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U.S. scientists say they have discovered a new but common neurodegenerative disease that primarily affects men over the age of 50.

The disorder — fragile X-associated tremor-ataxia syndrome — causes tremors, balance problems and dementia that become progressively more severe with age.

Randi Hagerman, medical director of the Medical Investigation of Neurodevelopmental Disorders Institute at the University of California, Davis, said FXTAS (pronounced fax-tass) affects about one in every 800 men in the general population and that roughly one-third of them (about one in 3,000 men) will actually develop symptoms.

She said the small mutation of the fragile X gene, one thought to have no health effects, can actually cause a serious disease, but one that is commonly misdiagnosed as Parkinson's or Alzheimer.

"FXTAS may be one of the most common causes of tremor and balance problems in the adult population, yet it is being misdiagnosed because neurologists who see adults with movement disorders are not aware that they need to look for a family history of fragile X in grandchildren or to check for the presence of the premutation in the fragile X gene," Dr. Hagerman said.

In a paper published in today's edition of the Journal of the American Medical Association, the research team says the disorder is common enough, and its symptoms severe enough, that all men over the age of 50 should be screened.

The condition can be accurately and easily identified with a standard DNA blood test. There is no cure, but some of the symptoms can be treated.

FXTAS is not to be confused with Fragile X Syndrome, the most common cause of mental retardation. There is, however, an interesting connection.

Dr. Hagerman, a pediatrician who has specialized in research and treatment of fragile X for more than 20 years, began looking for a connection between children and their grandfathers because several mothers of her fragile X patients were worried about their own fathers, who were falling down, becoming forgetful and experiencing other neurological problems.

To probe the possible link, the research team enlisted 192 individuals whose families belong to Fragile X Associations. They found a remarkably high number of men with FXTAS: 17 per cent of those in their 50s had FXTAS; 38 per cent of men in their 60s; 47 per cent of men in their 70s; and 75 per cent of men in their 80s.

"FXTAS is an enigma," Dr. Hagerman said. "The disorder appears later in life in men who are generally healthy throughout childhood and early-to-mid-adulthood and have normal to above-average intelligence, yet is caused by a defect in a gene known to cause mental retardation usually diagnosed in early childhood."

Initial signs of the disorder can include difficulty writing, using eating utensils, pouring water and walking. Other symptoms include short-term memory loss, anxiety, decreased sensation in the lower extremities to touch and vibration, lower-limb muscle weakness and parkinsonism (rigidity in movement). These initial symptoms grow progressively worse over years, until carrying out many of the tasks of daily living and walking without assistance becomes difficult or impossible.

The underlying cause of FXTAS is a mutation in the fragile X mental retardation 1 gene, or FMR1. Under normal conditions, this gene produces a protein that maintains the proper functioning of nerve cells in the brain. The gene causes both fragile X syndrome and FXTAS when a particular segment of DNA is repeated too many times.

An individual with a high number of repeats is considered a carrier of the premutation, which can lead to FXTAS later in life and to fragile X (the full mutation) in the next generations. Male carriers are at high risk of developing FXTAS, as well as for passing on the gene mutation to their daughters, who in turn are at risk to have children with fragile X syndrome.