Biology Reference
In-Depth Information
telangiectasia, and Werner's, Bloom's, and Cockayne syndromes. All told, this
process has a number of implications in a number of different pathological
processes.
1
HRR is a complex process that must have significant access to DNA and
involves the disruption of chromatin structure, at least temporarily. The first
step in HRR is the processing of DNA ends to produce 3
0
DNA ends for
RAD51 binding. Then, RAD51 induces the search for homology that must
disrupt base pairing and, by proxy, the chromatin structure. Next, the synthesis
portion of HRR requires significant access to DNA and will not be possible
without chromatin access, similar to that seen during replication. Finally, after
the entire process has occurred, there must be a process responsible for the
reorganization of the chromatin structure to resolve the enzymatic sequence of
events. It can be predicted that each of these steps has associated pathways to
regulate the intricate nature of modulating chromatin. In the current literature,
there are three basic types of regulation of chromatin described, including
histone variants, histone modifications, and ATP-dependent chromatin remo-
deling. In this section, we describe these events in their basic temporal activa-
tion during HRR.
Histone structure has been well described elsewhere, but in short, there
are the canonical replication inserted histones: H2A, H2B, H3, and H4.
Histone H2A has two variants that have been associated with DNA repair:
H2AX and H2AZ. The classical event in DNA DSB repair is phosphorylation of
histone H2AX, a variant of histone 2A, on Serine 139 (termed
g
-H2AX) in
mammals. This event has been shown to be mediated by Mec1 and Tel 1 in
Saccharomyces cerevisiae
or ATM, ATR, and DNA PKcs in mammalian cells.
g
-
H2AX phosphorylation results from the production of DSBs by extrinsic or
intrinsic factors such as ionizing radiation (IR), chemotherapeutic drugs, or
spontaneous cell damage as a result of replication fork arrest or oxidative
damage, and is involved in the recognition of DNA DSBs. In mammalian
cells, H2AX represents 2-25% of the H2A variants in the overall chromatin
depending on cell type. Yeast cells that express unmodifiable H2AX are hyper-
sensitive to treatment with drugs such as miracle mineral supplement (MMS)
or phleomycin, and mammalian stem cells lacking
g
-H2AX are sensitive to IR
and prone to genomic instability.
2
The exact function of
g
-H2AX is still not completely understood, though
several important roles have been well described.
g
-H2AX is not necessary for
the initial steps in repair, but is involved in localizing and concentrating repair
factors at the site of the DSB including chromatin modifiers such as the histone
acetyl transferases (HAT) NuA4, which is implicated in the relaxation of
chromatin surrounding a break. Further,
g
-H2AX is an important binding site
for MDC1, an adapter protein that serves as a landing bay of sorts for a plethora
of proteins including BRCA1, the MRN (Mre11/Rad50/Nbs1) complex, and
