Fragile X Syndrome
Also known as: Fragile X« Back to test list
The Fragile X syndrome is the commonest cause of familial intellectual disability. It is caused by a mutation in the FMR1 gene. This is an X-linked disorder, with males often being affected more severely than females.
Carrier women are often intellectually normal; some men can carry the mutation and be intellectually normal. Intellectually normal carriers can develop an ataxic/neurodegenerative disorder in late adult life (both genders) or have premature ovarian failure (women).
This test has diagnostic and familial implications.
In an intellectually disabled person of either gender, an abnormal result is diagnostic of the Fragile X syndrome. A normal result essentially excludes this diagnosis; the mutation is found in >99% of patients with this disorder.
In an intellectually normal person, an abnormal result indicates that they are at risk of having affected relatives, and that they may be at increased risk of ataxia or premature menopause in later life.
This is an assay for heritable mutations. It may raise issues of ethics or consent that are different from most other investigations ordered in the routine care of a patient. The interpretation of the result may also be dependent on the family history and the interpretation of other studies.
The mutation in this gene is abnormal repetition (or ‘expansion’) in the DNA sequence. Testing may require up to three steps to accurately determine the size of this repeated region in a patient.
This test can be requested by any medical practitioner. If the third step of analysis (Southern blotting) is ordered by a pathologist, the Medicare rebate may still apply.
4 mL blood in EDTA (separate tube required). Specimens may be collected by the requesting practitioner or at any Sonic Healthcare pathology collection centre.
To help ensure the quality of the test, a genetic test should be done with a dedicated sample whenever possible i.e. a sample collected specifically for that test rather than a sample that is used for multiple tests.
We also recommend that the patient or another adult check the labelling of request forms and sample tubes.
Please include statement about family history (or not) of fragile X in clinical details on referral form. If family history is present, please state relationship to patient.