Imprinting and epigenetic changes in the early embryo

Imprinted genes are epigenetically regulated so that only one allele is expressed in a parent-of-origin-dependent manner. Although they represent a small subset of the mammalian genome, imprinted genes are essential for normal development. The regulatory mechanisms underlying imprinting are complex and have been the subject of extensive investigation. DNA methylation is the best-established epigenetic mark that is critical for the allele-specific expression of imprinted genes. This mark must be correctly established in the germline, maintained throughout life, and erased and reestablished in the germline the next generation. These events coincide with the genome-wide epigenetic reprogramming that occurs during gametogenesis and early embryogenesis; therefore, the establishment and maintenance of DNA methylation must be tightly regulated. Studies on enzymes that participate in both de novo methylation and its maintenance (i.e., the DNMT family) have provided information on how methylation influences imprinting. However, many aspects of the regulation of DNA methylation are unknown, including how methylation complexes are targeted and the molecular mechanisms underlying DNA demethylation. In this review we focus on the epigenetic changes that occur in the germline and early embryo, with an emphasis on imprinting. We summarize recent findings on factors influencing DNA methylation establishment, maintenance, and erasure that have further elucidated the mechanisms of imprinting, while highlighting topics that require further investigation.