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Angelman syndrome

Angelman syndrome (AS) is neurological disorder in which severe learning difficulties are associated with a characteristic facial appearance and behavior. more...

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History

Dr. Harry Angelman, a pediatrician working in Warrington, Cheshire, first reported three children with this condition in 1965. It was initially presumed to be rare. In 1987, it was first noted that around half of the children with Angelman syndrome have a small piece of chromosome 15 missing (chromosome 15q partial deletion). Since this time the condition has been reported more frequently and the incidence is now thought to be 1 in 15,000 children.

Pathophysiology

Angelman syndrome is caused by the loss of the the normal maternal contribution to a region of chromosome 15, most commonly by deletion of a segment of that chromosome. Other causes include uniparental disomy, translocation, or single gene mutation in that region. A healthy person receives two copies of chromosome 15, one from mother, the other from father. However, in the region of the chromosome that is critical for Angelman syndrome, the maternal and paternal contribution express certain genes very differently. This is due to sex-related epigenetic imprinting; the biochemical mechanism is DNA methylation. If the maternal contribution is lost, the result is Angelman syndrome. (When the paternal contribution is lost, by similar mechanisms, the result is Prader-Willi syndrome.)

Angelman syndrome can also be the result of mutation of a single gene. This gene (Ube3a, part of the ubiquitin pathway) is present on both the maternal and paternal chromosomes, but differs in the pattern of methylation (Imprinting). The paternal silencing of the Ube3a gene occurs in a brain region-specific manner; the maternal allele is active almost exclusively in the hippocampus and cerebellum. The most common genetic defect leading to Angelman syndrome is an ~4Mb (mega base) maternal deletion in chromosomal region 15q11-13 causing an absence of Ube3a expression in the maternally imprinted brain regions. Ube3a codes for an E6-AP ubiquitin ligase, which chooses its substrates very selectively and the four identified E6-AP substrates have shed little light on the possible molecular mechanisms underlying the human Angelman syndrome mental retardation state.

Initial studies of mice that do not express maternal Ube3a show severe impairments in hippocampal memory formation. Most notably, there is a deficit in a learning paradigm that involves hippocampus-dependent contextual fear conditioning. In addition, maintenance of long-term synaptic plasticity in hippocampal area CA1 in vitro is disrupted in Ube3a -/- mice. These results provide links amongst hippocampal synaptic plasticity in vitro, formation of hippocampus-dependent memory in vitro, and the molecular pathology of Angelman syndrome.

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Prader lacks fader; Angelman misses mom? - Prader-Willi syndrome; Angelman syndrome
From Science News, 11/18/89 by R. Weiss

Prader lacks fader; Agelman misses mom?

At first glance, the two diseases have little in common. People with Prader-Willi syndrome display a lack of muscle tone, an odd facial structure, obesity and a low IQ. People with Angelman syndrome show severe retardation, puppet-like movements and uncontrollable bouts of laughter.

Yet new research suggests these diseases represent two sides of the same genetic coin. Moreover, geneticists examining the syndromes' molecular bases say Prader-Willi now appears to be the first concrete example in humans of a poorly understood genetic phenomenon called genetic imprinting, as well as the best evidence yet linking imprinting to human disease (SN: 5/20/89, p.312).

Most Prader-Willi cases occur when a particular segment of chromosome 15 gets deleted during embryonic development. Oddly, the syndrome results only when that deletion occurs in the chromosome 15 contributed by the father; the maternally contributed chromosome 15 in these patients is normal. But in about 40 percent of Prader-Willi cases, geneticists find no such deletion.

Robert D. Nicholls, Joan H.M. Knoll and Marc Lalande of the Children's Hospital in Boston and their co-workers performed genetic analyses on six families with Prader-Willi children who lack the characteristic chromosomal deletion. In all cases they found the patients had two maternal chromosome 15 segments and no paternal copy -- the result of a rare genetic error during early development. The syndrome provides "the first absolutely clear evidence in humans that it does make a difference which parent a gene comes from," comments geneticist Judith G. Hall of the University of British Columbia in Vancouver. Moreover, preliminary evidence suggests Angelman syndrome results when individuals inherit two paternal copies of the same segment on chromosome 15. The researchers reported their findings this week in Baltimore at the annual meeting of the American Society of Human Genetics and in the Nov. 16 NATURE.

Scientists have suspected that genetic imprinting -- a difference in gene expression dependent upon which parent contributed that gene -- might play a critical role in fetal and adult development, and may account for the unusual inheritance patterns of some diseases. Nicholls, now at the University of Florida College of Medicine in Gainesville, says the new findings should add to scientists' understanding of how genes regulate the exression of other genes -- knowledge that may someday aid in the diagnosis and treatment of genetic diseases.

COPYRIGHT 1989 Science Service, Inc.
COPYRIGHT 2004 Gale Group

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