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The p.Gly130Val mutation in the GJB2 gene: A familiar case of autosomal dominant non-syndromic hearing loss
International Journal of Pediatric Otorhinolaryngology 127 (2019) 109653
Contents lists available at ScienceDirect
International Journal of Pediatric Otorhinolaryngology journal homepage: www.elsevier.com/locate/ijporl
The p.Gly130Val mutation in the GJB2 gene: A familiar case of autosomal dominant non-syndromic hearing loss
T
Adelaide Bussinia, Rossana Righia, Chiara Pessinaa, Angelo Genonia, Eliana Cristofarib, Annalisa Melib, Paola Granataa, Emanuela Meronia, Francesco Broccoloc,∗, Rosario Casalonea,∗∗ a
Unit of Cytogenetics and Medical Genetic, ASST Settelaghi, Varese, Italy Unit of Audiology, ASST Settelaghi, Varese, Italy c Department of Medicine and Surgery (School of Medicine), University of Milano-Bicocca, Monza, Italy b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hearing loss GJB2 Connexin 26 p.G130V Skin diseases
Several forms of sensorineural hearing loss (SNHL) have been imputated to connexins mutations and prevalently to connexin 26 (Cx26), codified by the GJB2 gene (gap junction protein, beta 2). Here, we report the first familiar case (heterozygous p. G130V mutation) of non-syndromic (without any dermatological manifestation) dominant profound SNHL. Proband was a 6-years-old male with post-lingual bilateral profound SNHL, clinically identified at the age of 3 with diagnosis of severe SNHL. We confirm that the p. G130V variant of the GJB2 gene is causative of autosomal dominant form of SNHL, although it is not always associated with the presence of skin diseases.
1. Introduction Gap junctions are highly specialized membrane structures important for intercellular communication and are composed of members of the connexin protein family, expressed in several human tissues [1]. The tissue distribution of connexin isoforms increases possible structural variations of gap junctions. Genetic alterations of connexin proteins expressed in the epidermis and in the inner ear cause different forms of sensorineural hearing loss (SNHL) associated with skin diseases [2]. To date, more than 100 recessive GJB2 mutations causing NSHL have been reported [3]. Furthermore, about 20 GJB2 mutations are related to autosomal dominant non-syndromic or syndromic SNHL associated with different skin diseases [3]. Mutations of the GJB2 gene (gap junction protein beta2), which are located on chromosome 13q12 and encoding connexin 26 (Cx26), are involved in about 50% of autosomal recessive non-syndromic SNHL and sometimes in dominant forms [2]. Mutation c.35delG in homozygous or in compound-heterozygous condition is the most frequent cause of nonsyndromic autosomal recessive SNHL in European population [3]. Connexins are expressed in the human epidermis and are involved in the keratinocyte growth and differentiation. Cx26 is expressed in basal keratinocytes of palms and soles and occasionally in granular layer cells [4]. Eleven clinically defined cutaneous disorders have been
∗
correlated to connexins mutations [4]. Some GJB2 gene mutations are involved in different form of hearing loss (HL) associated with skin diseases such as Palmoplantar Keratoderma (PPK) deafness (OMIM 148350) [5]. The spectrum of the skin manifestations shows high phenotypic variability [4]. All GJB2 mutations causing syndromic HL associated with skin disorders are missense mutations and are inherited in a dominant manner [6]. The Cx26 polypeptide includes four transmembrane domains (TM1 to M4), two extracellular domains (E1, E2), a cytoplasmic loop (IC) and amino (-NH2) and carboxy-terminus (-COOH) which are located in the cytoplasm [1]. Generally, most of the mutations involved in HL and skin problems are clustered in the cytoplasmic N-terminal or in the first extracellular loop of Cx26 [6]. Conversely, the p. G130V mutation is localized in the second intracellular domain of the GJB2 gene. Therefore, further studies are needed to determine the clear link between the domains involved with the related phenotype. Regarding various studies which were performed, the p. G130V mutation is associated with a dominant syndromic form of HL but the severity of skin disease was variable [7,8]. Here, we report the first familiar case (heterozygous p. G130V mutation) of non-syndromic (without any sign of skin abnormality) dominant profound SNHL.
Corresponding author. Department of Medicine and Surgery (School of Medicine), University of Milano-Bicocca, 20090, Monza, Italy. Corresponding author. Director of the Cytogenetics and Medical Genetic Unit, ASST Settelaghi, Varese, Italy. E-mail addresses: [email protected] (F. Broccolo), [email protected] (R. Casalone).
∗∗
https://doi.org/10.1016/j.ijporl.2019.109653 Received 9 March 2019; Received in revised form 1 August 2019; Accepted 20 August 2019 Available online 22 August 2019 0165-5876/ © 2019 Elsevier B.V. All rights reserved.
International Journal of Pediatric Otorhinolaryngology 127 (2019) 109653
A. Bussini, et al.
Fig. 1. A) Pedigree of the family: black symbols denote patients affected by hearing loss and carriers of the p. Gly130Val mutation (proband is indicated by an arrow). (B) Pure tone audiometry threshold of the proband (III:1) and his father (II:2) both with bilateral severe-profound sensorineural hearing loss. (D) Conservation of p. Gly130 amino acid residues in GJB2 across species.
2. Case report
component showed HL or skin diseases. Blood samples were taken from proband, his father and other family members unaffected by HL (sibling, mother and gran parents). All family members were screened for GJB2 mutations by direct sequencing of amplified PCR fragments of the promoter region, 5’ UTR and exon 2. Sequence analysis showed heterozygous c.389G > T mutation (p.G130V), localized in the second intracellular domain of the GJB2 gene (NM_004004) in the proband (III:1) and in his father (II:2) (Fig. 1A and B). Exon 2 of the GJB3 and the GJA1 genes, transcript region of the MIR96 gene were also investigate. Deletions D13S1830 and D13S1854 on GJB6 gene with multiplex PCR were also investigated. The mutational analysis in the GJB2 gene did not reveal any aberration in the other family members without HL (Fig. 1A). No mutations in the other investigated genes (GJB3, GJA1, MIR96 and GJB6) were found. Use of different bioinformatics tools (SIFT, PROVEAN, PolyPhen-s, Mutation Taster, MutationAssessor, MutPred) shows that the G130 is conserved in protein structure (Fig. 1C) and due to unusual torsion angles at this position, only glycine is flexible enough to make these torsion angles; therefore, mutation in this position will force the local structure into an incorrect conformation and will disturb it. In conclusion, predictive in silico tolls reveal that this mutation could have a damaging impact and to because of disease (data not shown).
Proband was a 6-years-old male with post-lingual bilateral severeprofound SNHL, clinically identified at the age of 3. In Fig. 1A is showed the pedigree of the family with autosomal dominant severe hearing loss. His father (II:2) presents congenital bilateral profound SNHL, while the 3 years-old brother (III:2) and paternal grandparents (I:1 and I:2) do not show HL both. The audiological evaluation of the proband and his father (III:I, II:2, respectively) showed a severe-profound bilateral SNHL (Fig. 1A). Of note, proband and his father do not present any skin involvement or nails and hair abnormalities. For all cases, pure-tone audiometric test (PTA) for air and bone conduction at frequencies 250–8000 Hz was performed to clarify HL severity as follows: mild: 21–45 dB; moderate: 46–60 dB; moderately severe: 61–75 dB; severe: 76–96 dB; and profound: ≥97 dB. Also, HL severity of all patients with conventional auditory brainstem response (ABR) testing was performed by certified audiologists (which are not shown in this study). Both proband (III:1) and his father (II:2) were undergone to complete audiological tests: otoscopic evaluation, PTA, vocal audiometry (speech tracking), tympanometry, ABR, Slow Vertex Responses (SVR), subjective hearing related disability test (Sander's test). The audiological evaluation of the 3-years-old proband has identified a bilateral severe SNHL, normal tympanometry, absence of otoacoustic emission and ABR. SVR were evocable. A reevaluation at 6years has shown a worsening of clinical features from severe (85 dB) to profound HL (≥97 dB) and poor communicative performances at speech tracking test. The father's (II: 2) audiological tests have shown a symmetric severe-profound bilateral HL, a normal tympanometry, absence of otoacoustic emission and ABR. The patient has shown poor communicative performances at speech tracking test. No other family
3. Discussion Mutations in the GJB2 gene are causative of about 50% of pre-lingual recessive Non Syndromic Sensorineural Deafness, (DFNB1). Moreover, several mutations in the GJB2 gene have been linked to syndromic SNHL associated with variably expressed skin disorders [4]. One of these mutations is a G to T substitution in 389 position of the GJB2 gene, which produces alterated protein with glycine changed into 2
International Journal of Pediatric Otorhinolaryngology 127 (2019) 109653
A. Bussini, et al.
[11]. Our data are in accordance with the recent investigations by Talbi et al. carried out in algerian familes with non syndromic HL [12].
valine at amino acid position 130 (p.G130V) (Fig. 1B). P.G130V mutation is located on the second intracellular domain of the GJB2 gene and it has been reported in patients with HL associated with Vohwinkel syndrome or palmoplantar keratoderma. Snoeckx et al. [7] in 2005 reported a family case in which son and father showed profound HL and palmoplantar keratoderma interpreted in the father as Vohwinkel syndrome; Iossa et al. [8] in 2009 described a son and a mother with bilateral profound HL, palmoplantar keratoderma and mild ungueal dystrophy. Both families reported presented dominant transmission of the GJB2 variant in association with severe/profound HL and palmoplantar keratoderma. Non-syndromic dominant SNHL associated with GJB2 mutations is early-onset, moderate to severe, and (in contrast to autosomal recessive GJB2 related deafness), typically progressive. Also in our case, proband was early identified at the age of 3 with diagnosis of severe SNHL. A reevaluation at 6-years showed a worsening of clinical features from severe to profound SNHL and poor communicative performances at speech tracking test. Here, we report a father and a son affected by dominant profound SNHL, carrying heterozygous p. G130V mutation in the GJB2 gene, with no dermatological disease. The clinical characteristics of our patients do not confirm the steady association of the p. Gly130Val mutation with cutaneous manifestations, as previously reported, but only to dominant profound bilateral SNHL. Furthermore, Huang S et al., in 2014, seem to have reported another p. G130V deafness case without dermatologic involvement, though not very clearly [9]. This may be at least partially explained by redundancy in connexin expression or by a specific regulation at the transcriptional level as an alternative splicing [10]. The lack of associated skin disorders in cases of non-syndromic SNHL shows that the function and development of the epidermis is not affected by the simple loss of Cx26 function. In this case a likely explanation is that one or more of the several other connexins expressed in epidermis can compensate for loss of Cx26 from epidermis (but not in the inner ear) but they cannot overcome the effects of the dominant mutation (redundancy effect tissue specific). So, mutations in a connexin can affect one organ expressing this protein, but these mutations do not affect other tissues expressing the same protein. The variable penetrance of skin manifestations or their absence, as demonstrated by our case, may be due also to various phenomena, such as: additional mutations in genes involved in cell skin development co-segregating in linkage with GJB2; genes modifying the effect of the p. G130V variant; complex altered emichannel functions, induction of cell death, transdominant effect on other connexins. In our study, we are reporting that the p. G130V mutation can result in HL without any sign of skin abnormality and therefore the genotypephenotype correlation remains unclear. Finally, Other genes have been investigated with negative results. In particular, GJB6 (encoding connexin 30) was analyzed for two pathogenic deletions (D13S1854 and D13S1830), which represent the second most frequent cause in DFNB1 patients Spain, France, Brazil and Cuba
4. Conclusion We confirm that the p. G130V variant of the GJB2 gene is causative of autosomal dominant form of SNHL, although this mutation is not always associated with the presence of palmoplantar keratoderma or other skin disease. Conflicts of interest The authors declare no conflicts of interest. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Acknowledgments We thank the patients and the “La Gemma Rara Onlus” for their contribution to this paper. References [1] G. Meşe, G. Rihard, T.W. White, Gap junctions: basic structure and function, J. Investig. Dermatol. 127 (2007) 2516–2524. [2] D.P. Kelsell, J. Dunlop, H.P. Stevens, et al., Connexin 26 mutations in hereditary non-syndromic sensorineural deafness, Nature 387 (1997) 80–83. [3] A.E. Shearer, M.S. Hildebrand, R.J.H. Smith, M.P. Adam, H.H. Ardinger, R.A. Pagon, et al. (Eds.), Hereditary Hearing Loss and Deafness Overview 1999, GeneReviews, University of Washington, Seattle, Seattle (WA), 1993-2018. [4] L. Avshalumova, J. Fabrikant, A. Koriakos, Overview of skin diseases linked to connexin gene mutations, Int. J. Dermatol. 53 (2014) 192–205. [5] G. Richard, T.W. White, L.E. Smith, et al., Functional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma, Hum. Genet. 103 (1998) 393–399. [6] S. Iossa, E. Marciano, A. Franzé, GJB2 gene mutations in syndromic skin diseases with sensorineural hearing loss, Curr. Genom. 12 (2011) 475–485. [7] R.L. Snoeckx, D.M. Hassan, N.M. Kamal, et al., Mutation analysis of the GJB2 (connexin 26) gene in Egypt, Hum. Mutat. 26 (2005) 60–65. [8] S. Iossa, V. Chinetti, G. Auletta, et al., New evidence for the correlation of the p.G130V mutation in the GJB2 gene and syndromic hearing loss with palmoplantar keratoderma, Am. J. Med. Genet. 149A (2009) 685–688. [9] S. Huang, B. Huang, Y. Yuan, G. Wang, P. Dai, The study of GJB2 dominant mutaion distribution in Chinese deafness patient and the analysis of phenotype, Lin Chung Er Bi Yan Hou Tou Jing Wai Ke Za Zhi 28 (2014) 1744–1747 (Chinese). [10] G.M. Essenfelder, G. Larderet, G. Waksman, J. Lamartine, Gene structure and promoter analysis of the human GJB6 gene encoding connexin 30, Gene 350 (2005) 33–40. [11] I. del Castillo, et al., A deletion involving the connexin 30 gene in nonsyndromic hearing impairment, N. Engl. J. Med. 346 (4) (2002) 243–249. [12] S. Talbi, C. Bonnet, F. Boudjenah, M.T. Mansouri, C. Petit, F. Ammar Khodja, The spectrum of GJB2 gene mutations in Algerian families with nonsyndromic hearing loss from Sahara and Kabylie regions, Int. J. Pediatr. Otorhinolaryngol. 124 (2019) 157–160, https://doi.org/10.1016/j.ijporl.2019.05.036.
3
Contents lists available at ScienceDirect
International Journal of Pediatric Otorhinolaryngology journal homepage: www.elsevier.com/locate/ijporl
The p.Gly130Val mutation in the GJB2 gene: A familiar case of autosomal dominant non-syndromic hearing loss
T
Adelaide Bussinia, Rossana Righia, Chiara Pessinaa, Angelo Genonia, Eliana Cristofarib, Annalisa Melib, Paola Granataa, Emanuela Meronia, Francesco Broccoloc,∗, Rosario Casalonea,∗∗ a
Unit of Cytogenetics and Medical Genetic, ASST Settelaghi, Varese, Italy Unit of Audiology, ASST Settelaghi, Varese, Italy c Department of Medicine and Surgery (School of Medicine), University of Milano-Bicocca, Monza, Italy b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hearing loss GJB2 Connexin 26 p.G130V Skin diseases
Several forms of sensorineural hearing loss (SNHL) have been imputated to connexins mutations and prevalently to connexin 26 (Cx26), codified by the GJB2 gene (gap junction protein, beta 2). Here, we report the first familiar case (heterozygous p. G130V mutation) of non-syndromic (without any dermatological manifestation) dominant profound SNHL. Proband was a 6-years-old male with post-lingual bilateral profound SNHL, clinically identified at the age of 3 with diagnosis of severe SNHL. We confirm that the p. G130V variant of the GJB2 gene is causative of autosomal dominant form of SNHL, although it is not always associated with the presence of skin diseases.
1. Introduction Gap junctions are highly specialized membrane structures important for intercellular communication and are composed of members of the connexin protein family, expressed in several human tissues [1]. The tissue distribution of connexin isoforms increases possible structural variations of gap junctions. Genetic alterations of connexin proteins expressed in the epidermis and in the inner ear cause different forms of sensorineural hearing loss (SNHL) associated with skin diseases [2]. To date, more than 100 recessive GJB2 mutations causing NSHL have been reported [3]. Furthermore, about 20 GJB2 mutations are related to autosomal dominant non-syndromic or syndromic SNHL associated with different skin diseases [3]. Mutations of the GJB2 gene (gap junction protein beta2), which are located on chromosome 13q12 and encoding connexin 26 (Cx26), are involved in about 50% of autosomal recessive non-syndromic SNHL and sometimes in dominant forms [2]. Mutation c.35delG in homozygous or in compound-heterozygous condition is the most frequent cause of nonsyndromic autosomal recessive SNHL in European population [3]. Connexins are expressed in the human epidermis and are involved in the keratinocyte growth and differentiation. Cx26 is expressed in basal keratinocytes of palms and soles and occasionally in granular layer cells [4]. Eleven clinically defined cutaneous disorders have been
∗
correlated to connexins mutations [4]. Some GJB2 gene mutations are involved in different form of hearing loss (HL) associated with skin diseases such as Palmoplantar Keratoderma (PPK) deafness (OMIM 148350) [5]. The spectrum of the skin manifestations shows high phenotypic variability [4]. All GJB2 mutations causing syndromic HL associated with skin disorders are missense mutations and are inherited in a dominant manner [6]. The Cx26 polypeptide includes four transmembrane domains (TM1 to M4), two extracellular domains (E1, E2), a cytoplasmic loop (IC) and amino (-NH2) and carboxy-terminus (-COOH) which are located in the cytoplasm [1]. Generally, most of the mutations involved in HL and skin problems are clustered in the cytoplasmic N-terminal or in the first extracellular loop of Cx26 [6]. Conversely, the p. G130V mutation is localized in the second intracellular domain of the GJB2 gene. Therefore, further studies are needed to determine the clear link between the domains involved with the related phenotype. Regarding various studies which were performed, the p. G130V mutation is associated with a dominant syndromic form of HL but the severity of skin disease was variable [7,8]. Here, we report the first familiar case (heterozygous p. G130V mutation) of non-syndromic (without any sign of skin abnormality) dominant profound SNHL.
Corresponding author. Department of Medicine and Surgery (School of Medicine), University of Milano-Bicocca, 20090, Monza, Italy. Corresponding author. Director of the Cytogenetics and Medical Genetic Unit, ASST Settelaghi, Varese, Italy. E-mail addresses: [email protected] (F. Broccolo), [email protected] (R. Casalone).
∗∗
https://doi.org/10.1016/j.ijporl.2019.109653 Received 9 March 2019; Received in revised form 1 August 2019; Accepted 20 August 2019 Available online 22 August 2019 0165-5876/ © 2019 Elsevier B.V. All rights reserved.
International Journal of Pediatric Otorhinolaryngology 127 (2019) 109653
A. Bussini, et al.
Fig. 1. A) Pedigree of the family: black symbols denote patients affected by hearing loss and carriers of the p. Gly130Val mutation (proband is indicated by an arrow). (B) Pure tone audiometry threshold of the proband (III:1) and his father (II:2) both with bilateral severe-profound sensorineural hearing loss. (D) Conservation of p. Gly130 amino acid residues in GJB2 across species.
2. Case report
component showed HL or skin diseases. Blood samples were taken from proband, his father and other family members unaffected by HL (sibling, mother and gran parents). All family members were screened for GJB2 mutations by direct sequencing of amplified PCR fragments of the promoter region, 5’ UTR and exon 2. Sequence analysis showed heterozygous c.389G > T mutation (p.G130V), localized in the second intracellular domain of the GJB2 gene (NM_004004) in the proband (III:1) and in his father (II:2) (Fig. 1A and B). Exon 2 of the GJB3 and the GJA1 genes, transcript region of the MIR96 gene were also investigate. Deletions D13S1830 and D13S1854 on GJB6 gene with multiplex PCR were also investigated. The mutational analysis in the GJB2 gene did not reveal any aberration in the other family members without HL (Fig. 1A). No mutations in the other investigated genes (GJB3, GJA1, MIR96 and GJB6) were found. Use of different bioinformatics tools (SIFT, PROVEAN, PolyPhen-s, Mutation Taster, MutationAssessor, MutPred) shows that the G130 is conserved in protein structure (Fig. 1C) and due to unusual torsion angles at this position, only glycine is flexible enough to make these torsion angles; therefore, mutation in this position will force the local structure into an incorrect conformation and will disturb it. In conclusion, predictive in silico tolls reveal that this mutation could have a damaging impact and to because of disease (data not shown).
Proband was a 6-years-old male with post-lingual bilateral severeprofound SNHL, clinically identified at the age of 3. In Fig. 1A is showed the pedigree of the family with autosomal dominant severe hearing loss. His father (II:2) presents congenital bilateral profound SNHL, while the 3 years-old brother (III:2) and paternal grandparents (I:1 and I:2) do not show HL both. The audiological evaluation of the proband and his father (III:I, II:2, respectively) showed a severe-profound bilateral SNHL (Fig. 1A). Of note, proband and his father do not present any skin involvement or nails and hair abnormalities. For all cases, pure-tone audiometric test (PTA) for air and bone conduction at frequencies 250–8000 Hz was performed to clarify HL severity as follows: mild: 21–45 dB; moderate: 46–60 dB; moderately severe: 61–75 dB; severe: 76–96 dB; and profound: ≥97 dB. Also, HL severity of all patients with conventional auditory brainstem response (ABR) testing was performed by certified audiologists (which are not shown in this study). Both proband (III:1) and his father (II:2) were undergone to complete audiological tests: otoscopic evaluation, PTA, vocal audiometry (speech tracking), tympanometry, ABR, Slow Vertex Responses (SVR), subjective hearing related disability test (Sander's test). The audiological evaluation of the 3-years-old proband has identified a bilateral severe SNHL, normal tympanometry, absence of otoacoustic emission and ABR. SVR were evocable. A reevaluation at 6years has shown a worsening of clinical features from severe (85 dB) to profound HL (≥97 dB) and poor communicative performances at speech tracking test. The father's (II: 2) audiological tests have shown a symmetric severe-profound bilateral HL, a normal tympanometry, absence of otoacoustic emission and ABR. The patient has shown poor communicative performances at speech tracking test. No other family
3. Discussion Mutations in the GJB2 gene are causative of about 50% of pre-lingual recessive Non Syndromic Sensorineural Deafness, (DFNB1). Moreover, several mutations in the GJB2 gene have been linked to syndromic SNHL associated with variably expressed skin disorders [4]. One of these mutations is a G to T substitution in 389 position of the GJB2 gene, which produces alterated protein with glycine changed into 2
International Journal of Pediatric Otorhinolaryngology 127 (2019) 109653
A. Bussini, et al.
[11]. Our data are in accordance with the recent investigations by Talbi et al. carried out in algerian familes with non syndromic HL [12].
valine at amino acid position 130 (p.G130V) (Fig. 1B). P.G130V mutation is located on the second intracellular domain of the GJB2 gene and it has been reported in patients with HL associated with Vohwinkel syndrome or palmoplantar keratoderma. Snoeckx et al. [7] in 2005 reported a family case in which son and father showed profound HL and palmoplantar keratoderma interpreted in the father as Vohwinkel syndrome; Iossa et al. [8] in 2009 described a son and a mother with bilateral profound HL, palmoplantar keratoderma and mild ungueal dystrophy. Both families reported presented dominant transmission of the GJB2 variant in association with severe/profound HL and palmoplantar keratoderma. Non-syndromic dominant SNHL associated with GJB2 mutations is early-onset, moderate to severe, and (in contrast to autosomal recessive GJB2 related deafness), typically progressive. Also in our case, proband was early identified at the age of 3 with diagnosis of severe SNHL. A reevaluation at 6-years showed a worsening of clinical features from severe to profound SNHL and poor communicative performances at speech tracking test. Here, we report a father and a son affected by dominant profound SNHL, carrying heterozygous p. G130V mutation in the GJB2 gene, with no dermatological disease. The clinical characteristics of our patients do not confirm the steady association of the p. Gly130Val mutation with cutaneous manifestations, as previously reported, but only to dominant profound bilateral SNHL. Furthermore, Huang S et al., in 2014, seem to have reported another p. G130V deafness case without dermatologic involvement, though not very clearly [9]. This may be at least partially explained by redundancy in connexin expression or by a specific regulation at the transcriptional level as an alternative splicing [10]. The lack of associated skin disorders in cases of non-syndromic SNHL shows that the function and development of the epidermis is not affected by the simple loss of Cx26 function. In this case a likely explanation is that one or more of the several other connexins expressed in epidermis can compensate for loss of Cx26 from epidermis (but not in the inner ear) but they cannot overcome the effects of the dominant mutation (redundancy effect tissue specific). So, mutations in a connexin can affect one organ expressing this protein, but these mutations do not affect other tissues expressing the same protein. The variable penetrance of skin manifestations or their absence, as demonstrated by our case, may be due also to various phenomena, such as: additional mutations in genes involved in cell skin development co-segregating in linkage with GJB2; genes modifying the effect of the p. G130V variant; complex altered emichannel functions, induction of cell death, transdominant effect on other connexins. In our study, we are reporting that the p. G130V mutation can result in HL without any sign of skin abnormality and therefore the genotypephenotype correlation remains unclear. Finally, Other genes have been investigated with negative results. In particular, GJB6 (encoding connexin 30) was analyzed for two pathogenic deletions (D13S1854 and D13S1830), which represent the second most frequent cause in DFNB1 patients Spain, France, Brazil and Cuba
4. Conclusion We confirm that the p. G130V variant of the GJB2 gene is causative of autosomal dominant form of SNHL, although this mutation is not always associated with the presence of palmoplantar keratoderma or other skin disease. Conflicts of interest The authors declare no conflicts of interest. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Acknowledgments We thank the patients and the “La Gemma Rara Onlus” for their contribution to this paper. References [1] G. Meşe, G. Rihard, T.W. White, Gap junctions: basic structure and function, J. Investig. Dermatol. 127 (2007) 2516–2524. [2] D.P. Kelsell, J. Dunlop, H.P. Stevens, et al., Connexin 26 mutations in hereditary non-syndromic sensorineural deafness, Nature 387 (1997) 80–83. [3] A.E. Shearer, M.S. Hildebrand, R.J.H. Smith, M.P. Adam, H.H. Ardinger, R.A. Pagon, et al. (Eds.), Hereditary Hearing Loss and Deafness Overview 1999, GeneReviews, University of Washington, Seattle, Seattle (WA), 1993-2018. [4] L. Avshalumova, J. Fabrikant, A. Koriakos, Overview of skin diseases linked to connexin gene mutations, Int. J. Dermatol. 53 (2014) 192–205. [5] G. Richard, T.W. White, L.E. Smith, et al., Functional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma, Hum. Genet. 103 (1998) 393–399. [6] S. Iossa, E. Marciano, A. Franzé, GJB2 gene mutations in syndromic skin diseases with sensorineural hearing loss, Curr. Genom. 12 (2011) 475–485. [7] R.L. Snoeckx, D.M. Hassan, N.M. Kamal, et al., Mutation analysis of the GJB2 (connexin 26) gene in Egypt, Hum. Mutat. 26 (2005) 60–65. [8] S. Iossa, V. Chinetti, G. Auletta, et al., New evidence for the correlation of the p.G130V mutation in the GJB2 gene and syndromic hearing loss with palmoplantar keratoderma, Am. J. Med. Genet. 149A (2009) 685–688. [9] S. Huang, B. Huang, Y. Yuan, G. Wang, P. Dai, The study of GJB2 dominant mutaion distribution in Chinese deafness patient and the analysis of phenotype, Lin Chung Er Bi Yan Hou Tou Jing Wai Ke Za Zhi 28 (2014) 1744–1747 (Chinese). [10] G.M. Essenfelder, G. Larderet, G. Waksman, J. Lamartine, Gene structure and promoter analysis of the human GJB6 gene encoding connexin 30, Gene 350 (2005) 33–40. [11] I. del Castillo, et al., A deletion involving the connexin 30 gene in nonsyndromic hearing impairment, N. Engl. J. Med. 346 (4) (2002) 243–249. [12] S. Talbi, C. Bonnet, F. Boudjenah, M.T. Mansouri, C. Petit, F. Ammar Khodja, The spectrum of GJB2 gene mutations in Algerian families with nonsyndromic hearing loss from Sahara and Kabylie regions, Int. J. Pediatr. Otorhinolaryngol. 124 (2019) 157–160, https://doi.org/10.1016/j.ijporl.2019.05.036.
3