Basic Neurosciences · Paper A
Fragile X syndrome
Because Fragile X syndrome is the commonest inherited cause of intellectual disability (learning difficulty present from birth), recognising its physical and behavioural pattern as a set matters for genetic counselling: e.g. a boy with a long face, prominent ears, and marked social anxiety with gaze avoidance shows the classic combination. Clinically, spotting the full pattern together, not any single feature, is what points toward the diagnosis.
Fragile X syndrome is caused by an expansion of a CGG trinucleotide repeat in the FMR1 gene on the X chromosome, with more than 200 repeats defining a full mutation that silences the gene and stops FMRP (fragile X mental retardation protein) being made. It is the commonest inherited cause of intellectual disability and a major genetic cause of autism spectrum disorder. The phenotype includes a long face, large or protruding ears, macro-orchidism (enlarged testes, appearing after puberty), joint hyperlaxity (excessively flexible joints), marked social anxiety, and gaze avoidance. The speech profile similarly spares grammatical syntax while showing perseverationPersistently repeating a word, phrase, or action after it is no longer relevant or has already been asked for.Read more →, tangential language, and echolaliaAutomatically repeating another person's spoken words.Read more →, often together with cluttering (rapid, disorganised speech). Like other trinucleotide-repeat disorders, it shows anticipation, meaning the repeat and the disease severity can worsen across generations: e.g. a teenage boy with intellectual disability, prominent ears, and macro-orchidism, whose maternal uncle has milder learning difficulties, fits this X-linked inheritance pattern. In practice, considering Fragile X specifically whenever intellectual disability is combined with these characteristic physical features avoids missing a diagnosis relevant to family planning.
A fresh doctor might treat intellectual disability as a single undifferentiated category. Fragile X syndrome earns its own exam topic because it is the commonest INHERITED cause of intellectual disability, has a recognisable physical and behavioural pattern, and carries distinct risks for adult carriers that intellectual disability alone would never predict. Hold the phrase INHERITED CAUSE through every section below.
Fragile X syndrome is caused by an expansion of a CGG trinucleotide repeat (three DNA letters, cytosine-guanine-guanine, repeated many times in a row) in the FMR1 gene on the X chromosome. When the repeat expands beyond roughly 200 copies (a "full mutation"), the gene is switched off (silenced), so FMRP (fragile X mental retardation protein, needed for normal brain development) is not produced. It is the commonest inherited cause of intellectual disability and a significant genetic cause of autism spectrum disorder.
A common mistake is diagnosing Fragile X syndrome from intellectual disability plus a vague description of "unusual facial features" alone. Several genetic syndromes overlap on facial dysmorphism and intellectual disability, so the specific COMBINATION (long face, large ears, macro-orchidism after puberty, and marked social anxiety with gaze avoidance) is what should be weighed together, not any single feature or intellectual disability in isolation.
Imagine a mother who carries a smaller, "premutation" repeat expansion (55 to 200 repeats) that does not itself cause Fragile X syndrome but can expand further when passed to her children. Because FMR1 sits on the X chromosome, sons who inherit the full mutation are typically more severely affected than daughters, who have a second, usually unaffected, X chromosome to partly compensate. Like other trinucleotide-repeat disorders, Fragile X shows ANTICIPATION: repeats tend to expand further with each generation, particularly through maternal transmission, so severity and likelihood of the full mutation can increase across a family tree.
The classic physical features include a long, narrow face, large or protruding ears, a prominent jaw, and macro-orchidism (enlarged testes), which typically appears only after puberty and is a particularly useful late-onset clue. Joint hyperlaxity (excessively flexible joints) and a high-arched palate are also described. Behaviourally, Fragile X syndrome is characterised by marked social anxiety, gaze avoidance (avoiding eye contact), and hand-flapping or hand-biting, and it carries a high rate of comorbid autism spectrum disorder and (attention-deficit hyperactivity disorder). This social-anxiety-plus-gaze-avoidance pattern is a useful discriminator from other genetic intellectual disability syndromes that present with an unusually sociable, outgoing manner instead.
Williams syndrome presents with an unusually sociable, overly friendly manner and cardiovascular abnormalities, the opposite social pattern to Fragile X's anxious gaze-avoidant profile: a child with intellectual disability who is markedly over-friendly points toward Williams syndrome, not Fragile X. Down syndrome (trisomy 21) has its own distinct facial phenotype and is the commonest genetic cause of intellectual disability overall (though not the commonest INHERITED cause, since most Down syndrome arises from a new, non-inherited chromosomal error). Prader-Willi and Angelman syndromes, both linked to chromosome 15, present with hyperphagia and obesity, or with a happy, easily-laughing demeanour with severe intellectual disability and , respectively, and neither shows the CGG repeat mechanism of Fragile X.
Diagnosis is confirmed by genetic testing for the FMR1 CGG repeat expansion, typically using PCR and Southern blot analysis to size the repeat and distinguish a premutation from a full mutation. Testing is prompted clinically by the combination described above, developmental delay or intellectual disability plus the characteristic physical and behavioural features, rather than by any single feature. Once a full mutation is confirmed in a child, cascade testing (offering testing to close relatives) is appropriate, since maternal relatives may unknowingly carry a premutation with its own distinct risks.
There is no cure; management is supportive and multidisciplinary, including speech and language therapy, educational support tailored to intellectual disability, and behavioural or pharmacological management of comorbid or anxiety where clinically significant, following standard child and adolescent psychiatry and paediatric practice. NICE-aligned genetics referral pathways guide family counselling once a full mutation is identified, since siblings and maternal relatives may carry premutations. Genetic counselling is central to management here in a way that is not true for most non-inherited intellectual disability, because it directly informs reproductive decisions and surveillance for premutation-carrier-specific conditions in the wider family.
Questions often describe an adult female relative of a boy with Fragile X syndrome presenting later in life with tremor and unsteady gait (): this describes FXTAS (fragile X-associated tremor/ataxia syndrome), a premutation-carrier-specific late-onset movement disorder, not the full childhood syndrome. Another trap describes a premutation-carrier woman with earlier-than-expected menopause, testing recognition of premature ovarian insufficiency as a distinct premutation-carrier risk, separate from intellectual disability, which only occurs with the full mutation.
Fragile X syndrome, from a CGG repeat expansion silencing FMR1, is the commonest inherited cause of intellectual disability, combining a long face, large ears, macro-orchidism after puberty, joint hyperlaxity, and marked social anxiety with gaze avoidance, plus high rates of autism spectrum disorder and , confirmed by genetic testing and managed with lifelong multidisciplinary and genetic-counselling support rather than a cure. When the vignette adds "enlarged testes after puberty" to intellectual disability, think Fragile X before anything else on the list.