Biology Reference
In-Depth Information
DNA replication
26,28
that escape the proofreading
activity of DNA polymerases. Such errors occur with
a frequency of about 1 in 10
9
Direct Repair (DR)
As mentioned previously, the DR pathway removes
alkyl groups by a direct transfer to MGMT in a one-
time reaction. It is notable because it repairs only one
type of lesion, and the repair tool is not an enzyme.
However, it may be the most efficient of all repair paths.
The cell must be able to continually manufacture more
MGMT to perform this function. If it does not, the lack
of MGMT can influence the functions of other repair
pathways.
13
e
10
10
base pairs per cell
division.
7
Nucleotide mispairing can also occur after
exposure to exogenous agents or endogenous reactive
species that may cause base modifications. Varying
recognition complexes are formed based on the type of
mismatch to be repaired. Historically, the repair was
thought to be completed specifically on the
new
strand;
but the most recent evidence indicates that MMR
repair can happen before, during, or after mitosis (see
Chapter 9).
7
Loss of MMR leads to a mutator phenotype;
this predisposes one to cancer and affects, among
other processes, DNA damage response, signaling,
and recombination.
7
That is why MMR inhibitors are
not currently in clinical development. However, new
tumor cell targeting approaches may overcome these
challenges.
6
Base Excision Repair (BER)
BER repairs subtle, non-bulky lesions produced by
alkylation, oxidation or deamination of bases. A hall-
mark of BER is its varied glycosylases; they can be
mono- or bi-functional; they act only on specific
substrates, and their action is to cleave the lesion,
leaving an abasic site that APE1 processes. Then APE1
recruits repair proteins to the site to complete the repair.
The BER pathway consists of two sub-pathways, called
short-patch and long-patch. The short-patch pathway
is used more frequently; it repairs normal AP sites.
The long-patch pathway preferentially repairs oxidized
and reduced AP sites, replacing sequences of 2 to
8 nucleotides. PARP1 is a component of the long-patch
pathway.
26
Because BER activity produces cytotoxic
intermediates, it is important for BER to complete its
repairs before cell replication starts.
Homologous Recombination (HR)
HR is the repair pathway used to fix double-strand
breaks that are detected during the S/G2 portion of
the cell cycle. Because it uses a homologous template
to reconstruct the damaged DNA strand, it is highly
accurate in its repairs.
28
Unrepaired SSBs can become
DSBs when cell replication begins; this activates one or
more DNA checkpoints. The checkpoint responses
finely regulate DSB ends processing, dictating which
DSBs should be repaired by HR versus NHEJ. This is
a crucial stage in the recombination process.
7
Nucleotide Excision Repair (NER)
NER repairs large adducts and bulky DNA lesions,
such as those induced by crosslinking agents and base-
damaging carcinogens.
26
NER works on this helix-
distorting damage when only one of the two DNA
strands is affected. Its multistep “cut-and-patch” process
involves more than 30 proteins.
5
Two sub-pathways
comprise NER: global genome repair (GGR) and tran-
scription coupled repair (TCR). The pathway chosen is
predicated on the protein complexes that initiate the
repair.
26
GGR repairs damage throughout the genome,
on both transcribed and non-transcribed strands of
active genes. TCR preferentially repairs transcribed
strands in active genes,
27
removing distorting lesions
that block elongating RNA polymerases.
5
NER's capacity
and the expression of genes related to this pathway can
be modulated by oxidative stress.
5
Non-Homologous End Joining (NHEJ)
NHEJ rejoins the ends of double-strand breaks
regardless of sequence homology. It works at the ends
of broken DNA without using an identical copy of
DNA as a template, which creates the possibility of
losing or adding bases in the process. This occurs during
the G1 phase of the cell cycle, before replication, which
tends to make it prone to errors such as loss of nucleo-
tides.
5,28
Inactivation of CDK1 increases NHEJ events
in the G2 phase of the cell cycle.
7
DNA strands that
are not repaired completely by NHEJ are subject to
repair by HR.
29
DNA repair inhibitors for most pathways are either in
preclinical or clinical trials. Many inhibitors affect more
than one pathway; for example, PARP inhibitors affect
both BER and double-strand break repair; APE1 inhibi-
tors affect BER as well as transcription of other proteins;
RAD51 inhibitors affect double-strand break repair;
ATM and ATR inhibitors affect cell cycle checkpoints
and double-strand break repair. The preclinical
history, reasoning, challenges, and successes involved
in discovering cancer biomarkers and developing
Mismatch Repair (MMR)
MMR recognizes and repairs single-base mismatches
and misaligned short nucleotide repeats, such as small
insertion/deletion loops introduced during normal
