TMTC4 is a hair cell–specific human deafness gene
Jiang Li, Byung Yoon Choi, Yasmin Eltawil, Noura Ismail Mohamad, Yesai Park, Ian R. Matthews, Jin Hee Han, Bong Jik Kim, Elliott H. Sherr, Dylan K. Chan
Jiang Li, Byung Yoon Choi, Yasmin Eltawil, Noura Ismail Mohamad, Yesai Park, Ian R. Matthews, Jin Hee Han, Bong Jik Kim, Elliott H. Sherr, Dylan K. Chan
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Research Article Otology

TMTC4 is a hair cell–specific human deafness gene

Abstract

Transmembrane and tetratricopeptide repeat 4 (Tmtc4) is a deafness gene in mice. Tmtc4-KO mice have rapidly progressive postnatal hearing loss due to overactivation of the unfolded protein response (UPR); however, the cellular basis and human relevance of Tmtc4-associated hearing loss in the cochlea was not heretofore appreciated. We created a hair cell–specific conditional KO mouse that phenocopies the constitutive KO with postnatal onset deafness, demonstrating that Tmtc4 is a hair cell–specific deafness gene. Furthermore, we identified a human family in which Tmtc4 variants segregate with adult-onset progressive hearing loss. Lymphoblastoid cells derived from multiple affected and unaffected family members, as well as human embryonic kidney cells engineered to harbor each of the variants, demonstrated that the human Tmtc4 variants confer hypersensitivity of the UPR toward apoptosis. These findings provide evidence that TMTC4 is a deafness gene in humans and further implicate the UPR in progressive hearing loss.

Authors

Jiang Li, Byung Yoon Choi, Yasmin Eltawil, Noura Ismail Mohamad, Yesai Park, Ian R. Matthews, Jin Hee Han, Bong Jik Kim, Elliott H. Sherr, Dylan K. Chan

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Figure 6

TMTC4 variants in human hearing loss.

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TMTC4 variants in human hearing loss. TMTC4 is the likely cause of nonsy...
TMTC4 is the likely cause of nonsyndromic progressive sensorineural hearing loss. (A) Three-generation pedigree consistent with autosomal recessive inheritance pattern. Black: hearing loss; white: normal hearing; cross: deceased. Genotyping at c.547 and c.575 loci of TMTC4 demonstrates cosegregation of compound heterozygous rare variants c.547 G>A and c.575 C>T with hearing loss in patients 788 and 595 (pathogenic variants in red). (B) Audiograms for patients 788 and 595 show noise-induced notch at 2 kHz and high-frequency–predominant sensorineural hearing loss. (C) Lymphoblastoid cell lines were established from 4 human family members. mRNA levels of CHOP and S-XBP1 (opposing proapoptotic and prohomeostatic effectors of the UPR, respectively) were measured by qPCR, and the CHOP/S-XBP1 ratio, which reflects the proapoptotic balance of the UPR, was calculated. n = 4 for each condition. One-way ANOVA followed by Tukey’s multiple-comparison test was performed, with pairwise comparisons relative to patient 789 (as a normal-hearing control) performed. At baseline (left), only patient 595 had significant elevation of the CHOP/S-XBP1 ratio. After treatment with 1 mM thapsigargin (TG) for 6 hours, cells from the 2 patients with hearing loss (788 and 595) had significant elevation of the CHOP/S-XBP1 ratio. **P < 0.001. (D) CHOP/S-XBP1 ratio for the 4 patients was correlated with hearing level (bilateral pure-tone average to 0.5–8 kHz tones). The correlation was not significant for baseline CHOP/S-XBP1 ratio (black line) but was statistically significant for thapsigargin-induced levels (red line; linear regression R2 = 0.94, P < 0.05). (E) HEK cells were generated harboring homozygous E183K or homozygous A192V mutations, and CHOP and S-XBP1 mRNA levels were measured as in C. E183K and A192K mutant cells exhibited elevated CHOP/S-XBP1 ratios at baseline (left). Upon UPR induction with 1 μm TG (right), E183K cells had a greater CHOP/S-XBP1 ratio, whereas A192V cells had a lower CHOP/S-XBP1 ratio, compared with WT cells. n = 4 for each condition. *P < 0.01; **P < 0.001 by 2-tailed unpaired Student’s t test relative to WT.