Hearing impairment is a common and highly heterogeneous sensory disorder. Genetic causes are thought to be responsible for more than 60% of the cases in developed countries.¹ In the majority of cases, non-syndromic hearing impairment is inherited in an autosomal recessive pattern.² Thirty eight different loci and 20 genes for autosomal recessive non-syndromic hearing impairment (ARNSHI) have been identified to date.³

In many populations, up to 50% of all cases of ARNSHI are caused by mutations in the DFNB1 locus (MIM 220290) on 13q12.⁴ This locus contains the GJB2 gene (MIM 121011), encoding connexin-26 (Cx26),⁵ which belongs to a family of transmembrane proteins with about 20 members in humans. Hexamers of connexins (connexons) are displayed in the plasma membrane. Docking of connexons on the surfaces of two adjacent cells results in the formation of intercellular gap junction channels.⁶ Several different connexins, including Cx26, have been shown to participate in the complex gap junction networks of the cochlea.^7,8 It has been postulated that these networks play a key role in potassium homeostasis, which is essential for the sound transduction mechanism.⁹

Given the high prevalence of DFNB1 deafness, molecular testing for GJB2 mutations has become the standard of care for the diagnosis of patients with non-syndromic hearing impairment of unknown cause.¹⁰ However, the finding of a large number of affected subjects with only one GJB2 mutant allele complicates the molecular diagnosis of DFNB1 deafness. In different studies, these have accounted for 10–50% of deaf subjects with GJB2 mutations.⁴ It was hypothesised that there could be other mutations in the DFNB1 locus but outside the GJB2 gene. This hypothesis gained support by the finding of a deletion in the DFNB1 locus outside GJB2 but truncating the neighbouring GJB6 gene (MIM 604418), which …