Prader-Willi syndrome is a rare genetic disorder with an estimated worldwide incidence of 1 in 10,000 to 1 in 30,000 live births. It is characterized by many physical and intellectual features, varying from one affected person to another. In infancy, signs and symptoms include poor muscle tone, developmental delay and failure to thrive.
More signs and symptoms appear in childhood, including sleep disturbances, intellectual impairment, distinctive facial features, and insatiable appetite. A person with Prader-Willi syndrome may become obese through overeating, which can have life threatening consequences. It cannot be cured at present, and so treatment is symptomatic.
Genes Involved in Prader-Willi Syndrome
People typically receive a set of chromosomes from each parent. Sometimes, gene expression depends on whether or not the chromosome was derived from the mother (maternal) or the father (paternal). This is referred to as genomic imprinting: the genes on one chromosome are turned on (active), and the genes on the other are not.
The genes involved in Prader-Willi syndrome are located on a region of the paternally contributed chromosome 15. Currently, these genes have not all been identified, but are known to exist on the chromosome’s long (q) arm. This region appears to be critical to people’s health and development.
Prader-Willi syndrome occurs because genetic material is missing, inactive, or abnormal. Genes are instructions for products such as proteins, so when genes are missing or not working properly, the products don’t get made or are faulty. This can lead to signs and symptoms in an affected person’s body.
Genetic Mechanisms in Prader-Willi Syndrome
The following mechanisms can cause Prader-Willi syndrome:
- A deletion, where a portion of the paternally contributed chromosome 15 is missing. This is the most common cause of the disorder, accounting for about 70% of cases.
- Uniparental disomy, where two maternal copies of chromosome 15 are inherited. Most other cases are the result of this mechanism.
- A chromosome rearrangement, such as a translocation. This is where part of a chromosome is in the wrong place, affecting its function. This is a rare cause of the disorder.
- Genetic mutation, where a gene or region on the paternally contributed chromosome 15 is faulty, causing genomic imprinting to fail. This is also a rare cause of the disorder.
The first three mechanisms are usually spontaneous errors that aren’t passed down from parent to child. Mutations can be random too, but in rare cases they are inherited.
Genetic Testing and Prader-Willi Syndrome
There are several genetic tests that can detect Prader-Willi syndrome, depending on the genetic mechanism involved. Prenatal testing is also available. For individuals and families looking for information and support on genetic disorders, genetic counseling may be an option.
Read Understanding Genetic Consultation and Understanding Genetic Testing in Healthcare for more details on genetic consultation and testing. For more information on the symptoms, genetic mechanisms and testing of Prader-Willi syndrome, visit the Prader-Willi Syndrome Association USA’s website.
Sources
"Prader-Willi syndrome", updated: 3rd September 2009, GeneReviews (NCBI) website, accessed: 13th May 2010, author: Cassidy, Suzanne and Schwartz, Stuart.
"Prader-Willi syndrome”, Genes and Disease (NCBI) website, accessed: 13th May 2010, no author specified.
"Prader-Willi syndrome", published: 9th May 2010, Genetics Home Reference website, accessed: 13th May 2010, no author specified.
Barbara Melville - Popular introductory articles: Useful Science Basics for Understanding GeneticsUseful Science Basics for Understanding EvolutionBasic ...
