Caused (mostly) by absence of maternal contribution to a region of the 15q chromosome. Paternal uniparental disomy is one way this can happen, although mostly there is de novo maternal deletion. The same region is also responsible for Beckwith-Wiedemann syndrome, but this syndrome is the result of a paternal deletion.
Characteristically “happy puppet” –
- severe speech and language delay
- learning disability and epilepsy
- Movement disorder esp ataxia, also tremor, hyperreflexia
- open mouthed expression, large mandible
- Excitability, paroxysmal laughter