Prader-Willi syndrome (PWS) is caused by a rare birth defect centered on chromosome 15. Characteristics of the syndrome include developmental delays, mental retardation, behavioral problems, and insatiable appetite leading to obesity. Affected individuals also experience incomplete sexual development, poormuscle tone, and short stature as adults.
PWS occurs in 1 in 12,000 to 15,000 births and is regarded as the most commongenetic cause of obesity. It affects both genders and all races. Although PWS arises from a genetic defect, it is not an inherited condition--it is a birth defect. The defect occurs spontaneously and specifically involves chromosome 15.
A person normally inherits one copy of chromosome 15 from each parent. In PWScases, the copy from the father either lacks a specific segment of DNA (70-75% of cases) or is missing altogether (25-30% of cases). If the father's chromosome 15 is absent, a person with PWS has two copies of the mother's chromosome 15. Although the individual has the proper number of chromosomes, inheriting two copies of a chromosome from one parent is an abnormal situation called uniparental disomy. If that parent is the mother, it is called maternal uniparental disomy.
Virtually all parents of individuals with PWS have normal chromosomes; fewerthan 2% of cases are linked to an inherited genetic mutation. In most cases,an error occurs during embryo development. This error leads to deletion of part of the father's chromosome 15 or to maternal uniparental disomy for chromosome 15. In either case, genes that should have been inherited from the father are missing and PWS develops.
Newborns with PWS have low birth weight, poor muscle tone, are lethargic, donot feed well, and generally fail to thrive. Their genitalia are abnormally small, a condition that persists lifelong. At about two to four years of age,children with PWS develop an uncontrollable, insatiable appetite. Left to their own devices, they will eat themselves to extreme obesity.
Motor development is delayed 1-2 years, and speech and language problems arecommon. Mild mental retardation is present in about 63% of cases; moderate mental retardation occurs in 31% of cases. Severe mental retardation is seen inthe remainder.
Individuals with PWS often develop behavior problems--ranging from stubbornness to temper tantrums--and are easily upset by unexpected changes. Other common characteristics include a high pain threshold, obsessive/compulsive behavior, dental problems, and breathing difficulties. About two-thirds of individuals cannot vomit even after consumption of spoiled food or other noxious substances.
Puberty may occur early or late, but it is usually incomplete. In addition tothe effects on sexual development and fertility, individuals do not undergothe normal adolescent growth spurt and are short as adults. Muscles often remain underdeveloped.
Symptoms can lead to a diagnosis of PWS. This diagnosis can be confirmed through genetic testing. PWS cannot be cured. Treatment involves speech and language therapy and special education. Stringent control of food intake is vitalto prevent obesity-related disease and death. A lifelong restricted-calorie diet accompanied by regular exercise is needed to control weight. Unfortunately, diet drugs do not work for individuals with PWS, but medications may be helpful in treating behavioral and psychological problems. Growth and development of secondary sexual characteristics can be achieved with hormone treatment, but decisions regarding such treatment are made on an individual basis.
Life expectancy for individuals with PWS may be normal if weight can be controlled. Individuals with PWS typically do best in settings that offer a stableroutine and restricted access to food. PWS currently cannot be prevented.