G. Shashidhar Pai, MD MUSC Children s Hospital Department of Pediatrics Division of Genetics
One of every 500 newborns has bilateral permanent sensorineural hearing loss 40 db which makes it the most common birth defect included in newborn screening By adolescence, prevalence increases to 3.5 per 1000
Conductive hearing loss results from abnormalities of the external ear and/or the ossicles of the middle ear. Sensorineural hearing loss results from malfunction of inner ear structures (i.e., cochlea) Mixed hearing loss is a combination of conductive and sensorineural hearing loss Central auditory dysfunction results from damage or dysfunction at the level of the eighth cranial nerve, auditory brain stem, or cerebral cortex.
The frequency of hearing loss is designated as: Low (<500 Hz) Middle (501-2000 Hz) High (>2000 Hz)
Severity Mild Moderate Moderately severe Severe Hearing Threshold in Decibels 26-40 db 41-55 db 56-70 db 71-90 db Profound 90 +
Heritable Causes Syndromic hearing impairment is associated with malformations of the external ear or other organs or with medical problems involving other organ systems. Over 400 syndromes have been delineated Nonsyndromic hearing impairment has no associated visible abnormalities of the external ear or any related medical problems; however, it can be associated with abnormalities of the middle ear and/or inner ear.
Autosomal dominant Autosomal recessive Pseudo dominant X-linked Mitochondrial
Approximately 50% of autosomal recessive nonsyndromic hearing loss can be attributed to the disorder DFNB1, caused by mutations in GJB2 (connexin 26) and GJB6 (connexin 30). The carrier rate in the general population for a recessive deafnesscausing GJB2 mutation is approximately 1 in 33.
Congenital, nonsyndromic sensorineural hearing loss consistent with autosomal recessive inheritance or Apparent "pseudo dominant" inheritance GJB2 and GJB6 mutations are most common (accounts for about 50% of DFNB) Usher syndrome should be suspected if GJB2 and GJB6 are negative or in the presence of retinitis pigmentosa. RP has onset later in childhood
Single-gene testing may be performed in a step wise fashion In children with congenital severe-to-profound nonsyndromic hearing loss and pedigree pattern consistent with AR or pseudo dominant inheritance sequencing GJB2 and GJB6 will be the first step If negative, reflex to Usher syndrome panel, particularly if developmental motor milestones for sitting and walking independently are delayed. If CT examination of the temporal bones discloses an enlarged/dilated vestibular aqueduct or Mondini dysplasia, consider mutation screening for Pendred Syndrome gene Multiple Gene screening of all known genes associated with AD or AR will be the next step
Locus Name Gene Symbol Onset Type Test Availability DFNB1 GJB2 Prelingual 1 Usually stable GJB6 DFNB2 MYO7A Prelingual, postlingual Unspecified DFNB3 MYO15 Prelingual Severe to profound; stable DFNB4 SLC26A4 Prelingual, postlingual Stable, progressive DFNB6 TMIE Severe to profound; Prelinqual DFNB7/11 TMC1 stable DFNB8/10 TMPRSS3 Postlingual 2, Prelingual Progressive, stable DFNB9 OTOF Prelingual Usually severe to profound; stable DFNB12 CDH23 Prelingual Severe to profound; stable DFNB16 STRC Prelingual Severe to profound; stable DFNB18 USH1C Prelingual Severe to profound; stable DFNB21 TECTA Prelingual Severe to profound; stable DFNB22 OTOA Prelingual Severe to profound; stable DFNB23 PCDH15 Prelingual Severe to profound; stable DFNB24 RDX Prelingual Severe to profound; stable DFNB25 GRXCR1 Prelingual Moderate to profound; progressive Research only DFNB28 TRIOBP Prelingual Severe to profound; stable DFNB29 CLDN14 Prelingual Severe to profound; stable DFNB30 MYO3A Prelingual Severe to profound; stable DFNB31 DFNB31 Prelingual DFNB32/82 GPSM2 Prelingual Severe to profound; stable Research only DFNB35 ESRRB Unknown Severe to profound DFNB36 ESPN Prelingual DFNB37 MYO6 Prelingual DFNB39 HGF Prelingual Severe to profound; downsloping DFNB49 MARVELD2 Prelingual Moderate to profound;
Gene Symbol Mutation Severity Penetrance MT-RNR1 961 different mutations 1494C>T 1555A>G 7445A>G Variable Highly variable, aminoglycoside induced MT-TS1 7472insC 7510T>C Highly variable 7511T MT-CO1 7444G>A Severe to profound Complete, aminoglycoside associated; associated with MT-RNR1 1555A>G
Finding % of Affected Individuals Sensorineural hearing loss 47%-58% Heterochromic irides 15%-31% Hypoplastic blue irides 15%-18% White forelock 43%-48% Early graying 23%-38% Leukoderma 22%-36% High nasal root 52%-100% Medial eyebrow flare 63%-73%
Gene Symbol Test Method Mutations Detected Mutation Detection Frequency by Test Method 1 Test Availability PAX3 Sequence analysis Deletion / duplication analysis 3 Sequence variants 2 >90% Exonic or whole-gene deletions ~6%
Cost: Individual gene tests may be expensive, often not covered by insurance, Medicaid restrictions on coverage for out of state testing. Situation improving Benefit: Establishing molecular diagnosis will allow 1. Accurate recurrence risk estimation, carrier detection 2. Prognostication 3. Prenatal or preimplantation genetic diagnosis for future pregnancies Limitation: Relatively few genotype phenotype correlations and therapeutic implications at this time
Technical Limitations Sequence analysis will not identify whole gene deletions Will require a separate order for deletion / duplication analysis Need to know the frequency of types of mutation based on prior studies No way to identify regulatory mutations / alterations or other epigenetic phenomenon Whole Exome Sequence does not capture all exons false negatives results are possible The gene responsible for a syndrome may not be known
Genetic Contribution to Hearing Loss is quite significant Molecular testing strategy requires a comprehensive approach that looks at Age o f onset, severity, type of hearing loss, inheritance pattern, presence or absence of syndromic features Knowledge of technical limitations of genetic testing and of rapidly changing technological innovations