Around 70% of genetic hearing loss are non-syndromic hearing loss. A number of gene mutations are thought to be responsible for non-syndromic hearing loss. Many of these have recently been identified. The disordered DFNB1, caused by mutation in the CJB2 gene which encodes for the protein connexin 30, accounts for 50% of autosomal recessive non syndromic hearing loss.
The carrier rate in the general population for a recessive deafness causing GJB2 mutation is about 1 in 33(Smith RJ etal)
It is current practice the families of all children with severe/ profound hearing loss are offered genetic screening for these two mutations.
Establishing a molecular diagnosis of GJb2- related deafness is important clinically since these children can avoid further diagnostic test. They are not at increased risk for medical co-morbidity. In general, bony abnormalities of the cochlea are not part of this deafness phenotype and developmental motor milestones and vestibular function are normal.
- The hearing loss associated with GJB2 related deafness can vary from mild to profound.
- The diagnostic yeild of GJB2 screen is significantly higher in patients with severe to profound sensorineural hearing loss(SNHL) than in all other groups.
- Preciado et al suggested that a cost effective approach to screening would be to test children with severe – profound SNHL c a GJB2 screen as opposed to those with milder SNHL who should undergo imaging as the initial testing step.
- For children with GJB2 related (connexin) severe to profound deafness sibling with identical GJB2 generate have a 91% chance of having severe to profound deafness and a 9% chance of having mild to moderate deafness.
- For children with CJB2 related deafness and mild to moderate deafness sibling with identical GJB2 genotype have a 66% chance of having mild to moderate deafness and 34% chance of having severe to profound deafness.
- If the deaf child does not have DFNB1 based on molecular genetic testing of GJB2 (which codes for the protein connexin 26) the recurrence risk is 14% for deafness unrelated to connexin 26.
