Prader-Willi syndrome: A syndrome characterized by severe hypotonia (floppiness), poor suck and feeding problems in early infancy followed later in infancy by excessive eating that, if unchecked, leads gradually to huge obesity. All children with Prader-Willi syndrome (PWS) show developmental delay and mild-to-moderate mental retardation with multiple learning disabilities. Hypogonadism is present in both females (with small labia minora and clitoris) and males (with underdeveloped scrotum and nondescent of the testes). Short stature and small hands and feet are common.
The basic cause of PWS is extraordinary. It is due to absence of the paternally contributed region on chromosome 15q11-q13. What is missing is not just any chromosome 15q11-13 region but specifically that from the father. The child can have two copies of chromosome region 15q11-13 but, if both are from the mother (a phenomenon called maternal disomy), the child will still have PWS for lack of the region from the father.
Several of the genes in region 15q11-13 are subject to genomic imprinting. This explains why PWS results only when the paternally-contributed region 15q11-13 is absent. When the maternally-contributed region 15q11-13 is missing, the result is a different disease called Angelman syndrome.
There is currently no specific treatment or cure for PWS. Parents are advised to limit consumption of high-calorie foods, and to use techniques such as special education, speech therapy, and physical therapy to maximize the child's potential. Severe psychiatric illness is common in PWS adults. Those with psychotic illness have a double maternal copy of 15q11-13, suggesting that genes in this region are important in causing psychotic illness.