Paternal uniparental disomy in monozygotic twins discordant for hemihypertrophy.

Hemihypertrophy, or hemihyperplasia, is a condition in which there may be asymmetrical overgrowth of the cranium, face, trunk, and/or limbs on one side of the body. There may also be asymmetrical visceromegaly on the ipsilateral or contralateral side. Hemihypertrophy may occur in isolation, hence the term “isolated hemihypertrophy (IH)”, or as part of a number of overgrowth syndromes in which other characteristic clinical features are present. Such syndromes include neurofibromatosis, Klippel-TrenaunayWeber syndrome, McCune-Albright syndrome, and BeckwithWiedemann syndrome (BWS). The incidence of IH is ∼1/86 000 live births, with a male:female ratio of 1:2. The aetiology of IH is unknown. A number of different chromosomal anomalies, including diploidtriploid mosaicism and trisomy 18 mosaicism, have been identified, and the causes of IH are likely to be heterogeneous. It has been suggested that IH may be one end of the spectrum of phenotypes of BWS, linked to the chromosomal locus 11p15. Here we report a case of apparently isolated hemihypertrophy in one of a pair of monozygotic twins, with paternal uniparental disomy in an area of the 11p15 locus associated with BWS.