Auditory Genetics
Hearing impairment represents the most common sensory deficit in children. Sensorineural hearing loss (SNHL) or "nerve deafness" represents the most common form of congenital hearing impairment and a significant health care problem.
Over 40,000 children are born in the United States with significant hearing impairment, with about 4,000 of these profoundly deaf. The incidence is estimated at about 1 in every 1000 live births. Therefore, the otolaryngologist and parents will frequently be challenged with deciding on the appropriate diagnostic tests for these children.
About 50% of hearing impairment is genetic (familial) in origin. The Auditory Genetics Laboratory of the Ear and Hearing Center at Cincinnati Children's Hospital Medical Center functions to provide current state-of-the-art diagnostic and research testing for children with hearing loss and their families.
Molecular Genetic Testing
On the horizon of our diagnostic evaluation in children with SNHL is molecular genetic testing. The ability to unlock genetic information that can be attributed to deafness is truly a revolutionary development. Any means to diminish the number of afflicted individuals who remain undiagnosed would appear to offer a distinct improvement to current medical care.
Most patients with hearing loss have not other medically related problems (i.e. nonsyndromic) and have few other family members (i.e. recessive inheritance) with a similar problem. Autosomal recessive (a genetic pattern in which the parents are carriers and each give a mutation to the child) nonsyndromic hearing loss accounts for 70% of all genetic types and typically have a severe form of hearing loss.
Dominant forms (only one mutation is required as one parent is a carrier for the mutation and passes it along to the hearing impaired child) of hearing loss typically have at least several affected persons in the family and they are usually not completely deafened. Syndromic forms of hearing loss are generally easier to identify, because of the other medical problems the patients exhibit. Although, in several types of syndromes, the hearing loss may be the initial problem that brings the patient to the attention of a physician. Over 70 types of nonsyndromic hearing loss have been localized, with only a small number of these have actually been completely identified.
Family Research Testing
If initial evaluation and testing cannot find a cause for the hearing loss, the laboratory can also evaluate certain families with specialized genetics techniques. This research testing is easy to perform and can possibly determine the cause of a families hearing loss. This type of research requires all (or as many as possible) family members or parents to sign a consent form, obtain a hearing test and have a small amount of blood drawn to isolate their genetic material (DNA).
This service is provided at little or no expense to the family. The DNA is tested against known causes of hearing loss and if none is found, an extensive search is then begun to identify a new form of genetic hearing loss. The results of the study may take several months to several years to complete. A designated family member will be updated on any new or significant findings. As for our routine diagnostic testing, counseling on the results is provided by the Ear and Hearing Center. For more information, contact John Greinwald, MD.
The Auditory Genetics Laboratory is currently engaged in research projects seeking the identification of the gene for DFNB17 (a rare cause of nonsyndromic recessive deafness), the interaction with mitochondrial mutations and animoglycoside antibiotic exposure.
Gene chip
Our project would represent the first large scale effort to utilize microarray based resequencing technology (VDA) in a translational format. When successfully constructed and validated, the Ear and Hearing Center at Cincinnati Children's would become the world leader in the diagnosis of pediatric sensorineural hearing loss (SNHL).
With the support of the Cincinnati Children's TRI grant, a VDA was developed with genes likely to be involved in early childhood hearing loss. Genes were prioritized based on the likelihood to cause newborn / infant SNHL.
Thirteen genes were selected good candidate genes and eight were placed on the first generation chip due to size constraints (GJB2, GJB6, MYO7A, OTOF, SLC26A4, KCNE1, KCNQ1, CDH23). We are currently partnering with Harvard to expand our ability to screen for these 13 good candidate genes.
In collaboration with Bioinformatics Division at Cincinnati Children's and Affymetrix Corporation, gene chips were constructed. A six-subject integrity study has been completed which showed the PCR and hydridization protocols to be successful (Figure).
Using the Affymetrix data interpretation software, approximately 80% of the base pairs could be called. When the "no calls" were directly inspected on the chip, over 75% of them could be clearly interpreted. Overall, a 95-98% call rate was observed in the integrity study. Although, no single PCR reaction was found to be consistently deficient, modifications are currently being made to several reactions to better optimize the VDA hybridization.
Figure. Example of a VDA result with a base pair call of C
These findings clearly show that the VDA has great potential to interrogate large amounts of DNA with a high accuracy. In collaboration with Bioinformatics at Cincinnati Children's, we are developing our own user-friendly software to enhance our base call rates.
In addition, these data highlight the importance of improving our ability to call bases beyond the capability of the Affymetrix software. The development of deafness VDA specific software will not only aid in the interpretation of base calls, but will be required to produce a clinically significant interpretation. Validation studies are currently beginning to compare VDA accuracy to standard sequencing.
