Supervisors Prof.Dr.Ahmad Sameh Farid Professor of ENT and Audiology Faculty of Medicine Cairo University

Transcription

1 Connexin 26 gene in non-syndromic hearing loss Thesis Submitted for partial fulfillment of MD degree in Audiology by Mona Ahmed El-Akkad M.B, B.CH., MSc. Supervisors Prof.Dr.Ahmad Sameh Farid Professor of ENT and Audiology Faculty of Medicine Cairo University Prof.Dr.Mohamed Ibrahim Shabana Professor of Audiology Faculty of Medicine Cairo University Prof.Dr.Maha Hassan Abou-Elew Professor of Audiology Faculty of Medicine Cairo University Prof. Dr.Olfat Gamil Shaker Professor of Medical Biochemistry Faculty of Medicine Cairo University 2012

2 بسم هللا الرحمن الرحيم و ههللا أ خ ر ج ك م م ن ب ط ون أ همه ات ك م ل ت ع ل م ون ش ي ئ ا و ج ع ل ل ك م ال هسم ع و ا ل ب ص ار و ا ل ف ئ د ة ل ع لهك م ت ش ك ر ون )سوره النحل ا ليه: ) 87

3 Acknowledgement I would like to express my gratitude to all my supervisors who gave me the possibility to complete this thesis. I would like to thank Professor Dr. Ahmad Sameh Farid, Professor of ENT, Cairo University for his deeply indebted supervision. He taught and trained me how to be a hard working student, not only that but also showed me how to consume time efficiently and useful to go to get the expected goal. Furthermore, he is a very understandable and patient professor. I would like to thank Professor Dr. Mohamed Ibrahim Shabana, Professor of Audiology, Cairo University: who gave me a great help, stimulating suggestions and encouragement helped me in all the time of research for and writing of this thesis. I would like to thank Professor Dr. Maha Hassan Abou Elew, Professor of Audiology, Cairo University: for her support, interest and encouragement to go ahead with my thesis. She always dedicates her time and efficiency in research valuable hints. I would like to thank Professor Dr. Olfat Gamil Shaker, Professor of Medical Biochemistry, Cairo University: I want to thank her for giving me all suitable materials to commence this thesis in the first instance, to do the necessary research work and to use departmental data, with her patient love enabled me to complete this work.

4 To.. Mom'Soul, Dad and my brothers Thank you

5 ABSTRACT Background: Mutations in GJB2 gene, encoding the gap-junction protein connexion 26 (Cx26) are the leading cause of deafness in autosomal recessive nonsyndromic sensorineural hearing loss (ARNSHL) and the 35delG mutation is the most common in many ethnic groups. Aim: To evaluate the extent of contribution of the 35delG mutation of GJB2 gene to ARNSHL in the Egyptian population as well as to correlate the clinical relevance of this mutation to the severity of hearing loss (HL). Methodology: Thirty seven patients with congenital ARNSHL and 30 control subjects with normal hearing were enrolled in the study. Patients had variable degrees of bilateral sensorineural hearing loss (SNHL) ranging in severity from mild to profound. Screening for 35delG mutation was performed by allele specific-pcr (AS-PCR). Results: The 35delG mutation was found in 7 patients (18.9 %), 5 homozygous and 2 heterozygous for this mutation. The frequency of the mutant allele among patients was 16.21% (12/74 chromosomes).genotype-phenotype relation of the 35delG mutation in our patients revealed that bilateral profound SNHL was detected in 5 patients: 3 homozygous and 2 heterozygous for the mutation. In the remaining 2 patients with homozygous alleles: one had severe and the other suffered from moderate SNHL. The allelic frequency of 35delG mutation among control group was 1.7% (1/60 chromosomes). Conclusion: The 35delG mutation of GJB2 gene is an important contributor to ARNSHL among Egyptian population. It has a considerable phenotypic variation on the degree of HL. Further studies are needed on larger scale to identify other mutations in GJB2 gene as well as other genes implicated in the development of ARNSHL. Keywords: Nonsyndromic autosomal recessive sensorineural hearing loss, GJB2, Connexin 26, 35delG mutation.

6 List of Abbreviations ABR Auditory brain stem response AD Autosomal dominant AN Auditory neuropathy AR Autosomal recessive ARNSHL Autosomal recessive non-syndromic hearing loss Array-CGH Microarray -Comparative Genomic Hybridisation AS-PCR Allele specific polymerase chain reaction ASSR Auditory steady state response ATP Adenosine triphosphate CAPD Central suditory processing disorders CLDN Claudin CMV Cytomegalovirus COCH Cochlin COL Collagen CT Computed tomography Cx Connexin db Decibel Del Deletion ddntp Dideoxy nucleotides DFN X-linked DFNA Autosomal dominant DFNB Autosomal recessive DFNB1 Autosomal recessive gene number 1 DGGE Denaturing gradient gel electrophoresis DHPLC Denaturing high performance liquid chromatography DNA Deoxy ribonucleic acid ECG Electrocardiogram EVA Enlarged vestibular aqueduct FISH Fluorescent insitu hybridization GJA1 Gap junction A1 subunit GJA3 Gap junction A3 subunit I

7 GJA4 Gap junction A4 subunit GJA5 Gap junction A5 subunit GJA6 Gap junction A6 subunit GJA7 Gap junction A7 subunit GJA8 Gap junction A8 subunit GJA10 Gap junction A10 subunit GJA12 Gap junction A12 subunit GJB2 Gap junction B2 subunit HHL Hereditary hearing loss HI Hearing impairement Hz Hertz IgM Immunoglobulin M IP3 Inositol triphosphate JCIH Joint Committee on infant Hearing KCNQ4 Potassium channel, voltage gated, subfamily Q, member 4 LVAS Large vestibular aqueduct syndrome MRI Magnetic resonance imaging NS Nonsyndromic NSHL Nonsyndromic hearing loss NSADHI Nonsyndromic autosomal dominant hearing impairement OHCs Outer hair cells OME Otitis media with effusion OTOF Otoferlin OTOA Otoancorin PCR Polymerase chain reaction PRES Prestin PTA Pure tone audiometry RNA Ribonucleic acid SNHL Sensorineural hearing loss SNP Single nucleotide polymorphism SSCP Single stranded conformational polymorphoism STRC stereocilin II

8 SNHL Sensorineural hearing loss TECTA Tectorin TMIE Transmembrane inner ear TORCH Toxoplasma gondii, other viruses, rubella, CMV and herpes simplex UNS Universal neonatal screening WFS1 Wolframin 35delG Deletion of Guanine at position delT Deletion of Thymine at position delC Deletion of Cytosine at position 235 III

9 Glossary Allele One of two or more alternate forms of a gene. Candidate gene A gene that, on the basis of known properties or protein product, is thought to be the gene causing a specific genetic disease. Carrier An individual who has a copy of a disease-causing gene but does not express the disease. The term is usually used to denote heterozygotes for a recessive disease gene. cdna A collection of segments of cdna cloned into vehicles such as plasmid. Characteristic An attribute or feature. trait Chromosome Tread like structure (literally "coloured body") consisting of chromatin. Genes are arranged along chromosomes. Codon A group of three mrna bases, each of which specifies an amino acid when translated. Compound heterozygote An individual who is heterozygous for two different disease-causing mutations at a locus. Compound heterozygous for recessive disease mutations are usually affected with the disorder. Congenital Present at birth. Deletion The loss of chromosome material. DNA (deoxyribonucleic acid) A double helix molecule that consists of a sugar phosphate backbone and four nitrogenous bases (A, C, G AND T). DNA bases encode messenger RNA (mrna), which in turn encodes amino acid sequences. DNA sequence The order of DNA bases along a chromosome. Dominant Allele that is expressed in the same way in single copy (heterozygotes) as in double copy (homozygotes). Elecetrophoresis A technique in which charged molecules are placed in a medium and exposed to an electric field, causing them to migrate through the medium at different rates, according to charge, length, or other attributes. Enhancer Regulatory DNA sequence that interacts with specific transcription factors to increase the transcription of genes. Exons Proteins of genes that encode amino acids and are retained after the primary mrna transcript is spliced. Founder effect A large shift in gene frequencies that results when a small "founder" population, which contains limited genetic variation, is derived from a larger population. It can be considered a special case of genetic drift. Gene A genetic factor (region of DNA) that helps determine a characteristic Gene family A group of genes that are similar in DNA sequence and have evolved IV

10 Gene therapy Genetic code Genetic counselling Genotype Genetic mapping Heterozygote Homozygote Intron Linkage Locus Map Marker Mendelian Messenger RNA(mRNA) Mitochondria Modifier gene Molecular genetic Mutation Mutation screening Nucleotide Penetrance Phenotype Point mutation Polymerase chain reaction (PCR) from a single common ancestral gene; may or may not be located in the same chromosome gene. The insertion or alteration of genes to correct a disease. The combinations of mrna codons that specify individual amino acids/ rules that determine which triplet of nucleotides codes for amino acids during protein synthesis. The delivery of information about genetic diseases (risk, natural history, management) to patients and their families. Set of alleles that an individual possesses The ordering of genes on chromosomes according to recombination frequency. An individual possessing two different alleles at a locus. An individual possessing two of the same alleles at a locus. DNA sequence found between two exons and spliced out in the formation of the mature mrna transcript. Describes two loci that are located close enough on the same chromosome that their recombination frequency is less than 50%. Specific place on a chromosome occupied by an allele. A chart showing the positions of genetic and/ or physical markers in a genome. A distinctive feature on a genome map. Referring to Gregor Mendel, descrives a trait that is attributable to a single gene. RNA molecule that is formed from the transcription of DNA. It proceeds to the cytoplasmwhere translated into amino acid sequence. Cytoplasmic organelles that are important in cellular respiration. They are their own unique DNA. A gene that alters the expression of a gene at another locus. Study the structure and function of genes at the molecular level. An alteration in DNA sequene. A set of techniques for determining if a DNA molecule contains a specific mutation. A basic unit of DNA or RNA, consisting of one deoxyribose, one phosphate group, and one nitrogenous base. In a population, the proportion of individuals possessing a disease causing genotype who express the disease phenotype. When this proportion is less than 100%, the disease genotype is said to have reduced or incomplete penetrance. The appearance or manifestation of a character. A mutation that results from a single nucleotide change in a DNA molecule. A technique for amplifying a large number of copies of a specific DNA sequence flanked by two oligonucleotide primers. The DNA is alternately heated and cooled in the presence of DNA polymerase and V

11 Polymorphism Primer Recessive Recurrence risk Ribonucleic acid (RNA) Ribosome Single stranded Sporadic Stop codon Syndrome Thymine Transcription Transcription factor Translation free nucleotides so that the specified DNA segment is denatured, hybridized with primers, and extended by DNA polymerase. A locus in which two or more alleles have gene frequencies greater than 0.01 in a population. An oligonucleotide sequence that flanks either side of the DNA to be amplified by PCR. An allele that is phenotypically expressed only in the homozygous state. The recessive allele is masked by a dominant allele when the two occur together in a heterozygote. The prpbability that another affected off-spring will be produced in families in which one or more affected offspring have already been produced. A single stranded molecule that consists of a sugar, phosphate group, and a series of bases (A, C, G and T). There are three basic types of RNA: messenger RNA, ribosomal RNA and transfer RNA. The site of mrna translation into amino acids. A single gene The occurance of a disease in a family with no apparent genetic transmission pattern (often the result of a new mutation). mrna base triples that specify the point at which translation of the mrna ceases. A pattern of multiple primary malformations or defects all due to a single underlying cause. One of the four DNA bases. The process in which mrna sequence is synthesized from a DNA template. Protein that binds to DNA to influence and regulate transcription. The process in which an amino acid sequence is assembled according to the pattern specified by the mature mrna transcript. VI

12 List of Figures Fig. (1) Principles of Mendel s first law of segregation of heritable characters for a dominant trait. Fig. (2) Schematic presentation of connexin topology 22 Fig. (3) Membrane topology of connexins 28 Fig. (4) Chromosomal localization of connexin genes. Diagram of the human chromosomes in which the distribution and localization of connexin genes has been indicated. The genes are designated by their protein names. Fig. (5) Structure of the connexin genes 31 Fig. (6) Location of the different components of the cochlea 34 Fig. (7) Environmental and genetic contributions to total congenital SNHL 39 Fig. (8) Guidelines for evaluating patients with HHL. 48 Fig. (9) Audiological assessment 52 Fig. (10) Care plan for management of progressive SNHL 53 Fig. (11) UV Transilluminator 64 Fig. (12) Apparatus Corporation 64 Fig. (13) Agarose gel electrophoresis 65 page Fig. (14) Fig. (15) Fig. (16) Fig. (17) Fig. (18) Percentage distribution of sex and 35delG data of study versus control groups Percentage distribution of consangunity and 35 del G data of study versus control groups Percentage distribution of family history and 35delG data of study versus control groups Percentage distribution of tinnitus and 35delG data of study versus control groups Percentage distribution of vertigo and 35delG data of study versus control groups Fig. (19) Course of hearing loss of patients included in the study 72 Fig. (20) Degree of hearing loss of patients included in the study 72 Fig. (21) Configuration of audiogram of patients included in the study 73 Fig. (22) Percentage distribution of demographic and 35delG data of study versus 75 VII

13 control groups Fig. (23) Sex of patients with negative versus positive 35delG mutation 78 Fig. (24) Family history of patients with negative versus positive 35delG mutation 78 Fig. (25) Consangunity of patients with negative versus positive 35delG mutation 79 Fig. (26) Tinnitus of patients with negative versus positive 35delG mutation 79 Fig. (27) Vertigo of patients with negative versus positive 35delG mutation 80 Fig. (28) Course of hearing loss of patients with negative versus positive 35delG 80 mutation Fig. (29) Degree of hearing loss of patients with negative versus positive 35delG 81 mutation Fig. (30) PTA configuration of patients with negative versus positive 35delG 82 mutation Fig. (31) Characteristic of patients with negative versus positive 35delG mutation 82 Fig. (32) Description of detailed data of patients with 35delG mutation of GJB2 gene 84 Fig.(33) Distribution of the degree of sensorineural hearing loss of patients included in the study in relation to the presence of the 35delG mutation 85 VIII

14 List of tables page Table (1) Genetic testing for nonsyndromic hearing impairment 16 Table (2) non syndromic hereditary hearing loss: Genes (as identified 21 by the end of 2001) according to their function and type of inheritance. Table (3) List of human connexins 26 Table (4) AD genes identified to date 35 Table (5) AR genes identified to date 36 Table (6) Risk factors from birth to 28 days according to Joint Committee on Infant Hearing JCIH, (2000) 38 Table (7) Percentage distribution of demographic data of study versus control groups 68 Table (8) Audiological clinical data of patients included in the study 71 Table (9) Distribution of 35delG mutation in the study versus control group 74 Table (10) Characteristic of patients with negative versus positive 35delG mutation 77 Table (11) Description of detailed data of patients with 35delG mutation of GJB2 gene 83 IX

15 Contents Introduction & Rationale 1 Aims of the work 4 Chapter (1): Introduction to genetics 5 Chapter (2) : Genetic hearing loss 10 - Genes involved in the homeostasis of the cochlea 14 - Genes involved in the structure &function of the hair cell 15 - Genes with atypical or poorly understood function 18 - Modifier genes for HI 19 Chapter (3): Connexins 22 - The Connexin family 22 - Life Cycle of Connexins 23 - Nomenclature 24 - Domain Structure 28 - Genomic Organization 29 - Connexin Gene Structures 30 - Connexins in the inner ear 31 - Gap junctions networks in the inner ear 32 - Connexin Mutations and Deafness : 33 - GJB2 (Connexin26) 34 - Connexin future 37 Chapter (4): Diagnosis and Management 38 - Risk factors 38 - History 40 - Clinical diagnosis 42 X

16 - Examination 43 - Investigations 44 - Diagnostic applications 48 - Differential Diagnosis 50 - Management 50 - Audiological assessment 52 - Surgical management 53 - Genetic counseling 57 - Gene therapy 58 - Prognosis 56 - Genetic Counseling 57 - Gene Therapy 58 - Prevention of NSHL 58 Patients and Methods 60 Results 67 Discussion 86 Conclusions and Recommendations 94 References 95 Summary 109 XI

17 Introduction Deafness, the inability to hear, is the most common sensory deficit in human populations with both genetic and environmental etiologies. It is estimated that it affects 1 in 1000 child births of which approximately 60% cases are attributed to genetic factors (Marazita et al., 1993). The etiology of hereditary hearing loss is extraordinarily complex. More than 400 genetic syndromes are associated with hearing loss and more than 140 genetic loci associated with nonsyndromic hearing loss have been mapped, with more than 60 genes identified to date. Hereditary hearing loss can be inherited as an autosomal dominant, autosomal recessive, X- linked or mitochondrial (maternally inherited) condition (Alford, 2011). Autosomal recessive non-syndromic hearing loss (ARNSHL) is the most frequent cause of hereditary defafness and often exhibits the most severe hearing phenotype. To date, 46 genes have been identified that cause nonsyndromic hearing loss, making it an extremely heterogeneous trait (Hilgert, 2009a). Presently, 85 recessive deafness (DFNB) loci have been registered and 46 of the corresponding genes have been identified as documented on the Hereditary Hearing Loss Homepage Interestingly, even though a substantial number of genes are known to cause ARNSHL, mutations at the first identified DFNB1 locus, account for up to 50% of all cases with this diagnosis in various populations. DFNB1 contains the GJB2 and GJB6 genes that respectively code for connexin 26 (C 26) and connexin 30 (C 30), which are gap junction binding proteins, most abundantly expressed in the cochlea. Although over 100 mutations have been reported in GJB2 (The Connexin-deafness homepage: a single mutation, c.35delg, is the most common cause of ARNSHL and can account for up to 85% of DFNB1 in various populations (Kenneson et al., 2002). 1

18 The overlapping audiologic phenotypes associated with many genes and the variability and/or reduced, sometimes age-related, penetrance of some phenotypic features of syndromic hearing loss can complicate the distinction between various genetic causes of nonsyndromic hearing loss and between nonsyndromic and syndromic hearing loss, especially in childhood. Testing for individual genes associated with nonsyndromic hearing loss, beyond GJB2 which encodes Connexin 26, can become expensive and, without specific phenotypic features to guide selection of genes for testing (such as enlarged vestibular aqueducts, low frequency hearing loss or auditory neuropathy), it is not likely to yield an etiology. Advances in DNA sequencing and the rapid decline in the cost of sequencing presage the availability of testing that can identify the etiology in the majority of cases of genetic hearing loss. However, until comprehensive genetic testing of hearing loss is clinically available and cost-effective, thorough phenotypic and audiologic evaluation and careful documentation of risk factors, infectious exposures and patient and family medical history will continue to be important to efforts directed toward etiologic diagnosis. The complexities associated with interpretation of genetic test results, genetic counseling and genetic risk assessment make consultation with medical geneticists important for many patients (Alford, 2011). Rationale Previous studies in Egypt (Mustafa, 2004, Abou-Elew et al., 2006, Mohamed et al., 2010 and Scnoeckx, 2005a) were sporadic and reported variable prevalence of mutation of the GJB2 gene in patients with NSHL as well as among Egyptian population. A mutation in this gene does not occur in the same frequencies across different ethnic populations. A higher carrier frequency of 35delG mutation was found in Southern Europe (Van Laer et al., 2001, Davis 2

19 1997, Hilgert et al., 2009a and Dalamón et al., 2005). The prevalence of this mutation with its correlation to the degree of hearing loss is not completely understood. This encourages us to accomplish this work. 3

20 Aims of the work 1-To detect 35delG mutation in children and adults with early onset of non syndromic autosomal recessive SNHL 2- Mutation correlation with degree and configuration of audiogram. 4