Imprinting mechanisms in extraembryonic tissues

An important part of gene regulation during development involves so-called epigenetic marks. These are chemical additions (eg methyl groups) to the DNA or the chromatin (the histone proteins) and instruct genes to be active or inactive. There is increasing evidence that faults in these epigenetic marks can lead to diseases in humans, particularly those affecting growth or behaviour. Recently it has been realised that defects in cloned animals and some rare diseases that occur in children born from IVF, can be explained by defective epigenetic marks. In this application, we describe work on a new epigenetic mechanism based on attachment of methyl groups to histones, rather than DNA. It is possible that the histone marks are less stable than the ones in DNA, and this could be relevant to the defects that occur in disease.

DNA methylation is thought to be the major mechanism of genomic imprinting. In mouse embryos lacking the maintenance methyltransferase Dnmt1, imprinting of all genes studied is disrupted. However we have recently found that imprinting of several genes in an imprinting cluster (on distal chromosome 7) is maintained faithfully in Dnmt1- mutants in extraembryonic tissues (placenta). Deletion of an imprinting control region which produces a non-coding antisense transcript (IC2), by contrast, results in loss of silencing. We find that in the placenta the silent genes are marked by repressive histone tail modifications, which are disrupted by IC2 deletion. Thus imprinting in extraembryonic tissues seems to involve epigenetic marks other than DNA methylation. We will determine the developmental sequence of establishment of these marks and imprinted expression of the genes, in relation to antisense RNA expression, using a conditional mutant of IC2. Trans-acting factors such as the polycomb gene Eed and G9a will be investigated. The discovery and characterization of a new imprinting mechanism will provide exciting insights into specific diseases and perhaps the association between epigenetic disorders and IVF , as well as cloning and cell plasticity.