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Fragile X and company: Finding the right diagnosis

Fragile X syndrome (FXS) is a common cause of inherited mental retardation in boys. (More information about FXS can be found on the next page.) It is caused by a defect in a gene located on the X chromosome that is inherited from the mother. The gene in the mother carries a “premutation” which, when passed on to her son, expands to a “full” mutation that produces the signs of fragile X syndrome. The mother who carries the “premutation” does not have mental retardation because she also carries a normal X chromosome. She may have inherited the premutation from her father. This grandfather does not have mental retardation either because the gene only carries a “premutation.” However, some of these grandfathers develop a progressive tremor (shaking) and gait ataxia (unsteadiness) later in life, known as fragile X tremor-ataxia syndrome (FXTAS). Even more rarely, the mother with the premutation may develop FXTAS.

This issue of Neurology has two papers that further describe FXTAS, as well as an editorial on the subject. One of the papers, by Hall et al.,1 is from the same group of researchers who originally reported this syndrome in 2001. In the clinics where the boys with FXS were treated, the mothers indicated that their fathers (the boys’ maternal grandfathers) were having neurologic problems. Sixty-two family members were found to have the fragile X premutation by genetic testing. “Definite” FXTAS was diagnosed if the patients had a tremor with movement (intention tremor) or ataxia when walking with either 1) a very typical abnormality on brain MRI scan or 2) the disease was confirmed on autopsy. Twenty of the 62 met this …