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FXTAS vs FXS

Fragile X syndrome (FXS) and fragile X- associated tremor ataxia syndrome (FXTAS) are not the same condition. Though they both are part of the family of Fragile X-associated Disorders, it is important to distinguish between these two very different conditions. Sometimes someone might be told they have “Fragile X,” not knowing which of the Fragile X-associated Disorders is present.

Both FXS and FXTAS are caused by mutations of the same gene, the FMR1 gene. But they are caused by different changes in this gene. FXS is caused by a full mutation  of the FMR1 gene, FXTAS by a premutation of the FMR1 gene.

Even more important in distinguishing between FXS and FXTAS are the different features, age of onset, and diagnostic criteria of the two conditions. Let’s compare:

FXS is present (though often not diagnosed) at birth. Children display features of the condition early in life, usually noticed by family members in infancy, toddlerhood or early childhood. These features include delays in cognitive, language and social development, behavioral issues, and physical characteristics such as large ears, large testes in males, flexible joints and a long face (more common as children get older).

FXTAS develops in adulthood—usually after age 50. The symptoms may appear slowly and develop over years or decades. Individuals with FXTAS usually are healthy, usually with normal cognitive skills prior to the onset of the condition. They have no unique physical characteristics and did not experience the developmental delays in childhood that are seen in FXS.

FXS is caused by a full mutation and methylation of the FMR1 gene.

FXTAS is caused by a premutation of the FMR1 gene, which is in its normally unmethylated state.

FXS is diagnosed by molecular (DNA) testing of the FMR1 gene. Virtually all boys with a full mutation have FXS and about 50 percent of girls with a full mutation have features of FXS.
FXTAS is diagnosed by fulfilling certain criteria. These include being an FMR1 premutation carrier (though not all individuals who are carriers will develop FXTAS), the appearance of neurological features such as ataxia (balance problems), tremors, and other symptoms. MRI findings are also important in diagnosing FXTAS.

Contributed by: Liane Abrams, MS, CGC

 

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