Genomic imprinting: intricacies of epigenetic regulation in clusters.

An intriguing characteristic of imprinted genes is that they often cluster in large chromosomal domains, raising the possibility that gene-specific and domain-specific mechanisms regulate imprinting. Several common features emerged from comparative analysis of four imprinted domains in mice and humans: (a) Certain genes appear to be imprinted by secondary events, possibly indicating a lack of gene-specific imprinting marks; (b) some genes appear to resist silencing, predicting the presence of cis-elements that oppose domain-specific imprinting control; (c) the nature of the imprinting mark remains incompletely understood. In addition, common silencing mechanisms are employed by the various imprinting domains, including silencer elements that nucleate and propagate a silent chromatin state, insulator elements that prevent promoter-enhancer interactions when hypomethylated on one parental allele, and antisense RNAs that function in silencing the overlapping sense gene and more distantly located genes. These commonalities are reminiscent of the behavior of genes subjected to, and the mechanisms employed in, dosage compensation.