Novel small molecule-mediated restoration of the surface expression and anion exchange activity of mutated pendrin causing Pendred syndrome and DFNB4

Variants in SLC26A4 (pendrin) are the most common reasons for genetic hearing loss and vestibular dysfunction in East Asians. In patients with Pendred syndrome and DFNB4 (autosomal recessive type of genetic hearing loss 4), caused by variants in SLC26A4, the hearing function is residual at birth and deteriorates over several years, with no curative treatment for these disorders. In the present study, we revealed that a novel small molecule restores the expression and function of mutant pendrin. High-throughput screening of 54,000 small molecules was performed. We observed that pendrin corrector (PC2-1) increased the surface expression and anion exchange activity of p.H723R pendrin (H723R-PDS), the most prevalent genetic variant that causes Pendred syndrome and DFNB4. Furthermore, in endogenous H723R-PDS-expressing human nasal epithelial cells, PC2-1 significantly increased the surface expression of pendrin. PC2-1 exhibited high membrane permeability in vitro and high micromolar concentrations in the cochlear perilymph in vivo. In addition, neither inhibition of Kv11.1 activity in the human ether-a-go-go-related gene assay nor cell toxicity in the cell proliferation assay was observed at a high PC2-1 concentration (30 μM). These preclinical data support the hypothesis of the druggability of mutant pendrin using the novel corrector molecule PC2-1. In conclusion, PC2-1 may be a new therapeutic molecule for ameliorating hearing loss and treating vestibular disorders in patients with Pendred syndrome or DFNB4.

The Genetic Background of Hearing Loss in Patients with EVA and Cochlear Malformation

The most frequently observed congenital inner ear malformation is enlarged vestibular aqueduct (EVA). It is often accompanied with incomplete partition type 2 (IP2) of the cochlea and a dilated vestibule, which together constitute Mondini malformation. Pathogenic SLC26A4 variants are considered the major cause of inner ear malformation but the genetics still needs clarification. The aim of this study was to identify the cause of EVA in patients with hearing loss (HL). Genomic DNA was isolated from HL patients with radiologically confirmed bilateral EVA (n = 23) and analyzed by next generation sequencing using a custom HL gene panel encompassing 237 HL-related genes or a clinical exome. The presence and segregation of selected variants and the CEVA haplotype (in the 5' region of SLC26A4) was verified by Sanger sequencing. Minigene assay was used to evaluate the impact of novel synonymous variant on splicing. Genetic testing identified the cause of EVA in 17/23 individuals (74%). Two pathogenic variants in the SLC26A4 gene were identified as the cause of EVA in 8 of them (35%), and a CEVA haplotype was regarded as the cause of EVA in 6 of 7 patients (86%) who carried only one SLC26A4 genetic variant. In two individuals with a phenotype matching branchio-oto-renal (BOR) spectrum disorder, cochlear hypoplasia resulted from EYA1 pathogenic variants. In one patient, a novel variant in CHD7 was detected. Our study shows that SLC26A4, together with the CEVA haplotype, accounts for more than half of EVA cases. Syndromic forms of HL should also be considered in patients with EVA. We conclude that to better understand inner ear development and the pathogenesis of its malformations, there is a need to look for pathogenic variants in noncoding regions of known HL genes or to link them with novel candidate HL genes.

Natural Course of Residual Hearing with Reference to GJB2 and SLC26A4 Genotypes: Clinical Implications for Hearing Rehabilitation

BACKGROUND

Understanding the characteristics of residual hearing at low frequencies and its natural course in relation to molecular genetic etiology may be important in developing rehabilitation strategies. Thus, we aimed to explore the characteristics and natural course of residual hearing at low frequencies associated with the two most frequent deafness genes: GJB2 and SLC26A4.

METHODS

Initially, 53 GJB2 and 65 SLC26A4 subjects were enrolled, respectively. Only those whose audiograms exhibited hearing thresholds ≤70 dB at 250 and 500 Hz, and who had at least 1-year follow-up period between the first and last audiograms, were included. Collectively, the clinical characteristics of 14 ears from eight subjects with GJB2 variants, and 31 ears from 22 subjects with SLC26A4 variants fulfilled the strict criteria. In this study, a dropout rate refers to an incidence of dropping out of the cohort by cochlear implant surgery due to severe hearing deterioration.

RESULTS

Among the ears with complete serial audiogram data set, significant residual hearing at low frequencies at the time of inclusion was observed in 18.8% of those with GJB2 variants (15 out of 80 ears) and 42.6% of those with SLC26A4 variants (46 out of 108 ears), revealing a difference between two deafness genes. Subsequently, ears with SLC26A4 variants (11 of 46 ears, 23.9%) turned out to have a higher dropout rate for cochlear implantation due to hearing deterioration within the first year than those with GJB2 variants (1 of 15, 6.7%), albeit with no statistical significance. Throughout the follow-up period (mean: 37.2 ± 6.8, range: 12 to 80 months), deterioration of residual hearing at low frequencies at 250 Hz (dB HL/y) and 500 Hz (dB HL/y) of those with GJB2 variants exhibited 3.1 ± 1.3 (range: 0 to 15) and 5.2 ± 1.6 (range: 0 to 20), respectively, suggesting the deterioration of residual hearing in GJB2 variants was rather slow and gradual. Specifically, GJB2 p.Leu79Cysfs*3 show less remarkable residual hearing at low frequencies, but then a relatively stable nature. In contrast, SLC26A4 variants demonstrated a significantly higher dropout rate due to severe hearing deterioration requiring cochlear implantation compared with the GJB2 variants. This trend was observed not only in the first-year follow-up period but also in the follow-up periods thereafter. The p.His723Arg;c.919-2A>G genotype of SLC26A4, in particular, was associated with a high propensity for sudden hearing deterioration, as indicated by the dropout rate, which was as high as 46.2% for cochlear implantation due to hearing deterioration during the first year follow-up period. Furthermore, the dropout rate for cochlear implantation was observed in 7.1% of those with GJB2 variants (one out of 14 ears) and 30.3% of those with SLC26A4 variants (10 out of 33 ears) throughout the entire follow-up period.

CONCLUSIONS

Our results suggest that there is a difference with respect to the progressive nature of residual hearing at low frequencies between the two most common genes responsible for hearing loss, which may provide clinical implications of having individualized rehabilitation and timely intervention.

DNAJC14 Ameliorates Inner Ear Degeneration in the DFNB4 Mouse Model

The His723Arg (H723R) mutation in SLC26A4, encoding pendrin, is the most prevalent mutation in East Asia, resulting in DFNB4, an autosomal recessive type of genetic hearing loss. Although the main pathological mechanism of H723R was identified as a protein-folding defect in pendrin, there is still no curative treatment for associated hearing loss. Here, we show that H723R-pendrin expression and activity are rescued by activation of the chaperonin DNAJC14. In vitro, DNAJC14 was activated via Japanese encephalitis virus (JEV) inoculation, and toxin-attenuated JEV rescued the surface expression and anion exchange activity of H723R-pendrin. Human H723R-pendrin transgenic mice (hH723R Tg) were established in a mouse slc26a4 knockout background, in which only hH723R-pendrin was expressed in the inner ear (Pax2-Cre dependent) to mimic human DFNB4 pathology. Crossing hH723R Tg with DNAJC14-overexpressing mice resulted in reduced cochlear hydrops and more preserved outer hair cells in the cochlea compared to those in hH723R Tg mice. Furthermore, the stria vascularis and spiral ligament were thicker and KCNJ10 expression was increased with DNAJC14 overexpression; however, hearing function and enlarged endolymphatic hydrops were not recovered. These results indicate that DNAJC14 overexpression ameliorates the cochlear degeneration caused by misfolded pendrin and might be a potential therapeutic target for DFNB4.

Lessons From an Analysis of Newborn Hearing Screening Data for Children With Cochlear Implants

OBJECTIVES

The aims of the study are to identify the limitations of the current newborn hearing screening (NHS) programs and provide recommendations for better protocols.

STUDY DESIGN

Retrospective analysis.

SETTING

Tertiary referral center.

PATIENTS

The study participants were 185 children who received cochlear implants (CIs) at ≤5 years of age.

INTERVENTIONS

Therapeutic and rehabilitative.

MAIN OUTCOME MEASURES

The results of NHS, screening tools used, age, and hearing thresholds at which hearing loss was confirmed, causes of the hearing loss, age of CI insertion, aided pure-tone audiogram findings, and language development level were analyzed.

RESULTS

NHS data was available for 109 children, and 24 patients (22.0%) had passed NHS for both ears. Hearing loss was confirmed considerably later in children who had passed NHS than in children who were referred for further evaluation (p < 0.01). The most common cause for hearing loss in the NHS-pass group was SLC26A4 mutations (41.7%). Patients in the NHS-pass group received CIs considerably later than those in the NHS-referred group (p < 0.01). Among patients with SLC26A4 mutations, the language development level was significantly lower in the NHS-pass group than in the NHS-referred group (p < 0.01).

CONCLUSIONS

Careful counseling regarding NHS results is necessary for parents to understand that the absence of hearing loss at birth does not mean that the child will not develop hearing loss later in life. Genetic testing for SLC26A4 mutations may be necessary in regions with a high incidence of these mutations, such as East Asia.

Congenital hearing loss

Congenital hearing loss (hearing loss that is present at birth) is one of the most prevalent chronic conditions in children. In the majority of developed countries, neonatal hearing screening programmes enable early detection; early intervention will prevent delays in speech and language development and has long-lasting beneficial effects on social and emotional development and quality of life. A diagnosis of hearing loss is usually followed by a search for an underlying aetiology. Congenital hearing loss might be attributed to environmental and prenatal factors, which prevail in low-income settings; congenital infections, particularly cytomegalovirus infection, are also a common risk factor for hearing loss. Genetic causes probably account for the majority of cases in developed countries; mutations can affect any component of the hearing pathway, in particular, inner ear homeostasis (endolymph production and maintenance) and mechano-electrical transduction (the conversion of a mechanical stimulus into electrochemical activity). Once the underlying cause of hearing loss is established, it might direct therapeutic decision making and guide prevention and (genetic) counselling. Management options include specific antimicrobial therapies, surgical treatment of craniofacial abnormalities and implantable or non-implantable hearing devices. An improved understanding of the pathophysiology and molecular mechanisms that underlie hearing loss and increased awareness of recent advances in genetic testing will promote the development of new treatment and screening strategies.