Linkage of Usher syndrome type I gene (USH1B) to the long arm of chromosome 11

Contig maps and genomic sequencing identify candidate genes in the usher 1C locus

Usher syndrome 1C (USH1C) is a congenital condition manifesting profound hearing loss, the absence of vestibular function, and eventual retinal degeneration. The USH1C locus has been mapped genetically to a 2- to 3-cM interval in 11p14-15.1 between D11S899 and D11S861. In an effort to identify the USH1C disease gene we have isolated the region between these markers in yeast artificial chromosomes (YACs) using a combination of STS content mapping and Alu-PCR hybridization. The YAC contig is approximately 3.5 Mb and has located several other loci within this interval, resulting in the order CEN-LDHA-SAA1-TPH-D11S1310-(D11S1888/KCNC1 )-MYOD1-D11S902D11S921-D11S 1890-TEL. Subsequent haplotyping and homozygosity analysis refined the location of the disease gene to a 400-kb interval between D11S902 and D11S1890 with all affected individuals being homozygous for the internal marker D11S921. To facilitate gene identification, the critical region has been converted into P1 artificial chromosome (PAC) clones using sequence-tagged sites (STSs) mapped to the YAC contig, Alu-PCR products generated from the YACs, and PAC end probes. A contig of >50 PAC clones has been assembled between D11S1310 and D11S1890, confirming the order of markers used in haplotyping. Three PAC clones representing nearly two-thirds of the USH1C critical region have been sequenced. PowerBLAST analysis identified six clusters of expressed sequence tags (ESTs), two known genes (BIR, SUR1) mapped previously to this region, and a previously characterized but unmapped gene NEFA (DNA binding/EF hand/acidic amino-acid-rich). GRAIL analysis identified 11 CpG islands and 73 exons of excellent quality. These data allowed the construction of a transcription map for the USH1C critical region, consisting of three known genes and six or more novel transcripts. Based on their map location, these loci represent candidate disease loci for USH1C. The NEFA gene was assessed as the USH1C locus by the sequencing of an amplified NEFA cDNA from an USH1C patient; however, no mutations were detected.

An introduction to the genetics of normal and defective hearing

The recent rapid development of molecular biology techniques applied to the genetics of normal and defective hearing shed a new light on old questions regarding hearing and deafness. Genes are DNA sequences that determine characteristics, normally by specifying the sequence of aminoacids in a protein. The majority of genes is located in the chromosomes (human chromosomes have perhaps 80,000 pairs of genes). In addition there are 37 mithochondrial genes which are inherited only from the mother. One method used to identify candidate genes based on their function or pattern of tissue expression involves the construction of cDNA libraries from the target organ or tissue, in this case from the cochlea. The construction and characterization of cochlear cDNA libraries from humans and other species provide an important resource for rapid identification of cochlear genes involved in normal hearing and hearing disorders. Studies of the molecular genetics of the inner ear are hampered by the relative inaccessibility of the cochlea, by the limited number of cochlear and vestibular cells, and by our inability to maintain many of these cell types in long-term cultures. Several rodent inner-ear cDNA libraries and a human foetal cochlear cDNA library have already been constructed. Human and rodent cochlea-subtracted cDNA libraries are very useful for identifying genes controlling the development and maintenance of hearing. cDNA libraries constructed at different stages of development, and subtracted from each other, could be instrumental in identifying genes important at each stage of cochlear development. In addition, these libraries have the potential of fostering the identification of other proteins unique to the cochlea and will contribute to the identification, characterization, and functional analysis of these cochlea-specific proteins. Another important application of cDNA libraries is in identifying hearing-loss genes. Once the candidate gene for a given type of hearing loss is cloned and decoded, the structure of its protein product can be determined. This will provide insights into the biochemical function of the gene product in normal cochlear tissue, and will show why the genetic mutation results in hearing loss, that is, the recent identification of the myosin VIIa gene in Usher type IB. In addition, through the use of homologous recombination and transgenic technology, in vivo mouse models of inner-ear genetic disorders can be created. To date, 350 different genetic conditions associated with hearing impairment have been described, and during the past five years several of the genes involved in these form have already been mapped and identified.

Mutation profile of all 49 exons of the human myosin VIIA gene, and haplotype analysis, in Usher 1B families from diverse origins

Usher syndrome types I (USH1A-USH1E) are a group of autosomal recessive diseases characterized by profound congenital hearing loss, vestibular areflexia, and progressive visual loss due to retinitis pigmentosa. The human myosin VIIA gene, located on 11q14, has been shown to be responsible for Usher syndrome type 1B (USH1B). Haplotypes were constructed in 28 USH1 families by use of the following polymorphic markers spanning the USH1B locus: D11S787, D11S527, D11S1789, D11S906, D11S4186, and OMP. Affected individuals and members of their families from 12 different ethnic origins were screened for the presence of mutations in all 49 exons of the myosin VIIA gene. In 15 families myosin VIIA mutations were detected, verifying their classification as USH1B. All these mutations are novel, including three missense mutations, one premature stop codon, two splicing mutations, one frameshift, and one deletion of >2 kb comprising exons 47 and 48, a part of exon 49, and the introns between them. Three mutations were shared by more than one family, consistent with haplotype similarities. Altogether, 16 USH1B haplotypes were observed in the 15 families; most haplotypes were population specific. Several exonic and intronic polymorphisms were also detected. None of the 20 known USH1B mutations reported so far in other world populations were identified in our families.

Usher syndrome in the Samaritans: strengths and limitations of using inbred isolated populations to identify genes causing recessive disorders

We have previously reported significant linkage between markers on 11q13.5 and Usher syndrome type 1 (USH1B) in a large Samaritan kindred. USH1B is an autosomal recessive disease characterized by profound congenital sensorineural deafness, vestibular dysfunction and progressive visual loss. A unique haplotype found only in all USH1B carriers and affected individuals implied that the disease-causing mutation probably entered the community from a single founder. Screening for mutations in a gene called GARP, which was mapped to the same genetic interval as USH1B, revealed a base substitution in the coding region of the gene, in a homozygous state in all affected individuals. This base substitution, which results in an arginine to tryptophane change, is not found in control individuals and occurs at an amino acid residue that is conserved across species, including mouse, gorilla, chimpanzee and macaque. This study emphasizes the strength of using an isolated inbred population for efficient identification of the primary linkage and for narrowing the disease interval, but also demonstrates its limitations in distinguishing between mutations causing the disease and those representing unique and private polymorphisms.

Clinical and molecular genetic characterisation of a family segregating autosomal dominant retinitis pigmentosa and sensorineural deafness

AIMS/BACKGROUND

To characterise clinically a large kindred segregating retinitis pigmentosa and sensorineural hearing impairment in an autosomal dominant pattern and perform genetic linkage studies in this family. Extensive linkage analysis in this family had previously excluded the majority of loci shown to be involved in the aetiologies of RP, some other forms of inherited retinal degeneration, and inherited deafness.

METHODS

Members of the family were subjected to detailed ophthalmic and audiological assessment. In addition, some family members underwent skeletal muscle biopsy, electromyography, and electrocardiography. Linkage analysis using anonymous microsatellite markers was performed on DNA samples from all living members of the pedigree.

RESULTS

Patients in this kindred have a retinopathy typical of retinitis pigmentosa in addition to a hearing impairment. Those members of the pedigree examined demonstrated a subclinical myopathy, as evidence by abnormal skeletal muscle histology, electromyography, and electrocardiography. LOD scores of Zmax = 3.75 (theta = 0.10), Zmax = 3.41 (theta = 0.10), and Zmax = 3.25 (theta = 0.15) respectively were obtained with the markers D9S118, D9S121, and ASS, located on chromosome 9q34-qter, suggesting that the causative gene in this family may lie on the long arm (q) of chromosome 9.

CONCLUSIONS

These data indicate that the gene responsible for the phenotype in this kindred is located on chromosome 9 q. These data, together with evidence that a murine deafness gene is located in a syntenic area of the mouse genome, should direct the research community to consider this area as a candidate region for retinopathy and/or deafness genes.

The genomic structure of the gene defective in Usher syndrome type Ib (MYO7A)

Usher syndrome type Ib is a recessive autosomal disorder manifested by congenital deafness, vestibular dysfunction, and progressive retinal degeneration. Mutations in the human myosin VIIa gene (MYO7A) have been reported to cause Usher type Ib. Here we report the genomic organization of MYO7A. An STS content map was determined to discover the YAC clones that would cover the critical region for Usher syndrome type Ib. Three of the YACs (802A5, 966D6, and 965F10) were subcloned into cosmids and used to assemble a preliminary cosmid contig of the critical region. Part of the gene encoding human myosin VIIa was found in the preliminary cosmid contig. A cosmid, P1, PAC, and long PCR contig that contained the entire MYO7A gene was assembled. Primers were designed from the composite cDNA sequence and used to detect intron-exon junctions by directly sequencing cosmid, P1, PAC, and genomic PCR DNA. Alternatively spliced products were transcribed from the MYO7A gene: the largest transcript (7.4 kb) contains 49 exons. The MYO7A gene is relatively large, spanning approximately 120 kb of genomic DNA on chromosome 11q13.

A new locus for autosomal recessive retinitis pigmentosa (RP19) maps to 1p13-1p21

Autosomal recessive retinitis pigmentosa (arRP) is characterized by considerable allelic and nonallelic heterogeneity. Mutations have been described in the rhodopsin gene (RHO), the genes encoding the alpha and beta subunits of rod phosphodiesterase (PDEA and PDEB), and the gene encoding the alpha subunit of the cGMP-gated channel (CNCG). In addition, linkage studies in single extended pedigrees have defined two new arRP loci, at 1q and 6p. To identify the disease gene in a Spanish consanguineous arRP family, a linkage analysis was undertaken. After testing 102 polymorphic markers, a significant positive lod score (Zmax = 3.64 at theta = 0) was obtained with marker D1S188 at 1p13-p21, the same region where the Stargardt and fundus flavimaculatus (FFM) loci were previously defined. Exhaustive ophthalmologic examination of the patients clearly distinguished the disease from the Stargardt and FFM phenotypes and revealed an atypical form of arRP with choroidal atrophy as a distinctive feature.

Usher's syndrome type IC: clinical studies and fine-mapping the disease locus

Usher's syndrome type I is a heterogeneous group of diseases characterized by severe to profound sensorineural hearing loss, absent vestibular function, and progressive pigmentary retinopathy. Other identifying clinical features have not been documented. In this study, we examined olfactory acuity, plasma levels of polyunsaturated fatty acids and sarcosine, and cilia ultrastructure in a homogeneous cohort of patients with Usher's syndrome type IC. The normal age-dependent decline in olfactory acuity was observed, and normal plasma levels of polyunsaturated fatty acids and sarcosine were found. However, the incidence of compound cilia in biopsies from the inferior turbinate was significantly higher than that reported in control populations. By reconstructing haplotypes in affected persons. D11S902 and D11S1310 were identified as flanking markers over an interval that contains a candidate gene, KCNC1. No mutations in the coding sequence of this gene could be demonstrated in affected persons.

Usher syndrome in the city of Birmingham--prevalence and clinical classification

AIMS

To estimate the prevalence of Usher syndrome in the city of Birmingham, and to establish a database of patients who have been classified into different clinical subtypes essential for future gene mutation analysis.

METHODS

Symptomatic cases of Usher syndrome (US) resident in the city of Birmingham in June 1994 were ascertained through multiple sources. Ophthalmic and audiological reassessment together with examination of medical records and patient questionnaires allowed classification of three subtypes, US 1, US 2, and US 3. In addition, family pedigrees were examined and blood was taken from index patients for DNA extraction.

RESULTS

In the population aged over 15 years the prevalence was 6.2 per 100 000 population for all US subtypes. The prevalence for US 1 and US 2 was 5.3 per 100 000 population. This is greater than previously reported. In the age group 30-49 years the prevalence approached 1 in 10 000. Clinical classification found 33% US 1, 47% US 2, and 20% US 3.

CONCLUSION

This higher prevalence rate and greater frequency of US 2 and US 3 may reflect a more complete ascertainment.

Sensorineural hearing loss in children

Hearing loss in infants and children may be sensorineural, conductive, or mixed. Severity varies from mild to profound. Educational initiatives aimed at children, parents, and primary health care providers could help prevent needless permanent hearing impairment. Effective programs aimed at education and hearing conservation among children and adolescents are overdue. The causes of sensorineural hearing loss, the concept of multidisciplinary team evaluation, and measurement of hearing are discussed. Advances in genetics of hearing loss are reviewed.

Stable and progressive hearing loss in type 2A Usher's syndrome

Audiograms were traced or additionally performed on 23 Usher's syndrome patients in 10 Dutch multi-affected families, all linked to chromosome 1q (USH2A locus). Serial audiograms, available in 13 patients, were used for a regression analysis of binaural pure tone average on age (follow-up, 9 to 32 years) to test for "significant progression," ie, a significant regression coefficient, here called the "annual threshold increase" (ATI, expressed in decibels per year). A significant ATI (> 1 dB/y) was observed in 3 patients. Analysis of variance of ATI demonstrated significant heterogeneity; hearing loss was either stable or progressive. This implies a significant clinical heterogeneity. A similar analysis performed on our progressive USH2A cases and "type III" cases previously reported by others (ATI of 1 to 5 dB/y), some of which were recently linked to chromosome 3q (USH3 locus), failed to show any significant heterogeneity in the progression of hearing loss.

Detection of cone alpha transducin mRNA in human fetal cochlea: negative mutation analysis in Usher syndrome

Cone alpha transducin (GNAT2), known to be expressed in photoreceptors, was found to be transcribed in human fetal cochlea. Due to the unexpected finding of expression of this gene in the inner ear and the success of the candidate gene approach in identifying mutations for a variety of heritable disorders, we investigated the possible role of this gene in Usher syndrome type I and type II. Single-strand conformation polymorphism (SSCP) was used to screen the GNAT2 coding region, as well as splice donor and acceptor sites, for mutations in a total of 140 unrelated patients. Two nucleotide changes leading to two silent amino acid changes and one rare polymorphism were found. In view of these results and those of a previously published Southern blot analysis, it is unlikely that mutations in GNAT2 are a common gene abnormality in Usher syndrome type I or type II.

Genetic heterogeneity of Usher syndrome type II in a Dutch population

The Usher syndromes are a group of autosomal recessive disorders characterised by retinitis pigmentosa (RP) with congenital, stable (non-progressive) sensorineural hearing loss. Profound deafness, RP, and no vestibular responses are features of Usher type I, whereas moderate to severe hearing loss and RP with normal vestibular function describe Usher type II. The gene responsible for most cases of Usher II, USH2a, is on chromosome 1q41; at least one other Usher II gene (as yet unlinked) is known to exist. Usher III presents with a progressive hearing loss that can mimic the audiometric profile seen in Usher II. A gene causing Usher III in a group of Finnish families, USH3, resides on chromosome 3q. Since the phenotypes for Usher II and III overlap, it is important to determine how frequently Usher IIa, Usher IIb, and Usher III occur in a clinical population of non-Usher I patients. DNA was collected from 29 Dutch families and genotyped with six DNA markers known to flank the USH2a gene closely, and with five markers that flank USH3. Results of haplotype and linkage analysis were consistent with linkage to the USH2a locus in 26 of these 29 Dutch families. Three families displayed no linkage to 1q41 markers, and one of these three families appeared unlinked to 3q markers as well; current haplotypes of the other two families are inconclusive for linkage with the USH3 locus without further genotyping. While an A test for heterogeneity of USH2a was statistically significant, no convincing evidence of linkage to USH3 was found in this Dutch sample. Consequently, the frequency of the unlinked variety of Usher IIa (Usher IIb) in The Netherlands was estimated as 0.104. To determine if marker alleles could be used to differentiate Usher type IIa from Usher IIb, parental chromosomes of the 26 Usher IIa families were analysed for significant non-random association of specific alleles from flanking loci with USH2a, but no linkage disequilibrium was observed in this Dutch population.

Neuroradiology and clinical aspects of Usher syndrome

We describe the neurological evaluation and MRI analysis of 30 patients, belonging to 16 families with Usher syndrome (US) type I and type II (US1 and US2). In addition to the classic visual and audiological abnormalities seen in these patients, we observed abnormal gait in 88.9% of US1 and in 66.7% of US2 patients and abnormal coordination in 33.4% of US1, and in 58.3% of US2. Borderline mental retardation, depression or bipolar affective disorder were observed in 16.7% of US1 and 33.3% of US2 patients. MRI analysis showed cerebellar abnormalities in 50% of US1 and 75% of US2 patients, but no clear correlation was observed between structural abnormalities and clinical findings. A pattern for the MRI classification of US patients is suggested.

Genetic mapping studies of 40 loci and 23 cosmids in chromosome 11p13-11q13, and exclusion of mu-calpain as the multiple endocrine neoplasia type 1 gene

Forty loci (16 polymorphic and 24 non-polymorphic) together with 23 cosmids isolated from a chromosome 11-specific library were used to construct a detailed genetic map of 11p13-11q13. The map was constructed by using a panel of 13 somatic cell hybrids that sub-divided this region into 19 intervals, a meiotic mapping panel of 33 multiple endocrine neoplasia type 1 (MEN1) families (134 affected and 269 unaffected members) and a mitotic mapping panel that was used to identify loss of heterozygosity in 38 MEN1-associated tumours. The results defined the most likely order of the 16 loci as being: 11pter-D11S871-(D11S288, D11S149)-11cen-CNTF-PGA-ROM1-D11S480-PYGM- SEA-D11S913-D11S970-D11S97- D11S146-INT2-D11S971-D11S533-11qter. The meiotic mapping studies indicated that the most likely location of the MEN1 gene was in the interval flanked by PYGM and D11S97, and the results of mitotic mapping suggested a possible location of the MEN1 gene telomeric to SEA. Mapping studies of the gene encoding mu-calpain (CAPN1) located CAPN1 to 11q13 and in the vicinity of the MEN1 locus. However, mutational analysis studies did not detect any germ-line CAPN1 DNA sequence abnormalities in 47 unrelated MEN1 patients and the results therefore exclude CAPN1 as the MEN1 gene. The detailed genetic map that has been constructed of the 11p13-11q13 region should facilitate the construction of a physical map and the identification of candidate genes for disease loci mapped to this region.

Clinical features of progressive bifocal chorioretinal atrophy: a retinal dystrophy linked to chromosome 6q

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.

Analysis of phosducin as a candidate gene for retinopathies

Phosducin, a retina-expressed gene mapped to chromosome 1q25-32.1, was analyzed as a candidate gene for retinopathies. The phosducin gene was cloned and characterized, and PCR primers were designed. Eighty-three patients with various retinopathies and 45 control subjects (24 American, 21 Japanese) were analyzed for mutations in the phosducin gene by PCR, denaturing gradient gel electrophoresis (DGGE), and sequencing. A heterozygous sequence variant changing a glycine to arginine at codon 178 was found in one Usher syndrome type II (USH2) patient, while the other USH2 patients did not show any coding sequence variant. A heterozygous sequence variant changing an asparagine to lysine at codon 174 was found in a patient with a severe retinal degeneration in the category of diseases known as acute zonal occult outer retinopathy (AZOOR). Three non-coding sequence variants were found. Two of these were always present together and found in 20.8% of American and 2.4% of Japanese control subjects, reflecting a difference in population pools. In conclusion, the phosducin gene did not show mutations consistent with it being the causative gene for USH2, but its possible pathogenicity in AZOOR or other retinopathies remains an open question which may be answered by further analysis.

Evidence for a fourth locus in Usher syndrome type I

Usher syndrome type I (US1) is an autosomal recessive condition in which three different genes have been already localised (USH1A, USH1B, and USH1C on chromosomes 14q32, 11q13, and 11p15 respectively). The genetic heterogeneity of US1 has been confirmed in a previous study by linkage analysis of 20 French pedigrees. Here, we report the genetic exclusion of the three previously reported loci in two large multiplex families of Moroccan and Pakistani origin, suggesting the existence of at least a fourth locus in Usher syndrome type I.

Ophthalmologic findings in Usher syndrome type 2A

Thirty-seven patients, comprising 24 familial cases and 13 isolated patients with Usher syndrome type II (USH2), underwent ophthalmologic examination. Based on the degree of hearing loss, normal vestibular function, and gene-linkage analysis, familial cases were assumed to have USH2A. An analysis of genetic heterogeneity failed to reveal the presence of a second locus in the Dutch population. Although the patients appear to belong to a genetically homogeneous group, remarkable ophthalmologic variability was found. Corrected visual acuity decreased with age and remarkable differences in visual acuity were found within one family. Fundoscopic findings were classified as type A if attenuated vessels and bone corpuscles in all quadrants were found or as type B if findings other than these were found. The prevalence of type A significantly increased with age.

Mouse model for Usher syndrome: linkage mapping suggests homology to Usher type I reported at human chromosome 11p15

Usher syndrome is a group of diseases with autosomal recessive inheritance, congenital hearing loss, and the development of retinitis pigmentosa, a progressive retinal degeneration characterized by night blindness and visual field loss over several decades. The causes of Usher syndrome are unknown and no animal models have been available for study. Four human gene sites have been reported, suggesting at least four separate forms of Usher syndrome. We report a mouse model of type I Usher syndrome, rd5, whose linkage on mouse chromosome 7 to Hbb and tub has homology to human Usher I reported on human chromosome 11p15. The electroretinogram in homozygous rd5/rd5 mouse is never normal with reduced amplitudes that extinguish by 6 months. Auditory-evoked response testing demonstrates increased hearing thresholds more than control at 3 weeks of about 30 decibels (dB) that worsen to about 45 dB by 6 months.

Clinical findings in obligate carriers of type I Usher syndrome

Seventeen obligate carriers from nine families with autosomal recessive Usher syndrome type I underwent otological, audiological, vestibular, and ophthalmological examination in order to identify possible manifestations of heterozygosity. Linkage studies were performed and six families showed linkage to chromosome region 11q13.5 while 3 families have so far failed to show linkage to the candidate regions. Eight obligate carriers had an abnormal pure-tone audiogram. Two different audiometric patterns could be distinguished when hearing loss was corrected for age and sex. Four carriers (24%) had significant sensorineural hearing loss (SNHL) which increased at higher frequencies. The other 13 carriers had SNHL of about 10 dB at 0.25 and 0.5 kHz, but less at higher frequencies. Vestibular findings were generally normal. Electro-oculography demonstrated a significant lower mean light peak/dark trough ratio in Usher type I carriers compared to normal control individuals. The methods used in this study were found not to be specific enough to clinically identify carriers of Usher type I syndrome. Nevertheless it is remarkable that a number of obligate carriers showed significant audiological and ophthalmological abnormalities.

Recoverin, a calcium-binding protein in photoreceptors

Recoverin is a Ca2+-binding protein found primarily in vertebrate photoreceptors. The proposed physiological function of recoverin is based on the finding that recoverin inhibits light-stimulated phosphorylation of rhodopsin. Recoverin interacts with rod outer segment membranes in a Ca2+-dependent manner. This interaction requires N-terminal acylation of recoverin. Four types of fatty acids have been detected on the N-terminus of recoverin, but the functional significance of this heterogeneous acylation is not yet clear.

Future directions for rhodopsin structure and function studies

NMR (nuclear magnetic resonance) may be useful for determining the structure of retinal and its environment in rhodopsin, but not for determining the complete protein structure. Aggregation and low yield of fragments of rhodopsin may make them difficult to study by NMR. A long-term multidisciplinary attack on rhodopsin structure is required.

More answers about cGMP-gated channels pose more questions

Our understanding of the molecular properties and cellular role of cGMP-gated channels in outer segments of vertebrate photo-receptors has come from over a decade of studies which have continuously altered and refined ideas about these channels. Further examination of this current view may lead to future surprises and further refine the understanding of cGMP-gated channels.

Cyclic nucleotides as regulators of light-adaptation in photoreceptors

Cyclic nucleotides can regulate the sensitivity of retinal rods to light through phosducin. The phosphorylation state of phosducin determines the amount of G available for activation by Rho*. Phosducin phosphorylation is regulated by cyclic nucleotides through their activation of cAMP-dependent protein kinase. The regulation of phosphodiesterase activity by the noncatalytic cGMP binding sites as well as Ca2+/calmodulin dependent regulation of cGMP binding to the cation channel are also discussed.

Long term potentiation and CaM-sensitive adenylyl cyclase: Long-term prospects

The type I CaM-sensitive adenylyl cyclase is in a position to integrate signals from multiple inputs, consistent with the requirements for mediating long term potentiation (LTP). Biochemical and genetic evidence supports the idea that this enzyme plays an important role inc LTP. However, more work is needed before we will be certain of the role that CaM-sensitive adenylyl cyclases play in LTP.

Modulation of the cGMP-gated channel by calcium

Calcium acting through calmodulin has been shown to regulate the affinity of cyclic nucleotide-gated channels expressed in cell lines. But is calmodulin the Ca-sensor that normally regulates these channels?

How many light adaptation mechanisms are there?

The generally positive response to our target article indicates that most of the commentators accept our contention that light adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in adaptation. Finally, additional aspects of cGMP involvement in adaptation are considered. We discuss each of these points in turn.

Gene therapy, regulatory mechanisms, and protein function in vision

Hereditary retinal degeneration due to mutations in visual genes may be amenable to therapeutic interventions that modulate, either positively or negatively, the amount of protein product. Some of the proteins involved in phototransduction are rapidly moved by a lightdependent mechanism between the inner segment and the outer segment in rod photoreceptor cells, and this phenomenon is important in phototransduction.

A novel protein family of neuronal modulators

A number of proteins homologous to recoverin have been identified in the brains of the several vertebrate species. The brainderived members originally contain four EF-hand domains, but NH2- terminal domain is aberrant. Many of these proteins inhibited light-induced rhodopsin phosphorylation at high [Ca2+], suggesting that the brain-derived members may act as a Ca2+-sensitive modulator of receptor phosphorylation, as recoverin does.

The structure of rhodopsin and mechanisms of visual adaptation

Rapidly advancing studies on rhodopsin have focused on new strategies for crystallization of this integral membrane protein for x-ray analysis and on alternative methods for structural determination from nuclear magnetic resonance data. Functional studies of the interactions between the apoprotein and its chromophore have clarified the role of the chromophore in deactivation of opsin and in photoactivation of the pigment.

Crucial steps in photoreceptor adaptation: Regulation of phosphodiesterase and guanylate cyclase activities and Ca2+-buffering

This commentary discusses the balance of phosphodiesterase and guanylate cyclase activities in vertebrate photoreceptors at moderate light intensities. The rate of cGMP hydrolysis and synthesis seem to equal each other. Ca2+ as regulator of both enzyme activities is also effectively buffered in photoreceptor cells by cytoplasmic buffer components.

The atomic structure of visual rhodopsin: How and when?

Strong arguments are presented by Hargrave suggesting that the crystallization of visual rhodopsin for high resolution analysis by X-ray crystallography or electron microscopy is feasible. However, the effort needed to achieve this goal will most likely exceed the resources of a single laboratory and a concerted approach to the research is necessary.

Molecular insights gained from covalently tethering cGMP to the ligand-binding sites of retinal rod cGMP-gated channels

A photoaffinity analog of cGMP has been used to biochemically identify a new ligand-binding subunit of the retinal rod cGMP-activated ion channel, as well as amino acids in contact with cGMP in the original subunit. Covalent tethering of this probe to channels in excised menbrane patches has revealed a functional heteogeneity in the ligand-binding sites that may arise from the two biochemically identified subunits.

Genetic and functional complexity of inherited retinal degeneration

Recent findings emphasize the complexity, both genetic and functional, of the manifold genes and mutations causing inherited retinal degeneration in humans. Knowledge of the genetic bases of these diseases can contribute to design of rational therapy, as well as elucidating the function of each gene product in normal visual processes.

Channel structure and divalent cation regulation of phototransduction

The identification of additional subunits of the cGMP-gated cation channel suggests exciting questions about their regulatory roles and about structure/functional relationships. How do the different subunits interact? How is the complex assembled into the plasma membrane? Divalent cations have been implicated in the regulation of adaptation. One often overlooked cation is magnesium. Could this ion play a role in phototransduction?

Structure of the cGMP-gated channel

The subunit structure of the cGMP-gated cation channel of rod photoreceptors is rapidly being defined, and in the process the mode of regulation by Ca2+-calmodulin unraveled. Intriguingly, early results suggest that additional subunits of unknown function are associated with the channel and remain to be identified.

Linking genotypes with phenotypes in human retinal degenerations: Implications for future research and treatment

Although undoubtedly it will be incomplete by the time it is published, the target article by Daiger et al. organizes mutations in genes that produce retinal degenerations in humans into categories of clinically relevant phenotypes. Such classifications should help us understand the link between altered photoreceptor cell proteins and subsequent cell death, and they may yield insight into methods for preventing consequent blindness.

Genetic and clinical heterogeneity in tapetal retinal dystrophies

Large scale DNA-mutation screening in patients with hereditary retinal diseases greatly enhances our knowledge about retinal function and diseases. Scientists, clinicians, patients, and families involved with retinal disorders may directly benefit from these developments. However, certain aspects of this expanding knowledge, such as the correlation between genotype and phenotype, may be much more complicated than we expect at present.

The determination of rhodopsin structure may require alternative approaches

The structure of rhodopsin is a subject of intense interest. Solving the structure by traditional methods has proved exceedingly challenging. It may therefore be useful to confront the problem by a combination of alternate techniques. These include FTIR (Fourier transform infrared spectroscopy) and AFM (atomic force microscopy) on the intact protein. Furthermore, additional insights may be gained through structural investigations of discrete rhodopsin domains.

Na-Ca + K exchanger and Ca2+ homeostasis in retinal rod outer segments: Inactivation of the Ca2+ efflux mode and possible involvement of intracellular Ca2+ stores in Ca2+ homeostasis

Inactivation of the Ca2+ extrusion mode of the retinal rod Na- Ca + K exchanger is suggested to be the mechanism that prevents lowering of cytosolic free Ca2+ to < 1 nM when rod cells are saturated for a prolonged time under bright light conditions. Under these conditions, Ca2+ fluxes across disk membranes can contribute significantly to Ca2+ homeostasis in rods.

Nuclear magnetic resonance studies on the structure and function of rhodopsin

Magic angle spinning (MAS) NMR methods provide a means of obtaining high resolution structural data on rhodopsin and its photoin termediates. Current work has focused on the structure of the retinal chromophore and its interactions with surrounding protein charges. The recent development of MAS NMR methods for measuring internuclear distances with a resolution of ∼0.2 will complement diffraction methods for addressing key mechanistic questions.

Glutamate accumulation in the photoreceptor-presumed final common path of photoreceptor cell death

Genetic abnormalities of three factors related to the photoreceptor mechanism have been reported in both animal models and humans. Apoptotic mechanism has also been suggested as a final common pathway of photoreceptor cell death. Our findings of increased level of glutamate in photoreceptor cells in rds mice suggest that amino acid might mediate between these two pathological mechanisms.

Unique lipids and unique properties of retinal proteins

Amino-terminal heteroacylation has been identified in retinal proteins including recoverin and α subunit of G-protein, transducin. The tissue-specific modification seems to mediate not only a proteinmembrane interaction but also a specific protein-protein interaction. The mechanism generating the heterogeneity and its physiological role are still unclear, but an interesting idea for the latter postulates a fine regulation of the signal transduction pathway by distinct N-acyl groups.

Further insight into the structural and regulatory properties of the cGMP-gated channel

Recent studies from several different laboratories have provided further insight into structure-function relationships of cyclic nucleotide-gated channel and in particular the cCMPgated channel of rod photoreceptors. Site-directed mutagenesis and rod-olfactory chimeria constructs have defined important amino acids and peptide segments of the channel that are important in ion blockage, ligand specificity, and gating properties. Molecular cloning studies have indicated that cyclic nucleotide-gated channels consist of two subunits that are required to reproduce the properties of the native channels. Biochemical analysis of the cGMP-gated channel of rodcells have indicated that the 240 kDa protein that co-purifies with the 63 kDa channel subunit contains both the previously cloned second subunit of the channel and a glutamic acid-rich protein. The regulatory properties of the cGMP-gated channel from rod cells has also been studied in more detail. Studies indicate that the beta subunit of the cGMP-gated channel of rod cells contains the binding site for calmodulin. Interaction of calmodulin with the channel alters the apparent affinity of the channel for cGMP in all in vitro systems that have been studied. The significance of these recent studies are discussed in relation to the commentaries on the target article.