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Journal of Biomedicine and Biotechnology
Volume 2006 (2006), Article ID 59746, 6 pages
http://dx.doi.org/10.1155/JBB/2006/59746
Review Article

Do LINEs Have a Role in X-Chromosome Inactivation?

Mammalian Genetics Unit, Medical Research Council (MRC), Harwell, Oxfordshire OX11 0RD, United Kingdom

Received 31 May 2005; Revised 22 November 2005; Accepted 4 December 2005

Copyright © 2006 Mary F. Lyon. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

There is longstanding evidence that X-chromosome inactivation (XCI) travels less successfully in autosomal than in X-chromosomal chromatin. The interspersed repeat elements LINE1s (L1s) have been suggested as candidates for “boosters” which promote the spread of XCI in the X-chromosome. The present paper reviews the current evidence concerning the possible role of L1s in XCI. Recent evidence, accruing from the human genome sequencing project and other sources, confirms that mammalian X-chromosomes are indeed rich in L1s, except in regions where there are many genes escaping XCI. The density of L1s is the highest in the evolutionarily oldest regions. Recent work on X; autosome translocations in human and mouse suggested failure of stabilization of XCI in autosomal material, so that genes are reactivated, but resistance of autosomal genes to the original silencing is not excluded. The accumulation of L1s on the X-chromosome may have resulted from reduced recombination or late replication. Whether L1s are part of the mechanism of XCI or a result of it remains enigmatic.