Trinucleotide Repeats: Fragile X Syndrome

DNA is the chemical that makes up your genes. It contains a "code" of 3-letter words known as codons or trinucleotides. Many genes normally contain trinucleotide sequences that repeat. These are called trinucleotide repeats. But when the number of trinucleotide repeats increases to a larger than normal number of copies, the DNA is changed. The gene may not work as it should. Or it may not work at all.

Researchers don't know exactly what causes a trinucleotide repeat to expand into more than the usual number of copies that should be in a gene. Sometimes a person may have more than the usual number of copies but not enough to change the function of the gene. These people are premutation carriers. But when they pass on these extra copies to a child, the extra trinucleotide repeats cause the DNA to become unstable. The result is that the child has a gene that is not working correctly (if at all). The child has the full mutation. An example of a trinucleotide repeat disorder is fragile X syndrome.

Fragile X syndrome causes learning problems. This can range from mild to moderate to severe, in both males and females. But males are often more severely affected than females. Symptoms of fragile X syndrome in childhood are not always specific. They overlap with other disorders, such as autism, Prader-Willi syndrome, and attention-deficit/hyperactivity disorder. Symptoms may include delays in development of speech, language, and motor skills. Autistic-like behavior, frequent tantrums, anxiety, trouble paying attention, and hyperactivity are also commonly seen with fragile X syndrome. Gaze aversion, or the inability to make and hold eye contact, is very common among males and females with fragile X syndrome.

The gene that causes fragile X syndrome is called FMR1., It is located on the X chromosome. Females are usually not as severely affected as males. That is because females have a normal X chromosome in addition to the X with the mutation. The FMR1 gene normally contains 5 to 44 trinucleotide repeats. Premutation carriers have about 55 to 200 trinucleotide repeats. People with fragile X syndrome full mutation have more than 200 repeats.