Children's Hospital Colorado

Fragile X Syndrome

What is fragile X syndrome? 

There is much variability in the severity and symptoms in fragile X, however the majority of males have intellectual disability and approximately 30 to 60% have autism spectrum disorders or autistic behaviors. Females with fragile X often have learning disabilities, and approximately 30% of females have intellectual disability. Fragile X syndrome is the most common inherited cause of intellectual disability and one of the most common genetic disorders associated with autism. The signs and symptoms of fragile X syndrome can vary between individuals, and symptoms in males are usually more significant than in females. The syndrome occurs in approximately 1 in 3,600 males and 1 in 4,000-6,000 females.

Early delays in speech and motor development are common in infants and toddlers of both sexes. Other common behavioral and emotional characteristics can include attention-deficit/hyperactivity and impulsivity (ADHD), anxiety, hand flapping, poor eye contact, temper tantrums and sensitivities to touch and sounds.

What causes fragile X and who gets it?

Fragile X syndrome is caused by a change or mutation in a gene on the X chromosome called the FMR-1 (fragile X mental retardation-1) gene. Chromosomes are made up of thousands of genes that are passed from generation to generation. Most individuals have 46 chromosomes,two of which are sex chromosomes. In males, the sex chromosomes are X and Y. In females there are two Xs. 

Each gene of a chromosome contains a specific code of DNA that determines the function of the gene in the body. The FMR-1 gene produces a protein called FMRP, which plays an important role in brain development and in the biological mechanisms involved in learning and memory.

A mutation in the FMR-1 gene causes fragile X syndrome by making the body unable to produce the FMRP protein. The lack of the FMRP protein causes fragile X syndrome.

Since males have only one X chromosome, if they have the fragile X mutation they will produce minimal or no FMRP. Females have two X chromosomes,so if they have the mutation on one of their X chromosomes, the other X chromosome still produces normal levels of FMRP in about half the cells of the body. This explains why males with fragile X syndrome are usually more affected than females.

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