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

Gap junctions

Gap junctions are aggregates of intercellular channels that permit direct cell-cell transfer of ions and small molecules. Initially described as low-resistance ion pathways joining excitable cells (nerve and muscle), gap junctions are found joining virtually all cells in solid tissues. Their long evolutionary history has permitted adaptation of gap-junctional intercellular communication to a variety of functions, with multiple regulatory mechanisms. Gap-junctional channels are composed of hexamers of medium-sized families of integral proteins: connexins in chordates and innexins in precordates. The functions of gap junctions have been explored by studying mutations in flies, worms, and humans, and targeted gene disruption in mice. These studies have revealed a wide diversity of function in tissue and organ biology.

Mutations in the connexin 29 gene are not a major cause of nonsyndromic hearing impairment in India

Mutations in the GJC3 gene are known to cause nonsyndromic hearing impairment (NSHI). In this study, we screened for mutations in the connexin 29 (Cx29) gene in peripheral blood collected from patients with NSHI. DNA was extracted from peripheral blood cells of 123 NSHI patients and 127 normal-hearing control subjects. Coding regions of Cx29 were amplified by polymerase chain reaction using primer pairs flanking both exons. Sequences were analyzed and compared with the published Cx29 sequence. On comparison with control subjects, only one patient and her normal-hearing mother showed a novel heterozygous variant in exon 1 c.569T>A (p. Ile190Asn), which most likely represents a rare polymorphism. From the study, we conclude that mutations in the Cx29 gene do not play a role in the causation of NSHI in Indian population.

Mutation R184Q of connexin 26 in hearing loss patients has a dominant-negative effect on connexin 26 and connexin 30

Hearing impairment is the most common sensory disorder worldwide. In a recent study, the authors have shown that a heterozygous missense mutation, p.R184Q, in the connexin 26 (Cx26) is causally related to hearing loss. However, the functional change in the Cx26R184Q mutant remains unknown. This study compared the intracellular distribution and assembly of mutant Cx26R184Q with that of the wild-type (WT) Cx26 and Cx30WT in tet-on HeLa cells and the effect that the mutant protein had on those cells. Fluorescent localization assay of WT Cx26 showed the typical punctuate pattern of gap junction channel between neighboring expression cells. Conversely, the p.R184Q missense mutation resulted in accumulation of the Cx26 mutant protein in the Golgi apparatus rather than in the cytoplasmic membrane. Cx26R184Q coexpressed with either Cx26WT or Cx30WT showed perinuclear localization by bidirectional tet-on expression system, suggesting the impairment of the ability of both WT proteins to intracellular trafficking and targeting to the plasma membrane. Therefore, we proposed that Cx26R184Q has a dominant-negative effect on the function of WT Cx26 and Cx30.

Identification of novel variants in the TMIE gene of patients with nonsyndromic hearing loss

OBJECTIVE

To determine whether variants of the TMIE gene are causes of nonsyndromic deafness in Taiwan.

METHODS

A genetic survey was made from 370 individuals, with 250 nonsyndromic hearing loss and 120 normal hearing individuals. Genomic DNA was extracted from peripheral blood leukocytes and then subjected to PCR to amplify selected exons and flanking introns of the TMIE gene; the amplified products were screened for base variants by autosequence. Data from the two groups were then compared using Fisher's two-tailed exact test and Armitage's trend test.

RESULTS

The analysis revealed 7 novel variants in the TMIE gene. Of the 7 variants, 5 variants were found in both nonsyndromic hearing loss and normal hearing group. Both allelic and genotype frequencies of these sequence changes did not differ significantly between patients and controls (P>0.05). However, a missense variant (c.257G>A) and one promoter variant (g.1-219A>T) were found in two patients with nonsyndromic hearing loss. Family study and microsatellite analysis found that c.257G>A variant is not inherited from his parents. The c.257G>A variant encodes a protein with glutamine at position 86 instead of arginine (p.R86Q), a residue that is conserved in mammals but different in fish, and predicted to be extracellular.

CONCLUSIONS

Despite the fact that the frequency of TMIE variants in our study subjects was low, we suggested that c.257G>A (p.R86Q) variant is a de novo and may be as a risk factor for the development of hearing loss in Taiwanese.

Novel mutations in the connexin43 (GJA1) and GJA1 pseudogene may contribute to nonsyndromic hearing loss

Connexins (CXs), a large family of membrane proteins, are key components of gap junction channels. Among a cohort of patients with nonsyndromic hearing loss, we have recently identified three novel missense mutations in the GJA1 gene and GJA1 pseudogene (rhoGJA1) as likely being causally related to hearing loss. However, the functional alteration of CX43 caused by the mutations of GJA1 and rhoGJA1 gene remains unclear. This study compares the intracellular distribution and assembly of three CX43 mutants expressed in HeLa cells with their wild-type (WT) counterparts and the effects of the mutant proteins on those cells. Localization assay of WT CX43 reveals a typical punctuate fluorescence pattern of a gap junction channel between neighboring expression cells. Additionally, immunoblotting analysis of the transfectants confirms the production of mutant proteins, in which their distributions along appositional membranes are determined using immunofluorescent staining procedures. Furthermore, dye transfer assay results demonstrate that gap junctional intercellular communication is less in HeLa cells carrying mutant GJA1 or rhoGJA1 gene than in WT-expressing cells. The results of this study suggest that the three mutations in GJA1 or rhoGJA1 that we previously reported result in at least partial loss of normal functions carried out by CX43, which may form a basis for the mechanism contributing to hearing loss in patients.

Gap junctions in inherited human disease

Gap junctions (GJ) provide direct intercellular communication. The structures underlying these cell junctions are membrane-associated channels composed of six integral membrane connexin (Cx) proteins, which can form communicating channels connecting the cytoplasms of adjacent cells. This provides coupled cells with a direct pathway for sharing ions, nutrients, or small metabolites to establish electrical coupling or balancing metabolites in various tissues. Genetic approaches have uncovered a still growing number of mutations in Cxs related to human diseases including deafness, skin disease, peripheral and central neuropathies, cataracts, or cardiovascular dysfunctions. The discovery of a growing number of inherited human disorders provides an unequivocal demonstration that gap junctional communication is crucial for diverse physiological processes.

Application of SNaPshot multiplex assays for simultaneous multigene mutation screening in patients with idiopathic sensorineural hearing impairment

OBJECTIVES/HYPOTHESIS

To develop a cost-effective and robust genetic diagnostic tool for patients with idiopathic nonsyndromic sensorineural hearing impairment.

STUDY DESIGN

Development of a diagnostic tool and validation in a prospective cohort.

METHODS

Twenty common sequence variants in GJB2, SLC26A4, and the mitochondrial 12S rRNA gene were selected based on our previous epidemiological study. These variants were analyzed using the SNaPshot technique. The efficacies of the SNaPshot multiplex assays were determined by using a prospective cohort composed of 214 unrelated Taiwanese patients with idiopathic sensorineural hearing impairment. The results of the assays were compared to the results obtained by direct sequencing.

RESULTS

We developed a diagnostic technique consisting of two consecutive panels of SNaPshot multiplex assays, with each panel screening 10 common sequence variants. Theoretically, this design can detect more than 98% of the known deafness-associated sequence variants in Taiwanese individuals. A total of 126 (58.9%) patients were diagnosed as having at least one sequence variant using the SNaPshot multiplex assays. In total, the SNaPshot assays yielded an accuracy of more than 99%.

CONCLUSIONS

The strengths of SNaPshot multiplex assays include high accuracy, high sensitivity, high flexibility (the examination panel can be easily expanded for additional mutations), low cost (less than US $10 per patient), and easy implementation for any institute with a DNA sequencer. Although only 20 to 30 mutations can be examined in two to three runs of the SNaPshot assay, this technology may be suitable for first-pass screening of deafness-associated mutations in populations with a relatively homogeneous ethnic background.

Identification of novel variants in the Myosin VIIA gene of patients with nonsyndromic hearing loss from Taiwan

OBJECTIVE

To determine whether variants of exons 7, 11, 22 and 28 of the MYO7A gene are causes of nonsyndromic deafness in Taiwanese.

METHODS

We screened a total of 331 unrelated Taiwanese individuals (age range, 4-22 years), including 231 patients with severe to profound nonsyndromic hearing loss and 100 individuals with normal hearing. Genomic DNA was extracted from peripheral blood leukocytes and then subjected to PCR to amplify selected exons and flanking introns of the MYO7A gene; the amplified products were screened for base mutations by autosequence. Data from the two groups were then compared using the chi-square (chi(2)) test.

RESULTS

The analysis revealed six variants in 3 out of 4 screened exons and flanking intronic sequences of the MYO7A gene (exons 7, 11, and 22). Three missense variants were found only in patients with hearing loss and were heterozygous, including Arg206Cys, Arg206His and Thr381Met. A variant, c.IVS22+58G>A, was found in intron 22 of the MYO7A gene from both patients and control group. Allele frequencies of c.IVS22+58G>A were shown to be significant between the two groups using chi(2) test (P<0.05).

CONCLUSION

Our results indicate that Arg206 and Thr381 residues in the motor head region of MYO7A protein are critical sites and the mutations of these residues may lead to the development of nonsyndromic deafness.

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.

Connexin-caused genetic diseases and corresponding mouse models

The human and mouse genomes contain 21 and 20 connexin genes, respectively. During the last 10-year period, genetic research on connexins has been stimulated by two parallel approaches: first, the characterization of genetic diseases that are caused by connexin mutations and, second, the generation and characterization of connexin knockout (null) mutated mice in which the coding region of nearly all connexin genes has been deleted. We summarize the current results of each of these two approaches. More recently, first results have been published in which connexin point mutations in human connexin genes were inserted at the corresponding position of the orthologous mouse gene. Under these conditions, the mutated connexin protein is expressed, in contrast to a connexin null mutation, and its interaction with other connexin isoforms or other connexin-binding proteins can be maintained. In this review, we discuss advantages and problems of such an approach and possible implications regarding the mechanism of the disease. The long-term goal is to understand the biologic function of each connexin isoform and the contribution of these proteins to the physiology of the corresponding organs in health and disease.

Gap-junction channels dysfunction in deafness and hearing loss

Gap-junction channels connect the cytoplasm of adjacent cells, allowing the diffusion of ions and small metabolites. They are formed at the appositional plasma membranes by a family of related proteins named connexins. Mutations in connexins 26, 31, 30, 32, and 43 have been associated with nonsyndromic or syndromic deafness. The majority of these mutations are inherited in an autosomal recessive manner, but a few of them have been associated with dominantly inherited hearing loss. Mutations in the connexin26 gene (GJB2) are the most common cause of genetic deafness. This review summarizes the most relevant and recent information about different mutations in connexin genes found in human patients, with emphasis on GJB2. The possible effects of the mutations on channel expression and function are discussed, in addition to their possible physiologic consequences for inner ear physiology. Finally, we propose that connexin channels (gap junctions and hemichannels) may be targets for age-related hearing loss induced by oxidative damage.

Genetics of congenital hearing impairment: a clinical approach

Hearing impairment (HI) is the most frequent sensory disorder, with a genetic etiology in >50% of all cases, due to mutations in >44 identified genes. Autosomal recessive inheritance explains the majority, with GJB2 (connexin 26) mutations accounting for 15-50% of paediatric HI. Delayed presentation of HI to 11-60 months in cases of biallelic GJB2 mutations is a concern, necessitating a good audiological follow-up in addition to neonatal hearing screening. Providing a genetic diagnosis in congenital HI has implications for the prognosis, the possible risk of associated medical manifestations, and precise genetic counseling of the family, and should be integrated into the medical examinations done in order to diagnose syndromic features. Large-scale mutation detection methods, such as micro arrays, are promising for wider genetic testing, but few studies on their clinical utility have been published, so far. Limitations of interpretation of genetic test results, combined with significant ethical issues, currently do not justify to institute genetic screening for GJB2 mutations in neonates before a diagnosis of HI is established.

Mouse models for human hereditary deafness

Hearing impairment is a frequent condition in humans. Identification of the causative genes for the early onset forms of isolated deafness began 15 years ago and has been very fruitful. To date, approximately 50 causative genes have been identified. Yet, limited information regarding the underlying pathogenic mechanisms can be derived from hearing tests in deaf patients. This chapter describes the success of mouse models in the elucidation of some pathophysiological processes in the auditory sensory organ, the cochlea. These models have revealed a variety of defective structures and functions at the origin of deafness genetic forms. This is illustrated by three different examples: (1) the DFNB9 deafness form, a synaptopathy of the cochlear sensory cells where otoferlin is defective; (2) the Usher syndrome, in which deafness is related to abnormal development of the hair bundle, the mechanoreceptive structure of the sensory cells to sound; (3) the DFNB1 deafness form, which is the most common form of inherited deafness in Caucasian populations, mainly caused by connexin-26 defects that alter gap junction communication between nonsensory cochlear cells.

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.