Epigenetics and the brain: Transcriptome sequencing reveals new depths to genomic imprinting

Transcriptome sequencing has identified more than a thousand potentially imprinted genes in the mouse brain. This comes as a revelation to someone who cut his teeth on the identification of imprinted genes when only a handful was known. Genomic imprinting, an epigenetic mechanism that determines expression of alleles according to sex of transmitting parent, was discovered over 25 years ago in mice but remains an enigmatic phenomenon. Why do these genes disobey the normal Mendelian logic of inheritance, do they function in specific processes, and how is their imprinting conferred? Next generation sequencing technologies are providing an unprecedented opportunity to survey the whole genome for imprinted genes and are beginning to reveal that imprinting may be more pervasive than we had come to believe. Such advances should lay the foundation for a definitive account of imprinting, but may also challenge accepted views on what it means to be imprinted.