Connexin mutations in hearing loss, dermatological and neurological disorders

Connexin26 Modulates the Radiosensitivity of Cutaneous Squamous Cell Carcinoma by Regulating the Activation of the MAPK/NF-κB Signaling Pathway

Previous studies have confirmed that the gap junction protein Connexin26 (Cx26) is specifically expressed in human skin tissue. Cx26 can transmit radiation-induced damage signals. However, no study has yet reported whether Cx26 expression affects the radiosensitivity of human skin squamous cancer cells or the mechanism by which this occurs. In this study, we found that human skin squamous cell carcinoma cells (A431 cells) expressed significantly more Cx26 and were more sensitive to radiation compared to normal human keratinocytes (HaCaT cells). Knockdown of Cx26 in A431 cells (A431Cx26-/-) decreased radiosensitivity relative to control cells and altered the expression of key proteins in the MAPK and NF-κB signaling pathways. These results demonstrate that Cx26 expression might play an important role in mediating radiation damage in A431 cells and could serve as a potential target for clinical radiotherapy for cutaneous squamous cell carcinoma.

Expression profiles of the connexin genes, Gjb1 and Gjb3, in the developing mouse cochlea

Several connexin genes (GJB1, GJB2, GJB3, GJB6 and GJA1) have been found mutated in patients with non-syndromic and/or syndromic deafness indicating an important role of these proteins in the auditory system. In order to better understand the function of the connexins in the inner ear we have analyzed the gene expression profiles of two connexin genes, Gjb1 (connexin 32) and Gjb3 (connexin 31), by in situ hybridization during the mouse cochlea organogenesis, from early otocyst up to the mature organ in adult. In the developing otocyst epithelium, some restricted domains expressed Gjb3 and Gjb1 whilst high levels of both transcripts were present in the surrounding mesenchymal tissue. As development proceeds, expression of these two genes was found in various subtypes of fibrocytes, either within the spiral limbus or along the spiral ligament, as well as in the basilar membrane cells, in the Reissner's membrane cells, and in subsets of the cellular elements of the cochlear ganglion. Gjb3 and Gjb1 expression was spatiotemporally modulated within the sensory hair cells and the various supporting cells that compose the developing organ of Corti. A transitory expression of Gjb1 was found in the basal and intermediate cells of the stria vascularis. In the adult cochlea Gjb1 transcripts disappeared while Gjb3 expression remained present in fibrocytes with specific expression patterns.

Connexins and gap junctions in the inner ear--it's not just about K⁺ recycling

Normal development, function and repair of the sensory epithelia in the inner ear are all dependent on gap junctional intercellular communication. Mutations in the connexin genes GJB2 and GJB6 (encoding CX26 and CX30) result in syndromic and non-syndromic deafness via various mechanisms. Clinical vestibular defects, however, are harder to connect with connexin dysfunction. Cx26 and Cx30 proteins are widely expressed in the epithelial and connective tissues of the cochlea, where they may form homomeric or heteromeric gap junction channels in a cell-specific and spatiotemporally complex fashion. Despite the study of mutant channels and animal models for both recessive and dominant autosomal deafness, it is still unclear why gap junctions are essential for auditory function, and why Cx26 and Cx30 do not compensate for each other in vivo. Cx26 appears to be essential for normal development of the auditory sensory epithelium, but may be dispensable during normal hearing. Cx30 appears to be essential for normal repair following sensory cell loss. The specific modes of intercellular signalling mediated by inner ear gap junction channels remain undetermined, but they are hypothesised to play essential roles in the maintenance of ionic and metabolic homeostasis in the inner ear. Recent studies have highlighted involvement of gap junctions in the transfer of essential second messengers between the non-sensory cells, and have proposed roles for hemichannels in normal hearing. Here, we summarise the current knowledge about the molecular and functional properties of inner ear gap junctions, and about tissue pathologies associated with connexin mutations.

Identification and genotype/phenotype correlation of mutations in a large German cohort with hearing loss

The prevalence of hearing impairment is estimated as approximately 1 on 1,000 newborn children. To assess a higher mutation detection rate in individuals with hearing loss a three-step mutation screening program consisting of GJB2 in first line, then GJB1, GJB3 and GJB6 (second step) and if tested negative or heterozygote, testing of GJA1, GJB4, SLC26A4 and PJVK (third) was performed. Audiograms were derived from all patients to characterize audiological features of GJB2 mutations especially. In 59 patients (31.3%) of the 188 probands, the hearing impairment was due to GJB2 mutations, 45 (23.9%) of these being homozygous for 35delG mutation and 14 (7.4%) compound heterozygous for GJB2 mutations in the coding region of exon 2 whereas no significant sequence variation was found in exon 1. In 22 (11.7%) additional patients a single recessive mutation in GJB2, GJB3, GJB6 and SLC26A4 without a second mutation on the other allele was identified, making genetic counseling difficult. Our study showed significant difference in hearing loss degree in the patients with GJB2-mutations. Forty-five (45.5%) GJB2-cases were identified in 99 individuals diagnosed with severe to profound hearing loss, 14 (17.7%) GJB2-cases were identified in 79 individuals with moderate deafness whereas no clear GJB2 mutation was found in 10 patients with mild hearing loss (p < 0.001). Revealing a high variability of hearing levels in identical genotypes (even intrafamilial), a significant genotype-phenotype correlation could not be established. Based on the identified mutations spectrum and frequencies, speaking mostly of GJB2, a step by step screening for mutations can be devised and in addition may lead to a better stratification of patients for specific therapeutical approaches.

An unusual mucocutaneous syndrome with sensorineural deafness due to connexin 26 mutations

Mutations of the GJB2 gene, which encodes connexin 26, are related to a range of conditions associated with sensorineural deafness and keratinization disorders. We present the case of a newborn girl with sensorineural deafness, erythematous hyperkeratotic plaques on intertriginous areas, and parakeratosis on the oral and esophageal mucosa. She had an F142L mutation in exon 1 of the GJB2 gene, which was described previously in a patient with a similar phenotype.

Inherited hearing loss: molecular genetics and diagnostic testing

BACKGROUND

Hearing loss is a clinically and genetically heterogeneous condition with major medical and social consequences. It affects up to 8% of the general population.

OBJECTIVE

This review recapitulates the principles of auditory physiology and the molecular basis of hearing loss, outlines the main types of non-syndromic and syndromic deafness by mode of inheritance, and provides an overview of current clinically available genetic testing.

METHODS

This paper reviews the literature on auditory physiology and on genes, associated with hearing loss, for which genetic testing is presently offered.

RESULTS/CONCLUSION

The advent of molecular diagnostic assays for hereditary hearing loss permits earlier detection of the underlying causes, facilitates appropriate interventions, and is expected to generate the data necessary for more specific genotype-phenotype correlations.

Cell degeneration is not a primary causer for Connexin26 (GJB2) deficiency associated hearing loss

Connexin26 (Cx26, GJB2) mutations can induce congenital deafness and are responsible for ∼50% of nonsyndromic hearing loss in children. Mouse models show that Cx26 deficiency induces cochlear development disorder, hair cell loss, and spiral ganglion (SG) neuron degeneration. Hair cell loss and cell degeneration have been considered as a primary causer responsible for Cx26 deficiency associated hearing loss. In this study, by coincidental examination of cochlear postnatal development with recording of auditory brainstem response (ABR) and hair cell function, we found that occurrence of hearing loss in Cx26 knockout (KO) mice was ahead of hair cell loss and cochlear cell degeneration. ABR was absent at the whole-frequency range (8-40 kHz) after birth. However, cochlear cells including SG neurons had no significant degeneration throughout postnatal development. Severe cochlear hair cell loss and SG neuron degeneration were only visible in middle and basal turns, i.e., in middle and high frequency regions, in the adult Cx26 KO mouse cochlea. Functional tests show that hair cells in Cx26 KO mice functioned normally; outer hair cells retained electromotility. These data suggest that cell degeneration is not a primary causer of Cx26 deficiency associated hearing loss. Some mechanisms other than cell degeneration, such as cochlear development disorders, may play an essential role in this common hereditary deafness.

Molecular cloning and evolutionary analysis of GJB6 in mammals

GJB6 plays a crucial role in hearing. In mammals, bats use ultrasonic echolocation for orientation and locating prey. To investigate the evolution of GJB6 in mammals, we cloned the full-length coding region of GJB6 from 16 species of bats and 4 other mammal species and compared them with orthologous sequences in 11 other mammals. The results show purifying selection on GJB6 in mammals, as well as in the bat lineage, which indicates an important role for GJB6 in mammal hearing. We also found one unique amino acid substitution shared by 16 species of bats and 10 shared by two species of artiodactyls. This positioned the artiodactyls at an abnormal location in the gene tree. In addition, the cytoplasmic loop and carboxy terminus were more variable than other domains in all the mammals. These results demonstrate that GJB6 is basically conserved in mammals but has undergone relatively rapid evolution in particular lineages and domains.

Connexin 30 is expressed in the mouse sino-atrial node and modulates heart rate

AIMS

This study aimed at characterizing expression and the functional role of the Gjb6 gene, encoding for connexin 30 (Cx30) protein, in the adult mouse heart.

METHODS AND RESULTS

The expression of the Gjb6 gene in the mouse heart was investigated by RT-PCR and sequencing of amplified cDNA fragments. The sites of Gjb6 expression were identified in the adult heart using transgenic mice with reporter genes (Cx30(LacZ/LacZ) and Cx30(LacZ/LacZ)/Cx40(EGFP/EGFP) mice), as well as anti-HCN4 (hyperpolarization activated cyclic nucleotide-gated potassium channel 4) or anti-connexin antibodies. Cine-magnetic resonance imaging and telemetric ECG recordings were used to evaluate the impact of Cx30 deficiency on cardiac physiology. Gjb6 was shown to be expressed in the sinoatrial (SA) node of the adult mouse heart. Eighty from 100 nuclei on average were LacZ-positive in the SA node of Cx30(LacZ/LacZ) mice. No significant LacZ expression was seen in other cardiac tissues. Cx30 protein was identified in low abundance in the SA node of wild-type mice, as indicated by immunofluorescence experiments. Telemetric ECG recordings indicated that Cx30-deficient mice displayed a mean daily heart rate (HR) that was 9% faster than that measured in control mice (572 +/- 38 b.p.m. vs. 524 +/- 23, P < 0.05). This moderate tachycardia was still observed after inhibition of the autonomic nervous system, demonstrating that Cx30 deficiency resulted in changes in the intrinsic electrical properties of the SA node. Consistent with this hypothesis, Cx30(LacZ/LacZ) displayed a significant reduction of SDNN (standard deviation of the interbeat interval) compared with control mice. Increase of both the cardiac index (20%) and the end-diastolic volume to body weight ratio (16%) with no deficiency in ejection fraction or stroke volume were observed in mutant mice. An increase in cardiac index was interpreted as being a direct consequence of high HR, whereas large end-diastolic volume may be an indirect consequence of prolonged high HR.

CONCLUSION

Cx30 is functionally expressed, in low abundance, in the SA node of the adult mouse heart where it participates in HR regulation.

Evidence that variation in the oligodendrocyte lineage transcription factor 2 (OLIG2) gene is associated with psychosis in Alzheimer's disease

Psychotic symptoms are common in individuals with Alzheimer's disease (AD), and define a phenotype associated with more rapid cognitive and functional decline. Evidence suggests that psychotic symptoms may be influenced by genetic factors, and recent studies in schizophrenia, bipolar affective disorder (BPAD) and Alzheimer's disease with psychosis (AD+P) suggest that psychosis susceptibility or modifier genes may act across diseases. We hypothesised that oligodendrocyte lineage transcription factor 2 (OLIG2), a regulator of white matter development and a candidate gene for schizophrenia, may also be associated with psychotic symptoms in AD. We genotyped 11 SNPs in OLIG2 previously tested for association with schizophrenia [L. Georgieva, V. Moskvina, T. Peirce, N. Norton, N.J. Bray, L. Jones, P. Holmans, S. Macgregor, S. Zammit, J. Wilkinson, H. Williams, I. Nikolov, N. Williams, D. Ivanov, K.L. Davis, V. Haroutunian, J.D. Buxbaum, N. Craddock, G. Kirov, M.J. Owen, M.C. O'Donovan, Convergent evidence that oligodendrocyte lineage transcription factor 2 (OLIG2) and interacting genes influence susceptibility to schizophrenia, Proc. Natl. Acad. Sci. U.S.A. 103 (33) (2006) 12469-12474] and tested these for association with AD and AD+P. Significant evidence for association of psychotic symptoms within cases was identified for two SNPs, rs762237 (allelic P=0.002, OR=1.42, corrected P=0.019) and rs2834072 (allelic P=0.004, OR=1.41, corrected P=0.05).

Diverse deafness mechanisms of connexin mutations revealed by studies using in vitro approaches and mouse models

Mutations in connexins (Cxs), the constitutive protein subunits of gap junction (GJ) intercellular channels, are one of the most common human genetic defects that cause severe prelingual non-syndromic hearing impairments. Many subtypes of Cxs (e.g., Cxs 26, 29, 30, 31, 43) and pannexins (Panxs) are expressed in the cochlea where they contribute to the formation of a GJ-based intercellular communication network. Cx26 and Cx30 are the predominant cochlear Cxs and they co-assemble in most GJ plaques to form hybrid GJs. The cellular localization of specific Cx subtypes provides a basis for understanding the molecular structure of GJs and hemichannels in the cochlea. Information about the interactions among the various co-assembled Cx partners is critical to appreciate the functional consequences of various types of genetic mutations. In vitro studies of reconstituted GJs in cell lines have yielded surprisingly heterogeneous mechanisms of dysfunction caused by various Cx mutations. Availability of multiple lines of Cx-mutant mouse models has provided some insight into the pathogenesis processes in the cochlea of deaf mice. Here we summarize recent advances in understanding the structure and function of cochlear GJs and give a critical review of current findings obtained from both in vitro studies and mouse models on the mechanisms of Cx mutations that lead to cell death in the cochlea and hearing loss.

Gap junctions and connexins in the inner ear: their roles in homeostasis and deafness

PURPOSE OF REVIEW

Mutations in GJB2 and GJB6, the genes encoding the gap-junction proteins connexin 26 and connexin 30, are the most common cause of autosomal recessive nonsyndromic deafness in many populations across the world. In this review, we discuss current ideas about the roles of gap junctions in the inner ear and the implications of connexin mutations on auditory function.

RECENT FINDINGS

In recent years, a complex picture of the roles of gap junctions in cochlear physiology emerged. Rather than being mere conduits for the circulation of potassium ions in the inner ear, gap junctions have been implicated in intercellular signaling among nonsensory cells and may be involved in the maintenance of the endothelial barrier in the stria vascularis. Studies of mutant channels and mouse models for connexin-related deafness have provided valuable insights into some of the mechanisms by which connexin dysfunction causes cochlear degeneration. They have also identified potential therapeutic interventions for specific connexin mutations, such as the restoration of normal connexin 26 protein levels in GJB6-associated deafness.

SUMMARY

Despite recent advances, a better understanding of the complexity of gap-junctional communication in the inner ear and the structure-function relationships of connexin proteins is required for the development of mechanism-based treatments of connexin-associated hearing loss.

Novel mutations in GJA1 cause oculodentodigital syndrome

Oculodentodigital syndrome (ODD) is a rare, usually autosomal-dominant disorder that is characterized by developmental abnormalities of the face, eyes, teeth, and limbs. The most common clinical findings include a long, narrow nose, short palpebral fissures, type III syndactyly, and dental abnormalities including generalized microdontia and enamel hypoplasia. Recently, it has been shown that mutations in the gene GJA1, which encodes the gap junction protein connexin 43, underlie oculodentodigital syndrome. Gap junction communication between adjacent cells is known to be vital during embryogenesis and subsequently for normal tissue homeostasis. Here, we report 8 missense mutations in the coding region of GJA1, 6 of which have not been described previously, in ten unrelated families diagnosed with ODD. In addition, immunofluorescence analyses of a developmental series of mouse embryos and adult tissue demonstrates a strong correlation between the sites of connexin 43 expression and the clinical phenotype displayed by individuals affected by ODD.

Closing the gap on autosomal dominant connexin-26 and connexin-43 mutants linked to human disease

Cells within the vast majority of human tissues communicate directly through clustered arrays of intercellular channels called gap junctions. Gene ablation studies in mouse models have revealed that these intercellular channels are necessary for a variety of organ functions and that some of these genes are essential for survival. Molecular genetics has uncovered that germ line mutations in nearly half of the genes that encode the 21-member connexin family of gap junction proteins are linked to one or more human diseases. Frequently, these mutations are autosomal recessive, whereas in other cases, autosomal dominant mutations manifest as disease. Given the broad and overlapping distribution of connexins in a wide arrangement of tissues, it is hard to predict where connexin-linked diseases will clinically manifest. For instance, the most prevalent connexin in the human body is connexin-43 (Cx43), yet autosomal dominant mutations in the GJA1 gene, which encodes Cx43, exhibit modest developmental disorders resulting in a disease termed oculodentodigital dysplasia. Autosomal recessive mutations in the gene encoding Cx26 result in moderate to severe sensorineural hearing loss, whereas autosomal dominant mutations produce hearing loss and a wide range of skin diseases, including palmoplantar keratoderma. Here, we will focus on autosomal dominant mutations of the genes encoding Cx26 and Cx43 in relation to models that link genotypes to phenotypic outcomes with particular reference to how these approaches provide insight into human disease.

The C-terminus of connexin43 adopts different conformations in the Golgi and gap junction as detected with structure-specific antibodies

The C-terminus of the most abundant and best-studied gap-junction protein, connexin43, contains multiple phosphorylation sites and protein-binding domains that are involved in regulation of connexin trafficking and channel gating. It is well-documented that SDS/PAGE of NRK (normal rat kidney) cell lysates reveals at least three connexin43-specific bands (P0, P1 and P2). P1 and P2 are phosphorylated on multiple, unidentified serine residues and are found primarily in gap-junction plaques. In the present study we prepared monoclonal antibodies against a peptide representing the last 23 residues at the C-terminus of connexin43. Immunofluorescence studies showed that one antibody (designated CT1) bound primarily to connexin43 present in the Golgi apparatus, whereas the other antibody (designated IF1) labelled predominately connexin43 present in gap junctions. CT1 immunoprecipitates predominantly the P0 form whereas IF1 recognized all three bands. Peptide mapping, mutational analysis and protein-protein interaction experiments revealed that unphosphorylated Ser364 and/or Ser365 are critical for CT1 binding. The IF1 paratope binds to residues Pro375-Asp379 and requires Pro375 and Pro377. These proline residues are also necessary for ZO-1 interaction. These studies indicate that the conformation of Ser364/Ser365 is important for intracellular localization, whereas the tertiary structure of Pro375-Asp379 is essential in targeting and regulation of gap junctional connexin43.

Connexin Levels Regulate Keratinocyte Differentiation in the Epidermis

To understand the role of connexin43 (Cx43) in epidermal differentiation, we reduced Cx43 levels by RNA-mediated interference knockdown and impaired its functional status by overexpressing loss-of-function Cx43 mutants associated with the human disease oculodentodigital dysplasia (ODDD) in rat epidermal keratinocytes. When Cx43 expression was knocked down by 50-75%, there was a coordinate 55-65% reduction in Cx26 level, gap junction-based dye coupling was reduced by 60%, and transepithelial resistance decreased. Importantly, the overall growth and differentiation of Cx43 knockdown organotypic epidermis was severely impaired as revealed by alterations in the levels of the differentiation markers loricrin and involucrin and by reductions in vital and cornified layer thicknesses. Conversely, although the expression of Cx43 mutants reduced the coupling status of rat epidermal keratinocytes by approximately 80% without altering the levels of endogenous Cx43 or Cx26, their ability to differentiate was not altered. In addition, we used a mouse model of ODDD and found that newborn mice harboring the loss-of-function Cx43(G60S) mutant had slightly reduced Cx43 levels, whereas Cx26 levels, epidermis differentiation, and barrier function remained unaltered. This properly differentiated epidermis was maintained even when Cx43 and Cx26 levels decreased by more than 70% in 3-week-old mutant mice. Our studies indicate that Cx43 and Cx26 collectively co-regulate epidermal differentiation from basal keratinocytes but play a more minimal role in the maintenance of established epidermis. Altogether, these studies provide an explanation as to why the vast majority of ODDD patients, where Cx43 function is highly compromised, do not suffer from skin disease.

Connexin 26 mutations in autosomal recessive deafness disorders: a review

This review explores the association between GJB2 gene mutations, encoding connexin 26 (Cx26), and nonsyndromic hearing loss. Connexins are proteins that form intracellular membrane channels and regulate ion movement between contiguous fluid spaces. A family of autosomal gene mutations has been identified that lead to abnormal connexin expression within the inner ear that are associated with hearing loss. The exact mechanism by which this link is elicited remains unclear. We aim to highlight the clinically underestimated prevalence of GJB2 gene mutations, to explore the influential role of ethnic diversity in mutation frequency, and to provide a framework for hearing specialists in considering the differential diagnosis of nonsyndromic hearing loss. By linking an observed phenotype associated with abnormal Cx26 expression to the current understanding of the biological and genetic basis underlying it will allow a more accurate clinical description of associated hearing loss, and therefore enable more effective patient management and genetic counselling.

Mutations inGJB2,GJB6, and mitochondrial DNA are rare in African American and Caribbean Hispanic individuals with hearing impairment

Autosomal recessive nonsyndromic sensorineural hearing impairment (ARNSHI) comprises 80% of familial hearing loss cases. Approximately half result from mutations in the connexin 26 (Cx26) gene, GJB2, in Caucasian populations. Heterozygous mutations in GJB2 occasionally co-occur with a deletion of part of GJB6 (connexin 30; Cx30). It is estimated that approximately 1% of deafness is maternally inherited, due to mutations in mitochondrial DNA (mtDNA). Few studies have focused on the frequency of mutations in connexins or mtDNA in African American (AA) and Caribbean Hispanic (CH) admixture populations. In this study, we performed bidirectional sequencing of the GJB2 gene and polymerase chain reaction (PCR) screening for the common GJB6 deletion, as well as PCR/RFLP analysis for three mutations in mtDNA (A1555G, A3243G, A7445G), in 109 predominantly simplex AA and CH individuals. Variations found were a 101T > C (M34T; 1/101 cases), 109G > A (V37I; 1/101), 35delG (mutation; 4/101, (3/4) of non-AA/CH ethnicity), 167delT (mutation; 1/101), 139G > T (mutation; E47X; 1/101 homozygote, consanguineous), -15C > T (1/101), 79G > A (V27I; 9/101), 380G > A (R127H; 4/101; Guyana, India, Pakistan ethnicity), 670A > C (Indeterminate; K224Q; 1/101), 503A > G (novel; K168R; 3/101) and 684C > A (novel; 1/101). All but one of the AA and CH patients had monoallelic variations. There were no hemizygous GJB6 deletions in those with monoallelic GJB2 variations. We also did not identify any patients with the three mutations in mtDNA. Bidirectional sequencing of the GJB2 gene was performed in 187 AA and Hispanic healthy individuals. Our results reveal that GJB2 mutations, GJB6 deletions, and mtDNA mutations may not be significant in these minority admixture populations.

Gap junction-mediated intercellular communication in the immune system

Immune cells are usually considered non-attached blood cells, which would exclude the formation of gap junctions. This is a misconception since many immune cells express connexin 43 (Cx43) and other connexins and are often residing in tissue. The role of gap junctions is largely ignored by immunologists as is the immune system in the field of gap junction research. Here, the current knowledge of the distribution of connexins and the function of gap junctions in the immune system is discussed. Gap junctions appear to play many roles in antibody productions and specific immune responses and may be important in sensing danger in tissue by the immune system. Gap junctions not only transfer electrical and metabolical but also immunological information in the form of peptides for a process called cross-presentation. This is essential for proper immune responses to viruses and possibly tumours. Until now only 40 research papers on gap junctions in the immune system appeared and this will almost certainly expand with the increased mutual interest between the fields of immunology and gap junction research.

Identification of mutations in members of the connexin gene family as a cause of nonsyndromic deafness in Taiwan

Connexins (Cx), a large family of membrane proteins, are key components of gap junction channels. These channels are critical intercellular pathways through which ions or small molecules are passed, regulating a variety of physiological and developmental processes. One of these processes is hearing. In the current study, a genetic survey was made on 380 Taiwanese individuals, 260 with nonsyndromic deafness and 120 with normal hearing. All the 380 Taiwanese were screened for the presence of mutations in 8 genes of the Cx gene family. These genes included Cx26 (GJB2), Cx29 (GJE1), Cx30 (GJB6), Cx30.3 (GJB4), Cx31 (GJB3), Cx32 (GJB1), Cx43 (GJA1) and pseudogene [rho] of Cx43 (rho GJA1). Mutations were identified in 7 out of the 8 screened genes of the Cx family from 62 of the 260 deaf subjects (23.85%). Of the 17 mutations observed in the Cx gene family, 11 were novel mutations. Fourteen polymorphisms that were not associated with hearing loss were identified in the Cx gene family. The first 2 most frequently occurring mutations were found in the Cx26 (28/62; 45.16%) and the rho Cx43 (17/62; 27.42%), respectively. Nine cases of mutations were found in the Cx30.3 (9/62; 14.52%). In the Cx30, 1 novel mutation was identified in 1 case (1/62; 1.61%). Two patients with mutations of each of Cx29 and Cx43 were found (2/62; 3.23%). One novel mutation of Cx31 was identified in 3 patients with nonsyndromic deafness (3/62; 4.84%). The Cx32 was the only gene without detecting any mutation or polymorphism.Our study provides information for understanding the importance of genetic factors in nonsyndromic deafness of the Taiwanese and may be of use in the improvement of genetic diagnosis of hearing loss in Taiwan.

DNA sequence analysis of GJB2, encoding connexin 26: observations from a population of hearing impaired cases and variable carrier rates, complex genotypes, and ethnic stratification of alleles among controls

Mutations in GJB2 are associated with hereditary hearing loss. DNA sequencing of GJB2 in a cohort of hearing impaired patients and a multi-ethnic control group is reported. Among 610 hearing impaired cases, 43 DNA sequence variations were identified in the coding region of GJB2 including 24 mutations, 8 polymorphisms, 3 unclassified variants (G4D, R127C, M163V), 1 controversial variant (V37I), and 7 novel variants (G12C, N14D, V63A, T86M, L132V, D159, 592_600delinsCAGTGTTCATGACATTC). Sixteen non-coding sequence variations were also identified among cases including the IVS1+1A>G mutation, 2 polymorphisms, and 13 novel variants. A diagnosis of GJB2-associated hearing loss was confirmed for 63 cases (10.3%). Heterozygous mutations were found in 39 cases (6.4%). Eleven cases carrying novel or unclassified variants (1.8 %) and 18 cases carrying the controversial V37I variant were identified (3%). In addition, 294 control subjects from 4 ethnic groups were sequenced for GJB2. Thirteen sequence variations in the coding region of GJB2 were identified among controls including 2 mutations, 6 polymorphisms, 2 unclassified variants (G4D, T123N), 1 controversial variant (V37I), and 2 novel variants (R127L, V207L). Nine sequence variations were identified among controls in the non-coding regions in and around GJB2 exon 2. Of particular interest among controls were the variability in carrier rates and ethnic stratification of alleles, and the complex genotypes among Asians, 47% of whom carried two to four sequence variations in the coding region of GJB2. These data provide new information about carrier rates for GJB2-based hearing loss in various ethnic groups and contribute to evaluation of the pathogenicity of the controversial V37I variant.

Different mechanisms preclude mutant CLDN14 proteins from forming tight junctions in vitro

Mutations in claudin 14 (CLDN14) cause nonsyndromic DFNB29 deafness in humans. The analysis of a murine model indicated that this phenotype is associated with degeneration of hair cells, possibly due to cation overload. However, the mechanism linking these alterations to CLDN14 mutations is unknown. To investigate this mechanism, we compared the ability of wild-type and missense mutant CLDN14 to form tight junctions. Ectopic expression in L mouse fibroblasts (LM cells) of wild-type CLDN14 protein induced the formation of tight junctions, while both the c.254T>A (p.V85D) mutant, previously identified in a Pakistani family, and the c.301 G>A (p.G101R) mutant, identified in this study through the screen of 183 Spanish and Greek patients affected with sporadic nonsyndromic deafness, failed to form such junctions. However, the two mutant proteins differed in their ability to localize at the plasma membrane. We further identified hitherto undescribed exons of CLDN14 that are utilized in alternative spliced transcripts. We demonstrated that different mutations of CLDN14 impaired by different mechanisms the ability of the protein to form tight junctions. Our results indicate that the ability of CLDN14 to be recruited to these junctions is crucial for the hearing process.

A novel N14Y mutation in Connexin26 in keratitis-ichthyosis-deafness syndrome: analyses of altered gap junctional communication and molecular structure of N terminus of mutated Connexin26

Connexins (Cxs) are transmembranous proteins that connect adjacent cells via channels known as gap junctions. The N-terminal 21 amino acids of Cx26 are located at the cytoplasmic side of the channel pore and are thought to be essential for the regulation of channel selectivity. We have found a novel mutation, N14Y, in the N-terminal domain of Cx26 in a case of keratitis-ichthyosis-deafness syndrome. Reduced gap junctional intercellular communication was observed in the patient's keratinocytes by the dye transfer assay using scrape-loading methods. The effect of this mutation on molecular structure was investigated using synthetic N-terminal peptides from both wild-type and mutated Cx26. Two-dimensional (1)H nuclear magnetic resonance and circular dichroism measurements demonstrated that the secondary structures of these two model peptides are similar to each other. However, several novel nuclear Overhauser effect signals appeared in the N14Y mutant, and the secondary structure of the mutant peptide was more susceptible to induction of 2,2,2-trifluoroethanol than wild type. Thus, it is likely that the N14Y mutation induces a change in local structural flexibility of the N-terminal domain, which is important for exerting the activity of the channel function, resulting in impaired gap junctional intercellular communication.

A nonsense mutation in the first transmembrane domain of connexin 43 underlies autosomal recessive oculodentodigital syndrome

BACKGROUND

Oculodentodigital syndrome (ODD) is a pleiotropic congenital disorder characterised by abnormalities of the face, eyes, dentition, and limbs. ODD, which is inherited as an autosomal dominant trait, results from missense mutations in the gap junction protein connexin 43.

OBJECTIVE

To analyse a family with a history of ODD which is inherited in an autosomal recessive manner

RESULTS

ODD in this family resulted from the homozygous mutation R33X in the first transmembrane domain of connexin 43.

CONCLUSIONS

The findings provide clear genetic evidence that ODD can be inherited in an autosomal recessive manner and that a dominant negative mechanism underlies autosomal dominant ODD.

Predicting candidate genes for human deafness disorders: a bioinformatics approach

BACKGROUND

There are more than 50 genes for autosomal dominant and autosomal recessive nonsyndromic hereditary deafness that are yet to be cloned. The human genome sequence and expression profiles of transcripts in the inner ear have aided positional cloning approaches. The knowledge of protein interactions offers additional advantages in selecting candidate genes within a mapped region.

RESULTS

We have employed a bioinformatic approach to assemble the genes encoded by genomic regions that harbor various deafness loci. The genes were then in silico analyzed for their candidacy by expression pattern and ability to interact with other proteins. Such analyses have narrowed a list of 2400 genes from suspected regions of the genome to a manageable number of about 140 for further analysis.

CONCLUSION

We have established a list of strong candidate genes encoded by the regions linked to various nonsyndromic hereditary hearing loss phenotypes by using a novel bioinformatic approach. The candidates presented here provide a starting point for mutational analysis in well-characterized families along with genetic linkage to refine the loci. The advantages and shortcomings of this bioinformatic approach are discussed.

Neurotological and neuroanatomical changes in the connexin-26-related HID/KID syndrome

The phenotype of the HID (hystrix-like ichthyosis, deafness)/KID (keratitis, ichthyosis, deafness) syndrome is primarily characterized by skin changes. However, the connexin 26 (Cx 26) autosomal dominant mutation underlying this syndrome is of special neurotological interest. In the present paper, the clinical pattern, audiovestibular and neuroimaging findings and the detailed genetic analysis of 4 patients with identical HID/KID-associated mutation D50N of Cx 26 are reported. The audiological test results demonstrated profound sensorineural hearing loss in all of the patients. Neurotological testing revealed inconsistent abnormalities in dynamic posturography (sensory organization test), but the vestibular ocular reflex upon caloric irrigation was normal in all patients. Vestibular-evoked myogenic potential testing for otolith function (saccule) showed a regular response in 1 patient and pathologic responses in 3 patients, while subjective haptic vertical (utricular function) testing was normal in all of the patients. CCT showed an extended (in length), but very thin (in diameter) bony lining between the basal portion of the internal auditory canal and the vestibule in the 3 scanned patients. Our study provides evidence for functionally intact semicircular canals and normal utricular function in subjects with the autosomal dominant D50N mutation of Cx 26, in contrast to saccular function which was generally compromised and hearing loss which was profound.

Manipulating gene expression in the mature inner ear

It is possible to manipulate gene expression in cochlear tissue, but technical issues have made this challenging in the mature in vivo inner ear. Generally, the most common reasons for such manipulations involve basic science or therapeutic quests. Examples of experimental studies are those designed to elucidate the role of a specific gene or a gene expression cascade or to understand the function of a particular cell type. Therapeutic goals may include replacing a defective gene or enhancing tissue protection, repair, or regeneration. This review summarizes the main technical approaches that are viable options for in vivo manipulation of gene expression in the mature inner ear, as well as major research and clinical issues likely to benefit from such genetic manipulations.

Properties of human connexin 31, which is implicated in hereditary dermatological disease and deafness

The connexins are a family of at least 20 homologous proteins in humans that form aqueous channels connecting the interiors of coupled cells and mediating electrical and chemical communication. Mutations in the gene for human connexin 31 (hCx31) are associated with disorders of the skin and auditory system. Alterations in functional properties of Cx31 junctions are likely to play a role in these diseases; nonetheless, little is known about the properties of the wild-type channels. Here we show that hCx31 channels, like other connexin channels, are gated by voltage and close at low pH and when exposed to long-chain alkanols. Single-channel conductance of the fully open channel is approximately 85 pS, and it is permeable to Lucifer yellow, Alexa Fluor(350), ethidium bromide, and DAPI, which have valences of -2, -1, +1, and +2, respectively. In contrast to what has been reported for mouse Cx31, hCx31 appears to form functional heterotypic channels with all four connexins tested, Cx26, Cx30, Cx32, and Cx45. These findings provide an important first step in evaluating the pathogenesis of inherited human diseases associated with mutations in the gene for Cx31.

Human hair keratin-associated proteins (KAPs)

Elucidation of the genes encoding structural proteins of the human hair follicle has advanced rapidly during the last decade, complementing nearly three previous decades of research on this subject in other species. Primary among these advances was both the characterization of human hair keratins, as well as the hair keratin associated proteins (KAPs). This review describes the currently known human KAP families, their genomic organization, and their characteristics of expression. Furthermore, this report delves into further aspects, such as polymorphic variations in human KAP genes, the role that KAP proteins might play in hereditary hair diseases, as well as their modulation in several different transgenic mouse models displaying hair abnormalities.

Changes in gap junctional connexin isoforms during prostate cancer progression

BACKGROUND

Connexins have their traditional function as part of gap junction (GJ) structures, but have recently been shown to have GJ-independent roles. Although GJs and their connexin subunits are thought to be down-regulated in cancer, depending on the connexin examined, many times the expression level is preserved or even increased. This is further apparent by the importance of GJs in "bystander effects" of radiation and viral targeting treatments.

METHODS

We surveyed connexin isoforms in prostate cancer cell lines and tissue with RT-PCR and immunohistochemistry. Upon modulating GJ function, we observed prostate epithelial cell behaviors.

RESULTS

Advanced cells within PC-3 and LNCaP prostate cancer progression models exhibit elevated connexin 26 (Cx26) levels-a trend validated in clinical samples. When GJs were inhibited, adhesion was not affected, but invasion and migration were strikingly decreased. A link between the expression of Cx26 and integrin adhesion-linked functions are suggested by Cx26's direct interaction with focal adhesion kinase (FAK).

CONCLUSIONS

These results suggest a novel mechanism for adhesion regulation by a GJ-independent Cx26 function that correlates with prostate disease progression. The increased Cx26 expression during prostate cancer progression plays a role in adhesion regulation possibly through its interaction with FAK.

Expression and function of connexins in the epidermis, analyzed with transgenic mouse mutants

Eight different connexins are expressed in mouse epidermis with overlapping expression patterns in different epidermal layers. Analyses of mice with deficiency or modifications of distinct connexins yielded insights into the large variety of connexins in the epidermis. Connexin43 (Cx43) deficiency in mouse epidermis resulted in a significant acceleration of wound closure. Truncation by 125 amino acid residues of the Cx43 C-terminal region led to an altered epidermal expression pattern of Cx43 and defective development of the epidermal water barrier in transgenic mice, although the truncated Cx43 protein could still form open gap junctional channels in transfected HeLa cells. Thus, the phenotypic abnormalities observed in mice with truncated Cx43 protein (Cx43K258Stop) are more likely due to defective regulation of this protein rather than the closed Cx43 channel. Our studies of connexin-deficient mice revealed an extensive redundancy of connexins expressed in mouse epidermis. Epidermal connexins seem to form two functional groups in which deficiency of one connexin isoform can be compensated by other connexin isoforms of the same group.

Gap junctional communication in tissue inflammation and repair

Local injury induces a complex orchestrated response to stimulate healing of injured tissues, cellular regeneration and phagocytosis. Practically, inflammation is defined as a defense process whereby fluid and white blood cells accumulate at a site of injury. The balance of cytokines, chemokines, and growth factors is likely to play a key role in regulating important cell functions such as migration, proliferation, and matrix synthesis during the process of inflammation. Hence, the initiation, maintenance, and resolution of innate responses depend upon cellular communication. A process similar to tissue repair and subsequent scarring is found in a variety of fibrotic diseases. This may occur in a single organ such as liver, kidneys, pancreas, lung, skin, and heart, but fibrosis may also have a more generalized distribution such as in atherosclerosis. The purpose of this review is to summarize recent advances on the contribution of gap junction-mediated intercellular communication in the modulation of the inflammatory response and tissue repair.

GJB2 mutations in Turkish patients with ARNSHL: prevalence and two novel mutations

Mutations in the connexin 26 gene (GJB2) cause a significant proportion of prelingual non-syndromic autosomal recessive deafness in all populations studied so far. To determine the percentage of hearing loss attributed to GJB2 in northeast Turkey, 93 unrelated patients with autosomal recessive non-syndromic hearing loss (ARNSHL) were screened. Seven different mutations were found in 29 of the patients with severe to profound hearing loss. The 35delG mutation was the most common mutation, accounting for 76% of all mutant GJB2 alleles. Four already described mutations, W24X, 310del14, delE120 and R184P and two novel mutations, Q80K and P173S, were identified. The allelic Delta(GJB6-D13S1830), which can cause hearing loss in combination with GJB2 mutations, was not present in our patients. Our results are comparable to those reported in other regions in Turkey and indicate that GJB2 mutations account for about 30% of Turkish patients with ARNSHL. Besides 35delG, W24X and delE120 occur more than once in the Turkish ARNSHL population with a frequency of about 5%.

Cochlear gap junctions coassembled from Cx26 and 30 show faster intercellular Ca2+signaling than homomeric counterparts

The importance of connexins (Cxs) in cochlear functions has been demonstrated by the finding that mutations in Cx genes cause a large proportion of sensorineural hearing loss cases. However, it is still unclear how Cxs contribute to the cochlear function. Recent data ( 33 ) obtained from Cx30 knockout mice showing that a reduction of Cx diversity in assembling gap junctions is sufficient to cause deafness suggest that functional interactions of different subtypes of Cxs may be essential in normal hearing. In this work we show that the two major forms of Cxs (Cx26 and Cx30) in the cochlea have overlapping expression patterns beginning at early embryonic stages. Cx26 and Cx30 were colocalized in most gap junction plaques in the cochlea, and their coassembly was tested by coimmunoprecipitation. To compare functional differences of gap junctions with different molecular configurations, homo- and heteromeric gap junctions composed of Cx26 and/or Cx30 were reconstituted by transfections in human embryonic kidney-293 cells. The ratio imaging technique and fluorescent tracer diffusion assays were used to assess the function of reconstituted gap junctions. Our results revealed that gap junctions with different molecular configurations show differences in biochemical coupling, and that intercellular Ca2+signaling across heteromeric gap junctions consisting of Cx26 and Cx30 was at least twice as fast as their homomerically assembled counterparts. Our data suggest that biochemical permeability and the dynamics of intercellular signaling through gap junction channels, in addition to gap junction-mediated intercellular ionic coupling, may be important factors to consider for studying functional roles of gap junctions in the cochlea.

Connexin disorders of the skin

Over the past decade, the molecular basis of most disorders of cornification has been unveiled. Among these, a distinct group has emerged because of primary defects in cell-cell communication due to faulty gap junction proteins also known as connexins. This review aims to delineate the cutaneous connexin disorders and to highlight intriguing genotype-phenotype correlations and emanating clinical implications.

Connexins and their environment: effects of lipids composition on ion channels

Intercellular communication is mediated through paired connexons that form an aqueous pore between two adjacent cells. These membrane proteins reside in the plasma membrane of their respective cells and their activity is modulated by the composition of the lipid bilayer. The effects of the bilayer on connexon structure and function may be direct or indirect, and may arise from specific binding events or the physicochemical properties of the bilayer. While the effects of the bilayer and its constituent lipids on gap junction activity have been described in the literature, the underlying mechanisms of the interaction of connexin with its lipidic microenvironment are not as well characterized. Given that the information regarding connexons is limited, in this review, the specific roles of lipids and the properties of the bilayer on membrane protein structure and function are described for other ion channels as well as for connexons.

Innexins: members of an evolutionarily conserved family of gap-junction proteins

Gap junctions are clusters of intercellular channels that provide cells, in all metazoan organisms, with a means of communicating directly with their neighbours. Surprisingly, two gene families have evolved to fulfil this fundamental, and highly conserved, function. In vertebrates, gap junctions are assembled from a large family of connexin proteins. Innexins were originally characterized as the structural components of gap junctions in Drosophila, an arthropod, and the nematode Caenorhabditis elegans. Since then, innexin homologues have been identified in representatives of the other major invertebrate phyla and in insect-associated viruses. Intriguingly, functional innexin homologues have also been found in vertebrate genomes. These studies have informed our understanding of the molecular evolution of gap junctions and have greatly expanded the numbers of model systems available for functional studies. Genetic manipulation of innexin function in relatively simple cellular systems should speed progress not only in defining the importance of gap junctions in a variety of biological processes but also in elucidating the mechanisms by which they act.

The effects of a mutant connexin 26 on epidermal differentiation

To elucidate the mode of action of dominant mutant connexins in causing inherited skin diseases, transgenic mice were produced that express the true Vohwinkel syndrome-associated mutant Cx26 (D66H), from a keratin 10 promoter, specifically in the suprabasal epidermal keratinocytes. Following birth, the transgenic mice developed keratoderma similar to that of human carriers of Cx26 (D66H). Expression of the transgene resulted in a loss of Cx26 and Cx30 at intercellular junctions of epidermal keratinocytes and accumulation of these connexins in the cytoplasm. Injection of primary mouse keratinocytes with Lucifer Yellow showed no difference in terms of dye spreading between transgenic and non transgenic keratinocytes in vitro. Expression of the mutant Cx26 (D66H) did not interfere with the formation of the epidermal water barrier during late embryonic development. Attempts to produce transgenic mice expressing the wild type form of Cx26 from the K10 promoter failed to produce viable animals although transgenic embryos were recovered at days 9 and 12 of gestation, suggesting that the transgene might be embryonic lethal.

Expanding The Phenotypic Spectrum of Cx26 Disorders: Bart–Pumphrey Syndrome is Caused by a Novel Missense Mutation in GJB2

Bart-Pumphrey syndrome (BPS) is an autosomal dominant disorder characterized by sensorineural hearing loss, palmoplantar keratoderma, knuckle pads, and leukonychia, which show considerable phenotypic variability. The clinical features partially overlap with Vohwinkel syndrome and Keratitis-Ichthyosis-Deafness syndrome, both disorders caused by dominant mutations in the GJB2 gene encoding the gap junction protein connexin-26, suggesting an etiological relationship. We report here a novel GJB2 mutation N54K segregating in a family with BPS, which was not detected in 110 control individuals of Northern European ancestry. This non-conservative missense mutation lies within a cluster of pathogenic GJB2 mutations affecting the evolutionary conserved first extracellular loop of Cx26 important for docking of connexin hemichannels and voltage gating. Immunostaining of Cx26 in lesional palmar and knuckle skin was weak or absent, although its adnexal expression appeared normal and the punctate membrane staining of Cx26 and other epidermal connexins was not altered. Nevertheless, the widespread immunostaining of Cx30 throughout the spinous cell layers suggested a compensatory overexpression. Our results emphasize that pleiotropic GJB2 mutations are responsible for at least 5 overlapping dermatological disorders associated with syndromic hearing loss and cover a wide range of severity and organ involvement.

Claudin-1 gene mutations in neonatal sclerosing cholangitis associated with ichthyosis: a tight junction disease

BACKGROUND AND AIMS

Most human and animal cholestatic disorders are associated with changes in hepatocyte cytoskeleton and tight junctions (TJs). These changes are usually secondary and nonspecific phenomena, both in intra- and extrahepatic cholestasis. Recently, missense mutations in TJ protein 2 (ZO-2) have been identified in patients with familial hypercholanemia. In the liver, TJs separate bile flow from plasma and are composed of strands of claudins and occludin. We previously assigned a syndrome associating ichthyosis and neonatal sclerosing cholangitis (NISCH syndrome) to chromosome 3q27-q28. We considered claudin-1 to be a strong candidate gene based on its mapping to the minimum interval and on the expression pattern of the mouse ortholog.

METHODS

The 4 exons and intron-exon junctions of claudin-1 gene were amplified using standard polymerase chain reaction protocols and specific primers. Western blot analysis on cultured fibroblasts and immunohistochemistry on liver tissue section were performed using rabbit anti-claudin-1 antibodies.

RESULTS

We described in 4 patients, of 2 inbred kindred of Moroccan origin, a 2-bp deletion (200-201 TT) in exon 1 of the claudin-1 gene arising in a premature stop codon and resulting in total absence of claudin-1 protein in the liver and skin.

CONCLUSIONS

Lack of claudin-1 in NISCH syndrome may lead to increased paracellular permeability between epithelial cells. Bile duct injury may be related to the absence of claudin-1 expression in cholangiocytes. Our observation, in conjunction with ZO-2-associated hypercholanemia, emphasizes the role played by TJ components in hereditary cholestasis.

Progress in understanding GJB2-linked deafness

Mutations in the GJB2 gene (encoding for Connexin 26 protein) represent a leading cause of genetic hearing impairment. Extensive epidemiological and molecular studies have been reported, describing GJB2 mutations type, frequency and distribution. Moreover, several aspects of GJB2 mutations pathogenic effects have been elucidated taking advantage of in vitro and in vivo experimental approaches. Progress through reported studies is reviewed, highlighting recent major achievements in this field. Attention is focused on different unresolved questions regarding GJB2 deafness pathogenesis and genotype-phenotype relationships. Clarification of these important clues will significantly increase our understanding of the molecular basis of hearing loss and will improve the effectiveness of diagnosis and counselling of this frequent disease.

Altered gating properties of functional Cx26 mutants associated with recessive non-syndromic hearing loss

Connexins (Cx) form gap junctions that allow the exchange of small metabolites and ions. In the inner ear, Cx26 is the major gap junction protein and mutations in the Cx26-encoding gene, GJB2, are the most frequent cause of autosomal recessive non-syndromic hearing loss (DFNB1). We have functionally analyzed five Cx26 mutations associated with DFNB1, comprising the following single amino-acid substitutions: T8M, R143W, V153I, N206S and L214P. Coupling of cells expressing wild-type or mutant Cx26 was measured in the paired Xenopus oocyte assay. We found that the R143W, V153I and L214P mutations were unable to form functional channels. In contrast, the T8M and N206S mutants did electrically couple cells, though their voltage gating properties were different from wild-type Cx26 channels. The electrical coupling of oocytes expressing the T8M and N206S mutants suggest that these channels may retain high permeability to potassium ions. Therefore, deafness associated with Cx26 mutations may not only depend on reduced potassium re-circulation in the inner ear. Instead, abnormalities in the exchange of other metabolites through the cochlear gap junction network may also produce deafness.

Hexachlorobenzene, a dioxin-like compound, disrupts auditory function in rat

Hexachlorobenzene (HCB) is a dioxin-like compound widely distributed in the environment. In this study, we investigated the effects of HCB on the cochlea. Conscious free-moving rats were given HCB per os daily for 4 weeks at doses of 0.16, 4 or 16 mg/kg in olive oil, whereas the control group received olive oil only. The effects of HCB were evaluated at various time intervals, by measuring auditory nerve acoustic thresholds and plasma thyroid hormone concentration by radioimmunoassay. Histological evaluation involved surface preparation and scanning electron microscopy observations of cochlear hair cells. At a dose of 0.16 mg/kg, HCB induced no loss of acoustic sensitivity, whereas at 4 mg/kg, it induced cochlear sensitivity deficits at the mid-frequencies (2-16 kHz) with complete recovery once treatment was stopped. At a dose of 16 mg/kg, permanent threshold shifts were observed at all frequencies tested (from 1 to 32 kHz). Morphological studies showed no cochlear hair cell loss or alteration of stereocilia. HCB treatment reduced circulating thyroxine concentrations. Thyroidectomy had no effect on cochlear sensitivity in control animals. Thus, HCB is a potent oto-toxicant, and its ototoxicity may be independent of its thyroidal effects.