STAGE-SPECIFIC AND CELL-TYPE-SPECIFIC ASPECTS OF GENOMIC IMPRINTING EFFECTS IN MAMMALS

Recent studies have revealed that maternal and paternal alleles of som e imprinted genes are differentially expressed from the earliest time of expression, with virtually no expression from one of the two allele s, while for other imprinted genes the normally silent allele can be t ranscribed during early development. In addition, a number of imprinte d genes manifest their imprints only in select tissues. These observat ions indicate that the marks that denote parental chromosome origin ne ed not directly determine allele expression, but rather bias later epi genetic modifications toward a particular allele. Thus, factors expres sed at specific stages or in specific cell types are required to silen ce one parental allele or another. Stage-dependent and tissue-specific epigenetic modifications include the progressive establishment of the mature adult parental allele-specific DNA methylation patterns. These changes resemble and may share a common mechanistic basis with other epigenetic modifications that occur during development. Understanding the mechanisms by which these post-fertilization epigenetic modificati ons are mediated and regulated will be essential for understanding how genomic imprinting leads to differences in parental allele expression .