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Medical Service and Geriatric Research Education and Clinical Center Veterans Affairs Palo Alto Health Care System, and Division of Endocrinology, Gerontology, and Metabolism Department of Medicine Stanford University Palo Alto, California 94304
The adjacent genes, insulin-like growth factor 2
(Igf2) and H19, are imprinted in both mouse and
human. While Igf2 is expressed from the paternal allele,
H19 is transcribed exclusively from the maternal allele. To
explore the underlying mechanism of Igf2 and
H19 imprinting, we studied the effect of DNA demethylation
on allelic expression by injecting mice with the demethylating agent
5-azacytidine (5-aza-C). We observed a
2-fold increase in the
abundance of Igf2 mRNA in liver from treated mice compared
with that of control mice. There was no significant change in
Igf2 or H19 expression in brain. In the
5-aza-C-treated mice, there was dramatic modulation of Igf2
imprinting. In some tissues, Igf2 was expressed
biallelically, while in other tissues, the paternal allele was silenced
and the normally imprinted maternal allele was expressed, an example of
allelic switching. There was no change in the normal biallelic pattern
of Igf2 expression in brain. H19, on the other
hand, remained imprinted in all tissues in mice treated with 5-aza-C.
These results provide the first example of a pharmacological
manipulation of genomic imprinting of an endogenous gene in
vivo and further implicate DNA methylation as an important factor
in maintaining the differential allelic expression of the
Igf2 gene.
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