Effect of H19 mutations on H19 and Igf2 imprinting in
mouse endodermal tissues.
(a) On the maternal chromosome (top line), the enhancers (two filled circles)
drive the expression of the H19 gene (transcription is designated by
a horizontal arrow) and on the paternal chromosome, the enhancers drive the
expression of Igf2. The epigenetic modifications on the wild-type H19
alleles are depicted: chromatin hypersensitivity is indicated by small vertical
open arrows and hypermethylation shown as CH3.
(b-e) All deletions at the endogenous mouse locus are targeted mutations.
(b) In the absence of 13kb of sequence that includes the H19 transcription
unit, promoter and 10 kb of upstream flanking sequences (H19Æ13),
the enhancers drive the expression of Igf2 on the maternal allele but
at a lower level (smaller horizontal arrow) than the wild-type paternally-derived
Igf2 expression.
(c) Substitution of the luciferase gene sequences for H19 does not perturb
Igf2 imprinting (H19luc).
However, luciferase is expressed at varying levels on the paternal chromosome:
the expression is inversely correlated with the degree of methylation in the
promoter and luciferase gene coding sequence.
(d) Deletion of the DMD results in a loss of imprinting of the H19 and
Igf2 genes (H19ÆDMD).
(e) When the two endodermal enhancers are moved from their normal location and
inserted between H19 and Igf2 on the maternal allele, Igf2
is activated (ENHmov).
DMD = differentially methylated domain
from Brannan, C.I., and M.S. Bartolomei 1999. Mechanisms of genomic imprinting. Current Opinion in Genetics and Development 9:164-170.