Hearing loss is generally one of the most common diseases that occur in childhood, with a frequency of approximately 1 in 1000 births. It is divided into acquired (due to environmental factors, but also to fetal infections, e.g. toxoplasmosis, rubella, cytomegalovirus), of genetic etiology-inherited and of yet unknown etiology.
Hearing loss with a genetic etiology may be part of a syndrome (over 400 syndromes include hearing loss) and non-syndromic, following all three possible types of inheritance: recessive, dominant and X-linked.
Non-syndromic hearing loss and particularly the form that occurs before the development of language communication in children, is in >50% of cases a genetic disease and is mainly due to recessive mutations with an increased carrier frequency in the general population.
It is very important for the proper management of patients, but also for genetic counseling for future pregnancies, that we can distinguish inherited forms of deafness from acquired (non-genetic) forms.
This is achieved by combining various clinical tests recommended by the specialist otolaryngologist, but also with appropriate genetic tests.
More than 100 genes are estimated to be involved in hearing loss and deafness. However, mutations in two of these genes, GJB2 (connexin 26) and GJB6 (connexin 30), represent more than 65% of non-syndromic recessive deafness and about 35% of sporadic forms, with a carrier frequency of 1 in 33 in the general population.
In practice, this means that about 1 in 7 children with deafness harbor mutations in these two genes. Specifically for the GJB2 gene, a single mutation called 35delG constitutes ~90% of all pathogenic mutations of the gene, with a carrier frequency in the general population of about 3.5% (1 out of 28 individuals).