[Genetic obesity in childhood].

Polygenic obesity is increasingly frequent in childhood. Rare monogenic, non syndromic forms are of special physiopathological interest. They are due to recessive or dominant leptinlreceptor deficits, pro-opiomelanocortin deficit, or 4-melanocortin deficit. Genetic obesity syndromes include the Bardet-Biedl, Cohen, Albright, Alstrom, MOMA and Prader- Willi syndromes. The latter, first described in 1956, is the most frequent (1/10 000). It is associated with severe early-onset obesity, morbid hunger, and ante and postnatal hypotonia. It is due to microdeletion of paternal chromosome 15 (15qll-ql3) or to uniparental maternal disomy. This is the föirst human example of genomic imprinting and monoallelic gene expression. Early diagnosis is essential for effective therapeutic management.