Prader-Willi and Angelman syndromes (genomic imprinting)

What it is

Prader-Willi syndrome and Angelman syndrome both sit at 15q11-13, and whether the paternal or the maternal allele is lost decides which of the two appears.

• Prader-Willi syndrome: the paternal allele is deleted or imprinted (silenced) — methylation of paternal 15q11-13, or an imprinting defect.
• Angelman syndrome: the maternal allele is deleted or imprinted (silenced) — methylation of maternal 15q11-13, or an imprinting defect.

Prader-Willi syndrome is the most common syndromic form of obesity.

Causes and risk

Prader-Willi syndrome is sporadic. A paternal derived deletion 15q11-13 accounts for 80%, and maternal UPD for 20%.