Molecular mechanisms of genomic imprinting and clinical implications for cancer

Genomic imprinting is an epigenetic marking of genes in the parental germline that ensures the stable transmission of monoallelic gene expression patterns in a parent-of-origin-specific manner. Epigenetic marking systems are thus able to regulate gene activity independently of the underlying DNA sequence. Several imprinted gene products regulate cell proliferation and fetal growth; loss of their imprinted state, which effectively alters their dosage, might promote or suppress tumourigenic processes. Conversely, global epigenetic changes that underlie tumourigenesis might affect imprinted gene expression. Here, we review imprinted genes with regard to their roles in epigenetic predisposition to cancer, and discuss acquired epigenetic changes (DNA methylation, histone modifications and chromatin conformation) either as a result of cancer or as an early event in neoplasia. We also address recent work showing the potential role of noncoding RNA in modifying chromatin and affecting imprinted gene expression, and summarise the effects of loss of imprinting in cancer with regard to the roles that imprinted genes play in regulating growth signalling cascades. Finally, we speculate on the clinical applications of epigenetic drugs in cancer.