Overview

Did you know that Fragile X Syndrome (FXS) is the most common form of genetic intellectual disability and the most common single-gene cause of autism spectrum disorders (ASDs)?¹ It has an estimated prevalence worldwide of 1.4 per 10,000 individuals assigned male at birth (AMAB) and 0.9 per 10,000 individuals assigned female at birth (AFAB), although this could be an underestimation.²

Symptoms of FXS include:

• Intellectual disabilities, which can range from mild to severe in people with AMAB and mild to moderate in people with AFAB
• Behavioural traits, such as ASD and ADHD, are observed more often and severely in people AMAB than people AFAB
• Physical features in people with AMAB, such as a long face, large ears, macroorchidism, flat feet, hyper-flexible joints, soft skin, and scoliosis 

As you can see, FXS seems to be more common and severe in people with AMAB than in people with AFAB. Want to know why? Read on to find out!

Genetic basis of fragile x syndrome

The FMR1 gene in FXS

In FXS and associated disorders, genetic mutations occur in the FMR1 gene, which stands for Fragile X messenger ribonucleoprotein 1. The FMR1 gene product is a protein called FMRP, whose role is not yet fully understood. FMRP is expressed in all cells but is particularly abundant in nerve cells in the brain, where it is thought to participate in making and keeping cell-to-cell connections (called synapses). Furthermore, studies have shown that FMRP is also important in synaptic plasticity, which is when synapses reorganise themselves in new patterns – a process of fundamental importance for learning and memory.³

FXS is an x-linked condition

Human cells (excluding gametes) contain 46 chromosomes, organised in 23 pairs. The last chromosomal pair is known as the sex chromosomes because they determine your biological sex. People AMAB usually have X and Y chromosomes as their 23rd pair (XY), while people AFAB have two X chromosomes (XX). Therefore, when it comes to biological sex determination, you can only get X chromosomes from your mother, and either an X or Y chromosome from your father. That’s where the “X” from “Fragile X syndrome” comes from – it is because the FMR1 gene is located in the X chromosome. FXS is what is known as an “X-linked” condition, that is, associated with the inheritance of the X chromosome.

The different versions of the FMR1 gene

By now you’ve probably learned that FXS is caused by mutations in the FMR1 gene, located in the X chromosome, but what does that mean exactly? And more importantly, how does that translate to the different presentations of this condition?

CGG repeats

Genes are made of a sequence of 4 different molecules, which can be abbreviated into 4 letters - A, T, C, and G. In the case of FMR1, there’s a specific part at the beginning of the gene that has three letters - CGG -  repeated many times, one after the other. This short CGG sequence is sometimes called a CGG triplet or CGG repeat. 

The number of sequential CGG repeats found in the FMR1 gene is measured during FXS testing and can be classed as:⁴

• Typical
• Intermediate
• Premutation
• Full mutation

In the case of individuals with two X chromosomes, the chromosome with the highest number of repeats will be considered for the final classification. 

Typical

In this form, the FMR1 gene presents between 5 and 44 CGG repeats and is correctly expressed into the FMRP protein.

Intermediate

When the FMR1 gene contains 45 to 54 CGG repeats, it is classified as an “intermediate” and considered as a grey zone. This is a relatively stable form that infrequently may expand to a premutation in a single generation. This form of FMR1 does not cause FXS or any associated Fragile X disorders. 

Premutation

In this version, the FMR1 gene presents 55 to 200 CGG repeats. This is considered an unstable mutation which could further expand into a full mutation in the next generations. Individuals with FMR1 premutation are also at increased risk of developing Fragile X-associated disorders, such as Fragile X-associated tremor/ataxia syndrome (FXTAS). People who have ovaries can also develop Fragile X-associated primary ovarian insufficiency (FXPOI). 

Full mutation

A full mutation of the FMR1 gene contains more than 200 CGG repeats and will trigger some extent of methylation, leading to gene inactivation. It is not known how FMR1 genes with more than 200 CGG repeats trigger methylation events. The full mutation manifests as FXS.

AGG interruptions

The CGG expansion repeats found in FMR1 can be interspaced by AGG triplets, called AGG interruptions. In 1994, researchers first suggested that it is actually the length of uninterrupted CGG repeats that leads to instability and, eventually, full mutation.⁵

This was supported by a 2015 study, which showed that the higher the number of AGG interruptions, the higher the stability of the FMR1 gene. This means that individuals at the intermediate and premutation stages were less likely to expand toward full mutations if they had more AGG interruptions in their FMR1 genes.⁶

Indeed, sometimes FMR1 genes with roughly the same number of CGG repeats can vary wildly in instability, depending on the number of AGG interruptions present.⁷ 

Inheritance of FXS

The genetic basis of FXS can be quite tricky to understand, especially considering all the genetic variants of the FMR1 gene. 

Figure 1 helps illustrate some of the possible scenarios for inheritance of FXS. Overall, we can conclude that:

• Parents of AMAB that carry the premutation of FMR1 (with X^FY chromosomes) will pass it to all their daughters and none of their sons
• Parents AFAB that carry the premutation or full mutation of FMR1 (with X^FX chromosomes) have a 50% chance of passing it to their children in each pregnancy

Figure 1: Pedigree charts depicting the possible offspring of parents carrying different versions of the FMR1 gene.

The transition from premutation to full mutation

Scientists have determined that the expansion from premutation to full mutation occurs exclusively in the maternal line, probably during the maturation of egg cells.⁸ If a person AFAB has a number of CGG repeats on the higher end of the premutation range, that increases their chance of having eggs with the full mutation. Therefore, people AFAB with a premutation can pass to their children either a premutation or a full mutation.

This does not happen during sperm formation in people with AMAB carrying a premutation. Scientific evidence shows that sperm cells carrying the full mutation are selected during sperm cell formation.9 This means that a person AMAB can only pass permutations to their children.

Carriers

Generally, when talking about genetic disorders, a carrier is a person who has a specific mutation but does not exhibit any symptoms. In the case of the FMR1 gene, this technical definition is more nuanced, because carriers sometimes can show clinically relevant symptoms, as summarised below:

• Carriers AFAB with at least one copy of FMR1 with the premutation can develop FXTAS and/or FXPOI
• Carriers AFAB with at least one copy of FMR1 with the full mutation can develop learning, behavioural, social, or psychiatric disorders
• Carriers AMAB with the FMR1 premutation can develop FXTAS later in their lives

Summary

FXS is an X-linked condition caused by the full mutation of the FMR1 gene. Some of the symptoms are intellectual disabilities, behavioural conditions (such as ASD), and physical changes. Because it is linked to chromosome X, it is more common and severe in people with AMAB (i.e., with XY chromosomes). The FMR1 gene encodes a protein known as FMRP, which participates in brain formation and function. The FMR1 gene is presented in different forms - typical, intermediate, premutation, or full mutation - according to the number of CGG repeats present in its sequence. The higher the number of CGG repeats, the more unstable the gene becomes, which could trigger methylation events and gene inactivation. The number of AGG interruptions interspaced between the CGG repeats also contributes to the stability of FMR1. Although people with the premutation may not show any symptoms, they can still develop Fragile X-associated conditions, such as FXTAS and FXPOI. The premutation can expand into a full mutation during egg cell maturation, therefore people AFAB with a premutation can pass forward permutations or full mutations to their children. People AMAB with a premutation or full mutation will pass the premutation forward, but only to their daughters. 

FAQs

Does everyone have an FMR1 gene?

Yes, everyone has the FMR1 gene, which can be presented in 4 different forms: typical, intermediate, premutation, or full mutation.

What does the FMR1 gene do?

The FMR1 gene is responsible for the production of the FMRP protein, which participates in brain formation. 

How is Fragile X syndrome inherited?

The FMR1 gene is located in the X chromosome. Therefore, people AMAB with the premutation or full mutation will pass the premutation to 100% of their biological daughters and none of their biological sons. People AFAB with the premutation have a 50% chance of passing the premutation or full mutation to their biological children.

What is the FXS testing procedure in the UK?

You can request a DNA test for Fragile X on the NHS via your GP or paediatrician. They will refer you to a hospital specialist, who will then request the test to a genetic testing centre.