Deafness-dystonia-optic-neuronopathy syndrome (DDON), also known as Mohr-Tranebjaerg syndrome, is a disorder that causes progressive loss of hearing, visual acuity, and neurological control of the body’s actions as well as behavior. This syndrome is found exclusively in males as it is caused by a mutant recessive gene found on the X chromosome. Because males only have one X chromosome, they do not have the benefit of a second X chromosome to hide or mask the effects of this gene. There have been no cases of females with this disorder as it would take two recessive genes on both X chromosomes for this disorder to be expressed. However, females can be, and often are, the carriers of this recessive gene.
The gene associated with this disorder is called TIMM8A and is responsible for instruction making of proteins found inside the mitochodria. Normal function of this gene produces normal transport of proteins inside the mitrochondria. The mutated form of TIMM8A does not produce the normal transport of the proteins which is believed to be what causes DDON symptoms to begin occurring.
Symptoms of an affected male with DDON usually show up during childhood. The first symptom to appear is the unexplained loss of hearing due to nerve damage. This hearing loss will progress as the child reaches his teens and he may end up with total loss of hearing by the time he reaches adulthood.
The next noticeable symptom is the onset of dystonia, which is the involuntary tensing of the muscles. This begins to occur as the child becomes a teenager and is often accompanied with a loss of coordination (ataxia). Like the hearing loss, these symptoms worsen as the individual gets older.
Another symptom is the loss of sharpness of vision and a sensitivity to light (photophobia). Symptoms dealing with vision do not usually manifest until the individual is in his teens or early adulthood. The vision problems will gradually increase and the individual will be legally blind by middle age.
Other symptoms commonly associated with this disorder are changes in behavior that include changes in personality, aggression and paranoia. As the individual ages, thinking and reasoning abilities diminish and dementia develops.
The reproductive organs are not affected by this disorder making it possible for the affected men to father children. Because of the nature of the disorder’s inheritance pattern, all of the men’s daughters will be carriers of the disorder while none of his sons will inherit it. Women with one copy of the gene have a 25% chance of passing it on to her daughters and a 25% chance of having a son with the disorder. Genetic screenings for the mutated TIMM8A gene are available for pregnant mothers who may suspect or are carriers of DDON.
DDON is considered a very rare disorder as only 70 individuals in the world have been diagnosed with it. Those with severe cases of the disorder may only live to their teens while others with less severe forms of the disorder may live past 50 years of age. There is no cure for DDON but treatment options are available. Because the loss of hearing is caused by auditory nerves rather than the ear or parts of the inner ear, hearing aids are of limited use and many individuals with DDON will need to learn sign language in order to communicate. Other symptoms such as dystonia, ataxia and the behavioral problems may be treated with medication and routine psychological evaluations.