Biology Reference
In-Depth Information
DNA methylation and microRNAs. Genomic DNA can
be enzymatically methylated and demethylated at the
C5 position of cytosines in CpG dinucleotide sequences.
The CpG sequences are often found clustered at the 5
0
regulatory regions of many genes. Methylation of the
CpG dinucleotides in the promoter sequences shuts
down expression of the gene. Inappropriate DNA meth-
ylation may thus lead to the lowering of expression of
DNA repair genes and the potential for increased
genomic instability and tumorigenesis.
244
A few studies
have analyzed the methylation status of NHEJ genes in
tumor cells or in peripheral blood lymphocytes, and
shown evidence for hypermethylation of the ATM gene
promoter associated with sporadic breast cancer
245,246
and colorectal cancer
247
and the Ku70 promoter in lung
cancer.
248
To gain a more mechanistic insight into the
hypermethylation response, Leng
et al.
249
observed that
current and former smokers who exhibit a high level of
promoter methylation of a panel of genes associated
with elevated lung cancer risk show a marked reduction
in the mean level of DSB repair, as judged by the induc-
tion of chromosomal aberrations in bleomycin-treated
peripheral lymphocytes. Furthermore, they found that
individuals with multiple SNPs in several DSB repair
genes, including MRE11 and DNA-PKcs, were also
highly likely to show promoter hypermethylation of the
lung cancer risk-associated genes. These observations
support the possibility that lower levels of DSB repair
may be an important factor in anomalous promoter
hypermethylation. In addition, since the promoter meth-
ylation was measured in DNA isolated from sputum, it
may serve as a potential biomarker for predicting lung
cancer.
MicroRNAs (miRNAs) are short single-stranded oli-
goribonucleotides (usually about 22 nucleotides in
length) that can repress translation by binding to
complementary sequences of their target messenger
RNAs, followed by cleavage of the messenger RNA
mediated by the RNA-induced silencing complex.
250
It
has been estimated that about 60% of mammalian genes
are targeted by miRNAs.
251
There is increasing evidence
that miRNAs may play critical roles in tumorigenesis
including through the regulation of DNA repair path-
ways. For example, in colon cancer cells overexpression
of miR-155 can downregulate expression of the core
mismatch repair proteins hMSH2, hMSH6, and
hMLH1.
252
With regard to NHEJ, at least two miRNAs,
miR-421, and miR-101 can suppress
ATM
expression
by targeting the 3
0
-untranslated region of
ATM
messenger RNA,
253,254
and the latter miRNA similarly
reduces DNA-PKcs protein levels.
254
Interestingly,
expression of miRNA-421 is induced by N-Myc, an
oncogenic transcription factor frequently amplified in
neuroblastoma.
253
This raises the possibility that early
activation of N-Myc could accelerate the carcinogenic
process by reducing ATM expression and increasing
genomic instability.
NH
EJ AND CANCER THERA
PY
NHEJ Proteins as Prognostic Factors
Two critical considerations for cancer patients are
prognostic and predictive factors. A prognostic factor is
a situation, condition or biomarker that provides infor-
mation about the patients' overall cancer outcome,
regardless of therapy. A predictive factor is a condition
or biomarker that provides information regarding the
effect of a therapeutic intervention. Common prognostic
factors include age of the patient, tumor size, location,
grade (appearance of the tumor cells in comparison to
normal cells in biopsied material), spread of the disease
to other parts of the body, recurrence of the disease, and
biomarkers. Predictive factors tend to be biomarkers.
Biomarkers include hormones, metabolites, cell surface
and other proteins and DNA and RNA. They can be
obtained in numerous ways from body fluids, like
plasma, serum or urine, and by more invasive tech-
niques such as tumor tissue immunohistochemistry or
DNA and RNA analyses from biopsies. Some
biomarkers, such as hormone receptor status, can be
both prognostic and predictive factors. Mre11 protein
expression in bladder cancer patients serves as a good
example that illustrates the difference between a prog-
nostic and a predictive factor. In a recent study pretreat-
ment tissue samples from three cohorts of bladder cancer
patients were examined by immunohistochemistry for
expression of DSB repair-related proteins.
255
Patients in
two of the cohorts (test and validation cohorts) were
treated with curative (radical) radiotherapy while
patients in the other cohort underwent surgery to
remove all or part of the bladder (cystectomy). In the
patients in the radiation test cohort low Mre11 protein
levels in their tumor tissue was associated with poorer
cause-specific survival after 3 years than high expres-
sion, which was confirmed in the validation cohort. In
the surgical cohort, on the other hand, no such relation-
ship was observed. Thus, Mre11 expression is a predic-
tive marker for bladder cancer radiotherapy, but not
a prognostic factor for general bladder cancer survival.
Several of the NHEJ associated genes or their proteins
have been investigated as potential prognostic and
predictive biomarkers for a variety of cancers. ATM is
probably the most widely studied of the NHEJ associ-
ated genes to be analyzed, especially for breast and
hematological malignancies. In an early study, cytoge-
netic analysis using fluorescence
in situ
hybridization
of mononuclear cells from the blood of patients with
chronic lymphocytic leukemia revealed that patients
