Connexin 26 deafness

Overview of hearing problems

Hearing loss affects 1 in every 5 individuals above 50 years of age. It would, however, be wrong to associate hearing problems strictly with age. In the recent decade, small children, teens, and the young adults have also been found to be suffering from hearing loss. Serious cases of hearing loss which persist for an unusually long time may even lead to deafness.

A number of auditory disorders are present from birth and are therefore known as congenital defects. Connexin 26 deafness is a disease belonging to this category and is caused by some major chromosomal disorder, mainly certain unexplained genetic mutations. A complete cure for the disease is still an ongoing research area.

At present, nearly 360 million people worldwide suffer from serious forms of deafness. 328 million of these people are adults and 32 million are children.

What is Connexin 26 deafness?

Connexin 26 deafness is a form of sensorineural congenital hearing loss caused due to abnormalities of the Connexin 26 (Cx26) protein found in the GJB2 gene. Mutations in the Connexin 26 protein has led to about 20% of all genetic hearing loss and 10% of all childhood hearing loss in the recent past. Connexin 26 deafness has been more commonly studied in some particular ethnic groups. These are- the Caucasian and the Ashkenazi Jewish population.

Connexin 26 deafness may cause unilateral or bilateral auditory loss. The second type is more critical and is usually very difficult to cure.

Auditory role of Connexin 26

The Connexin 26 (Cx26) is a protein located on the GJB2 gene. The GJB2 gene contains the instructions for the manufacturing of a number of proteins, including the Connexin 26 proteins. In proper medical terms, the Connexin 26 proteins are known as “gap-junction” proteins, which facilitate intercellular communications. The deficiency of the Connexin 26 proteins impedes the flow of potassium from the hair cells in the cochlea. This results in an excess build-up of potassium in the Corti’s organ, which in turn leads to sensorineural hearing loss.

What are some of the symptoms of Connexin 26 deafness?

Connexin 26 deafness is ideally present at birth and so are the symptoms. The affected children may show lack of auditory development and will not respond to sounds normally. The disease may also develop over time and may manifest itself properly after a few months or even years.

The common symptoms of Connexin 26 deafness seen in child are as follows-

• Lack of response to sound
• Not turning the head or moving the eyes towards the source of sound
• Showing no signs of irritation even when a sound higher than the permissible limit is generated
• Lack of linguistic skills in the developing years (due to non-reception or ill-reception of sound)
• Poking the ears frequently (due to irritation caused by the accumulation of potassium in the cochlea)
• Lack of coordination with the environment (due to the lack of audio-lingual capacities)
• Learning disabilities

These children are usually admitted to the educational institutes that provide special education through assistive audio-visual devices. The common symptoms of Connexin 26 deafness in adults are listed below-

• Inability to perceive sound from the environment or other people
• Presence of background noise, such as a persistent ringing or buzzing sound in the ear
• Inability to develop language and communication skills in childhood
• Requesting people to repeat their words
• Inability to hear or communicate without using hearing aids
• The absence of reaction to sound
• Misinterpretation of speech
• Dizziness (occasional)
• Inability to trace a source of sound
• Occasional dizziness
• Losing focus while communicating with someone
• Delayed reaction to sound
• Ear-ache (occasional)

What are the different types of Connexin 26 deafness?

Connexin 26 deafness can be categorised into two types based on the inheritance pattern. These are discussed below-

Recessive: This is the commonly studied type of Connexin 26 deafness. A recessive inheritance of deafness implies that the affected child inherits one copy of the defective gene from each parent. The carriers of the disease-bearing only one copy of the defective gene do not exhibit signs of the disease. But they usually have a 50% chance of passing the non-functioning gene on to the future generation. The possibility of inheritance of the deafness becomes 25%, if the other parent is also a carrier of the mutated gene.

Dominant: This is a rarely found category and is usually known as non-syndromic autosomic dominant deafness. Five types of dominant Connexin 26 deafness have been identified so far. These are-

• M34T
• W44C
• W44S
• R75W
• D66H

What are the additional complexities associated with Connexin 26 mutations?

Recent research findings have shown that additional complexities may arise from Connexin 26 mutations. Till date, only 75% of the mutations predicted as Connexin 26 were actually diagnosed and accurately detected through genetic testing. Medical evidence have established that mutations of the genes adjacent to Cx26, also known as Connexin 30 (Cx30) at GJB6 may also contribute to Connexin 26 deafness. It has also been found that Connexin 26 and Connexin 30 are expressed in the same hair cells of the cochlea. In fact, Cx26 and Cx30 closely resemble each other in that they share about three-quarters of the same amino acid sequences. Hence many of the research workers in this field have suggested that mutations in the Cx30 may also contribute to Connexin 26 deafness.

Diagnosis and treatment of Connexin 26 deafness

A preliminary detection of the absence of auditory functions can be done by an ENT using an equipment called otoscope. In normal deafness, the colour of the ear-drum helps explain the root cause of hearing loss. In Connexin 26 deafness, a genetic test in the pre-natal or post-natal period clearly indicates the occurrence of the disease. There is no complete cure for the disease till date. Doctors recommend cochlear implant in many cases.

Treatment consists of hearing evaluation for determining the severity of the problem. Such examination can be helpful in analyzing the suitability of cochlear implantation or amplification in cases of profound hearing loss.

Moreover, assistive learning devices can also be beneficial in conjunction with cochlear implants and/or amplification.

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