Biology Reference
In-Depth Information
helix-distorting lesions. The following section describes
the most common DNA base damage repaired through
the BER pathway.
physiological conditions via a hydrolytic reaction,
a process enhanced by the presence of free radicals.
15
The 6-amino group of purine and the 4-amino group
in pyrimidine are the hydrogen donors in normal Wat-
son
Crick base pairings. Upon deamination, the amino
group is replaced by a keto group, which is a hydrogen
acceptor in a normal Watson
e
Oxidation
DNA of eukaryotic cells is continuously subjected to
reactive oxygen species (ROS) exposure which can be
derived from the external environment, but predomi-
nantly from endogenous byproducts of the oxidative
phosphorylation events which occur during mitochon-
drial respiratory chain of all aerobic organisms.
8
Another
major source of ROS production is represented by phago-
cytic NADPH oxidases during inflammatory responses
and by non-phagocytic NADPH oxidases, as determined
in different cell systems.
9
In its ground state, molecular
oxygen (O
2
) is relatively unreactive. However, during
normal metabolic activity, and as a consequence of
various environmental perturbation (e.g. radiation,
biotic, and abiotic stresses, xenobiotics and diseases) O
2
is capable of giving rise to frightfully reactive excited
states such as free radicals and derivatives.
10
All ROS
are extremely reactive and can cause molecular damage,
leading to cell death.
11
Purines undergo oxidation of the
ring atoms, leading to various chemical modifications.
The highly mutagenic guanine derivate 8-hydroxygua-
nine (8-oxoG) is formed in large quantities as a conse-
quence of the high oxidation potential of this base
(
Figure 3.1
).
12
The miscoding effect of 8-oxoG lesion is
due to DNA polymerase activity which inserts adenine
opposite to 8-oxoG, resulting in G:C to A:T transition
mutations, therefore generating a DNA base mutation.
The most frequent pyrimidine oxidation is repre-
sented by the formation of 5-hydroxycytosine (5-OHC)
which leads to the insertion of a thymine creating
a potential premutagenic lesion.
13
Other important
oxidation lesions are the formamidopyrimidine such
as faPyA and faPyG and the oxidized thymine glycol
(TG) (see
Figure 3.1
).
14
All these mutations are recog-
nized by different DNA glycosylases, which remove
the damaged nitrogenous base by cleaving the N-
glycosylic bond and generating an abasic (AP) site while
leaving the sugar-phosphate backbone intact. This reac-
tion represents the initiation step of the BER pathway
and forms the substrate for the AP endonuclease
enzyme which generates a nick in the phosphodiester
backbone of the AP site.
Crick base pairing. It is
therefore likely that all deamination products of DNA
bases are highly mutagenic.
16
The most common
product of pyrimidine deamination is uracil, whose
repair is initiated by a uracil DNA glycosylase (UNG)
enzyme family member. UNGs are ubiquitous, highly
expressed enzymes that recognize uracil in both single-
and double-stranded DNA, but will not recognize uracil
present in a RNA molecule. UNGs catalyze the hydro-
lytic cleavage of the N-glycosidic bond of deoxyuridine
in DNA, and thus initiates the first step for the uracil
base excision repair. 5-OH-Ura is generated by DNA
exposure to ionizing radiation and ROS-producing
agents.
17
This lesion is probably derived from the rela-
tively unstable cytosine glycol, which readily undergoes
deamination to yield uracil glycol and then dehydrated
to form 5-OH-Ura. This base modification is particularly
harmful because it is readily bypassed by DNA
polymerases, which predominantly inserts an A oppo-
site 5-OH-Ura.
18
Because 5-OH-Ura is an oxidation
product of cytosine, insertion of an A opposite 5-OH-
Ura is thus mutagenic and generates a G:C to A:T muta-
tion. 5-HMU is the product of ROS-induced thymine
oxidation and it is also found as a stable radiation
product. Despite the fact that in DNA 5-HMU pairs
like T producing the non-mutagenic pair 5-HMU:A, it
can also be formed from oxidation of the 5-methyl group
of 5-methylcytosine followed by deamination,
19
which
will generate a mutagenic 5-HMU:G pair. Hypoxanthine
and xanthine are the two major purine deamination
products derived from adenine and guanine, respec-
tively. Both of these lesions are highly mutagenic; the
presence of hypoxanthine within the DNA will cause
A:T to G:C transition mutation, while xanthine will
prevalently generate a G:C to A:T transition mutations.
15
e
Alkylation
Some of the most prevalent cytotoxic and mutagenic
lesions to DNA arise from the alkylation of its bases.
The most common form of nonenzymatic methylation
of DNA likely results from physiological exposure to
endogenous
S
-adenosyl methionine (SAM), which is
found in the nucleus and participates in targeted enzy-
matic DNA methylation.
13
Analternativesourceof
alkylating agents that generates DNA alkyl lesions
are environmental toxins
20
and monofunctional
(e.g. temozolomide, nitrosurea compounds, alkylsulfo-
nates) or bifunctional (e.g. cisplatin, mitomycin C,
Deamination
Another major base damage result is the formation of
deamination products such as uracil, 5-hydroxyuracil
(5-OH-Ura), 5-hydroxymethyluracil (5-HMU), hypoxan-
thine, and xanthine (see
Figure 3.1
). Deamination of
DNA bases
can
occur
spontaneously
under
