HHMI News-January 09,2004-New Insight into Control of Parental Gene expression in Eggs
Researchers have identified a crucial step in a genetic process required for the development of viable eggs. The process, known as imprinting, distinguishes the paternally inherited and the maternally inherited copies of a number of developmentally important genes.

The majority of mammalian GENEs are present in two copies, both of which are equally expressed and regulated. A small number of mammalian genes, however, are subject to special regulation by a process called gene imprinting. The imprint is a chemical mark, such as methylation, attached to genes during egg or sperm development. Imprinting physically marks genes in such a way that the parental origin of the two copies can be distinguished so that one parent"s copy is turned on while the other is silenced. Imprinted genes are the likely reason that maternal and paternal contributions are necessary for normal mammalian development.

Exploring the mechanisms underlying gene imprinting may provide insight into so-called epigenetic control of gene expression, in which the cellular machinery governs the expression of genes in the cell. The function of that machinery, which makes modifications to the genome, remains among the major mysteries in biology.

Howard Hughes Medical Institute investigator Marisa S. Bartolomei, Andrew Fedoriw, Paula Stein, Petr Svoboda and Richard Schultz at the University of Pennsylvania published their findings in the January 9, 2004, issue of the journal science.

The researchers sought to pinpoint the regulatory role of a protein known as CTCF, which is believed to attach to a control region near imprinted genes. By binding to this region, called the differentially methylated domain, CTCF blocks the site from the attachment of methyl groups to the DNA — chemical modifications that the Cell"s epigenetic machinery uses to silence genes.

Bartolomei and her colleagues focused on the role of CTCF in protecting a gene called H19, whose paternal version is silenced in the developing embryo while the maternal version remains active.

To determine whether CTCF might be protecting the maternal copy of H19 from DNA methylation, the researchers used a technique called RNA interference, in which they genetically engineered mouse eggs to produce RNA molecules that can interfere with a particular messenger RNA (mRNA). In this case, the mRNA for the CTCF protein was targeted. This technique in effect degrades the target mRNA, thereby reducing the level of the protein for which it codes.