Genome-wide survey of imprinted genes

The developmental failure of mammalian parthenogenote has been a mystery for a long time and posed a question as to why bi-parental reproduction is necessary for development to term. In the 1980s, it was proven that this failure was not due to the genetic information itself, but to epigenetic modification of genomic DNA. In the following decade, several studies successfully identified imprinted genes which were differentially expressed in a parent-of-origin-specific manner, and it was shown that the differential expression depended on the pattern of DNA methylation. These facts prompted development of genome-wide systematic screening methods based on DNA methylation and differential gene expression to identify imprinted genes. Recently computational approaches and microarray technology have been introduced to identify imprinted genes/loci, contributing to the expansion of our knowledge. However, it has been shown that the gene silencing derived from genomic imprinting is accomplished by several mechanisms in addition to direct DNA methylation, indicating that novel approaches are further required for comprehensive understanding of genomic imprinting. To unveil the mechanism of developmental failure in mammalian parthenogenote, systematic screenings for imprinted genes/loci have been developed. In this review, we describe genomic imprinting focusing on the history of genome-wide screening.