GJB2 Regulatory Element Deletion Testing (GJB6-D13S1830 and GJB6-D13S1854)

Pathogenic variants in GJB2 are one of the most common causes of hereditary nonsyndromic hearing loss (NSHL), typically exhibiting autosomal recessive (AR) inheritance and characterized by congenital, mild to profound sensorineural hearing loss. This test is specific for two previously reported GJB2 regulatory element deletions that have been documented to be pathogenic for AR hearing loss.

Hearing loss (HL) or deafness is the most common sensory deficit in humans, affecting an estimated 5% of the world's population (Azaiez et al. 2018. PubMed ID: 30245029). Molecular genetic testing is possible for many types of hearing loss and plays a prominent role in diagnosis, genetic counseling, evaluating hearing loss stability and potential of undiagnosed related clinical features (Shearer et al. 2017. PubMed ID: 20301607).

While the vast majority of GJB2 pathogenic variants for NSHL display AR inheritance, a small number of have been documented to cause autosomal dominant (AD) NSHL with early to late childhood onset, as well as several AD syndromic disorders with mild to severe skin features either with or without hearing loss (Smith et al. 2016. PubMed ID: 20301449).  

GJB2 variants causing AR NSHL span the variant effect spectrum including missense, premature termination, splicing, regulatory, insertion/deletion, and gross deletion variants. Only missense variants have been associated with AD NSHL or syndromic GJB2-related disorders (Smith and Jones. 2016. PubMed ID: 20301449; Del Castillo and Del Castillo. 2017. PubMed ID: 29311818).

The most common GJB2 pathogenic sequence variants for AR NSHL and respective allele frequencies are c.167del in Ashkenazi Jewish (1.6%), c.35del in European (0.97%), and c.235del in East Asian populations (0.64%) (Chan and Chang. 2014. PubMed ID: 23900770; Genome Aggregation Database).

A small number of large deletions upstream of but not containing the GJB2 gene have been documented to cause AR NSHL, spanning the GJB6 and CRYL1 genes. While originally attributed to digenic inheritance between GJB2 and GJB6, functional studies have shown that these deletions result in loss of expression of GJB2 on the same allele as the deletion. Therefore, these deletions contain an essential GJB2 cis-regulatory element (Rodriguez-Paris and Schrijver, 2009. PubMed ID: 19723508; Del Castillo and Del Castillo. 2017. PubMed ID: 29311818).). A 95 kb critical region for this GJB2 regulatory element has been established by comparing overlapping deletions in different patients (Tayoun et al. 2016. PubMed ID: 26444186), and contains the last 5 exons of CRYL1 as well as GJB6-CRYL1 intergenic sequence.

The most common GJB2 regulatory element deletions that have been reported are the 309 kb GJB6-D13S1830 and 232 kb GJB6-D13S1854 deletions, both spanning from GJB6 to CRYL1. In patients with NSHL and a single potentially pathogenic variant in GJB2, these two large deletions were detected in 0-42% and 0-25% of cases across different populations, respectively (del Castillo et al. 2003. PubMed ID: 14571368; del Castillo. 2005. PubMed ID: 15994881; Pandya et al. 2020. PubMed ID: 32067424). The GJB6-D13S1830 and GJB6-D13S1854 deletions account for 1-10% of variants in patients with GJB2-associated hearing loss of European ancestry, with the GJB6-D13S1830 deletion being more frequent (Bliznetz et al. 2017. PubMed ID: 28405014).

The GJB2 gene codes for the gap junction beta 2 protein, also known as connexin 26. Connexins form gap junctions that permit the transport of nutrients, ions, and signaling molecules between neighboring cells that are in contact with each other. Connexin 26 transports potassium ions and some small molecules. GJB2 gene variants alter gap junctions, which may disturb the level of potassium ions in the inner ear. Levels of potassium ions that are too high may affect the function and survival of cells that are needed for hearing. Connexin 26 also plays a role in the growth, maturation, and stability of the outermost layer of skin, the epidermis (Jagger and Forge. 2015. PubMed ID: 25381570; Smith and Jones. 2016. PubMed ID: 20301449).

The clinical sensitivity of this test is difficult to estimate given the genetic heterogeneity of hearing loss. This test will detect the two most common large deletions of GJB2 regulatory elements (GJB6-D13S1830 and GJB6-D13S1854), but will not detect deletions in this region with different breakpoints.

A PCR based deletion test is used to detect the two most common large deletions of GJB2 regulatory elements (GJB6-D13S1830 and GJB6-D13S1854).