Genomic imprinting and human disease.

In many epigenetic phenomena, covalent modifications on DNA and chromatin mediate somatically heritable patterns of gene expression. Genomic imprinting is a classical example of epigenetic regulation in mammals. To date, more than 100 imprinted genes have been identified in humans and mice. Many of these are involved in foetal growth and deve lopment, others control behaviour. Mono-allelic expression of imprinted genes depends on whether the gene is inherited from the mother or the father. This remarkable pattern of expression is controlled by specialized sequence elements called ICRs (imprinting control regions). ICRs are marked by DNA methylation on one of the two parental alleles. These allelic marks originate from either the maternal or the paternal germ line. Perturbation of the allelic DNA methylation at ICRs is causally involved in several human diseases, including the Beckwith-Wiedemann and Silver-Russell syndromes, associated with aberrant foetal growth. Perturbed imprinted gene expression is also implicated in the neuro-developmental disorders Prader-Willi syndrome and Angelman syndrome. Embryo culture and human-assisted reproduction procedures can increase the occurrence of imprinting-related disorders. Recent research shows that, besides DNA methylation, covalent histone modifications and non-histone proteins also contribute to imprinting regulation. The involvement of imprinting in specific human pathologies (and in cancer) emphasizes the need to further explore the underlying molecular mechanisms.