Imprinting defects in 15q11-q13 are a rare but significant cause of Prader-
Willi syndrome (PWS) and Angelman syndrome (AS). Patients with an imprintin
g defect have apparently normal chromosomes 15 of biparental origin, but ar
e recognised by uniparental DNA methylation at D15S63 (PW71) or SNURF-SNRPN
exon 1. We have investigated the methylation status of five additional loc
i in 12 such patients with or without a deletion in the imprinting Centre.
In each patient, the imprinting defect affected all loci tested. During rou
tine diagnostic testing we identified four patients who had a normal methyl
ation pattern at SNURF-SNRPN exon 1, but an abnormal pattern at D15S63. In
two of these patients, who were suspected of having PWS, this change was re
stricted to D15S63. In two patients suspected of having AS, several but not
all loci were affected. Using a newly developed methylation-specific PCR t
est for D15S63 we found that these methylation changes are rare in patients
suspected of having AS. Although we can not prove that the methylation cha
nges in the four patients are causally related to their disease, our findin
gs demonstrate that spatially restricted changes in methylation can occur.
In some cases, these changes may reflect incomplete imprint spreading.