Localization of two genes for Usher syndrome type I to chromosome 11

A defect in harmonin, a PDZ domain-containing protein expressed in the inner ear sensory hair cells, underlies Usher syndrome type 1C

Usher syndrome type 1 (USH1) is an autosomal recessive sensory defect involving congenital profound sensorineural deafness, vestibular dysfunction and blindness (due to progressive retinitis pigmentosa)1. Six different USH1 loci have been reported. So far, only MYO7A (USH1B), encoding myosin VIIA, has been identified as a gene whose mutation causes the disease. Here, we report a gene underlying USH1C (MIM 276904), a USH1 subtype described in a population of Acadian descendants from Louisiana and in a Lebanese family. We identified this gene (USH1C), encoding a PDZ-domain-containing protein, harmonin, in a subtracted mouse cDNA library derived from inner ear sensory areas. In patients we found a splice-site mutation, a frameshift mutation and the expansion of an intronic variable number of tandem repeat (VNTR). We showed that, in the mouse inner ear, only the sensory hair cells express harmonin. The inner ear Ush1c transcripts predicted several harmonin isoforms, some containing an additional coiled-coil domain and a proline- and serine-rich region. As several of these transcripts were absent from the eye, we propose that USH1C also underlies the DFNB18 form of isolated deafness.

Evaluation of the myosin VIIA gene and visual function in patients with Usher syndrome type I

Usher syndrome type I (USH1) is a recessively-inherited disorder consisting of retinitis pigmentosa, profound congenital deafness, and vestibular ataxia. It can be caused by mutations in at least six different loci (USH1A-1F). The gene encoding human myosin VIIA (MYO7A) is the USH1B locus. In this study, 66 unrelated patients with USH1 were evaluated for defects in MYO7A using single-strand conformation polymorphism analysis and direct genomic sequencing. Twenty-nine per cent of cases were found to have likely pathogenic MYO7A mutations. A total of 22 likely pathogenic changes were identified, 18 of which were novel. Cosegregation analysis of mutations in five available families showed that the MYO7A changes segregated with the disease in an autosomal recessive fashion. Average visual function as measured by visual acuity, visual field area, and ERG amplitude was not significantly different between the group of patients with likely pathogenic MYO7A changes and the group in which no likely pathogenic MYO7A changes were detected.

Genetic heterogeneity in familial exudative vitreoretinopathy; exclusion of the EVR1 locus on chromosome 11q in a large autosomal dominant pedigree

BACKGROUND/AIMS

Familial exudative vitreoretinopathy (FEVR) is associated with mutations in the Norrie disease gene in X linked pedigrees and with linkage to the EVR1 locus at 11q13 in autosomal dominant cases. A large autosomal dominant FEVR family was studied, both clinically and by linkage analysis, to determine whether it differed from the known forms of FEVR.

METHODS

Affected members and obligate gene carriers from this family were examined by slit lamp biomicroscopy, indirect ophthalmoscopy, and in some cases fluorescein angiography. Patient DNAs were genotyped for markers at the EVR1 locus on chromosome 11q13.

RESULTS

The clinical evaluation in this family is consistent with previous descriptions of FEVR pedigrees, but linkage analysis proves that it has a form of FEVR genetically distinct from the EVR1 locus on 11q.

CONCLUSION

This proves that there are at least three different loci associated with comparable FEVR phenotypes, a situation similar to that existing for many forms of retinal degeneration.

A sequence-ready map of the Usher syndrome type III critical region on chromosome 3q

Usher syndrome type 3 (USH3; MIM 276902) is an autosomal recessive disorder associated with progressive hearing loss and retinal degeneration. We recently refined the localization of USH3 to a 1-cM genetic interval between markers D3S1299 and D3S3625. We have now constructed a bacterial artificial chromosome contig over the region. Novel polymorphic markers were generated and physically fine-mapped, allowing further narrowing of the critical interval to a 250-kb genomic fragment. Of seven ESTs mapping to the initial critical region, WI-11588 and SHGC-133 represent the human SIAH2 gene, which was excluded as a candidate for USH3 by sequencing and subsequently, by its position. KIAA0001 and D3S3882 derive from the transcript of a putative G-protein-coupled receptor gene that was excluded as a candidate by sequencing of patient DNA. These data provide a basis for the sequencing and final characterization of the USH3 region and isolation of the disease gene.

The cochlear nuclei in two patients with Usher syndrome type I

HYPOTHESIS

Does long-term sound deprivation lead to degeneration of the cochlear nuclei in two Usher type I patients?

METHODS

The cochlear nuclei of these patients were morphometrically analyzed and compared with two age-matched controls. Routine autopsy of the brainstems was performed before the design of this study was known. During this procedure, the ventral cochlear nucleus (VCN) can easily be damaged. Five partially damaged VCN could nevertheless be analyzed for this study, including the right VCN of Usher patient 1 and both VCN of Usher patient 2. Using 15 microm thick serial paraffine sections of the cochlear nuclei, estimates of volume, neuronal densities, number of cells and mean cell diameter of the dorsal cochlear nucleus (DCN) and VCN were obtained.

RESULTS

This study presents unique material of the cochlear nuclei in two patients with Usher syndrome type I. Data regarding volume and total cell number of the VCN are influenced by the absence of a part of the VCN. Results suggest a decrease in mean cell diameter of the VCN in Usher patients. Other parameters of the VCN and DCN, however, showed no major differences between Usher type I patients and controls.

CONCLUSION

Only minor degenerative changes are apparent in the cochlear nuclei of two patients with Usher type I, who were deprived of acoustic stimuli since birth.

DelGEF, an RCC1-related protein encoded by a gene on chromosome 11p14 critical for two forms of hereditary deafness

We have cloned a human cDNA, DELGEF (deafness locus associated putative guanine nucleotide exchange factor), derived from a 225 kb genomic sequence of chromosome 11p14, critical for the Usher 1C syndrome and for DFNB18, a locus for non-syndromic sensorineural deafness. The amino acid sequence of the protein hDelGEF1 is homologous to the nucleotide exchange factor RCCI for the small GTPase Ran. hDelGEF2 is derived from the same DELGEF gene by alternative splicing. In addition, we have identified a murine homologue, mDelGEF. The ubiquitously expressed soluble protein hDelGEF1 is found both in the cyytoplasm and in the nucleus. Overexpressed hDelGEF2 colocalizes with mitochondria.

The role of mouse mutants in the identification of human hereditary hearing loss genes

The mouse is the model organism for the study of hearing loss in mammals. In recent years, the identification of five different mutated genes in the mouse (Pax3, Mitf; Myo7a, Pou4f3, and Myo15) has led directly to the identification of mutations in families with either congenital sensorineural deafness or progressive sensorineural hearing loss. Each of these cases is reviewed here. In addition to providing a powerful gateway to the identification of human hearing loss genes, the study of mouse deafness mutants can lead to the discovery of critical components of the auditory system. Given the availability of several mouse mutants that affect possible homologues of other human deafness genes, it is likely that the mouse will play a key role in identifying other human hearing loss genes in the years to come.

Retinitis pigmentosa and progressive sensorineural hearing loss caused by a C12258A mutation in the mitochondrial MTTS2 gene

Family ZMK is a large Irish kindred that segregates progressive sensorineural hearing loss and retinitis pigmentosa. The symptoms in the family are almost identical to those observed in Usher syndrome type III. Unlike that in Usher syndrome type III, the inheritance pattern in this family is compatible with dominant, X-linked dominant, or maternal inheritance. Prior linkage studies had resulted in exclusion of most candidate loci and >90% of the genome. A tentative location for a causative nuclear gene had been established on 9q; however, it is notable that no markers were found at zero recombination with respect to the disease gene. The marked variability in symptoms, together with the observation of subclinical muscle abnormalities in a single muscle biopsy, stimulated sequencing of the entire mtDNA in affected and unaffected individuals. This revealed a number of previously reported polymorphisms and/or silent substitutions. However, a C-->A transversion at position 12258 in the gene encoding the second mitochondrial serine tRNA, MTTS2, was heteroplasmic and was found in family members only. This sequence change was not present in 270 normal individuals from the same ethnic background. The consensus C at this position is highly conserved and is present in species as divergent from Homo sapiens as vulture and platypus. The mutation probably disrupts the amino acid-acceptor stem of the tRNA molecule, affecting aminoacylation of the tRNA and thereby reducing the efficiency and accuracy of mitochondrial translation. In summary, the data presented provide substantial evidence that the C12258A mtDNA mutation is causative of the disease phenotype in family ZMK.

Genomics and Hearing Impairment

Hearing impairment is clinically and genetically heterogeneous. There are >400 disorders in which hearing impairment is a characteristic of the syndrome, and family studies demonstrate that there are at least 30 autosomal loci for nonsyndromic hearing impairment. The genes that have been identified encode diaphanous (HDIA1), α-tectorin (TECTA), the transcription factorPOU4F3, connexin 26 (GJB2), and two unconventional myosins (MYO7A and MYO15), and four novel proteins (PDS,COCH, DFNA5, DFNB9). The same clinical phenotype in hearing-impaired individuals, even those within the same family, can result from mutations in different genes. Conversely, mutations in the same gene can result in a variety of clinical phenotypes with different modes of inheritance. For example, mutations in the gene encoding MYO7A cause Usher syndrome type IB, autosomal-recessive nonsyndromic hearing impairment (DFNB2), and autosomal-dominant nonsyndromic hearing impairment (DFNA11). Additionally, the mouse ortholog of theMYO7A gene is the shaker-1 gene. Mouse models such asshaker-1 have facilitated the identification of genes that cause hearing impairment in humans. The availability of high-resolution maps of the human and mouse genomes and new technologies for gene identification are advancing molecular understanding of hearing impairment and the complex mechanisms of the auditory system.

Detection of a novel Cys628STOP mutation of the myosin VIIA gene in Usher syndrome type Ib

A Spanish family with three Usher I syndrome-affected members was linked to markers located on chromosome 11q. A search for mutations on the myosin VIIA gene revealed a novel mutation (Cys628STOP) on exon 16 segregating with the disorder in a homozygous state. This nonsense mutation could be responsible for the disease since it leads to a truncated protein that presumably has no function.

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.

Usher syndrome type III (USH3) linked to chromosome 3q in an Italian family

We report an Italian family affected by Usher type III syndrome. Linkage study, performed using markers corresponding to the Usher loci already mapped, clearly showed linkage with markers on chromosome 3q24-25. Our data further support the presence of an Usher III locus on chromosome 3, as recently reported in a Finnish population.

Assembly of a high-resolution map of the Acadian Usher syndrome region and localization of the nuclear EF-hand acidic gene

Usher syndrome type 1C (USH1C) occurs in a small population of Acadian descendants from southwestern Louisiana. Linkage and linkage disequilibrium analyses localize USH1C to chromosome 11p between markers D11S1397 and D11S1888, an interval of less than 680 kb. Here, we refine the USH1C linkage to a region less than 400 kb, between genetic markers D11S1397 and D11S1890. Using 17 genetic markers from this interval, we have isolated a contiguous set of 60 bacterial artificial chromosomes (BACs) that span the USH1C critical region. Exon trapping of BAC clones from this region resulted in the recovery of an exon of the nuclear EF-hand acidic (NEFA) gene. However, DNA sequence analysis of the NEFA cDNA from lymphocytes of affected individuals provided no evidence of mutation, making structural mutations in the NEFA protein unlikely as the cellular cause of Acadian Usher syndrome.

Linkage analysis in Usher syndrome type I (USH1) families from Spain

Usher syndrome (USH) is an autosomal recessive hereditary disorder characterised by congenital sensorineural hearing loss and gradual visual impairment secondary to retinitis pigmentosa (RP). The disorder is clinically and genetically heterogeneous. With regard to Usher type I (USH1), several subtypes have been described, the most frequent being USH1B located on chromosome 11q13.5. Of 18 USH1 families studied by linkage analysis, 12 (67%) showed significant lod score values for locus D11S527 (Zmax=14.032, theta=0.000) situated on chromosome 11q. Our findings suggest considerable genetic heterogeneity in the Spanish USH1 population. It is important to note that one of our families linked to the USH1B locus shows interesting intrafamilial clinical variability. As regards the remaining six USH1 families, the linkage analysis did not provide conclusive data, although two of them show slight linkage to markers located on chromosome 3q (Zmax=1.880, theta=0.000 for D3S1279), the same location that had previously been assigned to some USH3 families.

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.

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.

The autosomal recessive isolated deafness, DFNB2, and the Usher 1B syndrome are allelic defects of the myosin-VIIA gene

Hereditary non-syndromic profound deafness affects about 1 in 2000 children prior to language acquisition. In 80% of the cases, the mode of transmission is autosomal recessive. The number of genes involved in these recessive forms of isolated deafness (DFNB genes) has been estimated to between 30 and 100. So far, ten DFNB genes have been mapped to human chromosomes, one of which has been isolated. By linkage analysis of a single family whose members were affected with profound deafness, some of them presenting with vestibular dysfunction, DFNB2 has been mapped to chromosome 11q13 (ref. 3). The gene responsible for a form of Usher syndrome type I, USH1B, has been assigned to the same chromosomal region. Usher syndrome associates profound congenital deafness and vestibular dysfunction with retinitis pigmentosa. In the homologous murine region are located the shaker-1 mutations responsible for deafness and vestibular dysfunction. It has been demonstrated that the murine shaker-1 and human USH1B phenotypes result from mutations in the gene encoding myosin-VIIA. Based on mapping data as well as on the similarities between the phenotypes of DFNB2-affected patients and shaker-1 mouse mutants, we have proposed that a defective myosin-VIIA may also be responsible for DFNB2 (ref. 1). Sequence analysis of each of the coding exons of the myosin-VIIA gene (MYO7A) was thus undertaken in the DFNB2-affected family. In the last nucleotide of exon 15, a G to A transition was detected, a type of mutation that is known to decrease the efficiency of splicing. Accordingly, this result shows that different mutations in MYO7A result in either an isolated or a syndromic form of deafness.

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.

Pendred syndrome: evidence for genetic homogeneity and further refinement of linkage

Pendred syndrome is the association between congenital sensorineural deafness and goitre. The disorder is characterised by the incomplete discharge of radioiodide from a primed thyroid following perchlorate challenge. However, the molecular basis of the association between hearing loss and a defect in organification of iodide remains unclear. Pendred syndrome is inherited as an autosomal recessive trait and has recently been mapped to 7q31 coincident with the non-syndromic deafness locus DFNB4. To define the critical linkage interval for Pendred syndrome we have studied five kindreds, each with members affected by Pendred syndrome. All families support linkage to the chromosome 7 region, defined by the microsatellite markers D7S501-D7S523. Detailed haplotype analysis refines the Pendred syndrome linkage interval to a region flanked by the marker loci D7S501 and D7S525, separated by a genetic distance estimated to be 2.5 cM. As potential candidate genes have as yet not been mapped to this interval, these data will contribute to a positional cloning approach for the identification of the Pendred syndrome gene.

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.

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.

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.

Construction of a YAC contig encompassing the Usher syndrome type 1C and familial hyperinsulinism loci on chromosome 11p14-15.1

The Usher syndrome type 1C (USH1C) and familial hyperinsulinism (HI) loci have been assigned to chromosome 11p14-15.1, within the interval D11S419-D11S1310. We have constructed a yeast artificial chromosome (YAC) contig, extending from D11S926 to D11S899, which encompasses the critical regions for both USH1C and HI and spans an estimated genetic distance of approximately 4 cM. A minimal set of six YAC clones constitute the contig, with another 22 YACs confirming the order of sequence-tagged sites (STSs) and position of YACs on the contig. A total of 40 STSs, including 10 new STSs generated from YAC insert-end sequences and inter-Alu PCR products, were used to order the clones within the contig. This physical map provides a resource for identification of gene transcripts associated with USH1C, HI, and other genetic disorders that map to the D11S926-D11S899 interval.

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.

Autosomal dominant zonular cataract with sutural opacities in a four-generation family

PURPOSE

We identified and examined four generations of a family with coexisting autosomal dominant zonular cataracts and sutural opacities and sought to determine their genetic basis.

METHODS

Twenty-four of the 48 members in the family were examined. Systemic and ocular histories were obtained, and a detailed ophthalmic examination was performed. From each individual, 20 ml of blood was drawn for linkage studies with microsatellite markers in regions to which zonular cataracts had previously been localized (chromosomes 1, 2, and 16).

RESULTS

Individuals of the first generation were reportedly asymptomatic. Several members of the second generation had morphologically identical zonular cataracts. Affected members of the third generation showed morphologic heterogeneity, with the zonular opacity varying from a uniform lamella to a segregation of dots. A high degree of consanguinity in the second generation suggested recessive inheritance with a pseudodominant inheritance pattern. However, examination of one member of the asymptomatic first generation disclosed senile cataractous changes superimposed on a faint zonular cataract enclosing sutural opacities and a pulverulent fetal nucleus. The latter findings were reconfirmed to be present in affected members of all generations, suggesting an autosomal dominant mode of inheritance. Initial efforts at linkage analysis excluded the gene locus causing this cataract from the Duffy, haptoglobin, and gamma-crystallin regions.

CONCLUSIONS

The cataract in this family is both phenotypically and genetically distinct from previously described and mapped cataracts.

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.

Giant retinal tear and retinal detachment with underlying retinitis pigmentosa and hearing loss

Few retinal detachments have been described in patients with retinitis pigmentosa and allied retinal disorders, with only two cases (both with hearing loss) reported in association with giant retinal tears. To further characterise clinical characteristics of giant retinal tear associated with retinitis pigmentosa, we reviewed the course of four eyes of three patients. Unexpectedly, all three individuals also suffered from congenital sensorineural hearing loss. One suffered from associated myopathy. Despite aggressive surgical management, three of the four eyes became blind. The diagnosis of retinal detachment preceded the diagnosis of photoreceptor dystrophy in two of the three patients. To date, giant retinal tears occurring with underlying retinitis pigmentosa have been described in five young individuals, all of whom had associated congenital sensorineural hearing loss.

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.