Prader-Willi Syndrome
Aidan and Trey

Nature of the Genetic Difficulty

Although the cause is complex it results from an abnormality on the 15th chromosome. The abnormality is the Chromosome partial deletion of the long ("q") arm of one copy of chromosome 15. It is either entirely deleted or partially deleted. If the mother's copy of the chromosomal arm is deleted, Angelman syndrome results. If the father's copy of the chromosomal arm is deleted, Prader-Willi syndrome results, this is the reason why Angelman and Prader-Willi are called sister diseases.The smallest observed region that can result in this syndrome when deleted is called the PWS/AS critical region (due to the closeness of the diseases) and is situated at 15q11-q13 on the chromosomal map. The Genes deleted in this region are the SNRPN and necdin genes along with clusters of snoRNAs: SNORD64, SNORD107, SNORD108 and two copies of SNORD109, 29 copies of SNORD116 (HBII-85) and 48 copies of SNORD115 (HBII-52) gene. I really don't know exactally what each of these genes do, but the symptoms that their absences cause will be explained in depth further on in the presentation. In addition to deletions, uniparental disomy of chromosome 15 also causes Prader-Willi syndrome (receiving two copies of Chromosome 15 from one parent and none from the other), this is just further proof that the genomic imprinting responsible for PWS must occur in this region.
Is there a point mutation, or any insertion/deletion entirely inside one gene? Is there a (gene or genes) missing? A whole chromosone extra, missing, or both? Or might the gene be extended in length?

Basis Facts of Disease


Prader-Willi syndrome (abbreviated PWS) is a very rare genetic disorder in which seven genes (or some subset thereof) on chromosome 15 (q 11-13) are deleted or unexpressed (chromosome 15q partial deletion) on the paternal chromosome. It was first described in 1956 by Andrea Prader, Heinrich Willi, Alexis Labhart, Andrew Ziegler, and Guido Fanconi of Switzerland. Sadly for all these guys, only Prader and Willi got their names on it...poor Fanconi.
PWS occurs in males and females equally and in all races. Prevalence estimates have ranged from 1:8,000 to 1:25,000 with the most likely figure being 1:15,000.
There are many symptoms associated with PWS, but the most common are, low muscle tone (hypotonia), short stature (if not treated with growth hormone), incomplete sexual development, and a chronic feeling of hunger that, coupled with a metabolism that utilizes drastically fewer calories than normal, can lead to excessive eating and life-threatening obesity (extreme childhood obesity is a sure sign of PWR). The Average IQ of a person with PWS is around 70, but ranges from as low as 20 to as high as 100 (average) although it is amply evident that PWS on a whole inhibits many of an individuals higher cognitive functions. Social and motor deficits also exist in a majority of persons afflicted with PWS.


At birth the infant typically has low birth weight for gestation, (weak muscles), and difficulty sucking due to the hypotonia which can lead to a diagnosis of failure to thrive. The second stage (“thriving too well”), has a typical onset between the ages of two and five, but can be later. The hyperphagia (extreme unsatisfied drive to consume food) lasts throughout the lifetime. Children with PWS have sweet and loving personalities, but this phase is also characterized by increased appetite, weight control issues, and motor development delays along with some behavior problems and unique medical issues. Obesity represents the greatest problem to health. Limiting caloric intake will control the obesity but the family, neighbors, school, and other institutions must cooperate closely as the child will attempt to obtain food wherever possible. Exercise can increase lean body mass in children with Prader-Willi syndrome.
Growth hormone has also been shown to improve physical strength and agility in patients with Prader-Willi syndrome. There have been some concerns regarding the effect of GH on lung function in children with this condition. Parents should discuss the possible side effects with the child's doctor.
A very small penis in the male infant may be corrected with testosterone.
Low levels of sex hormones may be corrected at puberty with hormone replacement.

Sources Cited