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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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Neurologic Disorder: Look for Congenital Syndrome
From OB/GYN News, 3/1/00 by Barbara Baker

WAIKOLOA VILLAGE, HAWAII -- Minor congenital anomalies can be an important tip-off that mental retardation or another neurologic disorder in childhood is caused by a congenital syndrome rather than a problem that occurred during labor and delivery.

It's therefore critical that a child be examined by a clinical geneticist or dysmorphologist whenever malpractice litigation cites fetal distress or perinatal asphyxia as the cause of a childhood neurologic disorder, Dr. Aubrey Milunsky advised at a conference on obstetrics, gynecology perinatal medicine, neonatology, and the law.

Minor congenital malformations may not be obvious, and a general pediatrician may not recognize them, said Dr. Milunsky, professor of human genetics, pediatrics, ob.gyn., and pathology at Boston University.

Examples of minor malformations include the absence of a hair whorl, extra nipples, frontal bossing, iris freckles, tapered fingers, and a cleft uvula. "The list is very long," he noted at the meeting sponsored by the university.

More than 2,000 congenital syndromes have been classified so far. "You don't have to know all of them," he said. But a clinical geneticist will be able to detect the most subtle of minor malformations, input them into a computer, and then search several databases to come up with a differential diagnosis.

About 10% of children with two minor malformations have a major abnormality such as a cardiac malformation or a neurologic disorder, such as mental retardation, developmental delay, seizures, spasticity, and psychosis. About 20% of children with three or more minor malformations have a major abnormality, he said.

The presence of minor malformations suggests that associated neurologic problems originated early in gestation, particularly at 6 to 12 weeks' gestation, rather than during labor and delivery.

"It's not surprising that abnormalities of the skin, hair, and nails may reflect a brain abnormality, given their shared embryological ectodermal origins," Dr. Milunsky said.

The cause of these syndromes may be inherited genes; a new gene mutation; or environmental factors, such as maternal exposure to an infection, drug, toxin, radiation, or heat.

Examples of congenital syndromes with neurologic dysfunction that could erroneously be attributed to perinatal asphyxia include fragile X syndrome, Prader-Willi syndrome, and Angelman syndrome, which resembles cerebral palsy.

Babies with these syndromes may very well have had problems during labor and delivery, "but it's axiomatic that bad babies, with congenital defects, do badly in labor. The mere fact that things are difficult in labor does not mean that the labor was the cause of the problem," Dr. Milunsky said.

During the discussion period, Dr. Maurice L. Druzin noted that babies with chromosomal and/or structural abnormalities often have abnormal antepartum and intrapartum fetal heart rate tracings. Normal fetal heart rate tracings in labor usually predict infants will be normal.

But if infants have normal fetal heart rate tracings during delivery and then suddenly decompensate afterward, it's critical to quickly think about a congenital anomaly, such as a diaphragmatic hernia or cardiac abnormalities, or group B strep sepsis, said Dr. Druzin, chief of maternal fetal medicine at Stanford (Calif.) University.

"Look for these causes so that the words 'perinatal asphyxia' do not appear on the chart. Once they appear, they're set in stone and you have to fight to disavow that diagnosis," he warned.

COPYRIGHT 2000 International Medical News Group
COPYRIGHT 2001 Gale Group

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