Table 2 lists alterations in genes controlling
DNA repair, oxidant status, or
apoptosis that result in increased life span.
The increases in life span found with
the genetic alterations in Table 2 are usually
an increase in the maximum life
span (not just the mean life span) by
about 30 to 40%. Mean life span can
be extended by reductions in tumorigenesis
or acute and sporadic diseases, not
generally regarded as a cause of aging.
The organisms with increased maximum
life span reported here showed longer
spans of normal vigorous activity (not
merely slowed metabolism, which can
also extend life span). The cellular roles
58 Aging and Sex, DNA Repair in
Tab. 2 Life span extension: increased life span from alterations in genes controlling DNA repair, apoptosis, or oxidant status.
Organism Genetic Pathway Aging Fertility Spontaneous Effect on
alteration phenotype phenotype cancer
phenotype Cellular Induced Spont. Induced
ROS DNA mutation apoptosis
damage
Mouse 100-fold excess
copies MGMT
gene
O6-meG DNA
repair
Life span
extension
n.t. Reduced n.t. Reduced
O6-meG
No effect at
H-ras
locus
n.t.
Fruit fly Excess SOD in
neurons or all
tissues or with
catalase in all
tissues
Removes ROS Life span
extension
No change n.t. Reduced
occurrence of
oxidized
proteins
n.t. n.t. n.t.
Fruit fly MsrA excess Methionine
sulfoxide
reductase
Life span
extension
Extended n.t. Repaired protein
oxidation
n.t. n.t. n.t.
Mouse p66shc defect Blocks oxidant
and apoptosis
parts of p53
pathways
Life span
extension
n.t. n.t. Reduced Reduced
oxidative
damage
n.t. Reduced
Human Higher specific
activity of PARP
BER Life span
extension
n.t. n.t. n.t. n.t. n.t. n.t.
Notes: ROS: reactive oxygen species; Spont.: spontaneous; MGMT: O6-methylguanine-DNA methyltransferase; n.t.: not tested; SOD: superoxide
dismutase.
Aging and Sex, DNA Repair in 59
of these genetic alterations are described
below.
1. MGMT. One frequent type of DNA
damage (see Table 1) isO6-methylguanine,
caused by low levels of alkylating agents
present in food, water, air, and tobacco
smoke, and formed by normal processes in
the body mediated by gastric bacteria and
macrophages.O6-methylguanine is specifically
repaired by a DNA repair enzyme
called O6-methylguanine-DNA methyltransferase
(MGMT). MGMT transfers the extra
methyl group from guanine in DNA to
a particular amino acid within itself and
becomes ‘‘used up’’ after the transfer occurs.
The MGMT gene codes for one of
the five DNA repair mechanisms listed
in Section 1.1. As indicated in Table 2,
when 100 copies of the MGMT gene were
inserted into the mouse genome, these
mice (under the usual conditions of mouse
maintenance) had their life span extended
and died at a considerably slower rate than
wild-type mice.
2. SOD. Another important type of
metabolically caused DNA damage is oxidative
damage, themost frequent damage
identified (Table 1). An apparently unavoidable
by-product of normal respiratory
metabolism is the production of reactive
oxygen species (ROS) from molecular
oxygen, and ROS cause oxidative damage.
ROS include free radicals (where
the symbol • indicates an unpaired electron):
the superoxide radical (O2
•−) and
the hydroxyl radical (OH•). Another oxygen
respiration by-product is hydrogen
peroxide (H2O2). H2O2, if not removed,
it diffuses fairly easily through the cell,
and when it encounters Fe2+ (the ferrous
ion), it can undergo the Fenton reaction
and produce OH• and other ROS.
ROS produce a number of lesions in
DNA, including base lesions, sugar lesions
(the deoxyribose sugar is in the
backbone of DNA), DNA–protein crosslinks,
single-strand breaks, double-strand
breaks, and abasic sites.
The major ROS produced by the cell
is O2
•−, formed in the mitochondria (the
energy-producing organelles of the cell).
Superoxide dismutase (SOD) occurs in
two forms, manganese SOD (MnSOD)
and copper/zinc SOD (Cu/ZnSOD). Both
forms of SODconvertO2
•− to the less damaging
H2O2, and then another enzyme,
catalase, converts H2O2 to molecular oxygen
and water. MnSOD occurs in the mitochondria
and Cu/ZnSOD occurs in the
cytoplasm.As shown in Table 2, a number
of investigators have found that inserting
genes producing higher than normal
levels of superoxide dismutase into the
fruit fly (Drosophila melanogaster) genome
results in life span extension. Insertion
of genes producing either MnSOD or
Cu/ZnSOD caused life span extension, although
the artificially inserted Cu/ZnSOD
only produced life span extension when
its expression was restricted to the motor
neurons, or solely to the adult phase of the
fruit fly life cycle.
Aging has been found to correlate with
increased levels of oxidative products, such
as protein carbonyls and 8-oxo-guanine in
DNA, and fruit flies lacking either catalase
or Cu/ZnSOD have a reduced life span.
Further, selection of a population of fruit
flies for increased life span correlates
with strongly increased expression of
both MnSOD and Cu/ZnSOD. Reverse
selection of these long life span flies to
a shorter life span resulted in reduced
expression of Cu/ZnSOD.
3. MsrA. In addition to DNA damage,
free radicals damage proteins, lipids,
and carbohydrates. Most proteins have
a short half-life (averaging about three
days in mouse liver). Oxidatively damaged
proteins and lipids are subject to both
60 Aging and Sex, DNA Repair in
degradation and some repair reactions.
If cellular genes that code for enzymes
involved in the replacement of damaged
proteins are themselves damaged, then
damaged proteins may not turn over
as rapidly, and protein damages may
become important as they accumulate with
age. Table 2 shows that insertion of an
extra gene encoding bovine methionine
sulfoxide reductase (MsrA) in the fruit fly
genome, which helps repair oxidatively
damaged proteins, leads to life span
extension. Consistent with this, MsrA,
when defective in the mouse, results in
early aging (Table 3).
4. p66Shc. The p53 gene has a central
role in response to DNA damage. The
p53 protein is directly active in three
forms of DNA repair (NER, BER, and
HRR).When there is no externally induced
DNA damage, p53 has a half-life of
only 5 to 40 minutes since specific
enzymes target p53 for degradation. Thus,
p53 is kept at a low level when there
is no DNA damage. However, upon
exposure of a cell to DNA-damaging
agents, p53 becomes metabolically stable
and, in addition, more copies of it are
produced in the cell. In the presence
of various types of DNA damage, p53
undergoes modifications at some of the
18 different sites within the protein. Some
of these modifications [phosphorylations,
acetylations, poly(ADP-ribosyl)ations, or
sumoylations (covalent attachments of
small ubiquitin-like proteins) allow the
p53 protein to act as a regulatory agent,
activating numerous other genes, carrying
out different responses to different kinds
or levels of DNA damage. The p53 protein
can regulate or act in at least four major
types of responses to DNA damage (acting
as a ‘‘master switch’’), and which action or
transactivation (regulating the induction
of other genes) it performs depends on
the level and type of DNA damage. p53
can (1) send the cell into cell cycle arrest
(to allow extra time for repair of DNA
damage); (2) act directly in DNA repair
(see Fig. 1 for where p53 acts in NER);
(3) cause the cell to switch into a cell
suicide mode (apoptosis); or (4) cause the
cell to produce higher levels of ROS
(apparently as a preliminary to entering
the cell suicide mode of apoptosis). When
acting to increase the internal level of ROS
and entry into apoptosis, p53 acts through
another gene it controls, p66Shc.
When a mouse embryo is produced
with both copies of its p66Shc gene
inactive (a p66Shc ‘‘knockout’’), mouse
embryo fibroblast cells derived from it
have intracellular levels of ROS reduced by
about 40%. Consistent with this reduction
in ROS, there is also greatly reduced
oxidative damage accumulation in both
nuclear and mitochondrial DNA of these
cells. A similar reduction in nuclear and
mitochondrial DNA damage is seen in
vivo in the tissues of lung, spleen, liver,
and skin in 3- and 24-month-old p66Shc
knockout mice, although there is no
reduction in the brain, where p66Shc is
not normally expressed. Cells of thesemice
are inhibited from undergoing apoptosis
after cellular oxidative damage (when
challenged with externally applied H2O2).
Knockout mice without p66Shc show
life span extension without any notable
increase in cancer or other pathological
defects (Table 2). Mice with a type of
overactive p53 (an increase in some
p53 functions) and intact p66Shc show
early aging (Table 3). On the other hand,
removal of all p53 functions (some of
which are protective in DNA repair) also
results in early aging (Table 3).
5. PARP. DNA damages caused by
alkylating agents (such as those that
methylate guanine, discussed above),
Aging and Sex, DNA Repair in 61
Tab. 3 Early aging: decreased life span from alterations in genes controlling DNA repair or protein oxidation.
Organism Genetic Pathway Aging Fertility Spontaneous Effect on
alteration phenotype cancer
Cellular Induced Spont. Induced
ROS DNA mutation apoptosis
damage
Human RECQ3 helicase and
exonuclease defect
(Werner syndrome)
HRR and NHEJ Early aging Reduced Increased n.t. Increased Increased n.t.
Human RECQ2 helicase defect
(Bloom syndrome)
HRR and NHEJ Early aging Reduced Increased n.t. Increased Increased n.t.
Human RECQ4 helicase defect
(Rothmund–Thomson
syndrome)
DNA repair
pathway,
unknown type
Early aging Reduced Increased n.t. n.t. Increased n.t.
Human and
Mouse
XPD helicase defective at
certain sites
(Trichothiodystrophy)
NER, also alters
transcription
initiation
Early aging Reduced No change n.t. n.t. n.t. Increased
Human CSB defect at 2 helicase
motifs or ATPase motif
(Cockayne syndrome)
BER if defective at
helicase motif V
or VI, TCR if
defective in
ATPase function
Early aging n.t. No change n.t. Increased n.t. Increased
Mouse Ku-80 (activator of Ku-70
helicase) defect
NHEJ Early aging n.t. Increased n.t. n.t. n.t.
Mouse Topoisomerase IIIβ
defect
Unknown, but
probably DNA
repair,
replication, or
recombination
Early aging n.t. n.t. n.t. n.t. n.t.
(continued overleaf )
62 Aging and Sex, DNA Repair in
Tab. 3 (continued)
Organism Genetic Pathway Aging Fertility Spontaneous Effect on
alteration phenotype cancer
Cellular Induced Spont. Induced
ROS DNA mutation apoptosis
damage
Mouse ERCC1 defect NER and interstrand
cross-link repair
(HRR)
Early aging Infertile No change n.t. Increased Increased
Mouse p53 overactivated Increases some p53
functions
Early aging n.t. Reduced n.t. n.t. n.t.
Mouse p53 defect Blocks all p53
functions,
including NER,
BER and HRR
Early aging Reduced Increased n.t. Increased Increased
Mouse MsrA Defect Methionine
sulfoxide
reductase
Early aging n.t. n.t. Increased
protein
oxidation
n.t. n.t.
Notes: ROS: reactive oxygen species; Spont.: spontaneous; n.t.: not tested.
Aging and Sex, DNA Repair in 63
ionizing radiation (which produces DNA
single- and double-strand breaks and
oxidative damages), and ROS result in
rapid activation of an enzyme called
poly(ADP-ribose) polymerase, or PARP.
PARP, similar to p53 discussed above,
has a role as a ‘‘master switch’’. PARP
can (1) act directly in one form of DNA
repair, BER, (2) control the function of
many other proteins by catalyzing the addition
of ADP-ribose branched polymers
onto them (either activating or repressing
their function), and (3) trigger apoptosis
(cell suicide). In addition, PARP controls
new transcription or activities of a number
of genes affecting survival or apoptosis,
including p53. It was found that centenarians
(humans who have lived for more
than 100 years) have a modified form
of PARP, which is more efficiently activated
than the PARP of noncentenarians
(Table 2), thereby apparently causing life
span extension. In addition, the maximal
poly(ADP-ribosyl)ation capacity (efficiency
of activation of PARP) in leukocytes of
13 mammalian species of different life
span was measured. There was a strong
correlation of PARP efficiency of activation
with species-specific life span.