Fragile X syndrome is a genetic disorder linked with intellectual disability, as well as a range of other symptoms and conditions, such as learning problems, hyperactivity and autism. This condition occurs in 1.4 in 10000 males and in 0.9 in 10000 females. There is no cure for this disorder, but several types of therapies and medication are available to help ease the discomfort and develop skills as much as possible.
What are the symptoms of fragile X syndrome?
The most prominent symptom of fragile X syndrome is intellectual disability, which affects all males and about two thirds of females. This symptom is more severe in males as well, and the average IQ for males who are affected by this condition is 40. On the other hand, females who are have fragile X syndrome tend to have a somewhat higher IQ, and some, who are tested positive for this disease, can also have a borderline or a normal IQ.
As for physical appearance, people with fragile X syndrome share a number of traits, such as an elongated face, large, protruding ears, flat feet, hyperextensible finger joints and thumbs, a high arched palate, soft skin, prominent jaw and forehead, large testicles, and low muscle tone.
People affected by this condition show some behavioral symptoms as well. The most common one is stereotyped movements, but it is also not uncommon for the following symptoms to appear:
- Shyness
- Avoiding eye contact
- Memory problems.
Panic attacks are common as well, and they are present in about half of cases.
Seizures are present in six percent of females, and 14 percent of males who are affected. Strabismus is also common in fragile X syndrome patients.
What is the cause of fragile X syndrome?
The easy answer is that fragile X syndrome is caused by a mutation on the X chromosome. For a more complicated answer, some basic facts about genetics are required. Genes are made of molecules called nucleotides. For the sake of keeping this story as simple as I can, let's say that there are four different types of nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). They are connected to one another, like a chain or beads.
If we imagine that each bead represents one nucleotide, then three beads would code an amino acid, which forms proteins. The X chromosome contains a gene called FMR1 (fragile X mental retardation 1). This gene codes a protein which is commonly found in the brain, and is responsible for cognitive development, as well as female reproductive function.
When a mutation on this gene occurs, it causes triplets of nucleotides (CGG) to repeat. As in, where there should be less than 44 CGG nucleotide combinations, multiple triplets of those nucleotides appear, one after another. While people who have between 55 and 200 of these triplets do not have this condition, they are the carriers of this illness. Patients with more than 200 CGG triplets have fragile X syndrome. The large number of these triplets causes the FMR1 gene to be silenced, leading to a lack of the protein it produces, which, consequentially, causes intellectual disability.
And why are males affected more often than females? Again, there's a simple answer and a complicated one. But the complicated one is too complicated to be explained here, so we'll keep it simple. Each cell needs one X chromosome to function properly. The male genotype contains an X and a Y chromosome. If the X chromosome carries the gene mutation, the person will suffer from fragile X syndrome. But, female genotype contains two X chromosomes, and, since only one X chromosome is needed for the cell to function, the other one inactivates.
This process happens randomly, and the amount of cells carrying an active mutated X gene determines how severe the condition will be in females. (As a little side note, saying that only the XY genotype represents males, and that only XX genotype represents females is an oversimplified and essentially wrong explanation. Sex and gender are more complex categories, and there are more than these two different combinations of sex chromosomes in cells.)
How are fragile X syndrome and autism connected?
Autism, or autism spectrum disorder, is a developmental condition. The most common symptoms include difficulties with social interaction, difficulties with communication, and showing repetitive behavior. There are many different causes to ASD, and one of them is fragile X syndrome. Up to 50 percent of people affected by fragile X syndrome are diagnosed with autism as well. Other studies suggest that up to 90 percent of FXS show some kind of symptoms which are also present in autism (such as repetitive behavior or avoiding eye contact).
More than 800 genes have been associated with autism. And often, autism is a result of a complicated corelation between different genes. As a matter of fact, only a few single gene mutations have been found. FRM1 is one of them. And it's the most common one, as well, with up to six percent of people on the spectrum suffering from fragile X syndrome as well.