XPOT Deficiency causes a human disorder through impaired tRNA nuclear export

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Abstract
Background: The transport of transfer RNAs (tRNAs) from the nucleus to the cytoplasm is a crucial step in the regulation of gene expression and protein synthesis. This process is mediated by specialized export molecules, among which XPOT (Exportin-t, XPO3) plays a central role by recognizing and transporting mature tRNAs through the nuclear pore complex. XPOT is not essential in RNA trafficking in the simple organisms, however the potential impact of XPOT deficiency in human health remains unresolved. Methods: We identified eight patients from five unrelated families with rare biallelic germline variants in XPOT resulting in putative loss-of-function. Functional analyses were carried out in patient-derived fibroblasts, lymphoblastoid cells and zebrafish models. Ex vivo immunohistochemical stainings for Xpot were performed in the mouse cochlea. xpot knockout zebrafish models were generated to assess the morphology and hearing ability. Results: All patients presented with a uniform clinical phenotype that included increased susceptibility to infection, bronchiectasis, severe sensorineural hearing loss, developmental delay, and growth retardation. We demonstrated a complete absence of XPOT protein expression in three patient-derived cell lines. XPOT deficiency leads to disruptions in protein synthesis of the cytokine TNFα pathway upon cellular stimulation. Additional XPO1 inhibition in XPOT deficient cells had little effect on cellular functions, suggesting alternative tRNA nuclear transporter pathways. Increased XPOT immunoreactivity was observed in type I spiral ganglion neurons and hair cells of the mouse cochlea, with enrichment in stereocilia. xpot knockout zebrafish model showed dysmorphic features, and reduced hearing, recapitulating key patient phenotypes. Conclusions: Our findings establish a direct connection between impaired XPOT-dependent tRNA export and human pathology. It illustrates that perturbations in nuclear export pathways lead to disease. It also raises the possibility that other nuclear transport receptors may play similarly underappreciated roles in human health and disease. The identification of XPOT as a disease-associated gene opens up new research directions and potential targets for therapeutic intervention.

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