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Molecular and Cellular Biology, July 1999, p. 5166-5169, Vol. 19, No. 7
Bayer-Chair Department of Molecular
Immunology and Allergology,
Received 26 January 1999/Returned for modification 16 February
1999/Accepted 30 March 1999
Sister chromatid exchange (SCE) frequency is a commonly used index
of chromosomal stability in response to environmental or genetic
mutagens. However, the mechanism generating cytologically detectable
SCEs and, therefore, their prognostic value for chromosomal stability
in mitotic cells remain unclear. We examined the role of the highly
conserved homologous recombination (HR) pathway in SCE by measuring SCE
levels in HR-defective vertebrate cells. Spontaneous and mitomycin
C-induced SCE levels were significantly reduced for chicken DT40 B
cells lacking the key HR genes RAD51 and RAD54
but not for nonhomologous DNA end-joining (NHEJ)-defective KU70 Symmetrical exchanges between newly
replicated chromatids and their sisters can be visualized cytologically
in vertebrate cells if the DNA of one chromatid is labelled with
5-bromodeoxyuridine (BUdR) during synthesis. Sister chromatid exchanges
(SCEs) can be induced by various genotoxic treatments (10),
suggesting that SCEs reflect a DNA repair process. Cytological
assessment of SCE levels in peripheral blood lymphocytes is used as an
index of the mutagenic potential of environmental factors. More
importantly, ~10 SCEs occur spontaneously in normally cycling human
cells (5, 8), suggesting a link between SCE and DNA
replication. Elevated spontaneous SCE levels are observed in cells from
Bloom syndrome patients (9), in mouse cells that lack
poly(ADP-ribose) polymerase (29) or KU70 (15),
and in hamster cells with defects in XRCC1 (28), but the
causal relationships between these enzymes and SCE are not clear.
While the phenomenon of SCE has long been established (27)
and many observations about the induction of SCEs have been made, their
molecular basis remains obscure. SCE is intimately associated with DNA
replication, and eukaryotic cells exposed to DNA-damaging agents in
G2 show elevated SCE levels only after completing a subsequent replication cycle (32). Homologous recombination (HR) was suggested as one of the mechanisms responsible (13, 14). While HR occurs between sister chromatids in yeast as a means to replicate around UV-induced lesions (12), it has
not been considered constitutively active during metazoan mitosis, perhaps because of the predominance of the nonhomologous DNA
end-joining (NHEJ) pathway (30). In addition, the lack of
recombinational repair mutants precluded direct testing of HR's
involvement in SCE, so other models evolved. It was proposed that SCEs
result from strand switching at stalled replication forks
(20). Another model involved topoisomerase II action at
coincident breaks at replication forks on both sister chromatids and
subsequent rejoining (4, 11, 19).
The two double-strand break (DSB) repair pathways of HR and NHEJ are
highly conserved between yeast and vertebrate cells. HR uses a
homologous chromosome or a sister chromatid as a template to effect
precise repair of a DNA lesion, while the NHEJ pathway carries out
repair with lower fidelity and no requirement for homology. To test the
idea that HR between sister chromatids is the primary mechanism for
SCE, we used reverse genetics in the hyperrecombinogenic chicken B-cell
line DT40 (3) to genetically ablate the HR enzymes Rad51
(23, 24) and Rad54 (1, 7) and then measured SCE
levels. We found that spontaneous and mitomycin C (MMC)-induced SCE
levels were significantly reduced in HR-deficient RAD51 Cells and cell culture.
The generation of
RAD54 Gene targeting assay.
The targeting construct for the
ovalbumin locus was prepared by insertion of an 8-kb
PmaCI-PshAI genomic fragment into pSP72 followed
by the insertion of a puromycin resistance cassette into the vector's
unique HindIII site (3). The targeting
construct was linearized by PvuI and electroporated (550 V,
25 µF) into 5 × 106 cells of each clone. Southern
analysis following selection was performed as described previously
(3).
Western blot analysis.
Western blot analysis of Rad51 and Ku
was performed as described previously (24). Briefly,
106 cells were lysed in 20 ml of sodium dodecyl sulfate
lysis buffer. Following sonication and boiling, aliquots were subjected
to sodium dodecyl sulfate-10% polyacrylamide gel electrophoresis.
After transfer to a nitrocellulose membrane (Schleicher & Schuell,
Dassel, Germany), proteins were detected by polyclonal rabbit
anti-human Rad51 polyclonal serum or rabbit anti-chicken Ku70
polyclonal serum (25) and horseradish peroxidase-conjugated
goat anti-rabbit immunoglobulin (Santa Cruz Biotechnology, Santa Cruz,
Calif.) with a Super Signal CL substrate (Pierce, Rockford, Ill.).
Measurement of SCE levels.
For BUdR labelling, cells were
cultured in the presence of 10 µM BUdR for 18 to 21 h (two cell
cycle periods) and pulsed with 0.1 µg of Colcemid per ml for the last
2 h (3 h in the case of RAD51 To measure the frequency of SCE, RAD54 Compared with wild-type cells (Fig. 3A; 3.0 SCEs/cell), the level of
spontaneous SCE was significantly reduced in
RAD54
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Copyright © 1999, American Society for Microbiology. All rights reserved.
Sister Chromatid Exchanges Are Mediated by
Homologous Recombination in Vertebrate Cells
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ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
/
cells. As measured by targeted
integration efficiency, reconstitution of HR activity by expression of
a human RAD51 transgene restored SCE levels to normal,
confirming that HR is the mechanism responsible for SCE. Our findings
show that HR uses the nascent sister chromatid to repair potentially
lethal DNA lesions accompanying replication, which might explain the
lethality or tumorigenic potential associated with defects in HR or
HR-associated proteins.
![]()
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
/
or RAD54
/
DT40 cells but not in NHEJ-defective KU70
/
cells. These findings suggest that HR is one of the principal mechanisms responsible for SCE in vertebrate cells.
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
/
,
RAD51
/
/tet-hRAD51+, and
KU70
/
mutant DT40 cells has been described
(1, 24, 25). RAD51
/
clone 104 was
obtained by the same method as 110 cells (24). Cells were
cultured in RPMI 1640 medium supplemented with 10
5 M
-mercaptoethanol, 10% fetal calf serum, and 1% chicken serum (Sigma, St. Louis, Mo.) at 39.5°C.
/
cells).
MMC (50 ng/ml) was added 8 h before harvest. The MMC sensitivities
of wild-type and RAD54
/
DT40 clones, as
measured by colony formation in methylcellulose plates (0.2% survival
relative to untreated cells), were comparable. Harvested cells were
treated with 75 mM KCl for 15 to 30 min and subsequently fixed with
methanol:acetic acid (3:1) for at least 30 min. Cells were fixed onto
wet (50% ethanol) glass slides and dried on a 40 to 42°C plate.
Dried slides were incubated with 10 µg of Hoechst 33258 per ml in
phosphate buffer (pH 6.8) for 20 min, followed by rinsing with
MacIlvaine solution (164 mM Na2HPO4, 16 mM
citric acid [pH 7.0]). Slides were irradiated with black light
(
= 352 nm) for 60 min and incubated in 2× SSC (1× SSC is
0.15 M NaCl plus 0.015 M sodium citrate) solution at 62°C for 1 h before staining with 3% Giemsa solution (pH 6.8) and subsequent microscopy.
![]()
RESULTS AND DISCUSSION
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
/
and KU70
/
DT40 cells were labelled with BUdR
for two cell cycle periods (18 h). Since RAD51 is an
essential gene, we used RAD51
/
cells
expressing a tet-repressible human RAD51
(hRAD51) transgene (24) and labelled cells with
BUdR simultaneously with repression of the transgene, so that the rapid
cell death following Rad51 depletion would not interfere with SCE
analysis; the time-dependent repression of this transgene is confirmed
in Fig. 1. The rate of spontaneous SCE in
wild-type DT40 cells is 3.0 ± 1.7 exchanges per metaphase (Fig.
2 and 3A),
while chicken and human cells display ~3 and ~10 exchanges per
mitosis on average, respectively (5, 8, 10, 18, 31). As the
human genome is three times larger than the chicken genome, the numbers
of SCEs per unit length of genomic DNA are comparable among DT40 cells,
chicken embryonic B lymphocytes, and human cells.

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FIG. 1.
Western blot analysis of Rad51 expression in DT40 cells.
Each lane contains 10 µg of protein visualized with anti-hRad51 and
anti-Ku70 antisera as described previously (24). Lanes: 1, parental DT40; 2, RAD51
/
cells expressing
high levels of hRad51 (clone 104: hRad51hi); 3, RAD51
/
cells expressing low levels of hRad51
(clone 110: hRad51lo); 4 to 7, clone 110 cells at 6, 12, 18, 21 h, respectively, after the repression of the
tet-controlled hRAD51 transgene.

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FIG. 2.
SCE in wild-type DT40 cells. Arrowheads indicate the
sites of SCE.

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FIG. 3.
Reduced levels of SCE in cells deficient in HR. Cells
were labelled with BUdR during two cell cycle periods with or without
MMC treatment (50 ng/ml) for the last 8 h. Spontaneous and
MMC-induced SCEs in the macrochromosomes of 200 metaphase cells were
counted. Histograms show the frequency of cells with the indicated
numbers of SCEs per cell. The mean number of SCEs per cell ± the
standard deviation is shown in the upper right corner of each
histogram; underlined values differ significantly (P < 0.002) from wild-type (wt) control SCE levels; statistical
significance was calculated by the Mann-Whitney nonparametric U test.
/
cells (Fig. 3E; 2.1 SCEs/cell,
P < 0.0001), which have a low level of HR as measured
by targeted integration frequency (Table 1) (1). Similarly, the
RAD51
/
clone 110, which expresses a low
level of hRAD51 transgene (24), showed
significantly reduced SCE frequency (Fig. 3C; 2.5 SCEs/cell, P = 0.0013) as well as a reduction in targeted
integration (Table 1) compared with wild-type cells. The inhibition of
the human RAD51 transgene with tetracycline further reduced
the level of SCE in 110 cells (Fig. 3D; 1.5 SCEs/cell, P < 0.0001). The reduced level of SCE found with RAD51 deficiency is
likely an underestimate, as this method detects SCE between chromatids
labelled during DNA synthesis, when diminishing amounts of Rad51 are
still present. As it does for spontaneous SCE, RAD51 or
RAD54 deficiency causes a statistically significant
reduction in MMC-induced SCE (Fig. 3I to K). The residual HR and SCE
activity in RAD51
/
and
RAD54
/
cells may be attributable to other
Rad51 and Rad54 homologues, such as XRCC2, XRCC3, Rad51B, and Rad54B.
Strikingly, overexpression of hRAD51 in the
RAD51
/
clone 104 restored both targeted
integration and spontaneous and induced SCE frequencies to wild-type
levels (Fig. 3B and H; Table 1). To assess whether NHEJ contributes to
SCE, we counted SCEs in KU70
/
DT40 cells
(25) (Fig. 3F and L) and found a slight increase in SCE
frequency, without statistical significance, despite a previous report
of elevated SCE in Ku70-deficient mouse fibroblasts (15). We
suggest that the elevated HR rate in DT40 cells (3) may mask
any minor effects of Ku70 deficiency on SCE. In summary, these
observations reveal that HR between sister chromatids is principally
responsible for SCE in higher eukaryotic cells.
TABLE 1.
Transfection and targeted integration frequencies in
wild-type and HR-deficient DT40 cells
The involvement of HR in SCE is a little surprising, because the
presence of spontaneous SCEs in vertebrate cells indicates the presence
of active HR during mitosis. However, the presence of Rad51 foci in
S-phase nuclei (26), as well as spontaneous chromosomal
breaks in HR-deficient RAD51
/
(24) and RAD54
/
(25)
cells, has suggested the essential involvement of HR in the maintenance
of chromosomal integrity in vertebrate cells. Spontaneous DSBs that
necessitate repair by the Rec proteins and occur during replication in
Escherichia coli (17, 22) and the recently
described formation and resolution of Holliday junctions in
Saccharomyces cerevisiae mitosis (33) demonstrate
that the role of HR in ensuring complete replication of the genome has been conserved throughout evolution.
DNA replication across a nick is likely to produce a DSB in one of the sister chromatids, while replication stalled at a damaged base may produce a single-strand gap between the damaged base and a new Okazaki fragment initiating downstream. However, the nature and origin of the DNA lesions generated during normal DNA replication and eventually repaired by HR, resulting in SCEs, remain to be determined. Chemical modifications of the genomic DNA, such as hydrolysis, oxidation, and nonenzymatic methylation, occur at significant rates in vivo (16). Although such covalently modified bases are usually repaired before DNA replication, it is possible that unrepaired lesions and nicks are encountered by replication forks and result in single-strand gaps and DSBs in one of the sister chromatids. Such strand discontinuities can be repaired postreplicationally by HR with the sister chromatid, as is the case for recombinational repair in yeast cells (12). Indeed, the modification of genomic DNA by 3-methyladenine specifically induces S-phase arrest, SCEs, and chromosome breaks (6). DSBs may also result from the breakage of arrested replication forks, following the forks' being impeded by DNA secondary structures and DNA-bound proteins (2).
The genetic analysis of recombination between sister chromatids in yeast, which is useful in detecting nonmutagenic carcinogens (21), can detect unequal SCE and gene conversion but not equal exchange of sister chromatids. The analysis of SCE in vertebrate cells provides a striking insight not only into the cellular response to potential carcinogens but also into the frequency of spontaneous lesions necessitating HR-mediated repair during DNA replication and the accuracy with which the HR machinery repairs them.
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ACKNOWLEDGMENTS |
|---|
We thank M. Hashishin, Y. Sato, O. Koga, and M. Hirao for their excellent technical assistance and Y. Ejima (Kyoto University) and T. Horiuchi (Okazaki National Institutes) for helpful discussions. We are indebted to J. Haber (Brandeis University) and W. F. Morgan (UCSF) for their critical readings of the manuscript.
C.M. is the recipient of a JSPS Postdoctoral Fellowship. The Bayer-Chair Department of Molecular Immunology and Allergology is supported by Bayer Yakuhin, Kyoto, Japan. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, by CREST of JST (Japan Science and Technology), and by a grant from the Mochida Memorial Foundation for Medical and Pharmaceutical Research.
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FOOTNOTES |
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* Corresponding author. Present address: Radiation Genetics, Faculty of Medicine, Kyoto University, Yoshida Konoe, Sakyo-ku, Kyoto 606-8315, Japan. Phone: 81-75-771-8159. Fax: 81-75-771-8184. E-mail: stakeda{at}mfour.med.kyoto-u.ac.jp.
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