|Classification and external resources|
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. The incidence of PWS is between 1 in 10,000 and 1 in 25,000 live births. The paternal gene origin is lost due to deletion and the maternal genes are silenced due to imprinting. PWS has the sister syndrome Angelman syndrome that includes maternally deleted and paternally imprinted genes in the same genetic region.
PWS affects approximately 1 in 10,000 to 1 in 15,000 newborns. There are more than 400,000 people who live with PWS around the world. It is characterized by hypotonia, short stature, polyphagia, obesity, small hands and feet, hypogonadism, and mild mental retardation.
Traditionally, PWS was diagnosed by clinical presentation. Currently, the syndrome is diagnosed through genetic testing; testing is recommended for newborns with pronounced hypotonia. Early diagnosis of PWS allows for early intervention as well as the early prescription of growth hormone. Daily recombinant growth hormone (GH) injections are indicated for children with PWS. GH supports linear growth and increased muscle mass, and may lessen food preoccupation and weight gain.
The mainstay of diagnosis is genetic testing, specifically DNA-based methylation testing to detect the absence of the paternally contributed Prader-Willi syndrome/Angelman syndrome (PWS/AS) region on chromosome 15q11-q13. Such testing detects over 97% of patients. Methylation-specific testing is important to confirm the diagnosis of PWS in all individuals, but especially those who are too young to manifest sufficient features to make the diagnosis on clinical grounds or in those individuals who have atypical findings. PWS can also be caused by trauma in the birth canal, resulting in individuals with atypical PWS.
Prader-Willi syndrome is often misdiagnosed as Down syndrome, simply because of the relative frequency of Down syndrome compared to PWS. Also, marked obesity can occur in Down syndrome due to behavioral problems. Adding to the confusion, parents of children who already carry a diagnosis of Prader-Willi syndrome may tell friends, family, and even physicians and nurses that their child has Down syndrome because more people have heard of that condition.
Clinical features and signs
Holm et al. (1993) describe the following features and signs as pretest indicators of PWS, although not all will be present.
General physical appearance (adults)
PWS is caused by the deletion of the paternal copies of the imprinted 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). These are on chromosome 15 located in the region 15q11-13. This so-called PWS/AS region may be lost by one of several genetic mechanisms which, in the majority of instances occurs through chance mutation. Other less common mechanisms include; uniparental disomy, sporadic mutations, chromosome translocations, and gene deletions. Due to imprinting, the maternally inherited copies of these genes are virtually silent, only the paternal copies of the genes are expressed. PWS results from the loss of paternal copies of this region. Deletion of the same region on the maternal chromosome causes Angelman syndrome (AS). PWS and AS represent the first reported instances of imprinting disorders in humans.
The risk to the sibling of an affected child of having PWS depends upon the genetic mechanism which caused the disorder. The risk to siblings is <1% if the affected child has a gene deletion or uniparental disomy, up to 50% if the affected child has a mutation of the imprinting control region, and up to 25% if a parental chromosomal translocation is present. Prenatal testing is possible for any of the known genetic mechanisms.
Studies of human and mouse model systems have shown that deletion of the 29 copies of the C/D box snoRNA SNORD116 (HBII-85) has been shown to be the primary cause of Prader-Willi syndrome. 
Individuals with PWS are at risk of learning and attention difficulties. Curfs and Frym (1992) conducted research into the varying degrees of learning disability found in Prader Willi Syndrome (PWS). Their results were as follows:
Cassidy found that 40% of individuals with PWS have borderline/low average intelligence, a figure higher than that found in Curfs and Frym's study (32%). However, both studies suggest that most individuals (50% – 65%) fall within the mild/borderline/low average intelligence range.
Children with PWS show an unusual cognitive profile. They are often strong in visual organization and perception, including reading and vocabulary, but their spoken language (sometimes affected by hypernasality) is generally poorer than their comprehension. A marked skill in completing jigsaw puzzles has been noted.
Auditory information processing and sequential processing are relatively poor, as are arithmetic and writing skills, visual and auditory short term memory and auditory attention span. These sometimes improve with age, but deficits in these areas remain throughout adulthood.
Prader-Willi syndrome is also frequently associated with an extreme and insatiable appetite, often resulting in morbid obesity. There is currently no consensus as to the cause for this particular symptom, although genetic abnormalities in chromosome 15 disrupt the normal functioning of the hypothalamus. Given that the hypothalamus regulates many basic processes, including appetite, there may well be a link. However, no organic defect of the hypothalamus has been discovered on post mortem investigation.
Prader-Willi syndrome patients have high ghrelin levels, which are thought to directly contribute to the increased appetite, hyperphagia, and obesity seen in this syndrome. Cassidy states the need for a clear delineation of behavioural expectations, the reinforcement of behavioural limits and the establishment of regular routines.
The main mental health difficulties experienced by people with PWS include compulsive behaviour (usually manifested in skin-picking) and anxiety (Udwin, 1998; Clark et al, 1995). Psychiatric symptoms, for example, hallucinations, paranoia and depression have been described in some cases (Udwin, 1998) and affect approximately 5 - 10% of young adults ((Cassidy, 1997). Psychiatric and behavioural problems are the most common cause of hospitalization (Cassidy et al, 1994).
There are several aspects of PWS that support the concept of growth hormone deficiency in individuals with PWS. Specifically, individuals with PWS have short stature, are obese with abnormal body composition, have reduced fat free mass (FFM), have reduced LBM and total energy expenditure, and have decreased bone density.
PWS is characterized by hypogonadism. This is manifested as undescended testes in males and benign premature adrenarche in females. Testes may descend with time or can be managed with surgery or testosterone replacement. Adrenarche may be treated with hormone replacement therapy.
Prader-Willi syndrome has no cure, however, several treatments are in place to lessen the condition's symptoms. During infancy, subjects should undergo therapies to improve muscle tone. Speech and occupational therapy are also indicated. During the school years, children benefit from a highly structured learning environment as well as extra help. The largest problem associated with the syndrome is severe obesity.
Prescription of daily recombinant growth hormone injections are indicated for children with PWS. GH supports linear growth and increased muscle mass, and may lessen food preoccupation and weight gain.
Because of severe obesity, obstructive sleep apnea is a common sequela, and a positive airway pressure machine is often needed.
A sufferer from Prader-Willi Syndrome featured in the episode entitled 'Dog Eat Dog' of the television series CSI: Crime Scene Investigation (aired on November 24, 2005).
21.Cassidy,SB. 'Prader Willi Syndrome'. American Journal of Medical Genetics 1997:34:917-923.
22.Cassidy,SB; Devi,A; Mukaida,C; 'Aging in PWS: 232 patients over age 30 years'. Proc. Greenwood Genetic Centre 1994: 13: 102-3.
23.Clark,DJ; Boer,H; Webb T. 'General and behavioural aspects of PWS: a review.' Mental Health Research 1995: vol.8, No. 195. Pp 38-49.
24.Udwin,O. 'PWS Update'. CaF Directory of Specific Conditions and Rare Syndromes 1998: 5th Ed.