Connexin (GJB2 Sequencing and DFNB1 (GJB6) Deletions) Test Details
Over half of isolated childhood hearing loss has a genetic etiology, with many genes involved. The most common genetic causes of autosomal recessive, non-syndromic hearing loss are pathogenic changes in theDFNB1 locus, which includes theGJB2 and GJB6 genes.Read More...
The mutation spectrum in connexin-related hearing loss includes sequence variants in the GJB2 gene or large deletions in the DFNB1 locus which may include all or part of the GJB6 gene. Previously reported deletions include the GJB6-D13S1830 deletion, the GJB6-D13S1854 deletion, and a 131-kb deletion reported by Wilch (2010; PMID: 20236118). These deletions are thought to result in the loss of a cis-regulatory element of GJB2, thereby resulting in a lack of expression of the GJB2 allele (PMID: 19723508). Therefore, connexin-related hearing loss is caused by either a homozygous or compound heterozygous pathogenic variants in the GJB2 gene, a homozygous or compound heterozygous DFNB1 deletion or compound heterozygous variants including one deletion of and one pathogenic variant in the GJB2 gene. The hearing loss is nonsyndromic, sensorineural, and typically congenital but can present in early childhood. The severity can range from mild to profound and over time about 50% of those with a connexin related hearing loss will progress in severity. This hearing loss is most commonly inherited in an autosomal recessive pattern meaning that each parent is unaffected but is a carrier of a single variant in the GJB2 gene. Two carriers have a 25% (or 1 in 4) risk for having an affected child.Less
|GJB2||Gap Junction protein, beta 2 (Connexin 26)||121011||13q12|
|GJB6||Gap Junction protein, beta 6 (Connexin 30)||604418||13q12|
Given the large contribution of the GJB2 and GJB6 genes to genetic causes of hearing loss, the testing of these two genes is often the first line strategy in a diagnostic work up. Genetic testing for these two genes would be recommended in a child born with hearing loss of any severity. This is true even if there is no family history of hearing loss, as this is the most common presentation. The Connexin Test alone gives parents the highest likelihood of obtaining a cause for their child's hearing loss. Furthermore, a positive test result can alleviate concerns that other medical problems may arise in their child due to the association of hearing loss with many other syndromes (e.g. Usher syndrome in which blindness can develop at a later age). If the results are negative then additional tests, including more comprehensive panels, such as the OtoGenome™ can be pursued.
In addition, this test can be offered to someone whose partner is a known carrier of a pathogenic variant in the GJB2 gene or a known GJB6 genes deletion. In this situation, the test can identify or rule out carrier status in a hearing individual and determine the risk for connexin-related hearing loss in future children.
- Sanger sequencing of exon 1 and exon 2 of the GJB2 (connexin 26 gene (NM_004004)
- DFNB1 (GJB6) Deletions: Droplet digital PCR (ddPCR) is performed using a probe at GRCh37 chr13:21003868-21003954 to test for the presence or absence of the previously reported deletions in the DFNB1 (GJB6 gene) region, including the GJB6-D13S1854 309kb deletion, the GJB6-D13S1854 232kb deletion, and the deletions reported by Wilch 2010 (PMID: 20236118) and Feldman 2009 (PMID: 19101659). Any deletions that are identified are further clarified using the ddPCR probes at the following locations: GRCh37 chr13:21089281-21089358, chr13:21055271-21055336, chr13:20794699-20794765, chr13:20805056-20805133.
Analytical and Clinical Sensitivity
This assay is more than 99.9% accurate in detecting variants in the GJB2 sequence analyzed. Two GJB2 variants are detected in approximately 20% of individuals with nonsyndromic sensorineural hearing loss. However, a negative result does not rule out a diagnosis of GJB2-related hearing loss because there may be other variants within the GJB2 gene (for example, within the promoter or other non-coding or regulatory regions) that are not detected by this test. The GJB6-D13S1830 deletion may be present in 2-67% of persons with a single GJB2 variant, the exact percentage depending on ethnicity. The presence of two copies of the deletion is rare.