Table 8 Summary of two disorders of imprinting, Angelman syndrome and Prader-Willi syndrome
Angelman syndromePrader-Willi syndrome
Key featuresModerate to severe intellectual disability (IQ generally in the range 25–54)
Jerky, puppet-like movements
Happy and sociable disposition
Mild to moderate intellectual disability (IQ generally in the range 60–70)
Insatiable appetite leading to morbid obesity
Behaviour problems
Frequency in the populationApproximately 1 per 20000Approximately 1 per 15000
Underlying genetic abnormality (note that in some cases, the underlying cause has not been determined)Maternal 15q11.2 deletion (approximately 70%)
Paternal UPD (approximately 4%)
Imprinting defect (approximately 8%)
Pathogenic variant in UBE3A (∼6%)
Paternal 15q11.2 deletion (approximately 70%)
Maternal UPD (approximately 20%)
Imprinting defect (approximately 5%)
Key genesUBE3A encoding a ubiquitin ligaseSNORD116 gene cluster encoding snoRNAs (other genes in the imprinted region may also influence the phenotype)