Aging and Sex, DNA Repair in
75
that has been unwound and looped out
to lend its information to cover the gap
in the broken chromosome) is not cut
to resolve the crossovers. These models,
called the
synthesis-dependent strand an-
nealing
or
migrating D-loop models
,w
e
r
e
proposed to account for the prominence
of mitotic gene conversion without re-
ciprocal exchange observed in replicating
cells. The standard model of HRR is
shown in Fig. 3, however, since it ap-
plies to nonreplicating somatic cells, the
major cells showing decreased function
with age, and to germ line cells, as dis-
cussed below.
As indicated in Table 3, four genes that
act directly in HRR (
BLM
,
WRN
,
p53
,and
ERCC1
), when defective, cause early ag-
ing. Oxidative damage to DNA includes
double-strand breaks. Thus the gene prod-
uct,
p66Shc
, which increases the level of
intracellular ROS, may cause more DNA
damage, needing HRR when it is ac-
tive. When
p66Shc
is absent, there are
fewer such DNA damages and there is life
span extension.
2.4
NHEJ (Nonhomologous End Joining)
As noted in Section 2.3, double-strand
breaks, interstrand cross-links, and DNA
damages blocking a replication fork can be
repaired by one of two pathways, HRR
or NHEJ. HRR is an accurate repair
pathway, obtaining information missing
within a damaged chromosome from a
homologous, undamaged chromosome.
NHEJ, alone, however, is inaccurate at the
nucleotide level because it involves end-
joining reactions between single-stranded
ends to form junctions containing re-
gions of microhomology of 1 to 10
base pairs within 20 base pairs of the
ends. However, coupled homologous and
nonhomologous repair can occur (using
short homologous regions on a nearby het-
erologous chromosome) to employ NHEJ
in a more accurate process, with short
regions of gene conversion from the un-
damaged chromosome. In addition, it
was recently shown that NHEJ, when the
number of double-strand breaks is small,
serves to properly reconnect the two bro-
ken ends of a given chromosome. In
the absence of NHEJ, more misrejoined
chromosomes (chromosomal aberrations)
are produced.
NHEJ
of
a
double-strand
break
is
illustrated in Fig. 4. Two Ku heterodimers
(Ku consists of Ku70 complexed with
Ku86) attach to two broken ends of a
chromosome. Ku then recruits DNA-PK
cs
,
which becomes activated by the interaction
with Ku and a DNA end. Activated
DNA-PK
cs
phosphorylates a wide range of
DNA binding proteins and also phospho-
rylates Werner syndrome protein (WRN).
WRN displaces DNA-PK
cs
and, through
interaction with Ku, activates its WRN
exonuclease function, trimming back sin-
gle strands on the chromosome ends.
The complex of Mre11, Rad50, and NBS1
(MRN), plus BRCA1, may interact at this
point with Ku to keep the end join-
ing fairly accurate. In the absence of
WRN, frequent gross chromosomal rear-
rangements occur. The Bloom syndrome
protein, BLM, is also needed to keep the
NHEJ fairly accurate. BLM may also as-
sist with the alignment of two nearby
ends for fairly accurate NHEJ (BLM is
drawn with a dashed oval in Fig. 4 to in-
dicate that its role in the NHEJ repair
process is not yet clearly established). Pol
µ
then associates with small gaps in the
aligned and stabilized joined ends, and
pol
µ
,ina
s
s
o
c
i
a
t
i
onw
i
thKu
,r
e
c
ru
i
t
s
ligase IV and XRCC4 (X-ray repair cross
complementing protein 4). Pol
µ
±lls in
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