Parental imprinting and Angelman syndrome

Angelman syndrome is an inherited disorder that includes severe mental retardation and epilepsy. Patients have no speech, puppet-like gait with jerky movements, hyperactivity, disturbed sleep, bouts of inappropriate laughter, a pronounced jaw, and widely spaced teeth. The syndrome results from deletion or mutation within maternal chromosome 15q11-q13. Considerable evidence suggests that the gene or genes responsible for Angelman syndrome are expressed only from the maternal chromosome 15, a situation known as parental imprinting. This epigenetic marking of certain regions of the parental genomes is characterized by parent-of-origin-specific allelic DNA methylation, allele-specific DNA replication timing, and physical pairing of the two chromosome 15 homologues. Imprinting is important for normal development, and its disregulation causes several human disorders. The epilepsy of Angelman syndrome has been studied and indicates a rather typical electroencephalographic abnormality with slowing and notched wave and spikes. Various types of seizures occur, usually including myoclonus and atypical absence. Variable severity among patients suggests potential molecular diversity in the genetic mechanism, possibly the involvement of more than one gene. Angelman syndrome can arise from the following molecular genetic defects: a deletion in 15q11-q13 that covers the Angelman gene or genes, mutations that alter imprinting, and paternal uni-parental disomy for the region. Another 20% or so of patients with clinical symptoms of Angelman syndrome have none of these three defects but are believed to have mutations in one or more genes in the region, and this may be familial. The UBE3A gene, which codes for the enzyme ubiquitin protein ligase involved in protein degradation and processing, has been found to be mutated in many but not all of patients with Angelman syndrome and can be considered a major Angelman candidate gene. Other potential candidate genes in the region include a cluster of three GABAA receptor subunits, which are involved in inhibitory synaptic transmission in the brain. The GABRB3 gene, which codes for the beta 3 subunit, is deleted in most persons with Angelman syndrome. The absence of this gene in mice causes craniofacial abnormalities and neurologic impairment with seizures. The exact role of UBE3A and GABRB3 in the syndrome and their imprinting status are under investigation.