Introduction and Epidemiology
- Recognized as the most common inherited cause of intellectual disability and autism spectrum disorder.
- Incidence is approximately 1 in 4000 to 5000 males and 1 in 8000 females.
- Accounts for 30-50% of X-linked intellectual disability.
Genetics and Pathogenesis
- Caused by a dynamic mutation involving the expansion of CGG trinucleotide repeats in the 5' untranslated region of the FMR1 gene located on chromosome Xq27.3.
- Exhibits anticipation, known as the Sherman paradox, where repeats expand in successive generations primarily through maternal transmission.
- Inheritance follows an X-linked dominant pattern with reduced penetrance in females secondary to X-inactivation.
- Hypermethylation of the expanded sequence causes transcriptional silencing and the subsequent absence of the fragile X mental retardation protein (FMRP).
- FMRP regulates mRNA translation at synapses; its loss impairs synaptic plasticity and dendritic spine maturation, leading to cellular hyperexcitability.
CGG Repeat Classification
| Allele Type | Repeat Number | Clinical Consequence |
|---|---|---|
| Normal | <45 | Unaffected phenotype. |
| Intermediate | 45-54 | Unaffected, but may exhibit instability during meiosis. |
| Premutation | 55-200 | Associated with RNA toxicity leading to Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) or Primary Ovarian Insufficiency (FXPOI). |
| Full Mutation | >200 | Classic Fragile X syndrome resulting from absent FMRP. |
Clinical Features
| Patient Group | Characteristic Manifestations |
|---|---|
| Affected Males | Moderate to severe intellectual disability (IQ 40-55), language delay, and echolalia. Autism spectrum features (60-80%), hand flapping, and hyperactivity. Long narrow face, prominent forehead, large protruding ears, and macrocephaly. Hallmark post-pubertal macroorchidism (>30-50 mL). Seizures (15-20%) and mitral valve prolapse. |
| Affected Females | Milder phenotype seen in 50% due to skewed X-inactivation. Mild intellectual disability (IQ 70-85), emotional lability, anxiety, and premature ovarian failure. |
| Premutation Carriers | Females face a 20-25% risk of premature menopause before age 40 (FXPOI). Males over 50 years risk developing FXTAS, characterized by tremor, ataxia, parkinsonism, and cognitive decline. |
Investigations
- Molecular genetic testing of FMR1 CGG repeat size and methylation status represents the gold standard for diagnosis.
- Triplet repeat primed PCR (TP-PCR) or capillary electrophoresis serves as the first-line test to detect all alleles including large expansions.
- Southern blot with methylation analysis confirms full mutations, evaluates methylation status, and identifies mosaicism.
- Standard PCR alone is inadequate as it fails to amplify large expansions.
Management Principles
- Requires multidisciplinary care emphasizing early intervention, including speech, occupational, and physical therapy.
- Behavioral therapy utilizing applied behavior analysis for autism features.
- Pharmacotherapy includes stimulants for attention deficit hyperactivity disorder, selective serotonin reuptake inhibitors for anxiety, and valproate or levetiracetam for seizures. Carbamazepine should be avoided if possible.
Genetic Counselling
- Affected males transmit the premutation to all daughters, confirming no male-to-male transmission.
- Carrier females face a 50% chance of transmitting the mutation; the risk of expansion to a full mutation corresponds with maternal repeat size, exceeding 90% if >100 repeats are present.
- Prenatal diagnosis is accessible via chorionic villus sampling or amniocentesis utilizing TP-PCR.