Epigenetic Regulation of the Molecular Clockwork - Progress in Molecular Biology and Translational Science - page 30

1. INTRODUCTION TO EPIGENETICS AND THE
CIRCADIAN CLOCK
Epigenetics, which involves changes in gene expression or activity
that are not due to fundamental alterations in DNA sequence that are passed
down, encompasses several levels of regulation. These include histone mod-
ifications, methylation of DNA, histone composition, and small RNA pres-
ence. 1-8 In most cases, epigenetic processes are mediated not by one but
several of these mechanisms at once.
Rhythmicity in gene expression falls under the umbrella of epigenetics in
the sense that changes in the temporal aspects of gene expression can greatly
alter a cellular response while the genetic material being expressed might be
identical to another cell or organism ( Fig. 2.1 ) . At least 10% of genes in a
given cell are oscillatory and are controlled directly by cyclical changes in
the chromatin structure and function at relevant gene promoters. 9-12 Post-
translational modifications at histone proteins account for much of the plas-
ticity in chromatin remodeling processes. 4 Circadian changes in chromatin
modifications have even recently been demonstrated as important for a
Figure 2.1 Genetics versus epigenetics. The mechanisms underlying genetic and epi-
genetic inheritance involve the passing on of different information, such as mutations,
found within the coding region of the DNA itself (genetics) and changes in the expres-
sion patterns of the same DNA code (epigenetics). Circadian fluctuations in DNA expres-
sion are considered to be epigenetic in nature.