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THE RECURRENCE OF DEAF CHILDREN IN FAMILIES IS HIGHER THAN TRADITIONALLY BELIEVED

A new study into genetic mutations found in deaf children and adults offers an opportunity for effective counseling to affected families

NEW ORLEANS--Since the 19th century, and before the work of Gregor Mendel, it has been known that deafness could run in families. Suspected causes at that time for the hearing loss included infection, intermarriage of near relations, dropsy of the brain, and even excessive use of rocking chairs. Some even believed that a deaf child could make mental impressions on an expectant mother.

A century later, medical science has advanced the explanation of deafness, but even today, the etiology, or cause, in over half of deaf patients remains unknown. In these circumstances, the reasons for deafness are listed as "sporadic" or "presumed hereditary." In these circumstances, it is difficult for the physician to provide effective counseling to the parents of a deaf child.

Determining the recurrence chance for deafness in another child has proven to be equally complicated. Early estimates of the cause for deafness due to genetic factors varied 20 to 76 percent. Traditionally, a recurrence chance of 9.8 percent was given to a family with a deaf child with no history of deafness in the family. However, this figure is based on surveys, some more than 100 years old, and therefore might not be relevant today.

Several dozen deafness genes are known to exist, but mutations in one gene, GJB2, are predominant in congenital and hereditary deafness. This gene encodes the gap junction protein Connexin 26 which is believed to be important for potassium recycling necessitated by the conversion of sonic energy to an electro-chemical signal which can be understood by the brain.

A team of otolaryngologist-head and neck surgeons collected data from their laboratory and others that revealed that this gene is found in half of all groups of siblings where more than one child is affected by deafness. This finding implies that half of hereditary deafness is due to mutations in this gene. More surprisingly, the researchers found that a single mutation in this gene, called 35delG, accounts for most of the mutations. According to the researchers, the prevalence of GJB2 mutations will enable more accurate genetic counseling data for deaf individuals and their families.

The authors of the study, "Recurrence Chance Determination for Congenital Deafness," are Glenn E. Green, MD, (presenter), Daryl A. Scott, MD, Wyman T. McGuirt, MD, and Richard J.H. Smith, MD, all from the University of Iowa Hospitals and Clinics, Department of Otolaryngology, Iowa City, IA.

The results were presented before the American Academy of Otolaryngology Head and Neck Surgery Foundation Annual Meeting and Oto Expo being held September 26-29 at the Ernest N. Morial Convention Center in New Orleans, LA. At the gathering, the Academy's 13,000 members will have the opportunity to hear the latest research in the diagnosis and treatment of disorders of the ear, nose, throat, and related structures of the head and neck.

Methodology.

To determine the prevalence of the deafness-causing mutations in the community at-large, the Iowa researchers screened 560 individuals for the most common mutation. Fifty two deaf patients, taken in sequence from different families, were screened for mutations in GJB2. Recurrence chances were then calculated through Bayesian analysis.

Results:

Key findings of the study included:

* Three percent of individuals in the community at-large carry mutations in GJB2 that can cause deafness. In that same group, 2.5 percent carry the most common mutation, 35delG.

* In families with two or more deaf children, 58 percent were found to have mutations in GJB2, and in families with only one deaf child, 28 percent were found to have mutations in GBJ2

* In families with one child, where that child is deaf, the probable recurrence chance to have another deaf child is 18 percent. The recurrence rate increases to 25 percent if deafness is due to GJB2 mutations and drops to 14 percent if GJB2-related deafness is excluded.

* The average chance for a deaf couple to have a deaf child is approximately 15 percent. However, the chance increases to 100 percent if both individuals have GJB2-related deafness but falls to five percent if there is GJB2-related deafness in one parent only and is about ten percent if neither parent has that genetic characteristic.

* The chance for the sibling of a deaf individual to have a deaf child is 25 percent if the person and their spouse are both carriers for GJB2-mutations, and less than one percent if the sibling's spouse is not a carrier or the sibling is not a carrier of the mutated gene. If a GJB2-mutation carrying sibling of a deaf person marries a deaf person with GJB2-related deafness, the recurrence chance would be 50 percent.

Conclusions:

Current epidemiologic trends state that the percentage of deafness which has a genetic cause is increasing. This study confirms that by identifying the genetic etiology of deafness, more precise counseling data can be provided to deaf individuals and their families.

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Editor's Note: For a copy of this study or an interview with Drs. Smith or Green, contact Ken Satterfield at 703-519-1563 (through 9-21); 9/23-29, call the Annual Meeting Â鶹´«Ã½room at 504-670-5409.