The genetics of deafness
Hearing loss is the most prevalent sensorial problem in the population. Its importance among congenital disorders is considerable, since it is present in a frequency that varies, depending on the studied sample and region, from 2 to 7 in each 1,000 live births. Moreover, many other children will manifest hearing deficit after birth and before they acquire language, and others will manifest progressive disorders up to the second decades of life (Russo, 2000). Finally, over 60% of the people older than 70 years present hearing loss at sufficient levels to require some type of intervention so that they can continue to communicate (Kalatis & Petit, 1998).
Gradually, rehabilitation approaches, both oral and manual-based theories, are improving in order to enable the best development and social integration to subjects who are affected. Even though it is still controversial, cochlear implant has been used in children and adults, especially those with profound deafness.
The study of genetic causes has advanced significantly in the past four years. There are some genes related to deafness that have been identified. Mutations in some of these genes can cause isolated hearing loss, the so-called non-syndromic forms. Deafness can be associated with other abnormalities, among which blindness, configuring different syndrome pictures. There are over 400 known syndromes of genetic etiology that present hearing loss, the so-called syndromic forms of deafness; many genes have been identified and isolated.
At the end of 1997, the gene GJB2 was isolated and cloned, and it codified the protein connexin 26, the first nuclear gene related to non-syndromic deafness (Kelsell et al., 1997). Initially, its level of involvement in the origin of the cases of hereditary deafness was not considered, mainly in those with autosomal recessive pattern. However, it is known today that this gene is involved in 80% of the cases in which we observe this type of heritage and that many mutations of connexin 26 gene can also determine dominant inherited deafness (Denoyelle et al., 1997). In addition, a specific mutation, the 35delG (deletion of a guanine at position 35 of the gene) is involved in 70% of the cases of recessive autosomal inherited deafness. Finally, it is believed today that mutations to connexin 26 are responsible for 10 to 20% of all sensorineural hearing losses (Wilcox et al., 2000).
Even though it has been determined that connexin 26 is expressed in the cochlea, its function has not been precisely clarified. It is believed that the protein is associated to cell communication, related to the so-called gap junctions, channels that enable the passage of small molecules and ions between cell membranes, enabling, for example, the recycling of potassium ions of the cochlear fluids. The involvement of different genes of human deafness of genetic origin has provided valuable information about normal functioning of auditory functions (Kikuchi et al., 1995).
In addition to connexin 26 gene, it is believed that over 100 other genes are involved in the etiology of non-syndromic sensorineural deafness (Sobe et al., 2000). Some of these genes can be associated with both non-syndromic and syndromic forms of deafness; among them, there are connexins 30 and 31, associated not only with non-syndromic deafness of autosomal dominant pattern, but also cases of deafness associated with skin disorders (Rabionet et al., 2000).
35delG mutation
35delG mutation at connexin 26 gene is not rare; in fact, its presence in heterozygote can be found in up to 3% of the subjects in some populations (Friderici et al., 2001). The study of this mutation in 620 newborns in a country side city of the state of São Paulo, Brazil, revealed the presence of 6 heterozygotes, which enabled the estimation of the frequency in approximately 1:100 (Sartorato et al., 2001). In Italy, however, it is approximately 1:32, in Portugal, it is about 1:40, and in Spain, 1:45. If these three European populations are grouped, from which a large proportion of the Brazilian population descend, we will see that the mean frequency of heterozygotes for 35delG mutation is 1:42, considering a random union of heterozygotes and a 25% likelihood of affected descendents, we will realize that in these regions 1 to each 5,069 children were born deaf caused by homozygote of 35delG mutation (Gasparini et al., 2000).
Preliminary studies indicated, therefore, that in cases of deafness with family recurrence or even those from undefined origin, the first hypothesis to be tested should be the existence of mutations of connexin 26 gene, especially of 35delG, which is considered the most frequent mutation in any gene study with Caucasian people. Actually, the prevalence of homozygote deaf people for 35delG should be greater than for those with phenylketonuria (1:10,000 to 1:20,000 births), which can be detected with the PKU test. Mutation of 35delG can also be diagnosed at birth by collecting a drop of blood on a stick, using the technique of allele-specific PCR (Lucotte et al., 2001).
Connexin 26 gene mutations
In addition to 35delG, the first mutation described for connexin 26 gene, over 50 other mutations have already been reported. At the homepage http://www.iro.es/deafness/ we can find a table with all the mutations associated with deafness with dominant pattern, as well as null and polymorphism mutations (Denoyelle et al., 1998; Morlé et al., 2001).
In a recent study conducted with a sample of Brazilian population, mutations of GJB2 gene were found in 22% of the families with non-syndromic sensorineural deafness, indicating once again that the molecular analysis of this gene in patients with hearing loss not associated with syndrome cases should be the first step in determining the causes of hearing loss in our countries (Oliveira et al., 2001). It is especially true for family cases, among which the frequency of mutations of this gene was about 50%, but also in sporadic cases, among which the frequency was somewhat higher than 11% (approximately 1:9).
Similarly to what Oliveira et al. (2001) observed, among families with non-syndromic deafness studied in the Mediterranean region (Estivill et al., 1998), 49% of the cases with recessive autosomal pattern presented mutations of GJB2 gene; in this population, however, the mutations were observed in a greater proportion than in sporadic cases (37%). In Brazil, lack of precise information by the families about the events during pregnancy, delivery and perinatal period normally hinder the etiologic diagnosis of the deafness cases from environmental origin; thus, it is likely that cases of genetic origin end up being classified as of undefined origin.
Clinical heterogeneity
To differentiate the genetic deafness from that of environmental cause is not a simple task in many cases; molecular studies have at the same time brought answers to many questions and aggregated to our dilemmas about the etiologic diagnosis of deafness. Each day, there are new cases of hearing loss in which mutations of connexin 26 gene are detected, but the clinical expression is completely different from the expected presentation, according to what is currently known. Generally speaking, homozygote subjects to mutations of connexin 26 gene present profound pre-lingual deafness; currently, it is known that the phenotype at birth may vary from normal hearing to profound deafness. Cases of late deafness, confirmed only in the third decade of life, have also been reported. In addition, different grades of deafness can be observed in subjects of the same family with the same genotype (Denoyelle et al., 1999).
Other mutations of connexin 26, such as M34T, are still very controversial concerning its phenotypic expression; some consider that it determines dominant deafness and others that it is a polymorphism or null allele (Kelsell et al., 1997; Houseman et al., 2001).
Generally speaking, the level of deafness can not be predicted based on mutations found in the connexin 26 gene, and the definition of prognosis and therapeutic measures also faces difficulty in defining genotype-phenotype correlations. In order to determine pathogenicity of different mutations it is necessary to conduct further studies analyzing different populations.
Difficulties concerning genetic counseling
The main difficulty concerning genetic counseling of subjects with mutations of connexin 26 gene is the fact that approximately 40% of them have the mutation detected in only one of the alleles. Only when the definite origin of deafness is established can the families receive genetic counseling (Marlin et al., 2001).
Various hypotheses have been developed to explain deafness associated to mutation in only one of the alleles: 1) existence of mutations in non-codifying regions of gene GJB2, affecting its expression; 2) mutations of other genes (including genes of the connexin family), interacting with normal allele of gene GJB2 and, therefore, resulting in a deficient phenotype; 3) causal relation, and non-causal one, between mutations of gene GJB2, which would not be related to deafness in such cases. It is believed that this last hypothesis is not very likely, since it has been observed segregation of this normal allele with deafness. It is possible that there is an interaction between the genes, nuclear and/or mitochondrial, suppressing the expression of the normal allele (Wilcox et al., 2000).
It is believed that these complex patterns of segregation are due to both complexity of the nature of mutations of connexin 26 gene, which can determine, as previously mentioned, both an inherited recessive pattern and a dominant one, concerning preferential marriages between affected subjects who live together in closed communities. In fact, educational and social segregation determined by the difficulty to communicate make these subjects identify one to the other, leading to a tendency of inter-group marriages.
Thanks to the advance in research studies in this area, the importance of studies in mutation of gene GJB2 became evident; owing to the easiness in detecting mutations of connexin 26, this is the first gene to be molecularly analyzed in families that present sensorineural hearing loss (Sobe et al., 2000). The large number of cases with mutations identified in this gene makes it increase the expectations concerning genetic counseling.
The feasibility and benefits of screening of mutations of connexin 26 gene tend to reflect public health. The use of molecular tests together with audiological tests help early detection of deafness, which is extremely important for handling of these patients, especially in cases of progressive deafness, since the language stimulation in its critical period makes children learn how to communicate before deafness becomes more severe. In addition, it is possible today to have predictive diagnosis, that is, the detection of subjects with mutations of connexin 26 gene, but with no manifestations of deafness yet. The consequences of such a prediction from a social and family perspective are enormous, be it towards preventing deafness, or to assist and reduce costs of special education, medical treatment and professional decisions required for these subjects (Sobe et al., 2000; Sartorato et al., 2001).
Considering a family in which a child with deafness had had the diagnosis of mutations in two alleles of gene GJB2, the risk of recurrence in the siblings is estimated in 25%. However, the degree of hearing loss in a new affected subject is unpredictable, since there are cases in families with variable expression, that is, subjects with the same genotype have different phenotypes concerning hearing.
In turn, the risk of family recurrence in the case of a hearing couple with one affected child by non-syndromic deafness with no history of hearing loss in the family and no mutations of GJB2 gene, was estimated by Prasad et al. (2000) as 14%, slight less than the risk estimated by these authors in cases with no molecular tracking performed (17%). In a situation in which one of the parents has normal hearing and mutation of gene GJB2, and the other, has normal hearing and the two alleles of the gene are normal, the risk of having a deaf child would be below 0.075% (Green et al., 1999).
In summary, although the study of “silence” genes have improved substantially the etiologic diagnosis in cases of deafness, it is important to instruct the professionals involved in the service of the deaf about the importance and the difficulties involved in the process of genetic counseling of these subjects. In addition to providing information about the risks of recurrence of deafness in the siblings and the issue of the affected subjects and other family members, it is important to include all ethical principles of counseling, which should never be direct or coercive.
References
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1 Ph.D. in human genetics – UNICAMP – CBMEG.
Address correspondence to: Cidade Universitária Zeferino Vaz
Barão Geraldo Campinas SP 13083-970
Tel: (55 19)3788-1147 Fax: (55 19) 3788-1089 – E-mail: sartor@unicamp.br
2 Ph.D. in human genetics – UNICAMP – FCM Cidade Universitária Zeferino Vaz
Barão Geraldo Campinas SP CEP 13083-970
Article submitted on June 01, 2001. Article accepted on September 19, 2002
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