Genetic heterogeneity of Usher syndrome type II

Proposed therapy, developed in a Pcdh15-deficient mouse, for progressive loss of vision in human Usher syndrome

Usher syndrome type I (USH1) is characterized by deafness, vestibular areflexia, and progressive retinal degeneration. The protein-truncating p.Arg245* founder variant of PCDH15 (USH1F) has an ~2% carrier frequency amongst Ashkenazi Jews accounts for ~60% of their USH1 cases. Here, longitudinal phenotyping in 13 USH1F individuals revealed progressive retinal degeneration, leading to severe vision loss with macular atrophy by the sixth decade. Half of the affected individuals were legally blind by their mid-50s. The mouse Pcdh15^R250X variant is equivalent to human p.Arg245*. Homozygous Pcdh15^R250X mice also have visual deficits and aberrant light-dependent translocation of the phototransduction cascade proteins, arrestin, and transducin. Retinal pigment epithelium (RPE)-specific retinoid cycle proteins, RPE65 and CRALBP, were also reduced in Pcdh15^R250X mice, indicating a dual role for protocadherin-15 in photoreceptors and RPE. Exogenous 9-cis retinal improved ERG amplitudes in Pcdh15^R250X mice, suggesting a basis for a clinical trial of FDA-approved retinoids to preserve vision in USH1F patients.

Dark-adapted threshold and electroretinogram for diagnosis of Usher syndrome

PURPOSE

To determine the utility of ophthalmology evaluation, dark-adapted threshold, and full-field electroretinogram for early detection of Usher syndrome in young patients with bilateral sensorineural hearing loss.

METHODS

We identified 39 patients with secure genetic diagnoses of Usher Syndrome. Visual acuity, spherical equivalent, fundus appearance, dark-adapted threshold, and full-field electroretinogram results were summarized and compared to those in a group of healthy controls with normal hearing. In those Usher patients with repeated measures, regression analysis was done to evaluate for change in visual acuity and dark-adapted threshold with age. Spherical equivalent and full-field electroretinogram responses from dark- and light-adapted eyes were evaluated as a function of age.

RESULTS

The majority of initial visual acuity and spherical equivalent results were within normal limits for age. Visual acuity and dark-adapted threshold worsened significantly with age in Usher type 1 but not in Usher type 2. At initial test, full-field electroretinogram responses from dark- and light-adapted eyes were abnormal in 53% of patients. Remarkably, nearly half of our patients (17% of Usher type 1 and 30% of Usher type 2) would have been missed by tests of retinal function alone if evaluated before age 10.

CONCLUSIONS

Although there is an association of abnormal dark-adapted threshold and full-field electroretinogram at young ages in Usher patients, it appears that a small but important proportion of patients would not be detected by tests of retinal function alone. Thus, genetic testing is needed to secure a diagnosis of Usher syndrome.

Implication of Contactins in Demyelinating Pathologies

Demyelinating pathologies comprise of a variety of conditions where either central or peripheral myelin is attacked, resulting in white matter lesions and neurodegeneration. Myelinated axons are organized into molecularly distinct domains, and this segregation is crucial for their proper function. These defined domains are differentially affected at the different stages of demyelination as well as at the lesion and perilesion sites. Among the main players in myelinated axon organization are proteins of the contactin (CNTN) group of the immunoglobulin superfamily (IgSF) of cell adhesion molecules, namely Contactin-1 and Contactin-2 (CNTN1, CNTN2). The two contactins perform their functions through intermolecular interactions, which are crucial for myelinated axon integrity and functionality. In this review, we focus on the implication of these two molecules as well as their interactors in demyelinating pathologies in humans. At first, we describe the organization and function of myelinated axons in the central (CNS) and the peripheral (PNS) nervous system, further analyzing the role of CNTN1 and CNTN2 as well as their interactors in myelination. In the last section, studies showing the correlation of the two contactins with demyelinating pathologies are reviewed, highlighting the importance of these recognition molecules in shaping the function of the nervous system in multiple ways.

Usher's Syndrome Type II: A Comparative Study of Genetic Mutations and Vestibular System Evaluation

Objective Usher's syndrome type II (USH2) is characterized by moderate to profound congenital hearing loss, later onset of retinitis pigmentosa, and normal vestibular function. Recently, a study investigating the vestibular function of USH2 patients demonstrated a pathologic response to vestibular tests. In this cross-sectional study we performed vestibular tests of a group patients with genetic diagnosis of USH2 syndrome to demonstrate if vestibular damage is present in USH2 patients. Study Design Cross-sectional study. Setting Tertiary referral center. Subjects and Methods Mutated genes of 7 patients with a clinical diagnosis of USH2 were evaluated. Vestibular function was investigated by audiometry, Fitzgerald-Hallpike caloric vestibular testing, cervical vestibular evoked myogenic potentials (C-VEMPs), ocular vestibular evoked myogenic potentials (O-VEMPs), and video head impulse test (v-HIT). Results Genetic tests confirmed the USH2 diagnosis in 5 of 7 patients examined, with 1 patient reporting a unique mutation on genetic tests. Four (80%) of the 5 patients with a genetic diagnosis of USH2 showed pathological O-VEMPs. Two patients (40%) reported bilateral absent or abnormal values of C-VEMPs. The superior semicircular canal presented a significant deficit in 2 (40%) patients. The same 2 cases showed a pathologic response of the v-HIT of the horizontal semicircular canal. Finally, the posterior semicircular canal presented a significant deficit in 4 (40.0%) patients. Conclusion A vestibular evaluation with vestibular evoked myogenic potentials and v-HIT seems to identify latent damage to the vestibular receptors of USH2 patients.

Phenotype of Usher syndrome type II assosiated with compound missense mutations of c.721 C>T and c.1969 C>T in MYO7A in a Chinese Usher syndrome family

AIM

To identify the pathogenic mutations in a Chinese pedigree affected with Usher syndrome type II (USH2).

METHODS

The ophthalmic examinations and audiometric tests were performed to ascertain the phenotype of the family. To detect the genetic defect, exons of 103 known RDs -associated genes including 12 Usher syndrome (USH) genes of the proband were captured and sequencing analysis was performed to exclude known genetic defects and find potential pathogenic mutations. Subsequently, candidate mutations were validated in his pedigree and 100 normal controls using polymerase chain reaction (PCR) and Sanger sequencing.

RESULTS

The patient in the family occurred hearing loss (HL) and retinitis pigmentosa (RP) without vestibular dysfunction, which were consistent with standards of classification for USH2. He carried the compound heterozygous mutations, c.721 C>T and c.1969 C>T, in the MYO7A gene and the unaffected members carried only one of the two mutations. The mutations were not present in the 100 normal controls.

CONCLUSION

We suggested that the compound heterozygous mutations of the MYO7A could lead to USH2, which had revealed distinguished clinical phenotypes associated with MYO7A and expanded the spectrum of clinical phenotypes of the MYO7A mutations.

Novel and recurrent MYO7A mutations in Usher syndrome type 1 and type 2

Usher syndrome (USH) is a group of disorders manifested as retinitis pigmentosa and bilateral sensorineural hearing loss, with or without vestibular dysfunction. Here, we recruited three Chinese families affected with autosomal recessive USH for detailed clinical evaluations and for mutation screening in the genes associated with inherited retinal diseases. Using targeted next-generation sequencing (NGS) approach, three new alleles and one known mutation in MYO7A gene were identified in the three families. In two families with USH type 1, novel homozygous frameshift variant p.Pro194Hisfs*13 and recurrent missense variant p.Thr165Met were demonstrated as the causative mutations respectively. Crystal structural analysis denoted that p.Thr165Met would very likely change the tertiary structure of the protein encoded by MYO7A. In another family affected with USH type 2, novel biallelic mutations in MYO7A, c.[1343+1G>A];[2837T>G] or p.[?];[Met946Arg], were identified with clinical significance. Because MYO7A, to our knowledge, has rarely been correlated with USH type 2, our findings therefore reveal distinguished clinical phenotypes associated with MYO7A. We also conclude that targeted NGS is an effective approach for genetic diagnosis for USH, which can further provide better understanding of genotype-phenotype relationship of the disease.

The immunoglobulin superfamily of neuronal cell adhesion molecules: lessons from animal models and correlation with human disease

Neuronal cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) play a crucial role in the formation of neural circuits at different levels: cell migration, axonal and dendritic targeting as well as synapse formation. Furthermore, in perinatal and adult life, neuronal IgCAMs are required for the formation and maintenance of specialized axonal membrane domains, synaptic plasticity and neurogenesis. Mutations in the corresponding human genes have been correlated to several human neuronal disorders. Perturbing neuronal IgCAMs in animal models provides powerful means to understand the molecular and cellular basis of such human disorders. In this review, we concentrate on the NCAM, L1 and contactin subfamilies of neuronal IgCAMs summarizing recent functional studies from model systems and highlighting their links to disease pathogenesis.

Novel syndrome of cataracts, retinitis pigmentosa, late onset deafness and sperm abnormalities: a new Usher syndrome subtype with X-linked inheritance?

Tissues of the auditory, ocular and reproductive systems have some similarities in their protein families and structures. Consequently, syndromes comprising these systems are described. Hearing loss alone is a component of more than 400 known syndromes and is a common nonsyndromic congenital disorder. Here we describe a syndrome in five brothers with the distinctive presentation of late-onset progressive hearing loss, cataracts, retinitis pigmentosa, sperm motility and shape problems in a family from the Kurdish population in Iran. The clinical findings of these patients are presented in detail and compared to the classical Usher syndromes.

Evidence-based overview of ophthalmic disorders in deaf children: a literature update

BACKGROUND

Deaf children are heavily reliant on the sense of vision in order to develop efficient communication skills and explore the world around them. Any ophthalmic disorder may thus negatively impact on this process, especially if it is unrecognised in the early years of life. These disorders may be correctable (such as myopia) or treatable (such as cataract), and their early identification is of the utmost importance to optimise language development (spoken or sign, or both) and develop social cognition. Those children with non-correctable and non-treatable visual disorders, like retinitis pigmentosa in Usher syndrome, require multiple environmental adaptations and appropriate support services and information.

AIM

: To review the accumulated scientific knowledge on ophthalmic disorders in deaf children and assess the quality of evidence published in the literature in order to contribute to better diagnosis and management of these conditions.

MATERIAL AND METHODS

The project reviewed more than 1000 published papers and other sources. 191 papers complied with the aims of the study and were used in the project. From these studies, 95% were based on type III or IV evidence (mainly descriptive studies or case reports). Only 3% were based on type II evidence and 2% on type I evidence.

RESULTS-CONCLUSIONS

The main conclusions of this project are: a) the overall quality of evidence in the literature concerning deaf children and their ophthalmic problems is very low, b) the prevalence of ophthalmic problems in deaf children is very high (approximately 40% to 60%) and these problems may remain undetected for years although they may have a serious impact on children's acquisition of communication skills, c) screening for ophthalmic problems in deaf children should be encouraged and specialist ophthalmic examination should be carried out as soon as the diagnosis of deafness is confirmed irrespective of age, and may need to be repeated at intervals following diagnosis, d) families should be informed about the nature of the screening process in discussion with the relevant professionals and appropriate information should be available in a range of formats and in different community languages, e) professionals administering the tests should be familiar with the needs of deaf children with ophthalmic problems and should be sensitive to the communication needs of the child, especially undertaking behavioural testing where their collaboration is needed, f) while orthoptists can perform the majority of psychophysical tests (visual and stereo acuity tests, ocular motility tests, etc.) a comprehensive opthalmologic assessment by slit lamp biomicroscopy, streak retinoscopy, direct and indirect ophthalmoscopy, intraocular pressure measurement etc is required. Electrophysiologic testing to help identification of Usher syndrome may also be required, and finally g) serial hearing assessments of children with dual sensory deficits are needed to monitor hearing thresholds, to optimise hearing aid use and to ensure timely referral for cochlear implantation for those who need it.

Audiological findings in Usher syndrome types IIa and II (non-IIa)

The aim was to define the natural history of hearing loss in Usher syndrome type IIa compared to non-IIa. People with Usher syndrome type II show moderate-to-severe hearing loss, normal balance and retinitis pigmentosa. Several genes cause Usher syndrome type II. Our subjects formed two genetic groups: (1) subjects with Usher syndrome type IIa with a mutation and/or linkage to the Usher IIa gene; (2) subjects with the Usher II phenotype with no mutation and/or linkage to the Usher IIa gene. Four hundred and two audiograms of 80 Usher IIa subjects were compared with 435 audiograms of 87 non-IIa subjects. Serial audiograms with intervals of > or = 5 years were examined for progression in 109 individuals Those with Usher syndrome type IIa had significantly worse hearing thresholds than those with non-IIa Usher syndrome after the second decade. The hearing loss in Usher syndrome type IIa was found to be more progressive, and the progression started earlier than in non-IIa Usher syndrome. This suggests an auditory phenotype for Usher syndrome type IIa that is different from that of other types of Usher syndrome II. Thus, this is to our knowledge one of the first studies showing a genotype-phenotype auditory correlation.

Genetic heterogeneity of Usher syndrome type II: localisation to chromosome 5q

Usher syndrome is a group of autosomal recessive disorders that includes retinitis pigmentosa (RP) with hearing loss. Usher syndrome type II is defined as moderate to severe hearing loss with RP. The USH2A gene at 1q41 has been isolated and characterised. In 1993, a large Usher II family affected with a mild form of RP was found to be unlinked to 1q41 markers. Subsequent linkage studies of families in our Usher series identified several type II families unlinked to USH2A and USH3 on 3q25. After a second unlinked family with many affected members and a mild retinal phenotype was discovered, a genome search using these two large families showed another Usher II locus on 5q (two point lod = 3.1 at D5S484). To date, we have identified nine unrelated 5q linked families (maximum combined multipoint lod = 5.86) as well as three Usher II families that show no significant linkage to any known Usher loci. Haplotype analysis of 5q markers indicates that the new locus is flanked by D5S428 and D5S433. Review of ophthalmological data suggests that RP symptoms are milder in 5q linked families; the RP is often not diagnosed until patients near their third decade. Enamel hypoplasia and severe, very early onset RP were observed in two of the three unlinked families; dental anomalies have not been previously described as a feature of Usher type II.

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.

Early diagnosis of Usher syndrome in children

PURPOSE

To screen severe to profound, preverbal hearing-impaired children for Usher syndrome by ophthalmologic examinations, including electroretinographic testing. These patients are especially good candidates for early cochlear implants, which will improve listening and spoken language skills.

METHODS

Consecutive patients over 2 years of age, given a diagnosis of severe to profound, preverbal hearing loss, were screened for Usher syndrome by a complete ophthalmologic examination including an electroretinogram.

RESULTS

Five of 48 patients screened (10.4%) were diagnosed with Usher syndrome and received cochlear implants.

CONCLUSION

All children with severe to profound, preverbal sensorineural hearing loss should be screened for Usher syndrome by ophthalmologic examination including electroretinogram.

Management of hereditary retinal degenerations: present status and future directions

Research on hereditary retinal degenerations has considerably improved our understanding of these disorders, although much remains to be learned about the exact mechanism involved in the pathogenesis. The advent of recombinant DNA technology will refine diagnostic capabilities, which have so far been based on the manifestations of the disease to localization of the molecular defects. The correlation of the molecular defects with the phenotype of the disease will result in better prognostic counseling for patients. In certain forms of retinitis pigmentosa, such as Refsum disease, gyrate atrophy of the choroid and retina, and abetalipoproteinemia, exact biochemical defects have been identified and specific treatments have been applied with some success. In other forms of retinitis pigmentosa, various investigations have suggested the possibilities of arresting the progress of degeneration by means such as the use of growth factors and controlling apoptosis. Efforts to alter the expression of the mutated gene or to introduce a normal gene into the genome are in their infancy, but results are encouraging. Vitamin A has been tried in patients with retinitis pigmentosa, and the results demonstrate statistically significant beneficial effects of this vitamin, suggesting that the course of the disease can be decelerated to some extent. Another interesting research area with potential for therapeutic application is the replacement of the retinal pigment epithelium or the degenerated neural retina by transplantation of the respective cell types. Clinical trials are being conducted both with retinal pigment epithelium and neuroretinal transplants.

Summary of ocular genetic disorders and inherited systemic conditions with eye findings

Of the close to 10,000 known inherited disorders that affect humankind, a disproportionately high number affect the eye. The total number of genes responsible for the normal structure, function, and differentiation of the eye is unknown, but the list of these genes is rapidly and constantly growing. The objective of this paper is to provide a current list of mapped and/or cloned human eye genes that are responsible for inherited diseases of the eye. The ophthalmologist should be aware of recent advances in molecular technology which have resulted in significant progress towards the identification of these genes. The implications of this new knowledge will be discussed herein.

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.

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.

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.

Evidence of genetic heterogeneity of Leber's congenital amaurosis (LCA) and mapping of LCA1 to chromosome 17p13

Leber's congenital amaurosis (LCA) is an autosomal recessive disease responsible for congenital blindness. It is the earliest and most severe inherited retinal dystrophy in human and its genetic heterogeneity has long been recognised. We have recently reported on the first localisation of a disease gene (LCA1) to the short arm of chromosome 17 by homozygosity mapping in five families of North African origin. Here, we refine the genetic mapping of LCA1 to chromosome 17p13 between loci D17S938 and D17S1353 and provide strong support for the genetic heterogeneity of this condition (maximum likelihood for heterogeneity, 17.20 in InL; heterogeneity versus homogeneity, P = 0.0002, heterogeneity versus no linkage, P < 0.0001)

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.

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.

Characterization of a novel 350-kilodalton nuclear phosphoprotein that is specifically involved in mitotic-phase progression

A gene assigned to human chromosome 1q32-41 encodes a novel protein of 3,113 amino acids containing an internal tandem repeat of 177 amino acids. The protein, which we have named "mitosin," was identified by direct binding to purified retinoblastoma protein in vitro with a region distantly related to the retinoblastoma protein-binding site of E2F-1. Mitosin is expressed throughout S, G2, and M phases of the cell cycle but is absent in G1. Its localization is dramatically reorganized from a rather homogeneous nuclear distribution in S phase to paired dots at the kinetochore/centromere region, to the spindle apparatus, and then to the midbody during M-phase progression. This spatial reorganization coincides closely with the temporal phosphorylation patterns of mitosin. Overexpression of N-terminally truncated mutants blocks cell cycle progression mainly at G2/M. These results suggest that mitosin may play an important role in mitotic-phase progression.

Mapping and cloning hereditary deafness genes

In the past two years, considerable progress has been made in the mapping and cloning of human deafness genes. Highlights are the chromosomal localization of at least five genes for autosomal forms of non-syndromic deafness and, more recently, the cloning of an X-linked deafness gene, DFN3, and the Usher syndrome type IB gene. This last gene encodes a myosin-like protein and was identified as the human homolog of the mouse shaker-1 gene. The DFN3 gene Brain 4 encodes a POU domain containing transcription factor that is involved in the development of the inner ear.

Usher's syndrome type 3 in Finland

Usher's syndrome, type 3 (USH3) is characterized by progressive hearing loss. Usher's syndrome, type 3 has been supposed to be rare, occurring in 2% to 4% of all patients with Usher's syndrome. In a nationwide study we collected data on 229 patients with Usher's syndrome in Finland. Definite cases of USH3 were found in 30 (13%) of the 229 patients. An additional 61 patients had clinical evidence of earlier progression of their hearing impairment. We suggest that 91 (40%) of the 229 patients with Usher's syndrome represent cases of USH3.

The Usher syndrome type 2A: clinical findings in obligate carriers

Ten obligate carriers of Usher syndrome type 2A from 5 different families with 2 affected persons all underwent audiologic, vestibular and ophthalmologic examinations. They had a sensorineural hearing loss which was in excess of that expected for their age at all of the frequencies (0.25-8 kHz) tested, however, only a 10 dB (average) excess in hearing loss at 0.25-0.5 kHz proved to be significant. The speech discrimination scores obtained conformed with the hearing thresholds. Tympanometry, acoustic reflex and brain stem auditory-evoked potential findings were generally normal. Some vestibular abnormalities were found in a minority of the carrier sample, but not beyond the level of false positivity. Ophthalmologic findings were essentially normal, although in 5 carriers there was a subnormal electrooculography (EOG). These findings are not sufficient specific for carrier detection.