Mutation of SLC7A14 Causes Syndromic Hearing Loss
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Hearing loss is the most common sensory deficit worldwide. Various pathogenic variants have been linked to sensorineural hearing loss, and transcriptomic analysis has also implicated novel mutations in hair cell genes. This study investigated Slc7a14, which is upregulated in adult mouse inner hair cells (IHCs) compared to outer hair cells (OHCs). Mutations in this gene are associated with autosomal recessive retinitis pigmentosa (RP) in humans. SLC7A14, a glycosylated transporter protein with 14 transmembrane domains, is predicted to mediate lysosomal uptake of cationic amino acids such as arginine. Gene and protein expression analyses confirmed increasing expression of SLC7A14 in IHCs during development with IHC-specific expression through adulthood. Conserved expression of Slc7a14 orthologs was also noted in zebrafish, turtle, chicken and rat inner ear hair cells. Based on these observations, I hypothesized that loss of SLC7A14 will disrupt IHC function and homeostasis resulting in progressive hearing loss. Morphological examination of Slc7a14-knockout mouse cochleae showed degeneration of IHC stereocilia bundles and IHC loss throughout the cochlea. Auditory brainstem response (ABR) thresholds were elevated in Slc7a14-knockout mice, indicative of progressive hearing loss. However, distortion product otoacoustic emission (DPOAE) measurements showed normal OHC function in wildtype and knockout mice. To confirm syndromic disease, alteration in auditory functions were examined in knockin mice with the missense mutation SLC7A14-p.(Gly330Arg). ABR thresholds, but not DPOAE thresholds, were elevated in the knockin mice at 3.5 months. Additionally, RP patients with the homozygous SLC7A14-p.(Gly330Arg) missense mutation exhibited elevated hearing thresholds indicative of sensorineural hearing loss. The SLC7A14 protein localized to intracellular membranes, including the endoplasmic reticulum, Golgi, and lysosomal both in vitro and in vivo. However, the missense mutant SLC7A14-p.(Gly330Arg) showed decreased lysosomal localization. SLC7A14-knockdown in vitro led to an increase in basal autophagy, with similar results observed in knockin and knockout IHCs. Collectively, these results show that SLC7A14 is necessary for the normal function of IHCs and that knockout or loss of function mutation may disrupt homeostatic mechanisms including autophagy and result in sensorineural hearing loss.
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