BackgroundAlmost 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.MethodWhole-exome sequencing was carried out on these patients after a failure to identify mutations in the DFNB genes frequently involved.ResultsA 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.ConclusionThis new DFNB form is likely to arise from abnormal hair bundles resulting in compromised detection of physiological sound pressures.

Usher syndrome (USH) is an autosomal recessive disorder characterized by a dual sensory impairment affecting hearing and vision. USH is clinically and genetically heterogeneous. Ten different causal genes have been reported. We studied the molecular bases of the disease in 18 unrelated Algerian patients by targeted-exome sequencing, and identified the causal biallelic mutations in all of them: 16 patients carried the mutations at the homozygous state and 2 at the compound heterozygous state. Nine of the 17 different mutations detected in MYO7A (1 of 5 mutations), CDH23 (4 of 7 mutations), PCDH15 (1 mutation), USH1C (1 mutation), USH1G (1 mutation), and USH2A (1 of 2 mutations), had not been previously reported. The deleterious consequences of a missense mutation of CDH23 (p.Asp1501Asn) and the in-frame single codon deletion in USH1G (p.Ala397del) on the corresponding proteins were predicted from the solved 3D-structures of extracellular cadherin (EC) domains of cadherin-23 and the sterile alpha motif (SAM) domain of USH1G/sans, respectively. In addition, we were able to show that the USH1G mutation is likely to affect the binding interface between the SAM domain and USH1C/harmonin. This should spur the use of 3D-structures, not only of isolated protein domains, but also of protein-protein interaction interfaces, to predict the functional impact of mutations detected in the USH genes.

Congenital deafness is certainly one of the most common monogenic diseases in humans, but it is also one of the most genetically heterogeneous, which makes molecular diagnosis challenging in most cases. Whole-exome sequencing in two out of three Algerian siblings affected by recessively-inherited, moderate to severe sensorineural deafness allowed us to identify a novel splice donor site mutation (c.5272+1G > A) in the gene encoding α-tectorin, a major component of the cochlear tectorial membrane. The mutation was present at the homozygous state in the three affected siblings, and at the heterozygous state in their unaffected, consanguineous parents. To our knowledge, this is the first reported TECTA mutation leading to the DFNB21 form of hearing impairment among Maghrebian individuals suffering from congenital hearing impairment, which further illustrates the diversity of the genes involved in congenital deafness in the Maghreb.