Abstract
Mutations in the GJB2 gene, which encodes connexin 26, are a frequent cause of congenital non-syndromic sensorineural hearing loss. Two large deletions, del(GJB6-D13S1830) and del(GJB6-D13S1854), which truncate GJB6 (connexin 30), cause hearing loss in individuals homozygous, or compound heterozygous for these deletions or one such deletion and a mutation in GJB2. Recently, we have demonstrated that the del(GJB6-D13S1830) deletion contributes to hearing loss due to an allele-specific lack of GJB2 mRNA expression and not as a result of digenic inheritance, as was postulated earlier. In the current study we investigated the smaller del(GJB6-D13S1854) deletion, which disrupts the expression of GJB2 at the transcriptional level in a manner similar to the more common del(GJB6-D13S1830) deletion. Interestingly, in the presence of this deletion, GJB2 expression remains minimally but reproducibly present. The relative allele-specific expression of GJB2 was assessed by reverse-transcriptase PCR and restriction digestions in three probands who were compound heterozygous for a GJB2 mutation and del(GJB6-D13S1854). Each individual carried a different sequence variant in GJB2. All three individuals expressed the mutated GJB2 allele in trans with del(GJB6-D13S1854), but expression of the GJB2 allele in cis with the deletion was almost absent. Our study clearly corroborates the hypothesis that the del(GJB6-D13S1854), similar to the larger and more common del(GJB6-D13S1830), removes (a) putative cis-regulatory element(s) upstream of GJB6 and narrows down the region of location.
Highlights
The DFNB1 locus at chromosome 13q11-q12 includes the GJB2 and GJB6 genes, which respectively encode connexin 26 (Cx26) and connexin 30 (Cx30)
The complete failure to amplify the 728 bp segment from the GJB2 cDNA indicates the complete absence of GJB2 expression from both del(GJB6-D13S1830) alleles (Fig 4)
The main purpose of this study was to demonstrate an effect on GJB2 expression by del(GJB6-D13S1854), which is internal to the larger and more prevalent del(GJB6-D13S1830) deletion, which we studied previously [13]
Summary
The DFNB1 locus at chromosome 13q11-q12 includes the GJB2 and GJB6 genes, which respectively encode connexin 26 (Cx26) and connexin 30 (Cx30). These connexin proteins are coexpressed and co-localized in the cochlea, where they create heteromeric gap junctions [1] and make important contributions to cochlear homeostasis [2]. The other two are private mutations, one of which (.930 kb) deletes both GJB2 and GJB6 [11] and the other (del(chr13:19,837,343– 19,968,698) does not affect either gene and is located upstream of GJB6 [10,12] (Fig 1)
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