Fragile X Syndrome and Behavioral Phenotype
A 5-year-old boy is referred for evaluation of developmental delay and behavioral problems. He has a history of delayed walking (18 months) and speech delay (first words at 3 years). He has frequent tantrums, hand-flapping, and poor eye contact. He is very active and has difficulty with transitions. On examination, he has a long face, prominent ears, and macroorchidism. He has mild intellectual disability on testing. His mother reports that her brother has similar features and was diagnosed with intellectual disability. There is no history of seizures. Genetic testing confirms a full mutation in the FMR1 gene (>200 CGG repeats) with methylation.
1. What is the diagnosis, and what is the inheritance pattern?
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The diagnosis is Fragile X syndrome (FXS), caused by a full mutation (>200 CGG repeats) in the FMR1 gene on the X chromosome, leading to hypermethylation and silencing of the gene. The inheritance pattern is X-linked dominant with reduced penetrance. Males with the full mutation are typically more severely affected than females. The mother is likely a carrier of a premutation (55-200 repeats) or full mutation. The maternal uncle with intellectual disability suggests the mutation is segregating in the family. Genetic counseling is essential for the family, including discussion of premutation-associated conditions (e.g., fragile X-associated tremor/ataxia syndrome in older carriers, primary ovarian insufficiency in female carriers).
2. What behavioral and medical comorbidities should be screened for in this child?
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Children with FXS are at increased risk for: (1) Autism spectrum disorder (about 30-50% meet criteria); (2) Attention-deficit/hyperactivity disorder; (3) Anxiety disorders, including social anxiety; (4) Seizures (in ~15-20%, often focal or generalized); (5) Recurrent otitis media; (6) Mitral valve prolapse and connective tissue problems; (7) Strabismus; (8) Sleep disturbances; (9) Aggression and self-injury. Screening should include audiology, ophthalmology, cardiac evaluation (echocardiogram if murmur), and EEG if seizures are suspected. Behavioral assessment should include autism-specific measures (e.g., ADOS-2) and ADHD rating scales. Early intervention and behavioral therapy are important.
3. What pharmacological options are available for the behavioral symptoms, and what cautions apply?
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Pharmacological options target specific symptoms: (1) Stimulants (e.g., methylphenidate) for ADHD, but with caution due to increased risk of irritability and sleep disturbance; (2) SSRIs (e.g., sertraline, fluoxetine) for anxiety and mood, starting low and going slow; (3) Atypical antipsychotics (e.g., risperidone, aripiprazole) for severe aggression and irritability, with monitoring for metabolic side effects; (4) Alpha-2 agonists (guanfacine, clonidine) for hyperactivity and sleep; (5) Melatonin for sleep. Avoid serotonergic agents in combination without caution due to potential serotonin syndrome. There are no FDA-approved medications for the core symptoms of FXS. Targeted treatments (e.g., mGluR5 antagonists) have not shown consistent benefit in trials. Behavioral interventions remain the cornerstone.