Fragile X Syndrome
Fragile X syndrome is an X chromosome-associated mental retardation disorder caused by a mutation in the FMR1 genetic factor located at Xq27.3. It is the most common after the down-syndrome. It is one of the inherited disorders of mild mental retardation, with an incidence of 1 in 2,000 boys and 1 in 4,000 to 8,000 girls. This syndrome is found in about 30% of men with impaired intelligence and about 10% of women.
Characteristic clinical symptoms in males are behavioral disturbances, impaired intelligence, large testicles, a long face, and a protruding chin, and large and distinct ears. And females usually show varying degrees of intellectual decline. In addition, with each generation, clinical symptoms become more severe and distinct.
Do healthy women also need to be tested ?
FXS is a disorder that is typically inherited from the mother. It has been reported that 1 out of 200 to 300 women are at high risk for giving birth to a child with FXS. Also, premutation is known to be a cause of premature menopause. Thus, the test can help to prevent the birth of an intellectually disabled child, early diagnose the premature menopause and allow the affected individual to better prepare for it.
The test is recommended to those with a family history (incl. family members and relatives) of autism, intellectual disability, development disorder or premature menopause.
How is the “premutation carrier test” performed ?
DNA is extracted from the patient’s peripheral blood (3~5ml) to analyze the number of CGG sequence repeats in the gene that can cause the fragile X syndrome.
The patient will be notified in 2 weeks if the result is “heterozygote” and in 4 weeks for “homozygote” (in this case, an additional test using the Southern Blot method is necessary).
If the woman is a premutation carrier, how can the fetus be tested for intellectual disability ?
Women diagnosed with premutation carriers are at risk of giving birth to an affected children. Carriers can receive genetic counseling and opt to undergo a thorough fetus examination including aminocentesis and chorionic villi sampling.