Fragile X syndrome is an inherited condition which is commonly recognizable cause of developmental delay. In these individuals learning disability, speech & language delay, autistic features, and physical appearance of large prominent ears & macrocephaly (overly large head) takes place. The syndrome takes place due to disease-causing genetic variant (difference in genes that make up our DNA). The variant is the expansion of repetitive DNA near the FMR1 gene. This FMR1 gene generates instructions for FMRP protein which is involved in normal brain development. When genetic mutations occur near this FMR1 gene, FMRP production is disrupted and hence normal development of nervous system is affected. This genetic test is therefore taken to evaluate FMR1 gene.
Why is fragile X syndrome lab test done?
It is basically done to evaluate FMR1 gene and the expansion which is causing fragile X syndrome and FMR1 related conditions. Also, very close to FMR1 gene is a section of DNA having multiple repeats of CCG (cytosine-guanine-guanine; DNA bases), which is also called CCG triple repeat. This test also determines no. of CCG triplet repeats. Hence, the test is done:
To determine carriers of FMR1 premutation or full mutation & risks of passing it in your children
For pre-natal diagnose of fragile X syndrome or FMR1 related diseases; but pre-natal diagnosis only done in females known to be carriers of FMR1 premutation.
Diagnosis of FXPOI (fragile X-associated premature ovarian insufficiency) causing infertility and early menopause
Diagnosis of FXTAS (fragile X tremor/ataxia syndrome)
Depending upon the reason for which diagnosis was referred, the sample is taken. If it is for normal diagnostic/carrier testing, then blood is drawn by inserting needle in vein in the arm. But if it is for prenatal diagnosis then either CVS (chorionic villus sampling) or Amniocentesis (for amniotic fluid) is done.
The sample collected in test tube or vial is then sent to the laboratory
The technician performs fragile X genetic testing on sample using PCR (polymerase chain reaction) and looks for FMR1 gene mutation.
Which tests help diagnose fragile X syndrome?
Genetic testing is done to evaluate FMR1 gene & determine number of CGG triplet repeats for diagnosis of fragile X syndrome.
How long will it take to get results of fragile X syndrome?
Fragile X testing results are ready in 2 weeks
Why fragile X syndrome testing is done in a person?
Specific indications of test include female or male having intellectual disabilities, developmental delay, autism, speech & language delay, or learning disabilities which are due to unknown causes.
Is fetus also tested for fragile X?
In those individuals who consider themselves carriers of fragile X or history of fragile X in family, undergo prenatal testing for the fetus.
INTERPRETATION OF THE RESULTS:
Depending upon CGG triplet repeats, the ranges are:
Normal “55 to 44 CGG repeats”: indicates that there is no fragile X syndrome or FMR1 gene condition. Also, individual is not a mutant gene carrier
Intermediate “45 to 54 CGG repeats”: No fragile X syndrome or FMR1 related condition. However, female has higher chance of passing larger triplet expansion (premutation) in children but it will not expand to full mutation in one generation.
Premutation “55 to about 200 CGG repeats”: Some females at risk of FXPOI. Female (17%) and male (47%) at risk of FXTAS. Also, chance of expanding to full mutation in child if passed from female premutation carriers. Carrier fathers pass it to their daughters.
Full mutation “more than 200 CGG repeats”: indicates that they are affected with fragile X syndrome. With each pregnancy, mother with full mutation has 50% chance of passing to child. In men will full mutations/diagnosis of fragile X syndrome, they don’t have children commonly, and if they have cannot pass it in children.
You get tested for diagnosing fragile X syndrome or FMR1 gene conditions. Also, for determination of fragile X gene variant carriers, and risks of passing variant gene in your child (causing fragile X syndrome).