Auditory defects
In the course of life an auditory defect affects 5 - 8 % of population. An inborn defect of hearing occurs in one in 1000 newborns. Almost in 3/4 of cases there is a genetic cause. Most of the population with a hearing defect due to a genetic cause have healthy parents and we can observe the auditory defects in one generation of brothers and sisters of both sexes. This is one of the basic patterns of an autosomal recessive (horizontal) inheritance, when the altered (mutated) gene was received by the affected patient from both sound parents - carriers. The auditory defects may be the only result (simplex) or they can occur in association with other defects in syndromes.
For example, Usher syndrome comprises of defects of hearing and sight.
In case of Pendredov syndrome there is, apart from other things, a thyroidea.
Stickler syndrome is characterized, apart from sensory defects, by affected joints.
An auditory defect, which originated before the speech development, is denoted as the prelingual. If it manifests itself only after the speech development, it is a defect of the postlingual type.
In case of more than a half of the patients with the isolated prelingual defect of hearing, both parents are healthy carriers of the gene mutation for the albumin connexin-26 (Cx26), which is located on the chromosome No.13 (13q12). The carriage of the mutation of the Cx26 gene may be connected to a higher sensitivity of the auditory apparatus and to an earlier onset of presbyakusia - senile hearing loss.
The albumins called connexins participate in the formation of connections between cells. The consequence of the gene mutation is an altered structure and function of connexin-26 and results in a defect of connection between the inner ear cells. The most frequent gene mutation for connexin 26 is known as 35delG – when a single DNA base - Guanine - is missing in the structure of the gene. This mutation is relatively common and is carried by about every thirtieth healthy man. Therefore, when the transmission of the mutation Cx26 gene is shown, it is suitable to carry out a molecular genetic examination of the partner as well.
However, the examination may find both the cause of the auditory defects and the symptom-free carrying of the mutation, and thus a danger of affection of other family members. In a case of a high genetic risk of the auditory defect, caused by the mutation of the Cx26 gene, the family may be offered a prenatal examination of the foetus. A large proportion of deaf parents, however, prefer the birth of deaf children, who are closer to them with respect to communication. Recently, the prenatal examination connected to the perspective of pregnancy interruption can be substituted with pre-implantation diagnostics (PGD). With this method, the mutation of the Cx26 gene is examined from one cell in more embryos, obtained using the in vitro fertilization (IVF). Only the embryo having a favourable gene combination for connexin-26 is put into the uterus.