Brian P. Chadwick
from: Epigenetics: A Reference Manual (Edited by: Jeffrey M. Craig and Nicholas C. Wong). Caister Academic Press, U.K. (2011)
The recent completion of several mammalian genome sequences makes obvious that we share a near-identical collection of genes. What defines us as human must therefore be encoded within regions of the genome where we differ, providing an added level of complexity that probably influences the spatial and temporal expression of genes. Most DNA sequence variation occurs within the repetitive DNA, once called 'Junk DNA' that accounts for at least half of the human genome, and evidence is mounting for its important role in genome function. Although some repeat elements are conserved to some extent between mammals, their precise copy number and genomic location typically are not. In addition, some repeats are not conserved, including the large tandem repeats. This chapter focuses on two large tandem arrays in the human genome that can adopt quite different chromatin configurations as a result of epigenetic changes; one as a direct consequence of X chromosome inactivation and the other in the context of disease susceptibility. Both cases highlight how alternate packaging of these unusual DNA sequences probably results in differing functions. In each instance, common denominators are the acquisition of the epigenetic organizer protein CTCF and a distinct change in transcripts originating from the array read more ...