Inner ear defects commonly underlie deafness. The inner ears of Foxi1-/- deaf/circling mice, as reported on p. 2013, contain large, irregular cavities that replace the compartments of the inner ear. This phenotype, Hulander et al. suggest, is caused by a defect in inner ear fluid homeostasis that causes the endolymph compartment of the inner ear to expand and rupture during development. In support of this, they found that Pds expression is lost in the endolymphatic duct and sac — structures that reabsorb endolymph — of Foxi1 mutants, indicating that Foxi1 might be an upstream regulator of Pds. Pds encodes a chloride/iodide transporter believed to function in maintaining the ionic composition of endolymph. Moreover, human PDS mutations cause Pendred syndrome-associated deafness, which the Foxi1 phenotype resembles, leading the authors to suggest that FOXI1 mutations might also underlie other forms of Pendred syndrome-like deafness in humans.
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