Conspectus
DNA is damaged
by various endogenous and exogenous
sources, leading
to a diverse group of reactive intermediates that yield a complex
mixture of products. The initially formed products are often metastable
and can react to yield lesions that are more biologically deleterious.
Mechanistic studies are frequently carried out on free DNA as the
substrate. The observations do not necessarily reflect the reaction
environment inside human cells where genomic DNA is condensed as chromatin
in the nucleus. Chromatin is made up of monomeric structural units
called nucleosomes, which are comprised of DNA wrapped around an octameric
core of histone proteins (two copies each of histones H2A, H2B, H3,
and H4).
This account presents a summary of our work in the
past decade
on the mechanistic studies of DNA damage and repair in reconstituted
nucleosome core particles (NCPs). A series of metastable lesions and
reactive intermediates, such as abasic sites (AP), N7-methyl-2′-deoxyguanosine
(MdG), and 2′-deoxyadenosin-N6-yl radical (dA•), have been independently generated in a site-specific manner in
bottom-up-synthesized NCPs. Detailed mechanistic studies on these
NCPs revealed that histones actively participate in DNA damage and
repair processes in diverse ways. For instance, nucleophilic residues
in the flexible histone N-terminal tails, such as Lys and N-terminal
α-amine, react with electrophilic DNA damage and reactive intermediates.
In some cases, transient intermediates are produced, leading to the
promotion or suppression of damage and repair processes. In other
examples, reactions with histones yield reversible or stable DNA–protein
cross-links (DPCs). Histones also utilize acidic and basic residues,
such as histidine and aspartic acid, to catalyze DNA strand cleavage
through general acid/base catalysis. Alternatively, a Tyr in histone
plays a vital role in nucleosomal DNA damage and repair via radical
transfer. Finally, the reactivity discovered during the mechanistic
studies has facilitated the development of new reagents and methods
with applications in biotechnology.
This research has enriched
our knowledge of the roles of histone
proteins in DNA damage and repair and their contributions to epigenetics
and may have significant biological implications. The residues in
histone N-terminal tails that react with DNA lesions also play pivotal
roles in regulating the structure and function of chromatin, indicating
that there may be cross-talk between DNA damage and repair in eukaryotic
cells and epigenetic regulation. Also, in view of the biased amino
acid composition of histones, these results provide hints about how
the proteins have evolved to minimize their deleterious effects but
maximize beneficial ones for maintaining genome integrity. Finally,
previously unreported DPCs and histone post-translational modifications
have been discovered through this research. The effects of these newly
identified lesions on the structure and function of chromatin and
their fates inside cells remain to be elucidated.