Novel autosomal dominant TMC1 variants linked to hearing loss: insight into protein-lipid interactions.

BACKGROUND: TMC1, which encodes transmembrane channel-like protein 1, forms the mechanoelectrical transduction (MET) channel in auditory hair cells, necessary for auditory function. TMC1 variants are known to cause autosomal dominant (DFNA36) and autosomal recessive (DFNB7/11) non-syndromic hearing loss, but only a handful of TMC1 variants underlying DFNA36 have been reported, hampering analysis of genotype-phenotype correlations. METHODS: In this study, we retrospectively reviewed 338 probands in an in-house database of genetic hearing loss, evaluating the clinical phenotypes and genotypes of novel TMC1 variants associated with DFNA36. To analyze the structural impact of these variants, we generated two structural models of human TMC1, utilizing the Cryo-EM structure of C. elegans TMC1 as a template and AlphaFold protein structure database. Specifically, the lipid bilayer-embedded protein database was used to construct membrane-embedded models of TMC1. We then examined the effect of TMC1 variants on intramolecular interactions and predicted their potential pathogenicity. RESULTS: We identified two novel TMC1 variants related to DFNA36 (c.1256T > C:p.Phe419Ser and c.1444T > C:p.Trp482Arg). The affected subjects had bilateral, moderate, late-onset, progressive sensorineural hearing loss with a down-sloping configuration. The Phe419 residue located in the transmembrane domain 4 of TMC1 faces outward towards the channel pore and is in close proximity to the hydrophobic tail of the lipid bilayer. The non-polar-to-polar variant (p.Phe419Ser) alters the hydrophobicity in the membrane, compromising protein-lipid interactions. On the other hand, the Trp482 residue located in the extracellular linker region between transmembrane domains 5 and 6 is anchored to the membrane interfaces via its aromatic rings, mediating several molecular interactions that stabilize the structure of TMC1. This type of aromatic ring-based anchoring is also observed in homologous transmembrane proteins such as OSCA1.2. Conversely, the substitution of Trp with Arg (Trp482Arg) disrupts the cation-π interaction with phospholipids located in the outer leaflet of the phospholipid bilayer, destabilizing protein-lipid interactions. Additionally, Trp482Arg collapses the CH-π interaction between Trp482 and Pro511, possibly reducing the overall stability of the protein. In parallel with the molecular modeling, the two mutants degraded significantly faster compared to the wild-type protein, compromising protein stability. CONCLUSIONS: This results expand the genetic spectrum of disease-causing TMC1 variants related to DFNA36 and provide insight into TMC1 transmembrane protein-lipid interactions.

Impact of environmental volatile organic compounds on otitis media in children: Correlation between exposure and urinary metabolites.

INTRODUCTION: Volatile organic compounds (VOCs) induce inflammatory responses. Tobacco smoke contains numerous VOCs and is a risk factor for otitis media effusion (OME); however, no previous studies have investigated the association between VOCs and OME. OBJECTIVES: We used urinary metabolites and exposure to environmental risk factors to investigate the association between VOC and polycyclic aromatic hydrocarbon exposure and recurrent OME in children. METHODS: Children with recurrent OME who visited the Otorhinolaryngology Department of Seoul National University Hospital between November 2014 and June 2015 were prospectively enrolled in the study. Recurrent OME was defined as more than two OME episodes over a 6-month period lasting longer than 2 months. The control group consisted of children without OME in the last year. Demographic information, including age, sex, and previous medical history was obtained, and endoscopic examinations of the tympanic membrane were performed. Urinary concentrations of 1-hydroxypyrene, 2-naphthol, hippuric acid, trans, trans-muconic acid (t,t-MA), mandelic acid, phenyl glyoxylic acid, and methyl hippuric acid were analyzed using high-performance liquid chromatography/tandem mass spectroscopy. Environmental factors assessed included house type, age, renovations, the presence of furniture <6 months old, proximity to a road, and exposure to passive smoking. RESULTS: We enrolled 11 children with OME and 39 controls. Age and sex did not differ between groups. Exposure to passive smoking was significantly more common in the OME group than in the controls (P < 0.001). Urinary concentrations of t.t.-MA were significantly higher in the OME group (126.33 µg/g cr) than in controls (52.661 µg/g cr; P = 0.003). Other metabolites including 1-hydroxypyrene, 2-naphthol, hippuric acid, mandelic acid, phenyl glyoxylic acid, and methyl hippuric acid did not demonstrated significant relation with the OME. CONCLUSIONS: Levels of t,t-MA, a biomarker of benzene exposure, were significantly higher in the OME group than in controls. Passive smoking was significantly more common in the OME group. Our findings suggest that high t,t-MA levels which were probably originated from passive smoking and other pollutants could be indicative OME in children.