Mutations in Grxcr1 Are The Basis for Inner Ear Dysfunction in the Pirouette Mouse

Abstract

Recessive mutations at the mouse pirouette (pi) locus result in hearing loss and vestibular dysfunction due to neuroepithelial defects in the inner ear. Using a positional cloning strategy, we have identified mutations in the gene Grxcr1 (glutaredoxin cysteine-rich 1) in five independent allelic strains of pirouette mice. We also provide sequence data of GRXCR1 from humans with profound hearing loss suggesting that pirouette is a model for studying the mechanism of nonsyndromic deafness DFNB25. Grxcr1 encodes a 290 amino acid protein that contains a region of similarity to glutaredoxin proteins and a cysteine-rich region at its C terminus. Grxcr1 is expressed in sensory epithelia of the inner ear, and its encoded protein is localized along the length of stereocilia, the actin-filament-rich mechanosensory structures at the apical surface of auditory and vestibular hair cells. The precise architecture of hair cell stereocilia is essential for normal hearing. Loss of function of Grxcr1 in homozygous pirouette mice results in abnormally thin and slightly shortened stereocilia. When overexpressed in transfected cells, GRXCR1 localizes along the length of actin-filament-rich structures at the dorsal-apical surface and induces structures with greater actin filament content and/or increased lengths in a subset of cells. Our results suggest that deafness in pirouette mutants is associated with loss of GRXCR1 function in modulating actin cytoskeletal architecture in the developing stereocilia of sensory hair cells.

Genetics of Nonsyndromic Recessively Inherited Moderate to Severe and Progressive Deafness in Humans

“…Splice site mutations were identified in GRXCR1 in two Dutch families while three missense and a nonsense mutation were identified in GRXRC1 in families from Pakistan and Iran which segregated with hearing loss (Odeh et al, 2010;Schraders et al, 2010a). Two mutations create alternative splice sites within GRXCR1 which on usage are predicted to create frameshifts in the open reading frame of the gene.…”

“…The audiometric profile varied from flat to a slight U-shape and was down-sloping (Schraders et al, 2010a). Data was not provided about the audiometric profile of affected individuals in families from Iran, although hearing loss was reported to be severe to profound in degree (Odeh et al, 2010).…”

“…In the inner ear, Grxcr1 is expressed in the hair cells and is localized along the lengths of stereocilia exhibiting a differential pattern in levels of expression in young and adult mice (Odeh et al, 2010). There are five mutant alleles of Grxcr1 which cause profound deafness in the pirouette mice (Odeh et al, 2010). The absence of GRXCR1 results in formation of relatively short and thin stereocilia and cytocauds indicating actin abnormalities.…”

“…The absence of GRXCR1 results in formation of relatively short and thin stereocilia and cytocauds indicating actin abnormalities. This suggests that GRXCR1 plays an active role in development of actin architecture in the stereocilia (Beyer et al, 2000;Odeh et al, 2010).…”

“…These domains take part in reversible Sglutathionylation of proteins by which they are predicted to control activity or localization of the proteins. In the inner ear, Grxcr1 is expressed in the hair cells and is localized along the lengths of stereocilia exhibiting a differential pattern in levels of expression in young and adult mice (Odeh et al, 2010). There are five mutant alleles of Grxcr1 which cause profound deafness in the pirouette mice (Odeh et al, 2010).…”