Hiding in plain sight: genetic deaf-blindness is not always Usher syndrome

Unraveling the genetic spectrum of inherited deaf-blindness in Portugal

BACKGROUND

Syndromic genetic disorders affecting vision can also cause hearing loss, and Usher syndrome is by far the most common etiology. However, many other conditions can present dual sensory impairment. Accurate diagnosis is essential for providing patients with genetic counseling, prognostic information, and appropriate resources. This study aimed to describe the genetic profile of combined inherited deaf-blindness in Portugal.

METHODS

This was a cross-sectional study conducted at a tertiary hospital in Portugal. Patients were identified through the national, web-based inherited retinal dystrophies registry (IRD-PT, retina.com.pt). Demographics, clinical, and genetic data were retrieved from individual patient records. Genetic variants were classified according to the American College of Medical Genetics and Genomics; only likely pathogenic or pathogenic variants were considered relevant for solved cases.

RESULTS

Eighty-four patients (58.3% males; mean age 40.0 ± 17.9 years) from 71 families were included. Usher syndrome was the most frequent etiology (71.4%) followed by Polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract syndrome (6.0%), Autosomal dominant optic atrophy plus (4.8%) and cone-rod dystrophy and hearing loss (4.8%). Other less frequent etiologies included Alport syndrome (2.4%), Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (2.4%), Heimler syndrome (2.4%), Senior-Loken syndrome (1.2%), Waardenburg syndrome (1.2%), Maternally inherited diabetes and deafness (1.2%), and Stickler syndrome (1.2%). The overall diagnostic yield of deleterious variants in our deaf-blind cohort was 73.2%. A total of 55 genetic variants were identified across 16 different genes; 11 of these variants are novel and herein reported for the first time.

CONCLUSIONS

This is the first study to describe the genetic profile of patients with dual sensory impairment in Portugal, highlighting the genetic heterogeneity associated with inherited deaf-blindness. Usher syndrome was the most prevalent cause in this cohort. Nevertheless, several other less frequent causes must also be considered. This study showed a high diagnostic yield and reported 11 novel genetic variants, thereby contributing to expand the mutational spectrum of these conditions.

Exploring the diverse clinical and variant spectrum of CEP78-associated syndrome: Novel pathogenic variants identified in a case series

Dual sensory impairment, commonly referred to as combined hearing and vision loss, can stem from a diverse spectrum of conditions, each presenting with its unique set of clinical characteristics. Our understanding of dual sensory impairment has expanded significantly in the past decade, broadening the scope of genetic differential diagnoses, including genes such as CEP250, ARSG, TUBB4B, CEP78, and ABHD12. A case series including three patients from two families with genetically diagnosed CEP78-associated cone-rod dystrophy was identified. We collected and reviewed their clinical records, imaging data, and genetic testing results. In addition, a comprehensive literature review was conducted on the phenotype and the genetic testing modality employed in all published CEP78 cases through a PubMed search using the keyword "CEP78." A retinal dystrophy panel detected a novel homozygous CEP78 pathogenic variant (c.1447C>T, p.Arg483*) in siblings-Cases 1 and 2-from Family 1. Both teenagers have a clinical diagnosis of cone-rod dystrophy with presumed normal hearing. Case 3 from Family 2, diagnosed with cone-rod dystrophy and early-onset hearing loss, was found to carry a CEP78 pathogenic variant (c.1206-2A>C) and a likely pathogenic variant (c.856_857del, p.Leu286Glyfs*12) also through panel-based genetic testing. Intriguingly, neither of these variants was reported in an affected sibling's clinical whole-exome sequencing (WES) report when performed in 2015. A review of CEP78-related literature unveiled that the initial report linking CEP78 to cone-rod dystrophy and hearing loss was published in September 2016. Any pathogenic variant found in CEP78 before 2016 would have been categorized as a "clearly disruptive variant in a gene of uncertain significance (GUS)" and might not have been reported in the WES report. It is important to acknowledge that our understanding of genotype-phenotype associations is undergoing rapid expansion. It is also crucial to recognize that repeat genetic testing may represent a fundamentally different approach, given the technological advancements not only in the coverage of the sequencing but also in the more comprehensive understanding of genotype-phenotype associations. This case series also enriches the existing CEP78 literature by providing phenotypic details of the youngest case of CEP78-associated retinopathy reported in the literature (Case 2), which expands our perspective on the natural history of disease in this disorder.

TUBB4B is essential for the cytoskeletal architecture of cochlear supporting cells and motile cilia development

Microtubules are essential for various cellular processes. The functional diversity of microtubules is attributed to the incorporation of various α- and β-tubulin isotypes encoded by different genes. In this work, we investigated the functional role of β4B-tubulin isotype (TUBB4B) in hearing and vision as mutations in TUBB4B are associated with sensorineural disease. Using a Tubb4b knockout mouse model, our findings demonstrate that TUBB4B is essential for hearing. Mice lacking TUBB4B are profoundly deaf due to defects in the inner and middle ear. Specifically, in the inner ear, the absence of TUBB4B lead to disorganized and reduced densities of microtubules in pillar cells, suggesting a critical role for TUBB4B in providing mechanical support for auditory transmission. In the middle ear, Tubb4b-/- mice exhibit motile cilia defects in epithelial cells, leading to the development of otitis media. However, Tubb4b deletion does not affect photoreceptor function or cause retinal degeneration. Intriguingly, β6-tubulin levels increase in retinas lacking β4B-tubulin isotype, suggesting a functional compensation mechanism. Our findings illustrate the essential roles of TUBB4B in hearing but not in vision in mice, highlighting the distinct functions of tubulin isotypes in different sensory systems.

Comprehensive analysis of two hotspot codons in the TUBB4B gene and associated phenotypes

Our purpose was to elucidate the genotype and ophthalmological and audiological phenotype in TUBB4B-associated inherited retinal dystrophy (IRD) and sensorineural hearing loss (SNHL), and to model the effects of all possible amino acid substitutions at the hotspot codons Arg390 and Arg391. Six patients from five families with heterozygous missense variants in TUBB4B were included in this observational study. Ophthalmological testing included best-corrected visual acuity, fundus examination, optical coherence tomography, fundus autofluorescence imaging, and full-field electroretinography (ERG). Audiological examination included pure-tone and speech audiometry in adult patients and auditory brainstem response testing in a child. Genetic testing was performed by disease gene panel analysis based on genome sequencing. The molecular consequences of the substitutions of residues 390 and 391 on TUBB4B and its interaction with α-tubulin were predicted in silico on its three-dimensional structure obtained by homology modelling. Two independent patients had amino acid exchanges at position 391 (p.(Arg391His) or p.(Arg391Cys)) of the TUBB4B protein. Both had a distinct IRD phenotype with peripheral round yellowish lesions with pigmented spots and mild or moderate SNHL, respectively. Yet the phenotype was milder with a sectorial pattern of bone spicules in one patient, likely due to a genetically confirmed mosaicism for p.(Arg391His). Three patients were heterozygous for an amino acid exchange at position 390 (p.(Arg390Gln) or p.(Arg390Trp)) and presented with another distinct retinal phenotype with well demarcated pericentral retinitis pigmentosa. All showed SNHL ranging from mild to severe. One additional patient showed a variant distinct from codon 390 or 391 (p.(Tyr310His)), and presented with congenital profound hearing loss and reduced responses in ERG. Variants at codon positions 390 and 391 were predicted to decrease the structural stability of TUBB4B and its complex with α-tubulin, as well as the complex affinity. In conclusion, the twofold larger reduction in heterodimer affinity exhibited by Arg391 substitutions suggested an association with the more severe retinal phenotype, compared to the substitution at Arg390.

Multidisciplinary approach to inherited causes of dual sensory impairment

PURPOSE

This article presents a review of the main causes of inherited dual sensory impairment (DSI) with an emphasis on the multidisciplinary approach.

METHODS

A narrative review of English literature published before January 2023 was conducted using PubMed, Medline, and Scopus databases. The different causes of inherited DSI are discussed from a multidisciplinary perspective.

RESULTS

There are a wide range of dual sensory impairment (DSI), commonly referred to as blindness and deafness. While Usher syndrome is the most frequent genetic cause, other genetic syndromes such as Alport syndrome or Stickler syndrome can also lead to DSI. Various retinal phenotypes, including pigmentary retinopathy as seen in Usher syndrome, vitreoretinopathy as in Stickler syndrome, and macular dystrophy as in Alport syndrome, along with type of hearing loss (sensorineural or conductive) and additional systemic symptoms can aid in diagnostic suspicion. A thorough ophthalmologic and otorhinolaryngologic examination can help guide diagnosis, which can then be confirmed with genetic studies, crucial for determining prognosis. Effective hearing rehabilitation measures, such as hearing implants, and visual rehabilitation measures, such as low vision optical devices, are crucial for maintaining social interaction and proper development in these patients.

CONCLUSIONS

While Usher syndrome is the primary cause of inherited dual sensory impairment (DSI), other genetic syndromes can also lead to this condition. A proper diagnostic approach based on retinal phenotypes and types of hearing loss can aid in ruling out alternative causes. Multidisciplinary approaches can assist in reaching a definitive diagnosis, which has significant prognostic implications.

Navigating the Usher Syndrome Genetic Landscape: An Evaluation of the Associations between Specific Genes and Quality Categories of Cochlear Implant Outcomes

Usher syndrome (US) is a clinically and genetically heterogeneous disorder that involves three main features: sensorineural hearing loss, retinitis pigmentosa (RP), and vestibular impairment. With a prevalence of 4-17/100,000, it is the most common cause of deaf-blindness worldwide. Genetic research has provided crucial insights into the complexity of US. Among nine confirmed causative genes, MYO7A and USH2A are major players in US types 1 and 2, respectively, whereas CRLN1 is the sole confirmed gene associated with type 3. Variants in these genes also contribute to isolated forms of hearing loss and RP, indicating intersecting molecular pathways. While hearing loss can be adequately managed with hearing aids or cochlear implants (CIs), approved RP treatment modalities are lacking. Gene replacement and editing, antisense oligonucleotides, and small-molecule drugs hold promise for halting RP progression and restoring vision, enhancing patients' quality of life. Massively parallel sequencing has identified gene variants (e.g., in PCDH15) that influence CI results. Accordingly, preoperative genetic examination appears valuable for predicting CI success. To explore genetic mutations in CI recipients and establish correlations between implant outcomes and involved genes, we comprehensively reviewed the literature to gather data covering a broad spectrum of CI outcomes across all known US-causative genes. Implant outcomes were categorized as excellent or very good, good, poor or fair, and very poor. Our review of 95 cochlear-implant patients with US, along with their CI outcomes, revealed the importance of presurgical genetic testing to elucidate potential challenges and provide tailored counseling to improve auditory outcomes. The multifaceted nature of US demands a comprehensive understanding and innovative interventions. Genetic insights drive therapeutic advancements, offering potential remedies for the retinal component of US. The synergy between genetics and therapeutics holds promise for individuals with US and may enhance their sensory experiences through customized interventions.

Variants of LRP2, encoding a multifunctional cell-surface endocytic receptor, associated with hearing loss and retinal dystrophy

Hereditary deafness and retinal dystrophy are each genetically heterogenous and clinically variable. Three small unrelated families segregating the combination of deafness and retinal dystrophy were studied by exome sequencing (ES). The proband of Family 1 was found to be compound heterozygous for NM_004525.3: LRP2: c.5005A > G, p.(Asn1669Asp) and c.149C > G, p.(Thr50Ser). In Family 2, two sisters were found to be compound heterozygous for LRP2 variants, p.(Tyr3933Cys) and an experimentally confirmed c.7715 + 3A > T consensus splice-altering variant. In Family 3, the proband is compound heterozygous for a consensus donor splice site variant LRP2: c.8452_8452 + 1del and p.(Cys3150Tyr). In mouse cochlea, Lrp2 is expressed abundantly in the stria vascularis marginal cells demonstrated by smFISH, single-cell and single-nucleus RNAseq, suggesting that a deficiency of LRP2 may compromise the endocochlear potential, which is required for hearing. LRP2 variants have been associated with Donnai-Barrow syndrome and other multisystem pleiotropic phenotypes different from the phenotypes of the four cases reported herein. Our data expand the phenotypic spectrum associated with pathogenic variants in LRP2 warranting their consideration in individuals with a combination of hereditary hearing loss and retinal dystrophy.