Angelman Syndrome

Angelman syndrome (AS) is a neurogenetic disorder caused by loss of function of the maternally inherited UBE3A gene on chromosome 15q11-q13. The UBE3A gene encodes ubiquitin protein ligase E3A, an enzyme critical for synaptic development, neuronal plasticity, and protein degradation in the central nervous system. In neurons, the paternal copy of UBE3A is normally silenced through genomic imprinting (the UBE3A antisense transcript silences the paternal allele), so neurons depend entirely on the maternal copy. Four genetic mechanisms cause AS: (1) de novo deletion of the maternal 15q11-q13 region (70% of cases, most severe phenotype), (2) paternal uniparental disomy — both copies of chromosome 15 inherited from father (2-3%), (3) imprinting center defects (3-5%), and (4) point mutations in UBE3A itself (10-15%). Loss of UBE3A disrupts ubiquitin-mediated proteasomal degradation of synaptic proteins, impairing GABAergic and glutamatergic neurotransmission. This produces the characteristic phenotype: severe intellectual disability, absent or minimal speech, seizures (beginning by age 3 in >80%), ataxic gait with characteristic jerky limb movements, and a uniquely happy demeanor with frequent laughing and smiling. The seizure disorder is typically refractory to monotherapy...