gene was screened in 400 unrelated subjects with autosomal dominant central and peripheral retinal dystrophies.

A number of changes involving the intronic as well as the coding sequence were observed. In exon 1 a T to C nucleotide change was observed leaving the tyrosine residue 57 unchanged. In exon 3 a 1 bp intronic insertion, a single nucleotide substitution G to A in the intron 3′ of this exon, and a GAG to GAT change at codon 155 were observed. This latter change results in a conservative change of glutamic acid to aspartic acid. In exon 4 a 7 bp intronic insertion, a single nucleotide A to G substitution in the intron 5′ of this exon, and a single base pair change C to G in the intron 3′ of exon 4 were seen. None of these changes would be expected to affect correct splicing of this gene. All these changes were observed in controls. The results of this study do not show any evidence so far that GCAP2 is involved in the pathogenesis of autosomal dominant retinal degeneration in this group of patients. All the changes detected were found to be sequence variations or polymorphisms and not disease causing.

Biochemical analysis of a dimerization domain mutation in RetGC-1 associated with dominant cone-rod dystrophy

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.

Disruption of a retinal guanylyl cyclase gene leads to cone-specific dystrophy and paradoxical rod behavior

One of two orphan photoreceptor guanylyl cyclases that are highly conserved from fish to mammals, GC-E (or retGC1) was eliminated by gene disruption. Expression of the second retinal cyclase (GC-F) as well as the numbers and morphology of rods remained unchanged in GC-E null mice. However, rods isolated from such mice, despite having a normal dark current, recovered from a light flash markedly faster. Unexpectedly, the a- and b-waves of electroretinograms (ERG) from dark-adapted null mice were suppressed markedly. Cones, initially present in normal numbers in the retina, disappeared by 5 weeks, based on ERG and histology. Thus, the GC-E-deficient mouse defines a model for cone dystrophy, but it also demonstrates that morphologically normal rods display paradoxical behavior in their responses to light.

Characterization of the gene for HRG4 (UNC119), a novel photoreceptor synaptic protein homologous to unc-119

HRG4 (HGMW-approved symbol UNC119) is a novel human photoreceptor-enriched gene coding for a 240-amino-acid protein. Initially, HRG4 was shown to be 57% homologous to a newly discovered Caenorhabditis elegans gene, mutated in a coordination mutant and involved in chemosensation. Recently, HRG4 has been localized to the photoreceptor synapses in the outer plexiform layer of the retina. The HRG4 gene was cloned and characterized to facilitate its analysis as a potential pathogenic gene. The gene consisted of five coding exons, spread over approximately 8 kb of genomic DNA. The transcriptional start site was 14 bp upstream of the cDNA, 68 bp upstream of the putative translational initiation codon. Five GC boxes were identified in a 100-bp upstream region, along with a photoreceptor conserved element 1-like sequence at -603. Another photoreceptor gene-associated sequence, Ret-1, was present in intron 1, 71 bp downstream of the exon 1/intron 1 border. A CpG island encompassing exon 1 and sequences just before and after it was present. The gene was fine mapped to 17q11.2, facilitating its future consideration as a candidate for retinal diseases mapped to the same region.

Coexpression of the KCNA3B gene product with Kv1.5 leads to a novel A-type potassium channel

Shaker-related voltage-gated potassium (Kv) channels may be heterooligomers consisting of membrane-integral alpha-subunits associated with auxiliary cytoplasmic beta-subunits. In this study we have cloned the human Kvbeta3.1 subunit and the corresponding KCNA3B gene. Identification of sequence-tagged sites in the gene mapped KCNA3B to band p13.1 of human chromosome 17. Comparison of the KCNA1B, KCNA2B, and KCNA3B gene structures showed that the three Kvbeta genes have very disparate lengths varying from >/=350 kb (KCNA1B) to approximately 7 kb (KCNA3B). Yet, the exon patterns of the three genes, which code for the seven known mammalian Kvbeta subunits, are very similar. The Kvbeta1 and Kvbeta2 splice variants are generated by alternative use of 5'-exons. Mouse Kvbeta4, a potential splice variant of Kvbeta3, is a read-through product where the open reading frame starts within the sequence intervening between Kvbeta3 exons 7 and 8. The human KCNA3B sequence does not contain a mouse Kvbeta4-like open reading frame. Human Kvbeta3 mRNA is specifically expressed in the brain, where it is predominantly detected in the cerebellum. The heterologous coexpression of human Kv1.5 and Kvbeta3.1 subunits in Chinese hamster ovary cells yielded a novel Kv channel mediating very fast inactivating (A-type) outward currents upon depolarization. Thus, the expression of Kvbeta3.1 subunits potentially extends the possibilities to express diverse A-type Kv channels in the human brain.

Rod outer segment membrane guanylate cyclase type 1 (ROS-GC1) gene: structure, organization and regulation by phorbol ester, a protein kinase C activator

At present there are two recognized members of the ROS-GC subfamily of membrane guanylate cyclases. They are ROS-GC1 and ROS-GC2. A distinctive feature of this family is that its members are not switched on by the extracellular peptide hormones; instead, they are modulated by intracellular Ca2+ signals, consistent to their linkage with phototransduction. An intriguing feature of ROS-GC1, which distinguishes it from ROS-GC2, is that it has two Ca2+ switches. One switch inhibits the enzyme at micromolar concentrations of Ca2+, as in phototransduction; the other, stimulates. The stimulatory switch, most likely, is linked to retinal synaptic activity. Thus, ROS-GC1 is linked to both phototransduction and the synaptic activity. The present study describes (1) the almost complete structural identity of 18.5 kb ROS-GC1 gene; (2) its structural organization: the gene is composed of 20 exons and 19 introns with classical GT/AG boundaries; (3) the activity of the ROS-GC1 promoter assayed through luciferase reporter in COS cells; and (4) induction of the gene by phorbol ester, a protein kinase C (PKC) activator. The co-presence of PKC and ROS-GC1 in photoreceptors suggests that regulation of the ROS-GC1 gene by PKC might be a physiologically relevant phenomenon.

Molecular genetics of human retinal dystrophies

Retinal dystrophies are a heterogeneous group of diseases in which the retina degenerates, leading to either partial or complete blindness. The severe and clearly hereditary forms, retinitis pigmentosa (RP) and various macular degenerations, affect approximately 1 in 3000 people, but many more suffer from aging macular dystrophy in later life. Patients with RP present with narrowing visual fields and night blindness, while those with diseases of the macula lose central vision first. Even before the advent of molecular genetics it was evident that these were heterogeneous disorders, with wide variation in severity, mode of inheritance and phenotype. However, with the widespread application of linkage analysis and mutation detection techniques, a complex underlying pathology has now been revealed. In total, 66 distinct non-overlapping genes or gene loci have been implicated in the various forms of retinal dystrophy, with more being reported regularly in the literature. Within the category of non-syndromic RP alone there are at least 22 genes (and probably many more) involved, with further allelic heterogeneity arising from different mutations in the same gene. This complexity presents a problem for those involved in counselling patients, and also compounds the search for therapies. Nevertheless, several lines of research raise the hope of generic treatments applicable to all such patients, while the greater understanding of normal visual function that arises from genetic studies may open up new avenues for therapy.