Editing Genomic imprinting (section)

===Imprinting mechanisms===
Imprinting is a dynamic process. It must be possible to erase and re-establish imprints through each generation so that genes that are imprinted in an adult may still be expressed in that adult's offspring. (For example, the maternal genes that control insulin production will be imprinted in a male but will be expressed in any of the male's offspring that inherit these genes.) The nature of imprinting must therefore be [[epigenetics|epigenetic]] rather than DNA sequence dependent. In [[germline]] cells the imprint is erased and then re-established according to the [[sex]] of the individual, i.e. in the developing sperm (during [[spermatogenesis]]), a paternal imprint is established, whereas in developing oocytes ([[oogenesis]]), a maternal imprint is established. This process of erasure and [[reprogramming]]<ref>{{cite journal |author1 = Reik W |author-link1 =Wolf Reik |author2 = Dean W|author3 = Walter J | title = Epigenetic reprogramming in mammalian development | journal = Science | volume = 293 | issue = 5532 | pages = 1089–1093 | date = August 2001 | pmid = 11498579 | doi = 10.1126/science.1063443 | s2cid = 17089710 }}</ref> is necessary such that the germ cell imprinting status is relevant to the sex of the individual. In both plants and mammals there are two major mechanisms that are involved in establishing the imprint; these are [[DNA methylation]] and [[histone]] modifications.

Recently, a new study<ref name="Court and Tayama 2014">{{cite journal | vauthors = Court F, Tayama C, Romanelli V, Martin-Trujillo A, Iglesias-Platas I, Okamura K, Sugahara N, Simón C, Moore H, Harness JV, Keirstead H, Sanchez-Mut JV, Kaneki E, Lapunzina P, Soejima H, Wake N, Esteller M, Ogata T, Hata K, Nakabayashi K, Monk D | display-authors = 6 | title = Genome-wide parent-of-origin DNA methylation analysis reveals the intricacies of human imprinting and suggests a germline methylation-independent mechanism of establishment | journal = Genome Research | volume = 24 | issue = 4 | pages = 554–569 | date = April 2014 | pmid = 24402520 | pmc = 3975056 | doi = 10.1101/gr.164913.113 }}</ref> has suggested a novel inheritable imprinting mechanism in humans that would be specific of [[placenta]]l tissue and that is independent of DNA methylation (the main and classical mechanism for genomic imprinting). This was observed in humans, but not in mice, suggesting development after the evolutionary divergence of humans and mice, ~80 [[Mya (unit)|Mya]]. Among the hypothetical explanations for this novel phenomenon, two possible mechanisms have been proposed: either a histone modification that confers imprinting at novel placental-specific imprinted ''loci'' or, alternatively, a recruitment of [[DNA methyltransferase|DNMTs]] to these loci by a specific and unknown [[transcription factor]] that would be expressed during early trophoblast differentiation.