Fragile X Carrier Screening
What is Fragile X syndrome?
Fragile X syndrome (FXS) is the most common inherited form of intellectual disability. FXS is caused by a defected FMR1 gene located on the X chromosome, in which there is an increased number of unstable trinucleotide repeats. Overexpansion of >200 repeats cause FXS with mental retardation and autism.
Carriers with an increased repeats may have variable degree of health problems.
Why do we need to screen for FXS carriers?
Depends on ethnics group, FXS may affects up to 1 in 4,000 males and 1 in 8,000 females. The prevalence of premutation in female is approximately 1 in 250-1,000 and in male is approximately 1 in 250-1,600.
A premutation carrier mother is at risk to transmit her premutation allele to her babies, which may then further expand_ to > 200 times (full mutation), resulting in a FXS baby.
The risk of such transmission is dependent on the number of repeats present in the carrier mother. The more repeats a carrier mother has, the higher chance has a full mutation being transmitted.
Who should consider carrier screening?
You may consider carrier screening:
- If you have family history of FXS/unexplained mental retardation or developmental delay/autism/ovarian failure.
- If you are pregnant or planning for pregnancy and worry about FXS, you may consider the screening after counseling.