Prader–Willi syndrome (PWS) is a genetic disorder due to loss of function of specific genes on chromosome 15. In newborns symptoms include weak muscles, poor feeding, and slow development. In childhood the person becomes constantly hungry which often leads to obesity and type 2 diabetes. There is also typically mild to moderate intellectual impairment and behavioral problems. Often the forehead is narrow, hands and feet small, height short, skin light in color, and they are unable to have children.
About 70% of cases occur when part of the father’s chromosome 15 is deleted. In another 25% of cases the person has two copies of chromosome 15 from their mother and none from their father. As parts of the chromosome from the mother are turned off they end up with no working copies of certain genes. PWS is not generally inherited but instead the genetic changes happen during the formation of the egg, sperm, or in early development. There are no known risk factors. Those who have one child with PWS have less than a 1% chance of the next child being affected. A similar mechanism occurs in Angelman syndrome except there is a defective chromosome 15 from the mother or two copies from the father.
Prader–Willi syndrome has no cure. Treatment, however, may improve outcomes, especially if carried out early. In newborns feeding difficulties may be supported with feeding tubes. Strict food supervision is typically required starting around the age of three in combination with an exercise program. Growth hormone therapy also improves outcomes. Counseling and medications may help with some behavioral problems. Group homes are often necessary in adulthood.
PWS affects between 1 in 10,000 and 1 in 30,000 people. Males and females are affected equally. The condition is named after Andrea Prader, Heinrich Willi, and Alexis Labhart who described it in detail in 1956. An earlier description occurred in 1887 by John Langdon Down.
SIGNS AND SYMPTOMS
More aspects seen in a clinical overview include hypotonia and abnormal neurologic function, hypogonadism, developmental and cognitive delays, hyperphagia and obesity, short stature, and behavioral and psychiatric disturbances.
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
- Feeding difficulties (due to poor muscle tone affecting sucking reflex)
- Difficulties establishing respiration
- Delayed milestones/intellectual delay
- Excessive sleeping
- Strabismus (‘crossed eyes’)
- Scoliosis (often not detected at birth)
- 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
- Extreme flexibility
- Infertility (males and females)
- Sparse pubic hair
- Hypotonia (low muscle tone)
- Learning disabilities/borderline intellectual functioning (but some cases of average intelligence)
- Prone to diabetes mellitus
- Extreme flexibility
- 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
- Delayed motor development
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.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, a figure higher than the 32% found in Curfs and Fryns’ study. 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, but this may be an effect of increased practice.
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 frequently associated with a constant, extreme, ravenous insatiable appetite which persists no matter how much the patient eats, often resulting in morbid obesity. Caregivers need to strictly limit the patients’ access to food, usually by installing locks on refrigerators and on all closets and cabinets where food is stored. It is the most common genetic cause of morbid obesity in children. 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. 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. Psychiatric symptoms, for example, hallucinations, paranoia and depression, have been described in some cases and affect approximately 5–10% of young adults. Patients are also often extremely stubborn and prone to anger. Psychiatric and behavioural problems are the most common cause of hospitalization.
It is typical for to 70–90% of affected individuals to develop behavioral patterns in early childhood. Aspects of these patterns can include stubbornness, temper tantrums, controlling and manipulative behavior, difficulty with change in routine, and compulsive-like behaviors.
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.
PWS is commonly associated with development of strabismus. In one study, over 50% of patients had strabismus, mainly esotropia.
PWS is an autosomal dominant disorder 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. 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.
A microdeletion in one family of the snoRNA HBII-52 has excluded it from playing a major role in the disease.
Studies of human and mouse model systems have shown deletion of the 29 copies of the C/D box snoRNA SNORD116 (HBII-85) to be the primary cause of Prader–Willi syndrome.
PWS affects approximately 1 in 10,000 to 1 in 25,000 newborns. There are more than 400,000 people who live with PWS around the world. 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. 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. Sometimes it is misdiagnosed as Down syndrome, simply because of the relative frequency of Down syndrome compared to PWS.
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 strength. 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. Access to food must be strictly supervised and limited, usually by installing locks on all food-storage places including refrigerators.
Because hypotonia can be a symptom of PWS, it is vital to provide proper nutrition during infancy. It is also very important to stress physical activity in individuals with PWS for all ages in order to optimize strength and promote a healthy lifestyle.
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. 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, 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. In December, 2015 Zafgen halted a Phase III clinical trial for Prader–Willi syndrome after second patient death in order to determine whether the deaths were treatment related.
Behavior and psychiatric problems should be detected early for the best results. These issues are best when treated with parental education and training. Sometimes medication is introduced as well. Serotonin agonists have been most effective in lessening temper tantrums and improving compulsivity.
SOCIETY AND CULTURE
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. 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. 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. 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.
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.”
This is part of the Rare Human Diseases Series to help you discover what is wrong with you.
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