Insulator Mechanism of Imprinting : (e.g. H19 & IGF2 genes)

The H19 and IGF2 genes, the essential regulatory elements coordinating their imprinted expression have been well characterized. Shared enhancers that regulate H19 and IGF2 in endodermal and mesodermal tissues have been delimited. An imprinting control region (ICR) has been well defined and consists of a CG-rich differentially methylated domain (DMD), located 22 to 24 kb relative to the H19 transcription start site. The human H19 DMD encompasses seven different binding sites for CTCF. Only the sixth CTCF binding site has been reported to act as a key regulatory domain for switching between H19 and IGF2 expression, whereas the other sites appear to be hypermethylated in a study by Takai et al.

This DMD is essential for establishing the pattern of imprinting by which H19, a non-coding RNA, is exclusively expressed from the maternal chromosome and IGF2, a fetal growth factor, is only active paternally.

DMD has at least two distinct regulatory roles on each of the parental chromosomes. The activity of the DMD is determined by its methylation status. On the maternal chromosome, the DMD is hypomethylated and binds the protein CTCF via four highly conserved CG-rich repetitive sites. The binding of CTCF creates a physical boundary on the maternal allele and inhibits interaction of downstream enhancers with IGF2 promoters.

Maternal binding of CTCF to the four sites within the DMD has been further hypothesized to protect actively the region from becoming methylated during oogenesis and development. The DMD is also essential for optimal H19 and IGF2 expression and is required on the maternal allele for the initiation of H19 expression. On the other hand, the paternal H19 DMD is hypermethylated and hence the CTCF is unable to bind to it, resulting in expression of IGF2 while H19 is silenced.