Hearing impairment is the most common sensory disorder worldwide. Approximately one to two children per 1000 are born with hearing loss. In 50–60% of cases of congenital hearing impairment impaired hearing is hereditary (HIH) and in 40–50% of cases hearing loss is acquired. The relative incidence of congenital hearing impairment is increasing, since acquired impaired hearing due to meningitis is decreasing as a consequence of antibiotic therapy and vaccination programmes. In up to 80% of cases of congenital hearing impairment, prelingual hearing loss occurs as the only trait (nonsyndromic). Prelingual hearing loss is transmitted via an autosomal recessive trait (75–80%), an autosomal dominant trait (10–20%), is X-linked (1–5%), or mitochondrial (0–20%), depending on the study population.
Of the 30,000–50,000 human genes, 1%, i.e. 300–500 genes, are estimated to be necessary for hearing. Today, approximately 120 independent genes for HIH, approximately 80 for syndromic and 41 for nonsyndromic HIH, have been identified, which is about one-third of the total.
However, nonsyndromic HIH represents extreme genetic heterogeneity, as over 90 loci have been mapped to various human chromosomes. To date, 38 of the responsible nuclear genes (15 causing autosomal dominant HIH, 15 autosomal recessive HIH, 6 both autosomal dominant and recessive HIH, and 2 causing X-linked HIH) and 3 of the mitochondrial genes have been identified.
Despite the high number of identified loci for autosomal recessive nonsyndromic HIH, the majority of cases (58–88%) are linked to DFNB1 on the chromosome 13q12 and are due to mutations in the GJB2 gene (MIM 121011), which encodes the gap junction protein connexin 26. The 35delG mutation in the GBJ2 gene is the commonest mutation in Caucasian populations. Its carrier frequency is 1 per 35 in Southern Europe and 1 per 79 in Central and Northern Europe. Mutation analysis of 35delG mutation in the GJB2 gene is available as a genetic diagnostic test. Unlike several forms of congenital deafness, GJB2-related deafness has no known comorbidity. Knowing the mutation status at the outset in a child with hearing impairment will save the time, effort and cost involved in performing different investigations.