Usher syndrome type III can mimic other types of Usher syndrome

Usher syndrome IIIA: a review of the disorder and preclinical research advances in therapeutic approaches

Usher syndrome (USH) is an autosomal recessive disorder characterized by sensorineural hearing loss, progressive pigmentary retinopathy, and vestibular dysfunction. The degree and onset of hearing loss vary among subtypes I, II, and III, while blindness often occurs in the second to fourth decades of life. Usher type III (USH3), characterized by postlingual progressive sensorineural hearing loss, varying levels of vestibular dysfunction, and varying degrees of visual impairment, typically manifests in the first to second decades of life. While USH3 is rare, it is highly prevalent in certain populations. RP61, USH3, and USH3A symbolize the same disorder, with the latter symbol used more frequently in recent literature. This review focuses on the clinical features, epidemiology, molecular genetics, treatment, and research advances for sensory deficits in USH3A.

Genetic Screening of the Usher Syndrome in Cuba

Background

Usher syndrome (USH) is a recessive inherited disease characterized by sensorineural hearing loss, retinitis pigmentosa, and sometimes, vestibular dysfunction. Although the molecular epidemiology of Usher syndrome has been well studied in Europe and United States, there is a lack of studies in other regions like Africa or Central and South America.

Methods

We designed a NGS panel that included the 10 USH causative genes (MYO7A, USH1C, CDH23, PCDH15, USH1G, CIB2, USH2A, ADGRV1, WHRN, and CLRN1), four USH associated genes (HARS, PDZD7, CEP250, and C2orf71), and the region comprising the deep-intronic c.7595-2144A>G mutation in USH2A.

Results

NGS sequencing was performed in 11 USH patients from Cuba. All the cases were solved. We found the responsible mutations in the USH2A, ADGRV1, CDH23, PCDH15, and CLRN1 genes. Four mutations have not been previously reported. Two mutations are recurrent in this study: c.619C>T (p.Arg207^∗) in CLRN1, previously reported in two unrelated Spanish families of Basque origin, and c.4488G>C (p.Gln1496His) in CDH23, first described in a large Cuban family. Additionally, c.4488G>C has been reported two more times in the literature in two unrelated families of Spanish origin.

Conclusion

Although the sample size is very small, it is tempting to speculate that the gene frequencies in Cuba are distinct from other populations mainly due to an “island effect” and genetic drift. The two recurrent mutations appear to be of Spanish origin. Further studies with a larger cohort are needed to elucidate the real genetic landscape of Usher syndrome in the Cuban population.

SD-OCT imaging as a valuable tool to support molecular genetic diagnostics of Usher syndrome type 1

A girl with profound congenital deafness and balance problems was found at 3.5 years of age to be a carrier of two novel compound heterozygous mutations in MYO7A that were predicted to be disease-causing. She also carried one known pathogenic mutation and one rare variant in USH2A. Fundus examination performed at 4.75 years revealed one small peripheral pigment deposit in the right eye, indicating probable retinal degeneration. Spectral domain optical coherence tomography (SD-OCT) showed a loss of photoreceptors throughout the macular area, except for the foveolar region, clearly confirming a diagnosis of Usher syndrome type 1. This case demonstrates that SD-OCT may be easily used in young children to confirm retinal disease, quantify the extent of retinal damage, and monitor disease progression.

Communication in deafblind adults with Usher syndrome: retrospective observational study

PURPOSE

To characterize the communication and the main mechanisms that facilitate interpersonal relationships of deafblind, especially in relation to communication and locomotion and the impact of these aspects on deafblindness.

METHODS

Report of a series of cases conducted from semi-structured interviews with questions relating to the functionality of communication, with Usher syndrome patients attended in a specialized clinic in a university service, in the year 2007. The sample consisted of 11 deafblind subjects, with Usher syndrome, aged between 20 and 57 years (mean age 43 years and SD=12.27), of which 7 (63.6%) were female. The responses were analyzed by qualitative-quantitative technique of the Discurso do Sujeito Coletivo (DSC).

RESULTS

All participants reported that visual and auditory symptoms began in childhood. Of the 11 interviewed, 6 reported that the disease has negatively affected their daily activities, 6 experienced difficulty at work, and 2 at leisure. Four reported that there was a change in family relationships, and 5 reported no change in the interaction with family and friends. In discourse analysis, almost 30% of respondents reported to use alternative forms of communication, 40% said move alone if the way is known before. Only 1 of 11 participants said they did not ask for help when needed.

CONCLUSION

Individuals diagnosed with Usher syndrome face challenging situations in daily activities, personal relationships, at work and at play. Alternative forms of communication are often used when verbal communication is not possible. The majority of respondents have independence of locomotion, or seeking ways to achieve it.

Extended mutation spectrum of Usher syndrome in Finland

PURPOSE

The Finnish distribution of clinical Usher syndrome (USH) types is 40% USH3, 34% USH1 and 12% USH2. All patients with USH3 carry the founder mutation in clarin 1 (CLRN1), whereas we recently reported three novel myosin VIIA (MYO7A) mutations in two unrelated patients with USH1. This study was carried out to further investigate the USH mutation spectrum in Finnish patients.

METHODS

We analysed samples from nine unrelated USH patients/families without known mutations and two USH3 families with atypically severe phenotype. The Asper Ophthalmics USH mutation chip was used to screen for known mutations and to evaluate the chip in molecular diagnostics of Finnish patients.

RESULTS

The chip revealed a heterozygous usherin (USH2A) mutation, p.N346H, in one patient. Sequencing of MYO7A and/or USH2A in three index patients revealed two novel heterozygous mutations, p.R873W in MYO7A and c.14343+2T>C in USH2A. We did not identify definite pathogenic second mutations in the patients, but identified several probably nonpathogenic variations that may modify the disease phenotype. Possible digenism could not be excluded in two families segregating genomic variations in both MYO7A and USH2A, and two families with CLRN1 and USH2A.

CONCLUSION

We conclude that there is considerable genetic heterogeneity of USH1 and USH2 in Finland, making molecular diagnostics and genetic counselling of patients and families challenging.

Cone structure in patients with usher syndrome type III and mutations in the Clarin 1 gene

OBJECTIVE

To study macular structure and function in patients with Usher syndrome type III (USH3) caused by mutations in the Clarin 1 gene (CLRN1).

METHODS

High-resolution macular images were obtained by adaptive optics scanning laser ophthalmoscopy and spectral domain optical coherence tomography in 3 patients with USH3 and were compared with those of age-similar control subjects. Vision function measures included best-corrected visual acuity, kinetic and static perimetry, and full-field electroretinography. Coding regions of the CLRN1 gene were sequenced.

RESULTS

CLRN1 mutations were present in all the patients; a 20-year-old man showed compound heterozygous mutations (p.N48K and p.S188X), and 2 unrelated women aged 25 and 32 years had homozygous mutations (p.N48K). Best-corrected visual acuity ranged from 20/16 to 20/40, with scotomas beginning at 3° eccentricity. The inner segment-outer segment junction or the inner segment ellipsoid band was disrupted within 1° to 4° of the fovea, and the foveal inner and outer segment layers were significantly thinner than normal. Cones near the fovea in patients 1 and 2 showed normal spacing, and the preserved region ended abruptly. Retinal pigment epithelial cells were visible in patient 3 where cones were lost.

CONCLUSIONS

Cones were observed centrally but not in regions with scotomas, and retinal pigment epithelial cells were visible in regions without cones in patients with CLRN1 mutations. High-resolution measures of retinal structure demonstrate patterns of cone loss associated with CLRN1 mutations.

CLINICAL RELEVANCE

These findings provide insight into the effect of CLRN1 mutations on macular cone structure, which has implications for the development of treatments for USH3.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00254605.

A novel 5-bp deletion in Clarin 1 in a family with Usher syndrome

BACKGROUND

To identify the genetic defect in a Lebanese family with two sibs diagnosed with Usher Syndrome.

MATERIALS AND METHODS

Exome capture and sequencing were performed on DNA from one affected member using Agilent in solution bead capture, followed by Illumina sequencing.

RESULTS

This analysis revealed the presence of a novel homozygous 5-bp deletion, in Clarin 1 (CLRN1), a known gene responsible for Usher syndrome type III. The deletion is inherited from both parents and segregates with the disease phenotype in the family. The 5-bp deletion, c.301_305delGTCAT, p.Val101SerfsX27, is predicted to result in a frameshift and protein truncation after 27 amino acids. Sequencing all the coding regions of the CLRN1 gene in the proband did not reveal any other mutation or variant.

CONCLUSION

Here we describe a novel deletion in CLRN1. Our data support previously reported intra familial variability in the clinical features of Usher syndrome type I and III.

An update on the genetics of usher syndrome

Usher syndrome (USH) is an autosomal recessive disease characterized by hearing loss, retinitis pigmentosa (RP), and, in some cases, vestibular dysfunction. It is clinically and genetically heterogeneous and is the most common cause underlying deafness and blindness of genetic origin. Clinically, USH is divided into three types. Usher type I (USH1) is the most severe form and is characterized by severe to profound congenital deafness, vestibular areflexia, and prepubertal onset of progressive RP. Type II (USH2) displays moderate to severe hearing loss, absence of vestibular dysfunction, and later onset of retinal degeneration. Type III (USH3) shows progressive postlingual hearing loss, variable onset of RP, and variable vestibular response. To date, five USH1 genes have been identified: MYO7A (USH1B), CDH23 (USH1D), PCDH15 (USH1F), USH1C(USH1C), and USH1G(USH1G). Three genes are involved in USH2, namely, USH2A (USH2A), GPR98 (USH2C), and DFNB31 (USH2D). USH3 is rare except in certain populations, and the gene responsible for this type is USH3A.

Phenotypes in defined genotypes including siblings with Usher syndrome

OBJECTIVE

To characterize visual function in defined genotypes including siblings with Usher syndrome.

METHODS

Thirteen patients with phenotypically different subtypes of Usher syndrome, including 3 families with affected siblings, were selected. Genetic analysis and ophthalmological examinations including visual fields, full-field electroretinography (ERG), multifocal electroretinography (mf ERG), and optical coherence tomography (OCT) were assessed. The patients' degree of visual handicap was evaluated by a questionnaire (ADL).

RESULTS

Twelve of thirteen patients were genotyped as Usher 1B, 1D, 1F, 2A, 2C or 3A. In 12 of 13 patients examined with ERG the 30 Hz flickering light response revealed remaining cone function. In 3 of the patients with Usher type 1 mf ERG demonstrated a specific pattern, with a sharp distinction between the area with reduced function and the central area with remaining macular function and normal peak time. OCT demonstrated loss of foveal depression with distortion of the foveal architecture in the macula in all patients. The foveal thickness ranged from 159 to 384 µm and was not correlated to retinal function. Three siblings shared the same mutation for Usher 2C but in contrast to previous reports regarding this genotype, 1 of them diverged in phenotype with substantially normal visual fields, almost normal OCT and mf ERG findings, and only moderately reduced rod and cone function according to ERG.

CONCLUSIONS

Evaluation of visual function comprising both the severity of the rod cone degeneration and the function in the macular region confirm phenotypical heterogeneity within siblings and between different genotypes of Usher syndrome.

CLRN1 is nonessential in the mouse retina but is required for cochlear hair cell development

Mutations in the CLRN1 gene cause Usher syndrome type 3 (USH3), a human disease characterized by progressive blindness and deafness. Clarin 1, the protein product of CLRN1, is a four-transmembrane protein predicted to be associated with ribbon synapses of photoreceptors and cochlear hair cells, and recently demonstrated to be associated with the cytoskeleton. To study Clrn1, we created a Clrn1 knockout (KO) mouse and characterized the histological and functional consequences of Clrn1 deletion in the retina and cochlea. Clrn1 KO mice do not develop a retinal degeneration phenotype, but exhibit progressive loss of sensory hair cells in the cochlea and deterioration of the organ of Corti by 4 months. Hair cell stereocilia in KO animals were longer and disorganized by 4 months, and some Clrn1 KO mice exhibited circling behavior by 5-6 months of age. Clrn1 mRNA expression was localized in the retina using in situ hybridization (ISH), laser capture microdissection (LCM), and RT-PCR. Retinal Clrn1 transcripts were found throughout development and adulthood by RT-PCR, although expression peaked at P7 and declined to undetectable levels in adult retina by ISH. LCM localized Clrn1 transcripts to the retinas inner nuclear layer, and WT levels of retinal Clrn1 expression were observed in photoreceptor-less retinas. Examination of Clrn1 KO mice suggests that CLRN1 is unnecessary in the murine retina but essential for normal cochlear development and function. This may reflect a redundancy in the mouse retina not present in human retina. In contrast to mouse KO models of USH1 and USH2, our data indicate that Clrn1 expression in the retina is restricted to the Müller glia. This is a novel finding, as most retinal degeneration associated proteins are expressed in photoreceptors, not in glia. If CLRN1 expression in humans is comparable to the expression pattern observed in mice, this is the first report of an inner retinal protein that, when mutated, causes retinal degeneration.

Usher syndrome IIIA gene clarin-1 is essential for hair cell function and associated neural activation

Usher syndrome 3A (USH3A) is an autosomal recessive disorder characterized by progressive loss of hearing and vision due to mutation in the clarin-1 (CLRN1) gene. Lack of an animal model has hindered our ability to understand the function of CLRN1 and the pathophysiology associated with USH3A. Here we report for the first time a mouse model for ear disease in USH3A. Detailed evaluation of inner ear phenotype in the Clrn1 knockout mouse (Clrn1(-/-)) coupled with expression pattern of Clrn1 in the inner ear are presented here. Clrn1 was expressed as early as embryonic day 16.5 in the auditory and vestibular hair cells and associated ganglionic neurons, with its expression being higher in outer hair cells (OHCs) than inner hair cells. Clrn1(-/-) mice showed early onset hearing loss that rapidly progressed to severe levels. Two to three weeks after birth (P14-P21), Clrn1(-/-) mice showed elevated auditory-evoked brainstem response (ABR) thresholds and prolonged peak and interpeak latencies. By P21, approximately 70% of Clrn1(-/-) mice had no detectable ABR and by P30 these mice were deaf. Distortion product otoacoustic emissions were not recordable from Clrn1(-/-) mice. Vestibular function in Clrn1(-/-) mice mirrored the cochlear phenotype, although it deteriorated more gradually than cochlear function. Disorganization of OHC stereocilia was seen as early as P2 and by P21 OHC loss was observed. In sum, hair cell dysfunction and prolonged peak latencies in vestibular and cochlear evoked potentials in Clrn1(-/-) mice strongly indicate that Clrn1 is necessary for hair cell function and associated neural activation.

The changing face of Usher syndrome: clinical implications

Usher syndrome is both genetically and phenotypically heterogeneous. Traditionally, the condition has been classified into three clinical types, differentiated by the severity and progression of the hearing impairment and by the presence or absence of vestibular symptoms. Recent advances in molecular genetics have enabled researchers to study the phenotypic expression in confirmed molecular groups of Usher. In response to the expansion of clinical and genetic information on Usher, we report an up to date review of the different clinical forms of Usher in known molecular groups and use the emerging evidence to appraise the diagnostic utility of the traditional classification of Usher. Our findings undermine the traditional view that the clinical types of Usher have distinct genetic causes. The pleiotropic effects of some of the major causes of Usher lead to considerable overlap between the different clinical types, with very little evidence for phenotypic-genotypic correlations. The novel synthesis emerging from this review suggests more productive approaches to the diagnosis of Usher in hearing-impaired children which would provide more accurate prognostic information to families.

Retinitis pigmentosa

Hereditary degenerations of the human retina are genetically heterogeneous, with well over 100 genes implicated so far. This Seminar focuses on the subset of diseases called retinitis pigmentosa, in which patients typically lose night vision in adolescence, side vision in young adulthood, and central vision in later life because of progressive loss of rod and cone photoreceptor cells. Measures of retinal function, such as the electroretinogram, show that photoreceptor function is diminished generally many years before symptomic night blindness, visual-field scotomas, or decreased visual acuity arise. More than 45 genes for retinitis pigmentosa have been identified. These genes account for only about 60% of all patients; the remainder have defects in as yet unidentified genes. Findings of controlled trials indicate that nutritional interventions, including vitamin A palmitate and omega-3-rich fish, slow progression of disease in many patients. Imminent treatments for retinitis pigmentosa are greatly anticipated, especially for genetically defined subsets of patients, because of newly identified genes, growing knowledge of affected biochemical pathways, and development of animal models.

The Usher lifestyle survey: maintaining independence: a multi-centre study

Patients with Usher syndrome face a special set of challenges in order to maintain their independence when their sight and hearing worsen. Three different types of Usher (I, II and III) are distinguished by differences in onset, progression and severity of hearing loss, and by the presence or absence of balance problems. In this study 93 Usher patients from seven European countries filled out a questionnaire on maintaining independence (60 patients type I, 25 patients type II, four patients type III and four patients type unknown). Results of Usher type I and II patients are presented. Following the Nordic definition of maintaining independence in deaf-blindness, three domains are investigated: access to information, communication and mobility. Research variables in this study are: age and type of Usher, considered hearing loss- and the number of retinitis pigmentosa-related sight problems. Usher type I patients tend to need more help than Usher type II patients and the amount of help that they need grows when patients get older or when considered hearing loss worsens. No patterns in results were seen for the number of retinitis pigmentosa related sight problems.

Mutation screening of USH3 gene (clarin-1) in Spanish patients with Usher syndrome: low prevalence and phenotypic variability

Usher syndrome type III is an autosomal recessive disorder clinically characterized by the association of retinitis pigmentosa (RP), variable presence of vestibular dysfunction and progressive hearing loss, being the progression of the hearing impairment the critical parameter classically used to distinguish this form from Usher syndrome type I and Usher syndrome type II. Usher syndrome type III clinical subtype is the rarest form of Usher syndrome in Spain, accounting only for 6% of all Usher syndrome Spanish cases. The gene responsible for Usher syndrome type III is named clarin-1 and it is thought to be involved in hair cell and photoreceptor cell synapses. Here, we report a screening for mutations in clarin-1 gene among our series of Usher syndrome Spanish patients. Clarin-1 has been found to be responsible for the disease in only two families: the first one is a previously reported family homozygous for Y63X mutation and the second one, described here, is homozygous for C40G. This accounts for 1.7% of Usher syndrome Spanish families. It is noticeable that, whereas C40G family is clinically compatible with Usher syndrome type III due to the progression of the hearing loss, Y63X family could be diagnosed as Usher syndrome type I because the hearing impairment is profound and stable. Thus, we consider that the progression of hearing loss is not the definitive key parameter to distinguish Usher syndrome type III from Usher syndrome type I and Usher syndrome type II.

Audiological and vestibular features in affected subjects with USH3: a genotype/phenotype correlation

The aims were to compare the genotype/phenotype relationship between USH3 mutations and the consequent hearing and vestibular phenotype; and to compare hearing loss (HL) progression between Usher syndrome types IB, IIA and USH3. Genetic, audiometric and vestibular examinations were performed in 28 subjects with USH3. Five different mutations in USH3 were identified. Severe HL was present from an early age (4 to 6 years) in 35% of subjects with USH3. Progression of HL begins in the first decade, and approximately 50% of subjects with USH3 become profoundly deaf by age 40. Various vestibular abnormalities were found in about half (10/22) of the tested subjects with USH3. Depending on the severity of HL, subjects with USH3 might be misdiagnosed as either Usher type IB or IIA. The results from this study can be used as discriminatory features in differential diagnosis of this syndrome.

Serial audiometry and speech recognition findings in Finnish Usher syndrome type III patients

Audiometric features, evaluated by serial pure tone audiometry and speech recognition tests (n = 31), were analysed in 59 Finnish Usher syndrome type III patients (USH3) with Finmajor/Finmajor (n = 55) and Finmajor/Finminor (n = 4) USH3A mutations. These patients showed a highly variable type and degree of progressive sensorineural hearing impairment: from normal to moderate USH2A-like hearing impairment at young ages to profound or even USH1B-like hearing impairment at more advanced ages. Compound heterozygous patients generally showed a milder phenotype. The highest progression was seen during the first two decades of life, gradually slowing down with further ageing. This type of non-linear progression may be unique amongst the Usher syndromes. Speech recognition started to deteriorate at highly variable ages. In some patients, it jeopardised normal speech and language development, whereas in others it was still remarkably good at advanced ages.