Angle man syndrome
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"Angelman syndrome"[MeSH Terms]
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I have all the information needed for a thorough answer. Here is a comprehensive overview of Angelman Syndrome (AS):
Angelman Syndrome
Overview
Angelman syndrome (AS) is a neurogenetic disorder caused by loss of function of the maternally derived UBE3A gene on chromosome 15q11-q13. It occurs in approximately 1 in 15,000 births and was formerly nicknamed "happy puppet syndrome" due to its behavioral phenotype.
Clinical Features
| Feature | Detail |
|---|---|
| Behavioral | Happy demeanor, unprovoked/inappropriate laughter, hand-flapping, fascination with water |
| Neurological | Severe intellectual disability, absent or minimal speech |
| Motor | Ataxic gait, tremulous limb movements, hypotonia |
| Seizures | Characteristic and often severe; begin in infancy/early childhood |
| Craniofacial | Microcephaly |
| Other | Sleep disturbance, feeding difficulties (infancy), scoliosis, strabismus, constipation, GERD |
Genetics and Molecular Basis
AS is a classic example of genomic imprinting - a parent-of-origin effect where gene expression depends on which parent transmitted the allele.
The key gene is UBE3A (ubiquitin-protein ligase E3A), located at 15q11.2-q13:
- In most tissues, UBE3A is expressed from both alleles
- In the brain (particularly hippocampus and Purkinje cells of the cerebellum), only the maternal copy is active - the paternal copy is imprinted (silenced) by an antisense RNA transcript
- When the maternal UBE3A is lost, there is no functional UBE3A in the brain
- Loss of UBE3A inhibits synapse formation and synaptic plasticity
Mechanisms (four distinct causes)
| Mechanism | Frequency | Detail |
|---|---|---|
| Deletion of maternal 15q11.2-q13 | ~70% | Most common; 5 Mb interstitial deletion |
| Paternal uniparental disomy (UPD) | 20-25% | Two paternal copies of chr 15; no maternal UBE3A |
| Imprinting centre (IC) defect | ~5% | Maternal chromosome carries paternal imprint; mildest phenotype; risk of recurrence |
| UBE3A point mutation | ~10% | Pathogenic variant in UBE3A on maternal allele |
| Unknown | ~10% | Molecular defect not identified |
The imprinting control region (ICR/AS ICR) maps upstream of SNURF/SNRPN. An antisense RNA from the paternal allele silences UBE3A in neurons - this is why paternal UPD (having two paternal chromosomes) also causes AS.
Comparison with Prader-Willi Syndrome
Both AS and Prader-Willi syndrome (PWS) involve the same 15q11-q13 region, making them the classic teaching example of genomic imprinting:
| Feature | Angelman Syndrome | Prader-Willi Syndrome |
|---|---|---|
| Deleted chromosome | Maternal | Paternal |
| Key gene | UBE3A (single gene) | Multiple genes (SNORP family) |
| Phenotype | Happy affect, seizures, ataxia, no speech | Obesity, hyperphagia, hypogonadism, short stature, hypotonia |
| UPD type causing disease | Paternal UPD | Maternal UPD |
Diagnosis
- Methylation testing - detects ~80% of cases (detects abnormal maternal imprinting, maternal deletion, or paternal UPD)
- UBE3A sequencing - detects point mutations (~10%)
- Together these identify nearly all cases
- Chromosomal microarray can identify deletions
Management
There is no cure; management is supportive:
- Seizures: Antiepileptic drugs (AEDs) - avoid carbamazepine, vigabatrin, and tiagabine as AS is associated with movement abnormalities that these can worsen
- Communication: Speech therapy focused on nonverbal communication (AS patients have severe speech impairment)
- Sleep: Sedatives for nighttime wakefulness (sleep disturbance is common)
- Surveillance: Monitor for scoliosis, strabismus, GERD, constipation, feeding issues, behavior problems
- Physiotherapy: For gait and motor difficulties
Recurrence Risk
- Deletion or UPD: Very low recurrence risk (<1%)
- IC defect: Appreciable recurrence if mother carries the defect (up to 50%); even if mother tests negative, gonadal mosaicism gives non-trivial risk
- UBE3A point mutation: If maternally inherited, 50% recurrence risk
Sources: Robbins Pathologic Basis of Disease, p. 174 | Emery's Elements of Medical Genetics and Genomics | Harriet Lane Handbook, 23rd ed. | Principles of Neural Science (Kandel)
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