Identification of novel variants in the TMIE gene of patients with nonsyndromic hearing loss

How Transmembrane Inner Ear (TMIE) plays role in the auditory system: A mystery to us

Different cellular mechanisms contribute to the hearing sense, so it is obvious that any disruption in such processes leads to hearing impairment that greatly influences the global economy and quality of life of the patients and their relatives. In the past two decades, transmembrane inner ear (TMIE) protein has received a great deal of research interest because its impairments cause hereditary deafness in humans. This evolutionarily conserved membrane protein contributes to a fundamental complex that plays role in the maintenance and function of the sensory hair cells. Although the critical roles of the TMIE in mechanoelectrical transduction or hearing procedures have been discussed, there are little to no review papers summarizing the roles of the TMIE in the auditory system. In order to fill this gap, herein, we discuss the important roles of this protein in the auditory system including its role in mechanotransduction, olivocochlear synapse, morphology and different signalling pathways; we also review the genotype-phenotype correlation that can per se show the possible roles of this protein in the auditory system.

Homozygous mutations in Pakistani consanguineous families with prelingual nonsyndromic hearing loss

Autosomal recessive nonsyndromic hearing loss (DFNB) is relatively frequent in Pakistan, which is thought to be mainly due to relatively frequent consanguinity. DFNB genes vary widely in their kinds and functions making molecular diagnosis difficult. This study determined the genetic causes in five Pakistani DFNB families with prelingual onset. The familial genetic analysis identified four pathogenic or likely pathogenic homozygous mutations by whole exome sequencing: two splicing donor site mutations of c.787+1G>A in ESRRB (DFNB35) and c.637+1G>T in CABP2 (DFNB93) and two missense mutations of c.7814A>G (p.Asn2605Ser) in CDH23 (DFNB12) and c.242G>A (p.Arg81His) in TMIE (DFNB6). The ESRRB and TMIE mutations were novel, and the TMIE mutation was observed in two families. The two missense mutations were located at well conserved sites and in silico analysis predicted their pathogenicity. This study identified four homozygous mutations as the underlying cause of DFNB including two novel mutations. This study will be helpful for the exact molecular diagnosis and treatment of deafness patients.

Phosphoinositol-4,5-Bisphosphate Regulates Auditory Hair-Cell Mechanotransduction-Channel Pore Properties and Fast Adaptation

Membrane proteins, such as ion channels, interact dynamically with their lipid environment. Phosphoinositol-4,5-bisphosphate (PIP₂) can directly or indirectly modify ion-channel properties. In auditory sensory hair cells of rats (Sprague Dawley) of either sex, PIP₂ localizes within stereocilia, near stereocilia tips. Modulating the amount of free PIP₂ in inner hair-cell stereocilia resulted in the following: (1) the loss of a fast component of mechanoelectric-transduction current adaptation, (2) an increase in the number of channels open at the hair bundle's resting position, (3) a reduction of single-channel conductance, (4) a change in ion selectivity, and (5) a reduction in calcium pore blocking effects. These changes occur without altering hair-bundle compliance or the number of functional stereocilia within a given hair bundle. Although the specific molecular mechanism for PIP₂ action remains to be uncovered, data support a hypothesis for PIP₂ directly regulating channel conformation to alter calcium permeation and single-channel conductance.SIGNIFICANCE STATEMENT How forces are relayed to the auditory mechanoelectrical transduction (MET) channel remains unknown. However, researchers have surmised that lipids might be involved. Previous work on bullfrog hair cells showed an effect of phosphoinositol-4,5-bisphosphate (PIP₂) depletion on MET current amplitude and adaptation, leading to the postulation of the existence of an underlying myosin-based adaptation mechanism. We find similar results in rat cochlea hair cells but extend these data to include single-channel analysis, hair-bundle mechanics, and channel-permeation properties. These additional data attribute PIP₂ effects to actions on MET-channel properties and not motor interactions. Further findings support PIP₂'s role in modulating a fast, myosin-independent, and Ca²⁺-independent adaptation process, validating fast adaptation's biological origin. Together this shows PIP₂'s pivotal role in auditory MET, likely as a direct channel modulator.

Research of genetic bases of hereditary non-syndromic hearing loss

AIM

Hearing loss is the most common sensory disorder that affects approximately one per 1000 live births. With this project, we aimed to identify gene variants that were common causes of hearing loss in Turkey to contribute to the planning of genetic screening programs for hearing loss, as well as to improve genetic counseling to affected families.

MATERIAL AND METHODS

Twenty-one families with at least two affected individuals and parental consanguinity who presented with non-syndromic severe-to-profound sensorineural hearing loss were included in this study. We first screened for mutations in GJB2 and mitochondrial DNA 12S RNA genes. Subsequently, we genotyped the TMIE c.250C>T and SNP markers flanking the SLC26A4, MYO7A, MYO15A, OTOF, CDH23, TMIE, TECTA, PCDH15, TMC1, TMPRSS3, TMHS genes in the remaining twelve families without mutations in GJB2.

RESULTS

Screening for mutations in GJB2 gene showed c.[35delG];[35delG] mutation in four families, c.[35delG];[507C>A] mutation in two families, c.[35delG];[-23+1G>A] mutation in one family, and c.457G>A heterozygous mutation in one family. Genotyping SNP markers showed the c.[250C>T];[250C>T] mutation in TMIE in one family. A homozygous region with SNP genotypes was detected with the OTOF gene in one family, the TMPRSS3 gene in another family, and also a homozygous region was detected with TMHS, OTOF, and TMPRSS3 genes in another family.

CONCLUSIONS

Further research will be required to determine the genetic bases of hearing loss in families with non-syndromic hearing loss.

Comprehensive Analysis of Deafness Genes in Families with Autosomal Recessive Nonsyndromic Hearing Loss

Comprehensive genetic testing has the potential to become the standard of care for individuals with hearing loss. In this study, we investigated the genetic etiology of autosomal recessive nonsyndromic hearing loss (ARNSHL) in a Turkish cohort including individuals with cochlear implant, who had a pedigree suggestive of an autosomal recessive inheritance. A workflow including prescreening of GJB2 and a targeted next generation sequencing panel (Illumına TruSight^TM Exome) covering 2761 genes that we briefly called as mendelian exome sequencing was used. This panel includes 102 deafness genes and a number of genes causing Mendelian disorders. Using this approach, we identified causative variants in 21 of 29 families. Three different GJB2 variants were present in seven families. Remaining 14 families had 15 different variants in other known NSHL genes (MYO7A, MYO15A, MARVELD2, TMIE, DFNB31, LOXHD1, GPSM2, TMC1, USH1G, CDH23). Of these variants, eight are novel. Mutation detection rate of our workflow is 72.4%, confirming the usefulness of targeted sequencing approach in NSHL.

The how and why of identifying the hair cell mechano-electrical transduction channel

Identification of the auditory hair cell mechano-electrical transduction (hcMET) channel has been a major focus in the hearing research field since the 1980s when direct mechanical gating of a transduction channel was proposed (Corey and Hudspeth J Neurosci 3:962-976, 1983). To this day, the molecular identity of this channel remains controversial. However, many of the hcMET channel's properties have been characterized, including pore properties, calcium-dependent ion permeability, rectification, and single channel conductance. At this point, elucidating the molecular identity of the hcMET channel will provide new tools for understanding the mechanotransduction process. This review discusses the significance of identifying the hcMET channel, the difficulties associated with that task, as well as the establishment of clear criteria for this identification. Finally, we discuss potential candidate channels in light of these criteria.

Non-syndromic hearing impairment in India: high allelic heterogeneity among mutations in TMPRSS3, TMC1, USHIC, CDH23 and TMIE

Mutations in the autosomal genes TMPRSS3, TMC1, USHIC, CDH23 and TMIE are known to cause hereditary hearing loss. To study the contribution of these genes to autosomal recessive, non-syndromic hearing loss (ARNSHL) in India, we examined 374 families with the disorder to identify potential mutations. We found four mutations in TMPRSS3, eight in TMC1, ten in USHIC, eight in CDH23 and three in TMIE. Of the 33 potentially pathogenic variants identified in these genes, 23 were new and the remaining have been previously reported. Collectively, mutations in these five genes contribute to about one-tenth of ARNSHL among the families examined. New mutations detected in this study extend the allelic heterogeneity of the genes and provide several additional variants for structure-function correlation studies. These findings have implications for early DNA-based detection of deafness and genetic counseling of affected families in the Indian subcontinent.

Subcellular localization of the transmembrane inner ear (Tmie) protein in a stable Tmie-expressing cell line

Mutations in the transmembrane inner ear (Tmie) gene, which encodes the Tmie protein, have been attributed to deafness autosomal recessive 6 (DFNB6), an autosomal nonsyndromic recessive hearing loss disorder. Although the Tmie gene was identified a few years ago, little is known about subcellular localization of the Tmie protein. In order to address this, we developed a stable cell line expressing Tmie protein. The expression of Myc-tagged Tmie protein was confirmed by Western blot analysis using an anti-Myc antibody and localization of the Tmie protein was confirmed by immunostaining, using the anti-Myc antibody as well as the anti-tmie antibody. Our study demonstrates that the Tmie protein is localized mostly in the cellular membrane and to a lesser extent in cytoplasm. These results suggest that our Tmie expressing stable cell line provides a suitable in vitro model to explore Tmie synthesis and functions.