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Fragile X Syndrome


Molecular Genetics Clinical Laboratory- Fragile X Syndrome

INDICATIONS FOR TESTING:
Fragile X Syndrome is an X-linked dominant disorder with variable penetrance characterized by developmental delay, mental retardation (severe in males and milder in females) and macroorchidism in post-pubertal males. The genetic defect is an expansion of a trinucleotide repeat (CGG) pattern that causes loss of function of the FMR1 gene localized in chromosome Xq27. Increases in clinical severity are frequently observed after parent-to-child transmission. Female carriers are usually mildly- or unaffected and there may be unaffected males that are carriers of a premutation.

TESTING OFFERED:
Over 99% of affected individuals have an expansion of the repeated CGG pattern in the 5' untranslated region of the FMR1 gene that is readily detected. Normal individuals have from 6 to 52 CGG repeats; repeats from 52 to 200 are considered premutations that are not pathogenic but are at risk for expansion in the next generation. Affected individuals have over 200 CGG repeats, with longer repeats suggesting increased severity. Rarely, patients may have FMR1 deletions or point mutations.

If the etiology of mental impairment is not known, Fragile X testing is usually offered as a combination test along with a routine karyotype in the Cytogenetics Laboratory. Cytogenetic studies are critical since constitutional abnormalities have been identified as frequently or more frequently than fragile X mutations in mentally retarded individuals referred for fragile X testing.

TURN-AROUND TIME:
Most testing is completed within a time of 21 days following specimen receipt. Prenatal testing is usually completed within 7 days from specimen receipt.

SPECIMEN REQUIREMENTS:
All specimens should be kept at room temperature and shipped overnight in an insulated container.

Adults and Children:
For DNA analysis ONLY, a tube of 4 cc whole blood collected in an EDTA (lavender top) tube is preferred. Whole blood collected with other anticoagulants is acceptable but not preferred.

For combination Molecular and Cytogenetic karyotype analysis, two tubes of whole blood are required.

1) A tube of 4 cc whole blood collected in an EDTA (lavender top) tube is preferred. Whole blood collected with other anticoagulants is acceptable but not preferred.

2) A second tube with 3 cc of blood collected in sodium heparin (green top) tube is required. Lithium heparin and other anticoagulants are not acceptable.

Neonates:
For DNA analysis ONLY, a the minimum specimen is 0.5 cc whole blood in an EDTA tube.

For combination Molecular and Cytogenetic karyotype analysis, the minimum specimen is 0.5 cc whole blood in an EDTA tube and 3 cc of blood collected in sodium heparin (green top) tube.

Prenatal:
Amniocentesis: 15 cc whole amniotic fluid submitted without culture. A backup culture should be arranged but is rarely required.
CVS: a single whole villus, dissected free of decidua.

COST:
The cost of testing is available upon request.

FOR MORE INFORMATION, PLEASE CONTACT:
Phone:
(727) 767-8985 (Laboratory)
            (727) 767-8611 (Director)
Fax: (727) 767-8367
E-mail: omuelle1@jhmi.edu

 


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