IMPRINTING OF IGF2 AND H19 FROM A 130 KB YAC TRANSGENE

A stringent test for imprint control elements is to examine their func tion at ectopic loci in transgenic experiments, Igf2 and H19 are part of a larger imprinting region and as a first step, we examined these r eciprocally imprinted genes in transgenic experiments using a 130 kb Y AC clone. After paternal inheritance, H19 was appropriately repressed and Igf2 was expressed, irrespective of copy number or genetic backgro und, After maternal inheritance H19 was consistently expressed, albeit with some variability, The levels of H19 expression per copy of the t ransgene inversely correlated with Igf2 (-lacZ) expression in cis. The consistent imprinting of H19 from this YAC contrasts with the previou sly described imprinting of mini-H19 transgenes, which only occurs at multi-copy loci, is inconsistent, and is prone to genetic background e ffects, We propose a novel model in which silencing of the H19 gene is the default state and its activation after maternal inheritance is th e key mechanistic event for imprinting in this region. In addition, in situ analysis of the Igf2-lacZ reporter indicates that additional mes oderm-specific enhancers are present within the YAC clone. No obvious phenotype was detected from the excess gene dosage of H19.