Fragile X Syndrome in children

Fragile X syndrome is the most common cause of mental disabilities in children


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Fragile X syndrome is one of the most common causes of hereditary intellectual disability.

The gene that is responsible for it is FMR1 and it is found in the X sex chromosome (the disorder is linked to the X chromosome). Therefore, transmission and the extent of affectation varies between men and women. Males with a mutation in this gene have the disorder (affected) whilst affectation is lower in women. However, they do pass it on to the next generation. 

What is fragile X syndrome?

Fragile X syndrome is a dominant disorder linked to the X chromosome. Its cause is expansion in a repeat region located in the 5’ untranslated region of the FMR1 gene.  The length of the expansion determines if a person has the disorder or not.

  • A length under 55 repetitions means that the individual is normal.
  • Over 200 repeats (full mutation) means that the individual has the disorder because such an elevated number of repetitions inactivates the FMR1 gene. Since males have a single copy of the gene (just one X chromosome), inactivation leads to an absence of FMR1 protein synthesis and this, in turn, causes the set of clinical symptoms known as fragile X syndrome.
  • A length between 55 and 200 repetitions (premutation) means that gene inactivation is not complete and there is still a presence of FMR1 protein. As such, the characteristic symptoms of the syndrome are not manifest. The problem is that the premutation can increase in size or expand from one generation to the next. This can lead to the birth of children with fragile X syndrome if they have more than 200 repetitions.

How common is fragile X syndrome?

One in every 4,000 males are affected by fragile X syndrome.

How is fragile X syndrome diagnosed?

In order to detect fragile X syndrome, a blood sample analysis needs to be performed so that the individual’s DNA can be obtained. Using molecular biology techniques, the length of the FMR1 gene region is determined in order to understand if the mutation that is responsible for the disorder is present.

What is the purpose of performing a fragile X analysis?

The purpose of performing a fragile X analysis is to understand the origin of mental retardation in children and to give family members appropriate genetic guidance (preconception counselling for future pregnancies and an analysis of family members in order to get to know what their status is in terms of carriers and taking any necessary measures).

When is a fragile X analysis advisable?

It is advised when children have clinical symptoms of mental retardation so that their diagnosis can be completed. It is also advised if the mother of a child is known to be a carrier of the gene premutation so that it can be determined if the child has inherited the whole mutation and, as such, has the disorder.