GJB2
The Clinical Diagnostics Service of the Molecular Otolaryngology Research Laboratory is a Joint Commission-approved CLIA-accredited diagnostic laboratory that offers mutation screening of several genes.

DFNB1 (OMIM#: 220290)
Two different genes cause hearing loss at this locus, which maps to chromosome 13q11-q12:GJB2, encoding Connexin 26; and GJB6, encoding Connexin 30.

GJB2 (encodes Connexin 26, CX26)
CX26 (OMIM#: *121011) is a gap junction protein expressed in the supporting cells of the cochlea. The gene, GJB2, contains 2 exons, the second of which encodes the 226 amino acid protein CX26. Mutations in GJB2 are found in ~50% of persons with autosomal recessive nonsyndromic hearing loss. Over 100 different mutations have been identified.

GJB6 (encodes Connexin 30, CX30)
CX30 (OMIM#: *604418) is also a gap junction protein. Lerer and colleagues identified a deletion upstream of GJB2, which included the first exon of GJB6 in 2001. Del Castillo and colleagues characterized this deletion as a 342kb fragment of chromosome 13 that included D13S1830 in addition to a portion of GJB6, and so this deletion is commonly known as the del(GJB6-D13S1830) mutation. It cosegregates with mutations in GJB2 to cause recessively inherited deafness at the DFNB1 locus. A recent multicenter study investigating the mutation spectrum in over 1500 deaf persons from 16 countries found this mutation to represent 1.67% of mutations at the DFNB1 locus (Van Camp, et al. 2005). A second smaller deletion, del(GJB6-D13S1854), also causes deafness at the DFNB1 locus.

MORL screening methodology
Over 97% of the identified mutations at the DFNB1 locus occur in exon 2 of GJB2 (Van Camp, et al 2005). We have adopted a tiered screening process focusing first on exon 2 of GJB2 and the two GJB6-containing deletions. The finding of two deafness-causing mutations is consistent with the diagnosis of hearing loss at the DFNB1 locus. If one only mutation is found, mutation screening for the splice site mutation in exon 1 of GJB2 (IVS1 + 1 G>A) is completed. If no deafness-causing mutations are found, the diagnosis of hearing loss at the DFNB1 locus is excluded based on today’s standards. (GJB2 mutation screening is performed by amplification of oligonucleotide primers that flank each exon followed by bi-directional sequencing. Screening for the del(GJB6-D13S1830) and del(GJB6-D13S1854) mutations is completed by PCR amplification of oligonucleotide primers flanking and within the deletion breakpoints. Products are run on agarose gel and sized to determine presence or absence of a deletion.)

Figure legend: Lane 1, 1Kb + ladder; lane 2, wild type control; lane 3, del(GJB6-D13S1830) homozygote; lane 4, del(GJB6-D13S1854) heterozygote; lanes 5-7, normal controls.

Sensitivity
Sensitivity is greater than 98%.

Turn-around time
Turn around time is approximately 8 weeks.

Cost: $300

Indications for screening
This test is appropriate for any patient with congenital hearing impairment with negative family history.

GeneTests  GeneReviews – Nonsyndromic Hearing Loss and Deafness, DFNB1

References
Kelsell, D. P. et al.: Connexin 26 mutations in hereditary non-syndromic sensorineural deafness. Nature 387: 80-83, 1997.
PubMed ID:  9139825
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Del Castillo, I. et al.: A deletion involving the connexin 30 gene in nonsyndromic hearing impairment. New Eng. J. Med. 346: 243-249, 2002.
PubMed ID: 11807148
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Del Castillo, I. et al.: Prevalence and evolutionary origins of the del(GJB6-D13S1830) mutation in the DFNB1 locus in hearing-impaired subjects: a multicenter study. Am J Hum Genet. 2003 Dec;73(6):1452-8. Epub 2003 Oct 21.
PubMed ID: 14571368
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Azaiez,H.et al.: GJB2: The spectrum of deafness-causing allele variants and their phenotype. Human Mutation 2004 Oct;24(4):305-11.
PubMed ID: 15365987
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Del Castillo, F. J. et al.: A Novel Deletion Involving the Connexin-30 Gene, del(GJB6-D13S1854), Found in trans with Mutations in the GJB2 Gene (Connexin-26) in Subjects with Autosomal Recessive Non-Syndromic Hearing Impairment. J Med Genet 2005; 42: 588-594.
PubMed ID: 15994881
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Snoeckx, R. L. et al.: GJB2 Mutations and Degree of Hearing Loss: A Multicenter StudyAmerican Journal of Human Genetics 77:945-957, 2005.
PubMed ID: 16380907
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