DETECTION OF THE GJB2 MUTATION IN IRANIAN CHILDREN WITH HEARING LOSS TREATED WITH COCHLEAR IMPLANTATION
Peyvandi AA1, Morovvati S2,*, Rabiee HR3, Ranjbar R3, Ajalloueyan M 4, Hassanalifard M4
*Corresponding Author: Saeid Morovvati, Research Center for Human Genetics, Baqiyatallah University of Medical Sciences, Tehran, POB: 19395/5487, Iran; Tel./Fax: +98-21-88620812; E-mail: morovvati@hotmail. com; morovvati@bmsu.ac.ir
page: 19

DISCUSSION

The 35delG mutation accounts for approximately 70% of GJB2 mutant alleles in Northern and Southern European, as well as American Caucasian populations, with a carrier frequency of 2-4%. In the Iranian population this mutation accounts for approximately 12-21% of all involved mutations in autosomal recessive non syndromic hearing loss [14-18]. In our study, the Cx26 genetic defect was seen in 14.3% of the investigated children with hearing impairment. Deafness in our patients had an autosomal recessive inheritance pattern and according to this pattern, we expected to see deafness only in children with homozygous mutations but in four patients, the mutations were heterozygous. In their study, del Castillo et al. (19) suggested that other mutations are present in a gene or genes in the same chromosomal region. This phenomenon could be explained on the basis of either a monogenic or a digenic pattern of inheritance. In the case of a monogenic mode of inheritance, there must be a regulatory element that is essential for the expression of the GJB2 gene in the inner ear. This hypothetical element would be located far upstream of GJB2, and the deletion of this element would suppress the level of expression of the GJB2 gene enough to produce a phenotype of hearing impairment. An alternative interpretation would be that the deletion inactivates a second gene whose protein is functionally related to Cx26. Inheritance pattern by single or double genes is responsible for this phenomenon. In our mutation assessment of the Cx26 gene, the 35delG mutation was more frequent, while in Ashkenazi Jewish subjects, the 167delT mutation was more frequent [20]. Fukushima et al. [21] in their study on seven Japanese deaf children reported that better speech performance after CI may be observed in persons with GJB2-related deafness compared to children without GJB2-related deafness. Some authors such as Jun et al. [22] based on the microscopic evaluation of temporal histopathology in deaf children with GJB2 mutations concluded that a greater number of functional cells in the spiral ganglion of these children could explain their better speech performance. The GJB2 deafness phenotype is considered as typical non syndromic hearing loss and differences in their response to CI might be due to differences in higher brain function abilities [21].



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