Download The role of the replication licensing system in cell proliferation and

Progress in Cell Cycle Research, Vol. 5, 287-293, (2003)
(Meijer, L., Jézéquel, A., and Roberge, M., eds.)
chapter 29
The role of the replication licensing system
in cell proliferation and cancer
S. Shreeram and J. Julian Blow*
Wellcome Trust Biocentre, University of Dundee, Dow Street, Dundee DD1 5EH, Scotland, UK
* To whom correspondence should be addressed
The precise duplication of chromosomal DNA during each cell cycle is essential for the maintenance of
genetic stability. Failure to correctly regulate chromosomal DNA replication could lead to losses or duplication
of chromosome segments. The precise duplication of chromosomes is normally achieved by correct regulation
of the replication licensing system. Here we review our current knowledge of the licensing system and how this
might be defective in cancer cells. We also review how detection of licensing components can be used for the
diagnosis and prognosis of cancer. Finally we discuss the potential of the replication licensing system as a novel
anti-cancer target.
3-5). In metazoans, inhibition also involves a number of
different pathways, although these differ from those in
yeast. These pathways are shown schematically in the
bottom part of Figure 2.
THE REPLICATION LICENSING SYSTEM
The precise duplication of chromosomal DNA is
ensured by dividing the process of chromosome replication into two discrete steps (Figure 1). In the first
stage, which occurs during late mitosis and early G1,
replication origins are licensed for replication by loading complexes of the Mcm2-7 proteins onto them to
form a "pre-replicative complex" (pre-RC). At the onset
of S phase, cyclin-dependent kinases (CDKs) are activated and they initiate replication forks only at these
licensed origins. As initiation occurs, Mcm2-7 are
displaced from the origins, probably moving along
with the replication forks, so that the origin cannot fire
again in the current cell cycle. Replicated origins are
prevented from becoming re-replicated by inhibiting
further licensing during late G1 through to early M
phase. This is achieved by the combined activity of
CDKs and an inhibitory protein called geminin.
ORC binds with a high affinity to chromatin
during early G1 but the binding then weakens following origin licensing (6-8). In mammals, the Orc1 subunit is released from DNA during S phase and mitosis.
A recent report has also shown that the released Orc1
undergoes ubiquitin-mediated proteolysis (9).
Although a significant amount of chromatin-bound
Cdc6 is observed throughout the cell cycle, some Cdc6
degradation during S phase has also been reported (10,
11). The non-chromatin bound form is translocated
from the nucleus to the cytoplasm during G1 when
CDKs are activated (12-14). Like Cdc6, Cdt1 is also
subject to proteolysis at the beginning of S phase (15).
It is also inhibited by binding to geminin, a small protein whose levels and activity are under cell cycle
control and peak in S, G2 and M phases (16-19).
For origins to load Mcm2-7 and become licensed
at least three other proteins are required (Figure 2, top)
(1-4). The Origin Recognition Complex (ORC), a complex of six proteins Orc1-6, first binds to each replication origin and then recruits two other proteins, Cdc6
and Cdt1. These three proteins then act in concert to
load multiple Mcm2-7 complexes and to form a functionally licensed origin. This reaction has recently been
reconstituted with purified proteins (2). ORC, Cdc6 and
Cdt1 are required only for the loading, not the maintenance, of functional Mcm2-7 at origins. One important
consequence of this is that re-licensing of replicated DNA
can be prevented by inhibition or removal of ORC, Cdc6
or Cdt1 once S phase has started, without displacing functional Mcm2-7 at licensed origins.
The licensing system is also down-regulated as
cells reversibly or irreversibly withdraw from the cell
cycle (Figure 1) (4). Evidence from a range of different
organisms and cell types suggests that both G0 and
permanently arrested cells no longer have Mcm2-7
bound to chromatin and are thus functionally unlicensed. It has been proposed that the lack of chromatinbound Mcm2-7 provides a meaningful definition of
withdrawal from G1 and the proliferative state (4). Not
only is the Mcm2-7 hexamer removed from DNA as
cells pass into G0, but the unbound protein is also lost
from the cells. A similar reduction in Cdc6 protein is
seen in G0 and permanently arrested cells, though ORC
and Cdt1 proteins may persist (4, 20).
Activation of CDKs in late G1 then appears to be
the major trigger for blocking further origin licensing.
There is evidence from a range of different organisms
that all four components of the licensing system can
potentially be inhibited by CDKs. The situation is clearest
in Saccharomyces. cerevisiae, where CDKs cause the
nuclear export of Cdt1 and unbound Mcm2-7, whilst
ORC is inhibited and Cdc6 levels are down-regulated (1,
RE-REPLICATION OF CHROMOSOMAL DNA IN
CANCER CELLS
Aberrations to mechanisms that ensure precise
replication of the genomic DNA during each cell cycle
can cause chromosomal instability and thus lead to the
development of cancer (21). Aneuploidy (aberrations
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S. SHREERAM AND J.J. BLOW
in chromosome number) is commonly observed in human cancer (22, 23). There is compelling evidence to show that aneuploidy
contributes to malignant transformation and
progression of cancer. Firstly, specific
chromosome aneusomies correlate with distinct tumour phenotypes. Secondly, aneuploid tumour cell lines have been reported to
show an elevated rate of chromosome instability, and a number of mitotic genes regulating chromosome segregation have been
found mutated in cancer cells, implicating
such mutations in induction of aneuploidy in
tumours.
Figure 1. Replication licensing and CDK activity through the cell cycle.
A small segment of chromosomal DNA is depicted at different stages of the
cell cycle, with bound or unbound Mcm2-7 (hexagons). The activities of the
replication licensing system (RLS, dark) and CDKs (light) at the different
stages are shown in the central circle.
Figure 2. Pathways for the inhibition of origin licensing in higher eukaryotes. At the top, a replication origin is shown with bound ORC, Cdc6,
Cdt1 and several Mcm2-7 complexes. Below are shown the different mechanisms by which CDKs and geminin can inhibit licensing activity. Cdt1 is
bound and inhibited by geminin. Orc1 and Cdc6 may be degraded, whilst
Cdc6 may also be exported from the nucleus into the cytoplasm.
288
Chromosomal instability involving gains
and losses of whole chromosomes or parts is
likely to occur in most human malignancies,
conferring a selective growth advantage to
the cancer cell over its normal counterpart.
Whole chromosomes are likely to be gained
or lost by failure of correct sister chromatid
segregation during anaphase. However, loss
or duplication of chromosome segments
might be driven by failures in regulating
chromosomal DNA replication. Under-replication of even small segments of chromosomes will lead to chromosome breakage at
these segments as the sister chromatids are
pulled apart at mitosis ( Figure 3, top panel).
Over-replication of chromosomal segments
represents an irreversible genetic change,
which might be resolved to form tandem
duplications (Figure 3, bottom panel). These
sorts of chromosomal defects are commonly
seen in cancer cells, though whether they are
generated by replication defects such as these
is currently unclear. This section will discuss
how re-replication or endoreplication may
play a role in this process.
There are two general mechanisms by which
cells can increase their chromosomal DNA
content, involving "endocycles" or "origin refiring" (4), as outlined in Figure 4. During
endoreplication cell cycles (endocycles), complete rounds of chromosomal DNA replication occur without intervening cell divisions
(24). In endocycles there are alternating periods of high and low CDK activity so that
essentially normal S phases occur. Re-licensing of origins occurs when the CDK activity
present in G2 or mitotic cells is abolished
before cytokinesis has occurred, thus leading
to a further round of chromosome replication
in a single cell to double the DNA content. In
contrast, origin re-firing results in re-licensing and re-firing of certain origins in the
continued presence of high CDK levels. This
sort of localised re-replication is depicted in
the lower panel of Figure 3.
CHAPTER 29 / THE REPLICATION LICENSING SYSTEM
Figure 3. The consequence of origin misfiring. A small segment of chromosomal DNA replicated from three origins is
shown during different stages of the cell cycle. In the top panel
DNA is under-replicated as a result of the failure of one of the
origins to fire. As sister chromatids are separated during anaphase, the chromosome is likely to be broken near the unreplicated section. The middle panel shows the successful duplication of chromosomal DNA. In the bottom panel chromosomal
DNA is over-replicated as a result of one of the origins firing a
second time in S phase. The local duplication of DNA in the
vicinity of the over-firing origin is likely to represent an irreversible genetic change and might be resolved to form a tandem duplication.
Figure 4. Different routes by which cellular DNA content can
increase. A small segment of chromosomal DNA is depicted at
different stages of the cell cycle. The hexagons represent
Mcm2-7 bound to origins in G1 and S phases. The black arrows
show the normal sequence of cell-cycle events. The grey
arrows show routes by which cellular DNA content is increased (endocycles and origin re-firing). CDK activity in S, G2 and
M phase is denoted by the outer black circle. Redrawn from (4).
Endoreplication
Genetic and biochemical evidence from yeast show
that an oscillation in CDK activity is sufficient to induce
endoreplication. In the yeast Schizosaccharomyces pombe,
CDK activity determines the dependency of S phase on M
phase (25). Deletion of p56cdc13, the major B-type cyclin in
S. pombe, causes cells to undergo repeated rounds of S
phase without progression into M phase. As a result the
ploidy can be increased to >32C (26). Likewise, overexpression of Rum1, a CDK inhibitor, leads to increased
DNA content and nuclear enlargement (27, 28).
Another less well understood route for endoreplication with more potential relevance for cancer occurs
in cells with defects in the retinoblastoma protein pRb.
When the CDK inhibitor p21Cip1/Waf1 was expressed in
cells possessing functional pRb, the cell cycle was
arrested in G1 (32). However, when p21Cip1/Waf1 was
expressed in cells lacking pRb activity, cells arrested in
G2 and underwent cycles of endoreplication. The observation that p21Cip1/Waf1 can promote endoreduplication in pRb-negative contexts has potential practical
implications for cancer therapy. For tumours that are
pRb negative but p53 positive and therefore capable of
inducing p21Cip1/Waf1 in response to DNA damage,
endoreduplication may be a consequence of radiation
therapy, chemotherapy or any other genotoxic stress.
This may lead, in some cells, to enhanced malignancy.
An example of such a situation is familial retinoblastoma, where one germline copy of the Rb gene is mutated. Rb-negative cells occur at a relatively high frequency due to loss of heterozygosity at the RB locus
and result in neoplasia, most notably in the retina (33).
However, such patients are at a greatly increased risk of
developing secondary tumours in surrounding mesenchymal tissue exposed to ionizing radiation therapy (3436). It is possible that induction of p21Cip1/Waf1 in pRb-negative cells within such tissues leads to endoreduplication,
genetic instability, and ultimately, secondary malignancy.
Studies in mammalian cells are also consistent
with the above model, suggesting that endoreplication
depends on transient loss of CDK activity to permit
replication origins to become re-licensed for an additional cycle of replication while preventing the completion of mitotic events (29). The most detailed description to date of how endoreplication occurs in mammals comes from studies of trophoblast giant cells in
rodent placenta. Giant cells undergo endoreplication
during terminal differentiation, reaching ploidies of more
than 1000C (30). During the transition from a mitotic cell
cycle to endoreplication, trophoblast cells reduce expression of cyclin B (29). Coincidentally, there is an increase in
cyclin A and cyclin E- associated CDKs, the levels of
which oscillate prior to and during S phase. Modulation of
G1 and S phase CDK activity in endoreduplicating trophoblast cells appears to involve the cyclic synthesis and
destruction of the CDK inhibitor Kip2 (31). This would
create two distinct phases: a G1-like period when Kip2
levels are high and there is low CDK activity when licensing can occur, and an S phase-like period when Kip2
levels are low and CDK activity is high (31). However,
there is as yet no clear evidence to suggest that this type of
endocycle occurs in cancer cells.
Origin re-firing
Under certain circumstances, pre-RC proteins
(ORC, Cdc6, Cdt1 or Mcm2-7) may not be able to
respond to the inhibitory signals that normally act in S
phase as depicted in Figure 2. Instead, origins are relicensed and re-initiated in the continuing presence of
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S. SHREERAM AND J.J. BLOW
Does origin over-firing contribute to the chromosome instability of cancer cells? Amplification of subchromosomal portions of the genome is frequently
seen in many human cancers (42). Amplification of
specific genes may be advantageous to cancer cells by
increasing the copy number of oncogenes or genes that
confer resistance to anti-cancer drugs (43). Amplified
regions of the genome occur either as homogeneously
staining regions or small acentric autonomously replicating double minutes (44-47). Several mechanisms for
initiating DNA sequence amplification in mammalian
cells have been proposed, which fall either into the
over-replication
or
non-disjunction
groups.
Amplification of genes in the non-disjunction group
occurs as a consequence of defects in chromosome
segregation during mitosis (reviewed in (48, 49)), and
is thus outside the scope of this review.
Figure 5. Diagnostic potential of Mcm2-7 detection. Frozen
sections of normal cervix, LSILs, and HSILs were immunostained for Mcm5 or PCNA by the immunoperoxidase method. In
normal cervix, surface cells likely to be sampled by cervical
smear examination are negative with both antibodies. In LSIL,
anti-PCNA antibodies stain basal and some parabasal nuclei
but surface cells are negative; in contrast, anti-Mcm5 antibodies
stain nuclei in superficial as well as basal epithelial layers. In
HSIL, anti-PCNA antibodies show focal staining of 10% of
nuclei in the surface layers, whereas anti-Mcm5 antibodies stain
virtually all nuclei. Reproduced from Proc Natl Acad Sci USA 95
(25), 14932-14937, (1998).
high CDK level. One of the best studied examples of
this occurs within ovarian follicle cells during
Drosophila oogenesis, where certain genes are amplified to
ensure sufficient production of chorion proteins that make
up the eggshell. This occurs by the repeated initiation of
replication origins close to the amplified genes in the presence of high CDK levels (24, 37). In S. pombe, over-expression of wild type or a phosphorylation-defective cdc6
mutant (SpCDC18) results in extensive over-replication
(38, 39). Similar results from Arabidopsis show that endoreduplicating cells up-regulate Cdc6, and ectopic expression of Cdc6 can induce endoreplication (40).
For over-replication to generate tandem duplications, it is proposed that multiple strands of over-replicated DNA are ligated together and subsequently
recombined to generate expanded chromosomal
regions containing amplified genes. Alternatively, the
over-replicated strands might circularise to form extrachromosomal elements. Consistent with these ideas,
several studies have shown that amplification frequencies can be increased by treatments that transiently
inhibit DNA synthesis, resulting in chromosomal breakage (50). This suggests that the substrate for breakage
might be a replication bubble encompassing the locus that
will be amplified. Acentric amplicons can be generated by
chromosome breakage within a segment of over-replicated DNA (51) or within a stalled replication bubble (50)
encompassing a particular gene and one or more replication origins. If over-replication is really an important route
to gene amplification in cancers, then mutations might be
expected in pre-RC components or geminin, to relieve the
repression of re-replication.
THE LICENSING SYSTEM IN CANCER DIAGNOSIS AND PROGNOSIS
The development of cell proliferation markers has
revolutionised the early detection of malignant cells.
One of the most widely used proliferation markers is
Ki67, a protein of unknown function which is present
in cycling cells. Ki67 shows a good correlation with
bromodeoxyuridine labelling, thus validating its reliability as a marker of proliferation (52). It is unclear,
however, whether the Ki67 labelling index provides
additional prognostic information and what levels of
Ki67 labelling distinguish between good and poor prognosis for tumours. Moreover, Ki67 may not be expressed in cells entering G1 from G0 (53) and its expression
may also be down-regulated in proliferating cells by
nutritional deprivation (54). Proliferating cell nuclear
antigen (PCNA), the auxiliary factor for DNA polymerase δ, has also found use as a marker for the assessment of proliferation (55). PCNA gives good immunostaining in nuclei during S phase, but because of its
role in DNA repair synthesis as well as chromosome
replication, can also give a signal when repair is occur-
Defective inhibition of Cdt1 by geminin could also
lead to origin re-firing. Experiments in Xenopus egg
extracts have shown that geminin is the major inhibitor of origin licensing in S, G2 and M phases (18, 19). A
recent study carried out in Drosophila cells shows that
silencing of geminin leads to cessation of mitosis and
asynchronous over-replication of the genome, producing cells containing single giant nuclei and partial
ploidy between 4N and 8N DNA content (41).
Although it therefore represents a potential tumour
suppressor gene, no defects in geminin expression
have been reported to date in cancer cells.
290
CHAPTER 29 / THE REPLICATION LICENSING SYSTEM
ing (56, 57), thus eroding the quantitative relationship
between PCNA expression and proliferation (58).
THE LICENSING SYSTEM AS A POTENTIAL
ANTI-CANCER TARGET
One limitation of PCNA and Ki67 is that they
effectively label cells actively engaged in later cell cycle
events, but do not label cells in G1. Since proliferating
cells may spend most of their time in G1, this significantly reduces the sensitivity of PCNA and Ki67 as
proliferation markers. Mcm2-7 and Cdc6 are present in
G1 as well as S phase, but are lost as cells withdraw
from the cell cycle (Figure 1) and are therefore potentially more sensitive markers of proliferative capacity.
We have recently proposed that the presence of Mcm27 can be used as a functional definition of cells that are
in G1 phase (4). Therefore Mcm2-7 provide markers for
cells that have high proliferative potential, even if they
are dividing only very slowly.
Not only are the components of the licensing system useful in early cancer detection, they can potentially be exploited for the design of novel anti-cancer
drugs which are highly target specific. This potential
arises from the fact that most normal proliferating cells
in the human body have the option of temporarily
withdrawing into the G0 (unlicensed) state. When stimulated to re-enter the cell cycle, these cells must relicense their origins before entry into S phase. Entry
into S phase with insufficient licensed origins would
be lethal. It is therefore plausible that cells would possess a "licensing checkpoint" to block entry into S
phase until licensing is complete. We have recently
provided evidence in support of the existence of such a
"licensing checkpoint" (67). When primary human cells
were forced to over-express a non-degradable form of
geminin, licensing was inhibited as evidenced by a reduction in the quantity of Mcm2 loaded onto chromatin.
These cells arrested in a G1-like state, with low cyclin ECdk2 levels and no evidence of attempts to initiate DNA
replication. In contrast, a variety of cancer-derived cell
lines behaved as though they had lost this "licensing
checkpoint". When forced to over-express the same geminin construct, they progressed into an unsuccessful S
phase and ultimately underwent apoptosis.
The diagnostic potential of Mcm2-7 detection is
revealed by analysis of cervical epithelium low- and
high-grade squamous intraepithelial lesions (LSILs
and HSILs), both of which are viewed as representing
a potential precursor of malignancy (59). As shown in
Figure 5, antibodies to both PCNA and Mcm5 stained
the basal proliferating layers of normal cervical squamous epithelium but did not stain the superficial
differentiating cells. This shows that terminal differentiation of the superficial cells is accompanied by downregulation of Mcm5. In LSIL and HSIL there are
increasing numbers of PCNA- and Mcm5-positive cells
seen among the superficial cells, showing that these
cells are failing to withdraw from the cell cycle as they
should. It is also clear that a much higher proportion of
cells stain for Mcm5 than stain for PCNA, as expected
from their cell cycle behaviour. A similar increase in the
Mcm-staining cells is also seen in pre-malignant lesions of
the lung (60) and in kidney tumours (61), whilst increased
Cdc6 staining was detected in brain tumours (62).
These results suggest that transient inhibition of the
licensing system would cause a transient cell cycle arrest
of normal cells whilst killing cancer cells. Inhibitors of the
replication licensing system may therefore have good
potential as anti-cancer drugs. At present, the only reported licensing inhibitor is geminin, which has a ~10 kDa
domain with inhibitory activity (16). Given the difficulty
in delivering a molecule of this size into cells, a cell-permeant small molecule that inhibits replication licensing is
required to fully test this idea.
Mcm2-7 staining may also have prognostic value in
cancers. In non-small-cell lung cancers, an increased
Mcm2 immuno-reactivity corresponded to poorer survival, whilst no such correlation was seen with Ki67 (63). A
similar correlation between high Mcm2 expression and
poor prognosis was reported in prostate cancers (64) and
in oligodendrogliomas (65). These correlations presumably reflect the increased proliferative capacity of licensed
cells. It would be of considerable interest to determine
whether Mcm2-7 expression also provides a marker for
responsiveness to particular treatment regimes.
ACKNOWLEDGEMENTS
We thank Anna M. Woodward and Anatoly Li for
comments on the manuscript. The authors were supported by a British Commonwealth scholarship to SS
and Cancer Research UK [CRC] grant SP2385/0101.
REFERENCES
1.
2.
An exception to the normal correlation between
the absence of Mcm2-7 and withdrawal from the cell
cycle proliferation occurs in glandular epithelial cells of
the breast. Even though the proliferative rate was very
low in the breast cells of non-pregnant and non-lactating
women, a large percentage (47-65%) stained positive for
Mcm2-7 (66). As the lack of origin licensing in non-proliferating cells may be an important barrier to their inadvertent re-entry into the cell cycle, these licensed but
slowly proliferating breast cells might therefore be at a
higher risk of undergoing malignant transformation.
3.
4.
5.
6.
7.
8.
291
Diffley, J.F.X. (2001). Current Biol 11, 367-370.
Gillespie, P.J., Li, A. and Blow, J.J. (2001). BioMed
Central Biochem 2, 15.
Lei, M. and Tye, B.K. (2001). J Cell Sci 114, 14471454.
Blow, J.J. and Hodgson, B. (2002). Trends Cell Biol
12, 72-78.
Nguyen, V.Q., Co, C. and Li, J.J. (2001). Nature 411,
1068-1073.
Rowles, A., Tada, S. and Blow, J.J. (1999). J Cell Sci
112, 2011-2018.
Natale, D.A., Li, C.J., Sun, W.H. and DePamphilis,
M.L. (2000). EMBO J 19, 2728-2738.
Kreitz, S., Ritzi, M., Baack, M. and Knippers, R.
S. SHREERAM AND J.J. BLOW
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
(2001). J Biol Chem 276, 6337-6342.
Mendez, J., Zou-Yang, X.H., Kim, S.Y., Hidaka,
M., Tansey, W.P. and Stillman, B. (2002). Mol Cell
9, 481-491.
Coverley, D., Pelizon, C., Trewick, S. and Laskey,
R.A. (2000). J Cell Sci 113, 1929-1938.
Mendez, J. and Stillman, B. (2000). Mol Cell Biol 20,
8602-8612.
Saha, P., Chen, J., Thome, K.C., Lawlis, S.J., Hou,
Z.H., Hendricks, M., Parvin, J.D. and Dutta, A.
(1998). Mol Cell Biol 18, 2758-2767.
Petersen, B.O., Lukas, J., Sorensen, C.S., Bartek, J.
and Helin, K. (1999). EMBO J 18, 396-410.
Delmolino, L.M., Saha, P. and Dutta, A. (2001). J
Biol Chem 276, 26947-26954.
Nishitani, H., Taraviras, S., Lygerou, Z. and
Nishimoto, T. (2001). J Biol Chem 276, 44905-44911.
McGarry, T.J. and Kirschner, M.W. (1998). Cell 93,
1043-1053.
Wohlschlegel, J.A., Dwyer, B.T., Dhar, S.K.,
Cvetic, C., Walter, J.C. and Dutta, A. (2000).
Science 290, 2309-2312.
Tada, S., Li, A., Maiorano, D., Mechali, M. and
Blow, J.J. (2001). Nat Cell Biol 3, 107-113.
Hodgson, B., Li, A., Tada, S. and Blow, J.J. (2002).
Current Biol 12, 678-683.
Rialland, M., Sola, F. and Santocanale, C. (2002). J
Cell Sci 115, 1435-1440.
Lengauer, C., Kinzler, K.W. and Vogelstein, B.
(1998). Nature 396, 643-649.
Sen, S. (2000). Curr Opinion Oncol 12, 82-88.
Bialy, H. (1998). Nat. Biotechnol 16, 137-138.
Edgar, B.A. and Orr-Weaver, T.L. (2001). Cell 105,
297-306.
Nurse, P. (1994). Cell 79, 547-550.
Hayles, J., Fisher, D., Woollard, A. and Nurse, P.
(1994). Cell 78, 813-822.
Moreno, S. and Nurse, P. (1994). Nature 367, 236-242.
Correa-Bordes, J. and Nurse, P. (1995). Cell 83,
1001-1009.
MacAuley, A., Cross, J.C. and Werb, Z. (1998). Mol
Biol Cell 9, 795-807.
Varmuza, S., Prideaux, V., Kothary, R. and
Rossant, J. (1988). Development 102, 127-134.
Hattori, N., Davies, T.C., Anson-Cartwright, L.
and Cross, J.C. (2000). Mol Biol Cell 11, 1037-1045.
Niculescu, A.B., 3rd, Chen, X., Smeets, M., Hengst,
L., Prives, C. and Reed, S.I. (1998). Mol Cell Biol 18,
629-643.
Gallie, B.L. (1994). New Engl J Med 330, 786-787.
Cordon-Cardo, C. (1995). Am J Pathol 147, 545-560.
Diatloff-Zito, C., Turleau, C., Cabanis, M.O. and
de Grouchy, J. (1984). Carcinogenesis 5, 1305-1310.
Hawkins, M.M., Wilson, L.M., Burton, H.S.,
Potok, M.H., Winter, D.L., Marsden, H.B. and
Stovall, M.A. (1996). J Natl Cancer Inst 88, 270-278.
37. Calvi, B.R., Lilly, M.A. and Spradling, A.C. (1998).
Genes Dev 12, 734-744.
38. Nishitani, H. and Nurse, P. (1995). Cell 83, 397-405.
39. Jallepalli, P.V., Brown, G.W., Muzi-Falconi, M., Tien,
D. and Kelly, T.J. (1997). Genes Dev 11, 2767-2779.
40. Castellano, M.M., del Pozo, J.C., Ramirez-Parra,
E., Brown, S. and Gutierrez, C. (2001). Plant Cell
13, 2671-2686.
41. Mihaylov, I.S., Kondo, T., Jones, L., Ryzhikov, S.,
Tanaka, J., Zheng, J., Higa, L.A., Minamino, N.,
Cooley, L. and Zhang, H. (2002). Mol Cell Biol 22,
1868-1880.
42. Benner, S.E., Wahl, G.M. and Von Hoff, D.D.
(1991). Anticancer Drugs 2, 11-25.
43. Horns, R.C., Jr., Dower, W.J. and Schimke, R.T.
(1984). J Clin Oncol 2, 2-7.
44. Cowell, J.K. (1982). Annu Rev Genet 16, 21-59.
45. Schimke, R.T. (1988). J Biol Chem 263, 5989-5992.
46. Hamlin, J.L., Leu, T.H., Vaughn, J.P., Ma, C. and
Dijkwel, P.A. (1991). Prog Nucleic Acid Res Mol Biol
41, 203-239.
47. Stark, G.R. (1993). Adv Cancer Res 61, 87-113.
48. Wahl, G.M. (1989). Cancer Res 49, 1333-1340.
49. Windle, B.E. and Wahl, G.M. (1992). Mutat Res
276, 199-224.
50. Windle, B., Draper, B.W., Yin, Y.X., O'Gorman, S.
and Wahl, G.M. (1991). Genes Dev 5, 160-174.
51. Schimke, R.T., Sherwood, S.W., Hill, A.B. and
Johnston, R.N. (1986). Proc Natl Acad Sci USA 83,
2157-2161.
52. Onda, K., Davis, R.L., Shibuya, M., Wilson, C.B.
and Hoshino, T. (1994). Cancer 74, 1921-1926.
53. Gerdes, J., Lemke, H., Baisch, H., Wacker, H.H.,
Schwab, U. and Stein, H. (1984). J Immunol 133,
1710-1715.
54. Verheijen, R., Kuijpers, H.J., van Driel, R., Beck,
J.L., van Dierendonck, J.H., Brakenhoff, G.J. and
Ramaekers, F.C. (1989). J Cell Sci 92, 531-540.
55. Prelich, G., Tan, C.K., Kostura, M., Mathews, M.B.,
So, A.G., Downey, K.M. and Stillman, B. (1987).
Nature 326, 517-520.
56. Toschi, L. and Bravo, R. (1988). J Cell Biol 107,
1623-1628.
57. Celis, J.E. and Madsen, P. (1986). FEBS Lett 209,
277-283.
58. Hall, P.A., Levison, D.A., Woods, A.L., Yu, C.C.,
Kellock, D.B., Watkins, J.A., Barnes, D.M., Gillett,
C.E., Camplejohn, R., Dover, R. and et al. (1990). J
Pathol 162, 285-294.
59. Williams, G.H., Romanowski, P., Morris, L.,
Madine, M., Mills, A.D., Stoeber, K., Marr, J.,
Laskey, R.A. and Coleman, N. (1998). Proc Natl
Acad Sci USA 95, 14932-14937.
60. Tan, D.F., Huberman, J.A., Hyland, A., Loewen,
G.M., Brooks, J.S., Beck, A.F., Todorov, I.T. and
292
Bepler, G. (2001). BioMed Central Cancer 1, 6.
61. Rodins, K., Cheale, M., Coleman, N. and Fox, S.B.
(2002). Clin Cancer Res 8, 1075-1081.
62. Ohta, S., Koide, M., Tokuyama, T., Yokota, N.,
Nishizawa, S. and Namba, H. (2001). Oncol Rep 8,
1063-1066.
63. Ramnath, N., Hernandez, F.J., Tan, D.F.,
Huberman, J.A., Natarajan, N., Beck, A.F.,
Hyland, A., Todorov, I.T., Brooks, J.S. and Bepler,
G. (2001). J Clin Oncol 19, 4259-4266.
64. Meng, M.V., Grossfeld, G.D., Williams, G.H.,
Dilworth, S., Stoeber, K., Mulley, T.W., Weinberg,
V., Carroll, P.R. and Tlsty, T.D. (2001). Clin Cancer
Res 7, 2712-2718.
65. Wharton, S.B., Chan, K.K., Anderson, J.R.,
Stoeber, K. and Williams, G.H. (2001). Neuropathol
Appl Neurobiol 27, 305-313.
66. Stoeber, K., Tlsty, T.D., Happerfield, L., Thomas,
G.A., Romanov, S., Bobrow, L., Williams, E.D. and
Williams, G.H. (2001). J Cell Sci 114, 2027-2041.
67. Shreeram, S., Sparks, A., Lane, D.P. and Blow, J.J.
(2002) Oncogene 21, 6624-6632.
293
294