Clinical and molecular genetics of Usher syndrome

Usher syndrome (USH) is the name given to a group of recessively inherited disorders characterised by hearing loss, progressive visual loss due to a retinal degeneration termed retinitis pigmentosa (RP) and in some cases vestibular dysfunction. It is the most common form of syndromic RP and is clinically and genetically heterogeneous. There are three clinical subtypes termed USH1, USH2 and USH3, which are defined by the severity of hearing loss and vestibular dysfunction with visual loss due to RP being common to each subtype. To date, mutations in nine genes have been associated with the three clinical subtypes of Usher syndrome as well as non-syndromic hearing loss and RP. This wide spectrum of clinical and genetic variability provides challenges to clinicians in making a diagnosis of Usher syndrome and delivering prognostic information to affected individuals, whilst the genetic heterogeneity presents problems to geneticists attempting to achieve a molecular diagnosis. This study aims to address these issues by determining the distribution of clinical and molecular subtypes of USH in the United Kingdom (UK). This study represents an original contribution to the knowledge of Usher syndrome, as it is the first prospective clinical study to sequence the coding regions of each of the nine genes associated with this disorder in 187 affected families regardless of their clinical subtype. Detailed ophthalmic phenotyping was performed in 219 individuals. This comprehensive strategy of molecular analysis afforded the opportunity to interrogate for the possibility of digenic effects for which no evidence was found. This strategy enabled the discovery of an atypical and novel phenotype associated with the USH1C gene. A molecular diagnosis was achieved in 80% of families with Usher syndrome and the ophthalmic phenotype of a large cohort of affected individuals with Usher syndrome has been further delineated. This study has resulted in a large cohort of UK patients with a confirmed molecular diagnosis and detailed ophthalmic phenotyping, which will provide a framework for subsequent longitudinal studies enabling the characterisation of how visual function progresses over time. Understanding the natural history of this disorder in genotyped individuals will help pave the way for subsequent gene-directed therapy studies in the future

Development of a genotyping microarray for Usher syndrome

BACKGROUND: Usher syndrome, a combination of retinitis pigmentosa (RP) and sensorineural hearing loss with or without vestibular dysfunction, displays a high degree of clinical and genetic heterogeneity. Three clinical subtypes can be distinguished, based on the age of onset and severity of the hearing impairment, and the presence or absence of vestibular abnormalities. Thus far, eight genes have been implicated in the syndrome, together comprising 347 protein-coding exons. METHODS: To improve DNA diagnostics for patients with Usher syndrome, we developed a genotyping microarray based on the arrayed primer extension (APEX) method. Allele-specific oligonucleotides corresponding to all 298 Usher syndrome-associated sequence variants known to date, 76 of which are novel, were arrayed. RESULTS: Approximately half of these variants were validated using original patient DNAs, which yielded an accuracy of >98%. The efficiency of the Usher genotyping microarray was tested using DNAs from 370 unrelated European and American patients with Usher syndrome. Sequence variants were identified in 64/140 (46%) patients with Usher syndrome type I, 45/189 (24%) patients with Usher syndrome type II, 6/21 (29%) patients with Usher syndrome type III and 6/20 (30%) patients with atypical Usher syndrome. The chip also identified two novel sequence variants, c.400C>T (p.R134X) in PCDH15 and c.1606T>C (p.C536S) in USH2A. CONCLUSION: The Usher genotyping microarray is a versatile and affordable screening tool for Usher syndrome. Its efficiency will improve with the addition of novel sequence variants with minimal extra costs, making it a very useful first-pass screening tool

Choroidal macrovessels: multimodal imaging findings and review of the literature

Background/aims: To describe clinical and multimodal imaging features in a cohort of choroidal macrovessels. Methods: Demographics and multimodal imaging features of 16 eyes of 13 patients with choroidal macrovessels were reviewed. The multimodal imaging included colour fundus photography, fundus autofluorescence (FAF), spectral domain enhanced depth imaging optical coherence tomography (OCT), en face OCT, OCT-angiography (OCT-A), B-scan ultrasonography (US), fluorescein angiography (FFA) and indocyanine green angiography (ICGA). Results: Three patients had bilateral involvement. On colour fundus photography, three patterns were evident (a clearly visible orange-red vessel; a track of pigmentary changes; spots of mild pigmentary changes). Vessel orientation was horizontal (11 eyes), oblique (4 eyes) or vertical (1 eye). In 2 eyes, the vessel was extra-macular. OCT in all cases showed a hyporeflective choroidal area with posterior shadowing and elevation of the overlying retina. Subretinal fluid was present in 4 eyes. FAF (12 eyes) was normal (7 eyes) or showed a hypofluorescent/hyperfluorescent track (4 eyes) or linear hyperautofluorescence (1 eye). En-face OCT (2 eyes) revealed the course of the macrovessel at the level of choroid and choriocapillaris. On OCT-A (2 eyes) the vessel had a reflectivity similar to surrounding vessels but larger diameter. B-scan US (8 eyes) showed a nodular hypoechogenic lesion. FFA (5 eyes) showed early focal hyperfluorescence (4 eyes) not increasing in later phases, or was normal (1 eye). ICGA (6 eyes) showed early hyperfluorescence of the vessel. Conclusions: Choroidal macrovessels can mimic other entities, leading to underdiagnosis. Appreciating relevant features on different imaging modalities will aid a correct diagnosi

An association between stellate nonhereditary idiopathic foveomacular retinoschisis, peripheral retinoschisis and posterior hyaloid attachment

PURPOSE: Stellate nonhereditary idiopathic foveomacular retinoschisis (SNIFR) is a disorder characterized by splitting of the retina at the macula, without a known underlying mechanical or inherited cause. This study investigates demographic, anatomical and functional characteristics of subjects with SNIFR, to explore potential underlying mechanisms. METHODS: In this single-site, retrospective and cross-sectional, observational study, data were collected from 28 eyes from 24 subjects with SNIFR. Descriptive statistics were reported, based on the observed anatomico-functional features. RESULTS: Visual acuity remained stable (median 20/20) in all subjects over a median follow-up of 17 months. All cases demonstrated foveomacular retinoschisis within Henle's fiber layer, at the junction of the outer plexiform and outer nuclear layers. This schisis cavity extended beyond the limits of the macular OCT temporally in all eyes. In the majority of affected eyes, there were documented features of peripheral retinoschisis and broad attachment of the posterior hyaloid at the macula. Functional testing in a cross-sectional subset demonstrated normal retinal sensitivity centrally but an absolute scotoma peripherally. CONCLUSIONS: Stellate nonhereditary idiopathic foveomacular retinoschisis appears to be associated with peripheral retinoschisis and anomalous or incomplete posterior hyaloid detachment. Despite chronic manifestation, this does not significantly affect central visual function, but can manifest with profound loss of peripheral visual function

Functional characterisation and serial imaging of abnormal fundus autofluorescence in patients with retinitis pigmentosa and normal visual acuity

AIM: To characterise and monitor abnormal fundus autofluorescence (AF) in patients with retinitis pigmentosa (RP) who have good visual acuity. METHODS: 21 patients with a clinical diagnosis of RP were examined. All had rod‐cone dystrophy (ISCEV standard electroretinograms (ERGs)), visual acuity of 6/9 or better, and manifested a parafoveal ring of high density fundus AF. Repeat AF imaging was performed after periods of between 2 years and 5 years in 12 patients. Pattern ERG (PERG) and multifocal ERG (mfERG) were performed in 20 cases. Visual fields (VF), photopic and scotopic fine matrix mapping and small field PERGs were performed in representative cases. RESULTS: The rings of high density AF varied in size between patients (from 4°–16° diameter). MfERGs showed relative preservation over the central macular area, correlating with the size of AF ring and with PERG and psychophysical data. Progressive constriction of the AF ring was demonstrated at follow up in three patients. Serial PERG, mfERG, and VFs, performed in one of these cases, showed evidence of deterioration concordant with ring constriction. CONCLUSIONS: High density rings of AF, seen in some patients with RP with good visual acuity, demarcate areas of preserved central photopic function. MfERGs correlate with the area encircled by high density AF and the PERG data. The size of the ring of AF can show progressive constriction accompanied by increasing macular dysfunction

Clinical characterisation of a family with retinal dystrophy caused by mutation in the Mertk gene

BACKGROUND/AIM: MERTK, a tyrosine kinase receptor protein expressed by the retinal pigment epithelium (RPE), is mutated in both rodent models and humans affected by retinal disease. This study reports a survey of families for Mertk mutations and describes the phenotype exhibited by one family. METHODS: 96 probands with retinal dystrophy, consistent with autosomal recessive segregation, were screened by direct sequencing. A family homozygous for a likely null allele was investigated clinically. RESULTS: A novel frame shifting deletion was identified in one of 96 probands. Other polymorphisms were detected. The deletion allele occurred on both chromosomes of four affected family members. Electrophysiology demonstrated early loss of scotopic and macular function with later loss of photopic function. Visual acuities and visual fields were preserved into the second decade. Perception of light vision was present in a patient in the fourth decade. A “bull's eye” appearance and a hyperautofluorescent lesion at the central macula were consistent clinical findings. CONCLUSIONS: Mutations in Mertk are a rare cause of ARRP in humans. The study extends the phenotypic characteristics of this retinal dystrophy and shows distinctive clinical signs that may improve its clinical identification. The moderate severity and presence of autofluorescence implies that outer segment phagocytosis is not entirely absent