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What is it?

Fragile X syndrome is a genetic condition that causes a range of developmental problems including learning disabilities and intellectual impairment. It is the most common known cause of inherited intellectual and developmental disability affecting about 1 in 4000 males and between 1 in 5000 and 1 in 8000 females. It is also the most common genetic cause of autism. The syndrome is more often diagnosed in males than females, and males with Fragile X syndrome are usually more severely affected. According to the Fragile X Association of Australia, every week in Australia one child is born who is fully affected and 20 children are born who are carriers.

Fragile X syndrome is caused by mutations in the FMR1 gene. The FMR1 gene contains the DNA code to produce a protein, FMRP. This protein is present in a variety of tissues but is especially important in the brain. FMRP is essential for proper brain development and seems to play a role in ensuring that the connections between nerve cells (known as synapses) develop and function properly.

The FMR1 gene is carried on the X chromosome.  Males have one X chromosome and one Y chromosome whereas females have two X chromosomes. This means males have one copy of the FMR1 gene in each cell in their bodies whereas females have two copies of the gene in each cell.  In all women, one of the X chromosomes in each cell is randomly inactivated. If a woman is a carrier and has the FMR1 mutation on one of their X chromosomes, then on average, females will have about half of their FMR1 genes operating normally and as a result are affected less than males.

About 98 per cent of cases of Fragile X syndrome are due to mutations in a part of the FMR1 gene that contains a sequence of repeated groups of the three nucleotides CGG. These are known as triplet repeats. In most people the number of repeats is short (usually between 6 and 44 repeats) and this repeat length remains stable when passed from parents to children. However, in some people, the number of CGG repeats in the gene is increased and this is called a “repeat expansion”. Depending on how many repeats there are, the expansion is called a  “pre-mutation” if there are 55-200 CGG repeats, and a “full mutation”  if there are more than 200 CGG repeats present. Pre-mutations and full mutations affect the normal function of the gene and therefore, can have clinical consequences. 
Full mutation
Fragile X syndrome occurs when there is a full mutation in the FMR1 gene. Because they have only one X chromosome, men who have the full mutation in their FMR1 gene (more than 200 CGG repeats in the FMR1 gene) will have Fragile X syndrome. Women who have the full mutation in the FMR1 gene have less predictable outcomes. Because they have two x chromosomes, the random inactivation of only one X chromosome in each cell means some cells will have the faulty gene operating and some will have the normal gene operating. The result is that some women will be affected while others remain unaffected. About 50-60 per cent of women will have some degree of learning disability that can range from mild to severe.
People with a pre-mutation in the FMR1 gene do not usually have intellectual disability or the other characteristics of Fragile X syndrome, as enough FMRP protein is still being produced. Nevertheless, both male and female pre-mutation carriers are at risk of developing certain health problems later in life. They are at increased risk of developing a neurodegenerative disorder after middle-age called Fragile X tremor/ataxia syndrome (FXTAS). The lifetime risk of developing FXTAS is much higher for male pre-mutation carriers compared to female pre-mutation carriers. Women with pre-mutations also have about a 20 per cent risk of developing premature ovarian failure and having early menopause. (For more, see Symptoms.)

Last Review Date: April 27, 2023

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