One in every 10 Americans is hearing impaired. There are many causes of hearing impairment; including loud music, loud sounds, antibiotics, and accidents. However, for many of the 28 million cases of hearing loss, genetics is frequently an undiagnosed cause. It is believed that up to 60% of hearing loss is genetic in origin. Many of the patients with family history of hearing loss are never examined for possible hereditary deafness or hearing loss. Hereditary hearing loss is becoming progressively important since early identification can yield better rehabilitative results. That is, hearing aids can be fit earlier, or medical and surgical treatment can yield better hearing sooner in life.

Genetic researchers have identified more than 50 genes that can cause deafness when they are defective, but each of those accounts for only a very small percentage of the cases. The major genetic causes of hearing loss have remained elusive. In the late 1990's, four major genetic causes of hearing loss have been identified and at least one of them may be responsible for a large portion of inherited hearing loss.

Genetic screening at birth may be the single most important test for deafness yet to be developed.

Two major forms of inherited deafness have been recognized and defined as syndromic and non-syndromic. Syndromic deafness is one of a defect in a single gene causing a variety of medical problems, one of which is deafness. Examples are Usher's syndrome, where the victims are deaf and blind and Waardenburg syndrome with deafness, whites forelocks of hair, and widely spaced eyes, which are often different colors. There are more than two dozen genes that cause syndromic deafness.

It is estimated that at least 70% of hereditary deafness is non-syndromic. In these cases, deafness is the only symptom of a genetic defect. Before 1997, only three genes for non syndrotnic deafness were known. Two of these are in the intracellular areas called the mitochondria. The third type is on the sex determining X chromosomes that makes up the genetic blueprint of a human being. All three of these mutations are considered rare.

The search for the cause of deafness has been frustrating because deaf people tend to marry other deaf people creating what is called assortative mating. Because the husband and wife in many cases owe the hearing loss to different genes, assortative mating makes it very difficult to track a gene through several generations of a family. Researchers have tried to find isolated families where it is likely a single gene has been passed down for long periods. Such a family was found in Costa Rica. The family traced its ancestry to Spanish immigrants that settled in the Americas in the 1600s. This family has been marked by deafness for at least eight generations. Nearly half of the 147 family members suffer from hearing loss that begins at about age ten, and leads to total deafness by age 30. This family's deafness has been reported to be on chromosome 5 of a gene called

DFNA1.

Large families have also been found in Tunisia, China, where a second deafness gene called MYOSINVIIA was reported. This has two types, which cause Usher's syndrome and a pure deafness pattern as well. That is the IIA protein. This protein plays a part in the accumulation of certain antibiotics within the hair cells of the ear, leading to deafness.

Pendred's syndrome was also found to be caused by a mutation of a gene on chromosome 7. This has different mutations, causing just hearing loss alone, and the belief by geneticist Eric D. Green, at the National Genome Research Institute, that this may cause 10% of all hereditary deafness.

The most common gene to be discovered, as of yet, is called Connexin 2. Connexin 2 was found by a team at the University of London and followed up by geneticist Christine Petit, of the Pasteur Institute. Half of the family she was studying had this defect. Finally, Dr. Xavier Estivill, of Barcelona believed that 1 in every 31 people carries a defective gene for hearing loss, which has a frequency rate higher than carriers of Cystic fibrosis.

Researchers do not know how each of these defects produces hearing impairment. Some of the genes may be important in maintaining the structure on the hair cells of the inner ear that bend when sound wave vibrate the fluid in the inner ear, thus impeding the electrical signals firing towards the auditory nerve. Pendred protein may be responsible for interfering with sulfate ions into cells.

References: http://dnalab-www.uia.ac.be/dnalab/hhh

 

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