THE ABSENCE OF ENHANCER COMPETITION BETWEEN IGF2 AND H19 FOLLOWING TRANSFER INTO DIFFERENTIATED CELLS

H19 and Igf2 are reciprocally imprinted genes that lie 90 kb apart on mouse chromosome 7, The two genes are coexpressed during development, with the H19 gene expressed exclusively from the maternal chromosome a nd Igf2 from the paternal chromosome, It has been proposed that their reciprocal imprinting is governed by a competition between the genes f or a common set of enhancers, The competition on the paternal chromoso me is influenced by extensive allele-specific methylation of the H19 g ene and its 5' flank, which acts to inhibit H19 transcription and thus indirectly leads to the activation of the Igf2 gene, In contrast, no allele-specific methylation has been detected on the maternal chromoso me, and the basis for the preference for H19 transcription on that chr omosome is unresolved, Int this investigation, the mechanism controlli ng the silencing of the Igf2 gene on the maternal chromosome was explo red by studying the transcriptional activity of a yeast artificial chr omosome (YAC) containing Igf2 and H19 following transfer into differen tiated tissue culture cells, Contrary to expectations, both H19 and Ig f2 were expressed from a single integrated copy of the YAC, Furthermor e, Igf2 expression appeared to be independent of the H19 locus, based on deletions of the H19 gene promoter and its enhancers, These results suggest that an active process is responsible for the transcriptional bias toward H19 on the maternal chromosome and that the hypomethylate d state of this chromosome cannot be viewed as a ''default'' state, Mo reover, the active process is not reproduced in a differentiated cell and may require passage through the female germ line.