EPS8, encoding an actin-binding protein of cochlear hair cell stereocilia, is a new causal gene for autosomal recessive profound deafness

Abstract

Background: Almost 90% of all cases of congenital, non-syndromic, severe to profound inherited deafness display an autosomal recessive mode of transmission (DFNB forms). To date, 47 causal DFNB genes have been identified, but many others remain to be discovered. We report the study of two siblings born to consanguineous Algerian parents and affected by isolated, profound congenital deafness.

Method: Whole-exome sequencing was carried out on these patients after a failure to identify mutations in the DFNB genes frequently involved.

Results: A biallelic nonsense mutation, c.88C > T (p.Gln30*), was identified in EPS8 that encodes epidermal growth factor receptor pathway substrate 8, a 822 amino-acid protein involved in actin dynamics. This mutation predicts a truncated inactive protein or no protein at all. The mutation was also present, in the heterozygous state, in one clinically unaffected sibling and in both unaffected parents, and was absent from the other two unaffected siblings. It was not found in 120 Algerian normal hearing control individuals or in the Exome Variant Server database. EPS8 is an F-actin capping and bundling protein. Mutant mice lacking EPS8 (Eps8-/- mice), which is present in the hair bundle, the sensory antenna of the auditory sensory cells that operate the mechano-electrical transduction, are also profoundly deaf and have abnormally short hair bundle stereocilia.

Conclusion: This new DFNB form is likely to arise from abnormal hair bundles resulting in compromised detection of physiological sound pressures.

Figure 1

Clinical and molecular data in the patients harbouring the nonsense mutation in EPS8. (A) Map of Algeria showing the province of Tiaret (in red). (B) Segregation of the nonsense EPS8 mutation in the family. (C) Air-conduction audiometric curves for patients IV.4 (open blue circles) and IV.5 (closed green diamonds) at the ages of 11 and 7 years, respectively. Identical audiometric curves were obtained for both ears in both patients. (D) DNA sequencing chromatograms showing the mutation (arrow). (E) Schematic representations of the human EPS8 gene and protein. * indicates the position of EPS8 exon 3. The protein (822 amino acids) contains a split pleckstrin homology (PH) domain (in blue), two proline-rich (Pro-rich) domains (in red), an SRC Homology 3 (SH3) domain (in green), and an F-actin-binding domain (in orange).

Figure 2

Immunolocalization of EPS8 in the mouse cochlea and macaque retina. (A-C) Immunolabeling for F-actin (A) and EPS8 (B) in the mouse cochlea. DAPI (D) was used to label cell nuclei. Immunolabeling of EPS8 (E and H), synaptophysin (F) and whirlin (I) in the macaque retina. A polyclonal antibody was used to detect EPS8. (G) Co-immunolabeling of EPS8 and synaptophysin (J) Co-immunolabeling of EPS8 and whirlin. Abbreviations: OS, outer segment; ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GNL, ganglion nuclear layer. Scale bars: 5 μm.