Genomic imprinting: the influence of differential methylation in the two sexes.

Genomic imprinting is an epigenetic form of gene regulation that entails differential sex-specific methylation of the alleles of a gene. Such methylation distinguishes male and female genomes and is inherited in a parent-of-origin-specific manner. Sex-specific imprints are established in the germline during gametogenesis and remain intact throughout embryonic and postnatal development. Reprogramming of methylation patterns in gametes is essential to sex-specific inheritance of imprinted genes and assures exclusive harboring of female- and male-specific imprinted patterns in maternal and paternal gametes, respectively. The consequences of genomic imprinting are manifested by its loss, which can lead to a variety of disorders, the most prominent ones being Prader-Willi and Angelman syndromes. Although a great deal of research has been carried out to examine various imprinting mechanisms, little is known about the establishment and regulation of imprinted genes. In the present paper, we describe several epigenetic mechanisms that have relevance in imprinting and that may have impact on embryonic development, fetal growth and animal cloning.