Mutations In Hearing Loss Research Articles

Tissue-specific effects of wild-type and mutant connexin 31: a role in neurite outgrowth

Channels formed by connexins (Cx), the major protein subunits of gap junctions, allow passage of ions and molecular messengers between cells to provide a mechanism of synchronized cellular response. Twenty human Cx isoforms have been identified and mutations in the gene GJB3 encoding the 31 kDa isoform, Cx31, can cause dominant or recessive skin disease, dominant or recessive deafness or dominant neuropathy with deafness. Cx31 is expressed in differentiating keratinocytes in skin. Here, we also demonstrate endogenous Cx31 expression in human neuronal cell lines, particularly in differentiated neurones. Exogenous Cx31 expression induced neurite outgrowth in human neuronal cell lines, but not differentiation in primary human keratinocytes. Though neither the neuropathy and hearing loss mutation (66delD)Cx31 nor the skin disease associated mutation (R42P)Cx31 is able to traffic to the plasma membrane, the R42P mutant induced neurite outgrowth to a level equal to wild-type Cx31. In contrast, there was significantly reduced neurite outgrowth after (66delD)Cx31 expression. In addition to indicating a potential disease mechanism for the neuropathy/deafness mutation, this work demonstrates a tissue-specific function for Cx31.

High Frequency of 35delG GJB2 Mutation and Absence of del(GJB6-D13S1830) in Greek Cypriot Patients with Nonsyndromic Hearing Loss

Mutations in the GJB2 (Connexin 26) gene are responsible for more than half of all cases of prelingual, recessive, inherited, nonsyndromic deafness in Europe. This paper presents a mutation analysis of the GJB2 and GJB6 (Connexin 30) genes in 30 Greek Cypriot patients with sensorineural nonsyndromic hearing loss compatible with recessive inheritance. Ten of the patients (33.3%) had the 35delG mutation in the GJB2 gene. Moreover, 9 of these were homozygous for the 35delG mutation, whereas 1 patient was in the compound heterozygous state with the disease causing E47X nonsense mutation. Another patient with severe sensorineural hearing loss was heterozygous for the V153I missense mutation. Finally, no GJB6 mutations or the known del(GJB6-D13S1830) were identified in any of the investigated Greek Cypriot nonsyndromic hearing loss patients. This work confirms that the GJB2 35delG mutation is an important pathogenic mutation for hearing loss in the Greek Cypriot population. This finding will be used toward the effective diagnosis of nonsyndromic hearing loss, improve genetic counseling, and serve as a potential therapeutic platform in the future for the affected patients in Cyprus.

Connexins link deafness and skin disorders

The connexins are proteins that form links between adjacent cells, called gap junctions. These allow the rapid movement of ions, second messengers and small metabolites between cells, enabling the coordination of cellular activities. There are many connexin proteins, divided into two subtypes, α- and β-, which assemble into homomeric multimers and form channels linking cells together. The connexin genes are expressed in a wide variety of tissues and have been postulated to play major roles in the developing embryo and adult.Two of the β-connexins had previously been implicated in human disease: Cx32 (encoded by the gap junction β-1 gene, GJB1) in X-linked Charcot–Marie-tooth disease and Cx26 (encoded by GJB2) in a severe, autosomal recessive form of non-syndromic sensorineuronal deafness. A third β-connexin, Cx31 (encoded by GJB3), has now been linked to the etiology of specific forms of hearing loss1xMutations in the gap junction protein β-3 associated with autosomal dominant hearing impairment. Xia, J. et al. Nat. Genet. 1998; 20: 370–373Crossref | PubMed | Scopus (313)See all References1 and of keratoderma2xMutations in the human connexin gene GJB3 cause erythrokeratodermia variablis. Richard, G. et al. Nat. Genet. 1998; 20: 366–369Crossref | PubMed | Scopus (257)See all References2 (a skin disorder). Additionally, mutations in GJB2 have been found in a family affected by both deafness and keratoderma3xFunctional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma. Richard, G. et al. Hum. Genet. 1998; 103: 393–399Crossref | PubMed | Scopus (199)See all References3.Two groups have cloned GJB3 by identifying EST sequences that were similar to other connexins. Xia et al.1xMutations in the gap junction protein β-3 associated with autosomal dominant hearing impairment. Xia, J. et al. Nat. Genet. 1998; 20: 370–373Crossref | PubMed | Scopus (313)See all References1 mapped GJB3 to the chromosomal region, 1p33–p35, and then screened 42 families with a variety of diseases linked to this chromosomal location (including 6 with hearing loss and 5 with keratoderma). Richard et al.2xMutations in the human connexin gene GJB3 cause erythrokeratodermia variablis. Richard, G. et al. Nat. Genet. 1998; 20: 366–369Crossref | PubMed | Scopus (257)See all References2 screened for mutations in 12 families with erythrokeratoderma variablis (EKV), which had previously been linked to a small region on 1p34–p35. Mutations in GJB3 were identified in two of the families with hearing loss and in four of the families with EKV; associated with deafness were a missense and a stop mutation, and the three different GJB3 alterations in the EKV families were missense mutations. Richard et al.3xFunctional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma. Richard, G. et al. Hum. Genet. 1998; 103: 393–399Crossref | PubMed | Scopus (199)See all References3 also identified a heterozygous GJB2 missense mutation in a family with deaf-mutism and palmoplantar keratoderma.At first, this appears to be an unlikely scenario but closer analysis suggests that distinct roles exist for the same connexin protein in different tissues. The genes encoding Cx26 and Cx31 are expressed in several tissues, including the cochlea and skin. Although mutations in GJB2 were originally associated with deafness, its protein clearly plays an important role in epidermal maintenance, as indicated by its considerable upregulation in psoriatic lesions. GJB3 mutations identified in these papers1xMutations in the gap junction protein β-3 associated with autosomal dominant hearing impairment. Xia, J. et al. Nat. Genet. 1998; 20: 370–373Crossref | PubMed | Scopus (313)See all References, 2xMutations in the human connexin gene GJB3 cause erythrokeratodermia variablis. Richard, G. et al. Nat. Genet. 1998; 20: 366–369Crossref | PubMed | Scopus (257)See all References cluster in different domains of the Cx31 protein: the EKV mutations in the first half and the hearing loss mutations in the second half, suggesting a reason for the two distinct phenotypes. Another explanation is suggested by the analysis of mutant proteins expressed in vitro. A Cx26 mutant implicated in deafness failed to form functional gap channels but did not interfere with the function of the wild-type protein when expressed in vitro; however, the mutant Cx26 protein found in the family with both deafness and keratoderma had a dominant-negative effect on gap-channel function3xFunctional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma. Richard, G. et al. Hum. Genet. 1998; 103: 393–399Crossref | PubMed | Scopus (199)See all References3.Further studies will help to clarify the role of Cx26 and Cx31 in deafness and keratoderma, in addition to identifying other connexins that might cause similar phenotypes. As our knowledge of these proteins increases, the number of diseases that are associated with alterations in connexin function will continue to grow. Indeed, several of the connexins have already been implicated in such diverse processes as cataract formation, left–right axis determination, heart development and malignancy.