Fragile X syndrome, a rare genetic neurodevelopmental disorder, can result in intellectual disability, language disorders, anxiety, hyperactivity, and sensory hypersensitivity. It is the most common genetic cause of autism spectrum disorders, with no current treatment options available.
From a visible “break” to a silenced gene
In 1969, geneticist Herbert Lubs described an unusual constriction on the X chromosome in a family affected by intellectual disability.1 This fragile spot would ultimately give the syndrome its name, though the underlying mechanism had yet to be revealed.
The breakthrough came in 1991 when the FMR1 gene was identified. This gene contains a sequence in which three DNA “building blocks” (called CGG) are repeated. In people with Fragile X syndrome, more than 200 repetitions are observed2. Beyond this number of repetitions, the FMR1 gene “locks up”, meaning that it is not destroyed, but it becomes inactive. It can then no longer produce—or produces insufficient amounts of—a crucial protein called FMRP3. It is this deficiency or insufficient production of the FMRP protein that underpins fragile X syndrome and causes the clinical manifestations in patients.
The name “Fragile X” originates from a microscopic image showing the end of an X chromosome that appeared as if it were about to break.
FMRP protein, a complex and essential role
Understanding the genetic cause of Fragile X syndrome is a vital first step, but it is not enough to explain what makes the condition so diverse. The FMRP protein is not merely a simple “switch” but plays a much more complex and fundamental role. By “binding” to numerous “messengers” in our cells (called messenger RNAs), it helps regulate the production of certain other proteins. This role is particularly important at synapses, which are the connecting points where neurons communicate with one another1.
When the FMRP protein is absent or does not function properly, it triggers a series of irregularities, including hyperexcitability of nerve cells, or neurons. This impacts several brain functions, including sensory processing, learning, and language.1 As a result, Fragile X syndrome can present with a wide variety of symptoms, such as intellectual disability, anxiety, hyperactivity, repetitive behaviors, heightened sensitivity to certain stimuli (hypersensitivities), or even seizures.
Fragile X is one of the leading known genetic causes of autism spectrum disorder (ASD), and, according to GeneReviews, 50 to 70% of people with the syndrome also have ASD.3 Although Fragile X syndrome is frequently associated with autism, difficulties with social interaction are not present in all individuals with the syndrome and often vary widely.
The location of the abnormality on the X chromosome accounts for another significant source of variability: Boys are generally more severely affected than girls. In girls, the presence of a second X chromosome often results in milder symptoms.
The exact prevalence of the syndrome is difficult to determine, as epidemiological studies are scarce and the syndrome is not routinely screened for at birth. The mildest forms may therefore go undiagnosed. Nevertheless, a systematic review conducted in 2014 estimated the frequency of the complete FMR1 gene mutation—which causes the condition—at approximately 1 in 7,000 men and 1 in 11,000 women.4
Understanding the true impact of the condition on patients and their families
Scientists are also working to identify what therapeutic advances could tangibly improve the lives of people with Fragile X syndrome. During a Patient-Focused Drug Development initiative held in the United States in 2021, patients and their loved ones highlighted anxiety, cognitive abilities (learning and thinking), communication, certain behaviors, and sensory difficulties (such as excessive sensitivity to noise or light).5 These aspects interact with or overlap with one another. For example, reducing hyperreactivity could improve attention or help with social interactions.
Currently, patient care is, at best, multidisciplinary and individualized, involving educational and behavioral interventions, speech therapy (for language), occupational therapy (for daily activities), and medication to manage certain symptoms. However, there is no approved treatment that specifically targets the mechanisms underlying the syndrome.1,3 This creates a paradox: The genetic diagnosis is accurate, but no specific therapy is yet available.
Improving our understanding of research into Fragile X syndrome
Research is advancing on several fronts to better comprehend and treat Fragile X syndrome. The main goal is often to restore FMRP protein production. To achieve this, various avenues are being explored:
Other research aims to compensate for the effects the lack of the FMRP protein has on the brain by replacing it with a small molecule that can restore its function in certain structures. The key objective of such strategies is to limit neuronal excitability.
Researchers use electroencephalography (EEG)—the measurement of brain electrical activity—to assess such hyperexcitability. This technique detects specific “signatures” in patients that serve as reproducible indicators of disturbances6. The signatures may help determine whether a new treatment has a significant biological effect, distinguish between different patient profiles, and complement clinical observations during controlled clinical trials. Although this does not compensate for the patient’s lived experience, it does establish a valuable link between the biology of the disease and objectively measurable clinical manifestations.
About Servier
In 2024, Servier made a defining strategic decision to establish rare neurology as its next growth driver. Since then, the Group has been expanding its research pipeline and bolstering expertise with the aim of becoming a global player in rare neurological disorders by 2030. Servier’s priority is to address the biological causes of neurological diseases that are primarily genetic in origin and severely debilitating. This led Servier to acquire, in September 2025, an asset from biotech Kaerus Bioscience Ltd. to potentially treat Fragile X syndrome, a condition on the autism spectrum.
As the Group’s first acquired neurology asset, it is part of a precision medicine approach, which involves identifying a specific genetic cause, exploring the consequences, and evaluating whether modulating said consequences can yield a therapeutic benefit.
Key Takeaways
Références
[1] Hagerman RJ, Berry-Kravis E, Hazlett HC, et al. Fragile X syndrome. Nat Rev Dis Primers. 2017;3:17065. doi:10.1038/nrdp.2017.65.
[2] Verkerk AJMH, Pieretti M, Sutcliffe JS, et al. Identification of a gene (FMR-1) containing a CGG repeat coincident with a breakpoint cluster region exhibiting length variation in fragile X syndrome. Cell. 1991;65(5):905-914. doi:10.1016/0092-8674(91)90397-H.
[3] Hunter JE, Berry-Kravis E, Hipp H, Todd PK. FMR1 Disorders. In: Adam MP, Bick S, Mirzaa GM, et al., eds. GeneReviews®. University of Washington, Seattle; 1998 [mise à jour le 16 mai 2024].
[4] Hunter J, Rivero-Arias O, Angelov A, Kim E, Fotheringham I, Leal J. Epidemiology of fragile X syndrome: a systematic review and meta-analysis. Am J Med Genet A. 2014;164A(7):1648-1658. doi:10.1002/ajmg.a.36511.
[5] National Fragile X Foundation. Voice of the Patient Report: A Patient-Focused Drug Development Meeting for Fragile X Syndrome. Version du 23 mars 2022.
[6] Ethridge LE, De Stefano LA, Schmitt LM, et al. Auditory EEG biomarkers in fragile X syndrome: clinical relevance. Front Integr Neurosci. 2019;13:60. doi:10.3389/fnint.2019.00060.