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Osu1243967282.Pdf (495.5 A CASE FOR ETIOLOGIC FOCUS IN AUDIOLOGY – GENETIC TESTING Thesis Presented in Partial Fulfillment of the Requirements for The Masters in Speech and Hearing Science in the Graduate School of The Ohio State University By Shelley Bloom, B.S. ***** The Ohio State University 2009 Capstone Committee: Approved by Christina Roup, PhD, Advisor Gail Whitelaw, PhD Christy Goodman, AuD _________________________ Advisor Speech & Hearing Sciences Graduate Program ABSTRACT Genetic testing can determine the etiology for hearing losses of genetic origin. The genes for many common forms of hearing loss and some uncommon forms have been discovered and mapped. When the cause of the hearing loss is known, the treatment can be targeted and enables more accurate, complete information regarding the prognosis. Therefore, it would be beneficial for audiologists to include a referral for genetic testing as a routine part of an audiologic evaluation. However, genetic counseling is crucial for enabling patient understanding of the genetic information and comprehending the benefits and risks of genetic testing. This fits into the goal of autonomy for the patient or caregivers by providing enough information to make an informed decision about treatment for the hearing loss. ii DEDICATION To my family and friends, who helped support me every step of the way. iii VITA April 12, 1979……………………………Born – TACOMA, WASHINGTON June, 2003………………………………..BACHELORS OF SCIENCE WITH DISTICTION, UNIVERSITY OF WASHINGTON – Minors in Japanese and Music 2007-2008………………………………..INFANT HEARING PROGRAM GRADUATE INTERN The Ohio Department of Health iv FIELDS OF STUDY Major Field: Speech & Hearing Sciences with specialization in molecular genetics v TABLE OF CONTENTS Page Abstract………………………………………………………………………………….... ii Dedication………………………………………………………………………………... iii Vita………………………………………………………………………………………..iv List of Abbreviations……………………………………………………………………...vii Introduction……………………………………………………………………………….. 1 Genetic Testing for Hearing Loss………………………………………………………… 3 GBJ2 Gene – Connexin 26………………...………………………………………………6 SCL26A4 Gene: Enlarged Vestibular Aqueduct and Pendred Syndrome...……………..12 Mitochondrial DNA Hearing Loss……………………………………………………….20 Mitochondrial Multisystemic Syndromes……………………………………………......22 Mitochondrial DNA Mutations and Non-Syndromic Hearing Loss……………………..26 Age Related Hearing Loss – Presbyacusis….…………………………………………...29 Usher Syndrome………………………………………………………………………….32 Neurofibromatosis Type 2………..……………………………………………………...37 Other Genetic Hearing Losses…………………………………………………………...41 Concerns and Objections About Genetic Testing…..…………………………………...43 Conclusion……………………………………………………………………………….49 References…………………………………………………………………………….….51 vi LIST OF ABBREVIATIONS ABI Auditory Brainstem Implant ABR Auditory Brainstem Response AN Auditory Nerve ARHL Age Related Hearing Loss ASL American Sign Language ATP Adenosine Triphosphate Bp Base Pair - CHO3 Bicarbonate CI Cochlear Implant CMV Cytomegalovirus CR Calorie Restriction CT Computed Tomography DPOAE Distortion Product Otoacoustic Emissions ENT Ear, Nose and Throat ERG Electroretinogram EVA Enlarged Vestibular Aqueduct GFP Green Fluorescent Protein GJB2 Gap Junction β 2 IAM Internal Acoustic Meatus INC Inner Hair Cells MELAS Mitochondrial Encephalopathy, Lactic Acidosis, and Strokelike Episodes MERRF Myoclonic Epilepsy and Ragged Red Fibers MIDD Maternally Inherited Diabetes and Deafness mL Milliliter MRI Magnetic Resonance Imaging mtDNA Mitochondrial Deoxyribonucleic Acid mV Millavolt NF2 Neurofibromatosis Type 2 ns-EVA Non-syndromic Enlarged Vestibular Aqueduct OHC Outer Hair Cells OTOF Otoferlin PEO Progressive External Ophthalmoplegia PS Pendred Syndrome PTA Pure Tone Average ROS Reactive Oxygen Species RP Retinitis Pigmentosa vii SNHL Sensorineural Hearing Loss USH1B, C, D, F Usher Syndrome Type 1B, 1C, 1D, 1F USH2A Usher Syndrome Type 2A USH3A Usher Syndrome Type 3A WT Wild Type viii INTRODUCTION Hearing loss is one of the most common birth defects. The incidence of severe to profound hearing loss at birth in the United States is 0.8 per 1000, while moderate or unilateral hearing losses have an incidence of 1.1 per 1000 births. Genetic etiologies account for about half of the cases of pre-lingual hearing loss, which is defined as hearing loss present at birth or early childhood (Eisen & Ryugo, 2007). The remaining cases are attributed to environmental causes including pre- and post-natal infection, such as congenital rubella, cytomegalovirus (CMV), meningitis, along with perinatal complications and other factors, such as prematurity, anoxia, low birth weight, hyperbilirubinemia, ototoxic drugs, noise exposure and head injury (Nance & Dodson, 2007). Additionally, genetics can also cause adult-onset hearing loss. These post-lingual onset hearing losses have purely genetic causes, or may be brought about by a mixture of genetic susceptibility and environmental factors, such as noise exposure and medication. Even age-related hearing loss (ARHL) or Presbyacusis has a genetic component, resulting in detrimental anatomical and physiological changes to the auditory system (Seidman et al., 2004; Dai et al., 2004). Currently, audiologic testing is limited to determining the presence or absence of a hearing loss. Once a hearing loss is identified, the type, degree and configuration are 1 determined. All of this testing does not necessarily give definitive answers of why the patient has hearing loss. This results in limited information availability for the medical professional and patient in terms of prognosis and treatment for aural rehabilitation and medical intervention. Audiology should now move from primarily diagnosing and categorizing hearing loss to the possibility of including the cause of a hearing loss by including referrals for genetic testing as part of an audiologic evaluation. Genetic testing, in conjunction with audiologic testing and medical evaluation can determine the etiology in hearing impairment that has a genetic cause. When the exact gene is ascertained, more accurate, comprehensive information can be given regarding prognosis and treatment. Consequently, patients and families can make more informed decisions about the management of the hearing loss. 2 GENETIC TESTING FOR HEARING LOSS Etiology of Hearing Loss Genetic testing can provide a definitive reason for hearing loss with a genetic cause. Fifty to sixty percent of congenital hearing loss has a genetic cause and genetic factors could also play a part in adult-onset hearing loss (Arnos, 1997). Genetic testing also can enable specific information about the prognosis and clinical course of the hearing loss (Arnos, 2003). Genetic testing is non-invasive (Schrijver, 2004; Liu et al., 2008) and can be obtained with buccal smears (Torkos et al., 2006). Once a genetic diagnosis is reached, future diagnostic invasive, costly or painful procedures may be reduced or eliminated (Kimberling & Lindenmuth, 2007) such as tests for diagnosing prenatal infections, electrocardiograms, thyroid testing, magnetic resonance imaging (MRI), computed tomography (CT) scans of inner ear structures and electroretinograms (ERG) (Arnos, 2003; Cohen, Bitner-Glindzicz, & Luxon, 2007). Treatment Planning Knowledge of the etiology of the hearing loss allows for timely, accurate treatment planning. For syndromic hearing losses, such as Usher Syndrome and Neurofibromatosis 2, genetic testing also yields relevant medical information and any accompanying conditions to the hearing loss. This awareness of syndromic hearing loss enables appropriate medical referrals, care and support services (Arnos, Cunningham, 3 Israel, & Marazita, 1992). It is important to monitor known medical complications that may arise in the present and in the future, such as ocular or renal deficiencies (Bitner- Glidzicz, 2002) along with the hearing loss. The genetic diagnosis assists the decision on whether the hearing loss would be better treated with medical management, hearing aids or both. When the etiology of the hearing loss is known, planning for education and communication needs can be anticipated and made early in life. Patients and family members can evaluate the educational facilities and support programs available in the area of residence. Additionally, social and employment opportunities vary across the country and may affect where they choose to live. An option may be to relocate for medical or educational possibilities (Arnos, Cunningham, et al., 1992). Once the genetic cause of hearing loss is established, the inheritance pattern, such as dominant, recessive, or mitochondrial can be determined, thereby addressing concerns regarding whether hearing loss or other associated conditions will be passed on to siblings, children and grandchildren (Arnos, Israel, Devlin, & Wilson, 1992). The reoccurrence risk is an estimate derived from empirical data of the likelihood that the progeny inherit the hearing loss. This estimate can be different from the actual reoccurrence risks when the genetic cause is known. Empirically, normal hearing parents with one deaf or hard-of-hearing child have a 5 to 17% chance of having another child who is deaf or hard-of-hearing. If one parent is deaf or hard-of-hearing, the chance is 7 to 20% that the child will be depending if the other parent is deaf. However, actual reoccurrence risks range from less than 1% to more than 50% or even 100% (Brunger, 4 Matthews, Smith, & Robin, 2001). The etiology allows for more precise estimate of recurrence risk counseling (Brunger et al., 2001).
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