An ICE pattern crystallizes

Abstract

Identification of a new imprinting control element (ICE) on mouse chromosome 12 brings the total to five. Common features of imprinting mechanisms suggest a general model of ICE function.

Figure 1

ICE action on the non-imprinted and imprinted chromosome. (a) The ICE is active on the non-imprinted parental chromosome and can silence multiple mRNAs and activate ncRNA in cis. (b) The ICE is inactivated on the imprinted chromosome by the imprint, which is DNA methylation for the five known ICEs. As a consequence, ncRNA is not activated and mRNAs are not silenced. (c) A complete ICE deletion results in the same expression pattern that is seen on the imprinted chromosome. The absence of an ICE means that this chromosome is not marked during gametogenesis and will show the same expression pattern irrespective of parental origin. (d) A partially deleted ICE on the imprinted chromosome will fail to attract a full methylation imprint. As a consequence, the ICE will be incompletely silenced on the normally imprinted chromosome, and this will result in partial mRNA repression and partial ncRNA activation on this chromosome. (e) A partially deleted ICE on the non-imprinted chromosome will fail to fully silence mRNA and to fully activate ncRNA. The resulting expression pattern will be similar to that in d, with reduced coexpression of mRNA and ncRNA. Solid line, expressed; dashed line, not expressed; cross-hatched line, partially expressed. Gray, mRNA; purple, ncRNA.