Trinucleotide Repeats: Fragile X Syndrome
What are trinucleotide repeats?
DNA (deoxyribonucleic acid) is the chemical that makes up our genes. It contains a
"code" of 3-letter words known as "codons" or "trinucleotides." Many genes normally
contain trinucleotide sequences that can be repetitive. 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 function 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 functioning correctly (if at all).
The child has the "full mutation." An example of a trinucleotide repeat disorder is
fragile X syndrome.
What is fragile X syndrome?
Fragile X syndrome causes intellectual disability. This can range from mild to moderate
to severe, in both males and females. However, males are typically 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 (ADHD). 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, called FMR1, 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 over 200