Prader–Willi syndrome

Prader–Willi syndrome (/ˈprɑːdər ˈvɪli/; abbreviated PWS) is a 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 (1919–2001), Heinrich Willi (1900–1971), Alexis Labhart (1916–1994), Andrew Ziegler, and Guido Fanconi of Switzerland.^[1] Characteristic of PWS is "low muscle tone, short stature, incomplete sexual development, cognitive disabilities, behavior problems, and a chronic feeling of hunger that can lead to excessive eating and life-threatening obesity."^[2] The incidence of PWS is between 1 in 25,000 and 1 in 10,000 live births.

The paternal origin of the genetic material that is affected in the syndrome is important because the particular region of chromosome 15 involved is subject to parent-of-origin imprinting, meaning that for a number of genes in this region, only one copy of the gene is expressed while the other is silenced through imprinting. For the genes affected in PWS, it is the maternal copy that is usually imprinted (and thus is silenced), while the mutated paternal copy is not functional.^[3] This means that while most people have one working and one silenced set of these genes, people with PWS have a non-working set and a silenced set. If the maternally derived genetic material from the same region is affected instead, the sister Angelman Syndrome is the result.^[4]

With the recent benefits of early diagnosis and ongoing interventions, the obesity rate among children with Prader–Willi Syndrome has decreased to be similar to the typical population. With behavioural therapy and other treatments, the effects of the syndrome can be reduced.^[5]

Signs and symptoms

There are many signs and symptoms of Prader-Willi Syndrome. The symptoms can range from poor muscle tone during infancy to behavioral problems in early childhood. Some symptoms that are usually found in infants, besides poor muscle tone, would be a lack of eye coordination; some are born with almond-shaped eyes; and due to poor muscle tone the infant may not have a strong sucking reflex. Their cry is weak, and they have difficulty waking up. Another sign of this disease is a thin upper lip.

Holm et al. (1993) describe the following features and signs as pretest indicators of PWS, although not all will be present.

Uterus and birth

• Reduced fetal movement
• Frequent abnormal fetal position
• Occasional polyhydramnios (excessive amniotic fluid)
• Often breech or caesarean births
• Lethargy
• Hypotonia
• Feeding difficulties (due to poor muscle tone affecting sucking reflex)
• Difficulties establishing respiration
• Hypogonadism

Childhood

• Delayed milestones/intellectual delay
• Excessive sleeping
• Strabismus ('crossed eyes')
• Scoliosis (often not detected at birth)
• Cryptorchidism
• Speech delay
• Poor physical coordination
• Hyperphagia (over-eating) begins between the age of 2 and 8, and continues on throughout adulthood. Noted change from feeding difficulties in infancy.
• Excessive weight gain
• Sleep disorders
• Delayed puberty
• Short stature
• Obesity
• Extreme flexibility

Adulthood

• Infertility (males and females)
• Hypogonadism
• Sparse pubic hair
• Obesity
• Hypotonia (low muscle tone)
• Learning disabilities/borderline intellectual functioning (but some cases of average intelligence)
• Prone to diabetes mellitus
• Extreme flexibility

Physical appearance

• Prominent nasal bridge
• Small hands and feet with tapering of fingers
• Soft skin, which is easily bruised
• Excess fat, especially in the central portion of the body
• High, narrow forehead
• Thin upper lip
• Downturned mouth
• Almond-shaped eyes
• Light skin and hair relative to other family members
• Lack of complete sexual development
• Frequent skin picking
• Striae
• Delayed motor development

Neuro-cognitive

Individuals with PWS are at risk of learning and attention difficulties. Curfs and Fryns (1992) conducted research into the varying degrees of learning disability found in PWS.^[6] Their results, using a measure of IQ, were as follows:

• 5%: IQ above 85 (high to low average intelligence)
• 27%: IQ 70–85 (borderline intellectual functioning)
• 39%: IQ 50–70 (mild intellectual disability)
• 27%: IQ 35–50 (moderate intellectual disability)
• 1%: IQ 20–35 (severe intellectual disability)
• <1%: IQ <20 (profound intellectual disability)

Cassidy found that 40% of individuals with PWS have borderline/low average intelligence,^[7] a figure higher than the 32% found in Curfs and Fryns' study.^[6] However, both studies suggest that most individuals (50–65%) fall within the mild/borderline/low average intelligence range.

Parents report that some children have IQs >110 and function normally in school.

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,^[8][9] but this may be an effect of increased practice.^[10]

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.^[8]

Behavioral

Prader–Willi syndrome is frequently associated with an extreme and insatiable appetite, often resulting in morbid obesity. It is the most common genetic cause of morbid obesity in children.^[11] There is currently no consensus as to the cause for this symptom, although genetic abnormalities in chromosome 15 disrupt the normal functioning of the hypothalamus.^[7] Given that the hypothalamus arcuate nucleus regulates many basic processes, including appetite, there may well be a link. In the hypothalamus of people with PWS, nerve cells that produce oxytocin, a hormone thought to contribute to satiety, have been found to be abnormal.

People with Prader–Willi syndrome 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 behavioral 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.^[8][12] Psychiatric symptoms, for example, hallucinations, paranoia and depression, have been described in some cases^[8] and affect approximately 5–10% of young adults.^[7] Psychiatric and behavioural problems are the most common cause of hospitalization.^[13]

Endocrine

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 lean body mass (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.

Ophthalmologic

PWS is commonly associated with development of strabismus. In one study,^[14] over 50% of patients had strabismus, mainly esotropia.

Genetics

PWS is caused by the deletion of the paternal copies of 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). These are on chromosome 15 located in the region 15q11-13.^[15][16][17] 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.^[18][19] 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.

A microdeletion in one family of the snoRNA HBII-52 has excluded it from playing a major role in the disease.^[20]

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.^{[21][22][23][24][25]}

Diagnosis

Prader-Willi syndrome phenotype at 15 years of age. Note absence of typical PWS facial features and presence of mild truncal obesity.

PWS affects approximately 1 in 10,000 to 1 in 25,000 newborns.^[26] There are more than 400,000 people who live with PWS around the world.^[27] It is traditionally characterized by hypotonia, short stature, hyperphagia, obesity, behavioral issues (specifically OCD-like behaviors), small hands and feet, hypogonadism, and mild intellectual disability.^[26] However, with early diagnosis and early treatment (such as with growth hormone therapy), the prognosis for persons with PWS is beginning to change. Like autism, PWS is a spectrum disorder and symptoms can range from mild to severe and may change throughout the person's lifetime. Various organ systems are affected.

Traditionally, Prader–Willi syndrome 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 cases. 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.

Prader–Willi syndrome is often misdiagnosed as other syndromes due to many in the medical community's unfamiliarity with PWS.^[11] Sometimes it is misdiagnosed as Down syndrome, simply because of the relative frequency of Down syndrome compared to PWS.^[11]

Treatment

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.^[28][29][30]

Because of severe obesity, obstructive sleep apnea is a common sequela, and a positive airway pressure machine is often needed. There may come a time when a person who has been diagnosed with PWS may have to undergo surgical procedures. One surgery that has proven to be unsuccessful for treating the obesity is gastric bypass. Patients with Prader–Willi syndrome have a very high tolerance to pain; therefore they may be experiencing significant abdominal symptoms such as acute gastritis, appendicitis, or cholecystitis and not be aware of it until later.

Zafgen, Inc. is currently conducting Phase 3 trials of beloranib relating to controlling the weight and appetite of Prader–Willi syndrome patients,^[31][32][33] with 2a results showing 8.1% reduced body fat after 4 weeks (at the highest study dose of 1.8 mg) and decreased appetite, despite a 50% increase in daily calorie allowance.^[34][35]

Society and culture

A 1680 painting by Juan Carreno de Miranda of Eugenia Martínez Vallejo, a girl presumed to have Prader-Willi syndrome^[36]

Despite its rarity, Prader–Willi syndrome has been often referenced in popular culture, partly due to the fascination surrounding the insatiable appetite and obesity that are symptoms.

Prader–Willi syndrome has been depicted and documented several times in television. A fictional individual with Prader–Willi syndrome featured in the episode "Dog Eat Dog" of the television series CSI: Crime Scene Investigation, which aired on November 24, 2005.^[37] In the UK media in July 2007, Channel 4 aired a 2006 documentary called Can't Stop Eating, surrounding the everyday lives of two people with Prader–Willi syndrome, Joe and Tamara.^[38] In the May 9, 2010 episode of Extreme Makeover: Home Edition, Sheryl Crow helped Ty Pennington rebuild a home for a family whose youngest son, Ethan Starkweather, was suffering from Prader–Willi syndrome.^[39] In the March 22, 2012 episode of Mystery Diagnosis on the Discovery Health channel, Conor Heybach, who has Prader–Willi syndrome, shared his story of how he was diagnosed with it.^[40]

In December 2011 the Taipei Times, in Taiwan, highlighted the tragedy of a taxi driver who had killed himself and his nine-year-old daughter who had the condition, in what police described as a "probable murder-suicide."^[41]

See also

• Genomic imprinting
• Angelman syndrome
• Epigenetics
• ROHHAD

References

External links

Wikimedia Commons has media related to Prader-Willi syndrome.

• Prader–Willi syndrome at DMOZ
• International PWS Organisation