The Importance of Imprinting in the Human Placenta

Figure 1. The human fetus and placenta.
Villous trophoblasts of the human placenta grow as a branched structure, maximising exchange with maternal blood. Extravillous trophoblast invade into the maternal endometrium, and some cells colonise maternal spiral arteries, expanding them to maximise blood flow. ♂ = Paternally expressed; ♀ = maternally expressed. Imprinted genes are important during fetal growth. Some, such as GNAS, UBE3A, and PLAGL, have physiological impact on the fetus only. Other genes may influence growth in utero via the placenta, or the fetus and placenta. The IUGR seen in SRS, and overgrowth in BWS are suggestive of a role of IGF2 in the human placenta


The Importance of Imprinting in the Human Placenta

Jennifer M. Frost*, Gudrun E. Moore
Clinical and Molecular Genetics Unit, Institute of Child Health, University College London, London, United Kingdom

Abstract
As a field of study, genomic imprinting has grown rapidly in the last 20 years, with a growing figure of around 100 imprinted genes known in the mouse and approximately 50 in the human. The imprinted expression of genes may be transient and highly tissue-specific, and there are potentially hundreds of other, as yet undiscovered, imprinted transcripts. The placenta is notable amongst mammalian organs for its high and prolific expression of imprinted genes. This review discusses the development of the human placenta and focuses on the function of imprinting in this organ. Imprinting is potentially a mechanism to balance parental resource allocation and it plays an important role in growth. The placenta, as the interface between mother and fetus, is central to prenatal growth control. The expression of genes subject to parental allelic expression bias has, over the years, been shown to be essential for the normal development and physiology of the placenta. In this review we also discuss the significance of genes that lack conservation of imprinting between mice and humans, genes whose imprinted expression is often placental-specific. Finally, we illustrate the importance of imprinting in the postnatal human in terms of several human imprinting disorders, with consideration of the brain as a key organ for imprinted gene expression after birth.

Citation: Frost JM, Moore GE (2010) The Importance of Imprinting in the Human Placenta. PLoS Genet 6(7): e1001015. doi:10.1371/journal.pgen.1001015

Editor: Anne C. Ferguson-Smith, University of Cambridge, United Kingdom

Published: July 1, 2010

Copyright: © 2010 Frost, Moore. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: JMF was funded by an MRC PhD Studentship, and currently holds a Wellcome Trust post-doctoral research post. GEM is funded by MRC, Wellcome Trust, Wellbeing of Women, and SPARKS. The funders had no role in the decision to publish or preparation of the article.

Competing interests: The authors have declared that no competing interests exist.

* E-mail: j.frost@ich.ucl.ac.uk

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