Prader-Willi Syndrome
What is Prader-Willi syndrome?
Prader-Willi syndrome is a complex genetic condition that affects many parts of the body. In infancy, this condition is characterised by weak muscle tone (hypotonia), feeding difficulties, poor growth, and delayed development. Beginning in childhood, affected individuals develop an insatiable appetite and chronic overeating (hyperphagia), obesity, and short stature. People with Prader-Willi syndrome typically also have mental retardation or learning disabilities and behavioural problems. Some affected individuals also have unusually fair skin and light-coloured hair.
How common is Prader-Willi syndrome?
Prader-Willi syndrome affects an estimated 1 in 10,000 to 25,000 people.
What are the genetic changes related to Prader-Willi syndrome?
Prader-Willi syndrome is a chromosomal condition related to chromosome 15.
The OCA2 gene is associated with Prader-Willi syndrome.
This condition is caused by the loss of active genes in a specific region of chromosome 15. People normally have two copies of this chromosome in each cell, one copy from each parent. Some genes on this chromosome, however, are active only when they are inherited from a person's father (the paternal copy). Prader-Willi syndrome occurs when the region of the paternal chromosome 15 containing these genes is missing.
Researchers are working to identify genes on chromosome 15 that are responsible for the characteristic features of Prader-Willi syndrome. They have not yet definitively connected many genes with specific signs and symptoms. Researchers have determined, however, that a deletion of the OCA2 gene on chromosome 15 is associated with unusually fair skin and light-coloured hair in some affected individuals.
Most cases of Prader-Willi syndrome (about 70 percent) occur when a segment of the paternal chromosome 15 is deleted. In about 25 percent of cases, a person with Prader-Willi syndrome has two copies of chromosome 15 inherited from his or her mother (maternal copies) instead of one copy from each parent. This phenomenon is called maternal uniparental disomy. Rarely, Prader-Willi syndrome can also be caused by a chromosomal rearrangement called a translocation, or by a mutation or other defect that abnormally inactivates genes on the paternal chromosome 15. Each of these genetic changes results in a loss of gene function in a critical region of chromosome 15.
Can Prader-Willi syndrome be inherited?
Most cases of Prader-Willi syndrome are not inherited, particularly those caused by a deletion in the paternal chromosome 15 or by maternal uniparental disomy. These genetic changes occur as random events during the formation of reproductive cells or in early fetal development. Affected people typically have no history of the disorder in their family.
Rarely, a genetic change responsible for Prader-Willi syndrome can be inherited. For example, it is possible for a genetic defect that abnormally inactivates genes on the paternal chromosome 15 to be passed from one generation to the next.
What other names do people use for Prader-Willi syndrome?
- Prader-Labhart-Willi syndrome
- PWS
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