Genetic Causes of Mental Retardation - Prader-Willi Syndrome

Prader-Willi syndrome, also referred to as hypotonia-hypomentia-hypogonadism-obesity syndrome (HHHO), is a genetic disorder involving a malfunction of chromosome 15. Some explanation is necessary to unpack this unwieldy term:

• Hypotonia describes a condition where muscle tone is weak. The abnormality is apparent in infancy and is indicative of a problem in the central nervous system.
• Hypomentia refers to mental retardation or learning disabilities.
• Hypogonadism describes a condition characterized by inefficient, weak secretion of male or female hormones, often resulting in delays in sexual development and infertility.
• Obesity pertains to excessive weight gain. Obese individuals are typically at least thirty pounds over the recommended weight for their height.

Prader-Willi syndrome is not biased to occur more or less based on gender, race, or socioeconomic group. Only 2 percent of cases appear to be inherited from parent to child, the rest being the result of random mutation.

Because chromosome 15 is closely tied to obesity, obesity is commonly associated with Prader-Willi syndrome. Affected infants do not gain weight appropriately due to weak facial muscles that prevent appropriate suckling behavior. As children grow, however, they develop a great interest in food and have difficulty controlling urges to binge-eat. They also fail to grow at a normal rate, making their weight gain even more pronounced in appearance.

The first clues that the condition may be present are based on characteristic appearance and behavior. Children suspected of having the disorder can then be genetically tested to confirm the diagnosis. Facial features associated with Prader-Willi syndrome include almond-shaped eyes that may be crossed, full cheeks, and a narrow face. The mouth is small and the corners of the mouth turn downwards. The small mouth is accentuated by a thin upper lip.

Medical problems usually revolve around obesity. These individuals often develop diabetes and heart problems. Muscle mass is insufficient and individuals who have Prader-Willi syndrome are often given a growth hormone to help bulk their muscles. These individuals are also treated with reasonable diets, exercise, and weight maintenance programs. Prader-Willi syndrome is tied to borderline intellectual functioning and mild mental retardation. Behavior problems include tantrums, obsessive/compulsive tendencies, and skin plucking.