Moderate expression of TRF2 in the hematopoietic system increases development of large cell blastic T-cell lymphomas
Abstract
The telomeric repeat binding factor 2 (TRF2) plays a central role in the protection of chromosome ends by inhibiting telomeres from initiating a DNA damage cascade. TRF2 overexpression has been suggested to induce tumor development in the mouse, and TRF2 levels have been found increased in human tumors. Here we tested whether moderate expression of TRF2 in the hematopoietic system leads to cancer development in the mouse. TRF2 and a GFP-TRF2 fusion protein were introduced into hematopoietic precursors, and tested for function. TRF2 overexpressing cells were integrated into the hematopoietic system of C57BL/6J recipient mice, and animals were put on tumor watch. An increase in the development of T-cell lymphomas was observed in secondary recipient animals, however, overexpression of the TRF2 transgene was not detectable anymore in the tumors. The tumors were characterized as large cell blastic T-cell lymphomas and displayed signs of genome instability as evidenced by chromosome fusions. However, the rate of lymphoma development in TRF2-overexpressing animals was low, suggesting the TRF2 does not serve as a dominant oncogene in the system used.
Introduction
Telomeres are protective caps at
chromosome ends that consist of G rich repeats and associated proteins [1,2]. Their
major functions are to buffer replication associated shortening, and to protect
chromosome ends from being processed as double stranded breaks by the cellular
repair machinery. Telomeres can lose their protective function by excessive
erosion of the telomeric DNA tracts, as demonstrated in mouse models where the
RNA subunit or the catalytic subunit of telomerase have been subjected to
targeted deletion [3,4]. When
telomeres become critically short they fail to form a protective structure and
are recognized as DNA damage,
leading to cell cycle arrest, repair, or cell death [1,5]. Repair
of critically short telomeres, mostly accomplished by the non-homologous end
joining (NHEJ) machinery, results in covalent fusion of chromosome ends [6]. When a cell
passes through mitosis with fused chromosomes they break randomly, leading to
unequal distribution of DNA to the daughter cells, and hence to genome
instability. It has been shown extensively that telomere dysfunction results in
unstable chromosomes, and can therefore lead to neoplastic transformation [1,5,7,8].
The
core complex of telomere associated proteins is termed shelterin, and consists
of TRF1, TRF2, RAP1, TIN2, TPP1 and POT1, and plays a crucial role in telomere
protection and the regulation of telomere length homeostasis [2]. Disruption
of the complex leads to telomere dysfunction, often without extensive loss of
telomeric double stranded DNA. It has been well established that many levels of
protection exist and that they interact to inhibit the DNA damage machinery at
natural chromosome ends. It is challenging to assign individual roles to the
proteins in the complex, since disruption of any component might cause
destabilization of shelterin. For example, deletion of TRF1 or TIN2 in mice
causes embryonic lethality independent of telomerase dependent telomere length
regulation [9,10]. Since
TIN2 interacts with both TRF1 and TRF2 [11-13], it is
unclear at this point what the exact pathway to lethality is. Similarly,
suppression of POT1 led to partial loss of the telomeric 3' single stranded
overhang, and a transient detection of telomeres by the DNA damage machinery [14]. The
protective effects of POT1 are dependent on its interactions with TPP1 [15], again
demonstrating the interdependence of the members of shelterin. POT1 also
protect telomeres by preventing activation of the ATR dependent DNA damage
response machinery [16]. Inhibition
of TRF2 by a dominant negative allele or by targeted deletion leads to
extensive loss of the single stranded overhang and to dependent chromosome
fusion by NHEJ [6,17-20]. TRF2
also represses the ATM dependent DNA damage response [16],
potentially by directly interacting with the kinase [21].
Relatively
little is known about the role the shelterin components play in tumorigenesis.
TRF1, TRF2 and TIN2 have been found up-regulated occasionally during in gastric
carcinomas and during hepatocarcinogenesis [22,23].
Mutations in TIN2 have been demonstrated to lead to abnormally short telomeres,
and to be associated with dyskeratosis congenita and ataxia-pancytopenia,
diseases associated with an increased cancer disposition [24-26].
Despite
the fact that shelterin components interact with proteins involved in many
repair processes [2,27-30], no
general trend for de-regulation in tumors has been observed. In an effort to
study TRF2 overexpression in a mammalian organism mTRF2 has been overexpressed
under the K5 promoter in basal and stem cells of the epidermis [31,32]. This
led to XPF dependent telomere loss and increased skin cancer levels in the
animals [31], a
phenotype that was accelerated by telomerase abrogation [32].
TRF2 has also been demonstrated to
directly interact with ATM, and overexpression of TRF2 can partially prevent
ATM phosphorylation and the activation of the ATM dependent DNA damage response
[21,33]. Based on this finding we set out to test whether modest TRF2
overexpression in the murine hematopoietic system leads to a suppression of the
DNA damage response and to lymphoma development, as demonstrated for mice
lacking ATM [34]. Here we
show that TRF2 and GFP-TRF2 can be overexpressed in the hematopoietic system of
C57BL/6J mice, and that the transgenic TRF2 localizes to telomeres.
Approximately 15% of animals that were secondary recipients of TRF2
overexpressing hematopoietic precursors developed T cell lymphomas. Although
lymphoma incident was elevated in this cohort, most mice did not develop cancer
during their life-span, suggesting that TRF2 is not a dominant oncogene in this
system.
Results
and discussion
Overexpressed
mTRF2 and GFP-mTRF2 localize to telomeres
Wild
type mouse TRF2 (mTRF2), a fusion of GFP and mTRF2, as well as a GFP control
were introduced into lentiviral constructs under the control of the CAG
promoter (Figure 1A), and the constructs were transfected into murine 3T3
fibroblasts. Indirect immunofluorescence of control 3T3 cells with antibodies
against mTRF1 and mTRF2 revealed telomeric co-localization of the two proteins
(Figure 1B, upper panel). The GFP-mTRF2 fusion protein also co-localized with
endogenous mTRF1, demonstrating that the fusion protein localizes to telomeres
(Figure 1B, middle panel). Similarly, overexpressed mTRF2 localized to
telomeres (Figure 1B, lower panel), as detected by immunofluorescence with
antibodies against mTRF1 and mTRF2. To test whether mTRF2 was also expressed
and localizes to telomeres in hematopoietic precursors, high titer lentiviruses
were generated, and a liquid culture of CD45.1 donor bone marrow was infected
with the lentiviruses, and expression and localization was tested by immuno-fluorescence.
Co-localization of mTRF1 and GFP-mTRF2 demonstrated the telomeric localization
of the fusion protein (Figure 1C), as well as the wild type TRF2 allele (data
not shown). The cell lines expressing the transgenes did not display altered
growth rates or cell death (data not shown), suggesting that the expressed TRF2
alleles do not interfere with telomere protection.
Figure 1. Lentiviral expression of mTRF2 and GFP-mTRF2. (A) Schematic of transgene constructs. GFP, mouse TRF2
(mTRF2) and a GFP-mTRF2 fusion were cloned into a lentiviral vector system [35] under the control of a CAG
promoter [38]. (B) Indirect
immunofluorescence of 3T3 cells. 3T3 control cells (top panel), 3T3 cells
transfected with GFP-mTRF2 and cells transfected with the mTRF2 construct
were stained with antibodies against mTRF1 or mTRF2. GFP was visualized by
autofluorescence. DNA has been stained with DAPI, and the merge of the red,
green and blue channels has been provided on the right. (C) Indirect
immunofluorescence of CD45.1 donor bone marrow cells. Cells were infected
with GFP-mTRF2 expressing lentiviruses and GFP-mTRF2 was visualized by GFP
autofluorescence. TRF1 was detected by a mTRF1 specific antibody, the DNA
was counterstained with DAPI And the merge of the three colors is
indicated.
TRF2
constructs integrate and express in bone marrow and spleen of transgenic
C57BL/6J mice
Donor
bone marrow was infected in two independent sets of experiments, where either
GFP positive donor cells or CD45.1 expressing donor cells were used for infection.
This approach allows identification of the transplanted cells in the recipient bone marrow by FACS
analysis later. In the first set GFP positive donor bone marrow was infected
with lentiviruses expressing wild type mTRF2 and then re-introduced into the
tail veins of 13 lethally irradiated C57BL/6J donor mice. Alternatively, CD45.1
bone marrow was infected with GFP-mTRF2 expressing lentiviruses, and was
injected into the tail veins of 19 lethally irradiated C57BL/6J donor mice
(Figure 2A, left panel).
In
the second set only CD45.1 donor bone marrow was used, and 17 mice were
generated that expressed a GFP control, 19 that received cells that expressed
wild type mTRF2, and 17 that were transduced with cells expressing the
GFP-mTRF2 fusion (Figure 2A, right panel).
After
recovery and repopulation of the bone marrow with donor cells, we tested the
presence of the transgene in DNA isolated form whole blood of the recipient
animals. Using a PCR based approach (Figure 2B) followed by southern analysis
26 mice of the first infection-set tested positive for the presence of the
transgene in blood, as well as 15 animals of the second set. Mice transplanted
with bone marrow that was infected with GFP control viruses did not give a
signal in the PCR-southern analysis. In summary, we generated 41 animals that
expressed mTRF2 or mTRF2-GFP transgenes in their bone marrow.
Transplantation
of bone marrow from primary recipients into secondary C57BL/6J recipient mice
To
further promote tumor progression in recipient mice, we transplanted bone
marrow from primary recipient mice that were successfully transduced with
transgenic TRF2. Primary bone marrow from both primary sets was isolated four
months post infection and transplanted into lethally irradiated C57BL/6J
secondary recipients. A total of five secondary recipient populations were
generated: two populations with a total of 26 animals received bone marrow
expressing mTRF2, another two populations with a total of 28 animals received
bone marrow expressing GFP-mTRF2. As negative control a population of 30
animals received bone marrow transduced with the GFP transgene.
To
test for functionality of the TRF2 alleles in the integrated cell populations
we isolated bone marrow as well as splenocytes from secondary recipient
animals. Immunofluorescent staining demonstrated clear co-localization of
GFP-TRF2 with TRF1 in bone marrow cells (Figure 2C, upper panel) and
splenocytes (Figure 2C, lower panel), suggesting telomeric localization and
functionality. In summary, transgenic mTRF2 integrates into donor cells, which
keep expressing the transgene after repopulating the bone marrow of lethally
irradiated recipient animals.
Figure 2. Generation of primary recipient mice by lentiviral transduction of transgenic TRF2 into GFP and CD45.1 donor bone marrow. (A) A primary #1 mouse colony (Set 1) was
generated by the transduction of GFP donor bone marrow (BM) with the mTRF2
transgene and CD45.1 donor bone marrow with the GFP-mTRF2 transgene. For
the primary #2 colony (Set 2) only CD45.1 donor bone marrow was used and
transduced with a GFP-control, the mTRF2 or the GFP-mTRF2 transgene. Recipient
mice were C57BL/6J. (B) Genotyping of primary recipient mice
transduced with transgenic GFP-mTRF2 and GFP by nested PCR and subsequent Southern
analysis. Examples for Set 1 and Set 2 are displayed. As a negative control
(-) genomic DNA from a C57BL/6J mouse was used. As a positive PCR control
(+) genomic DNA of GFP-mTRF2 expressing HeLa cells was included. (C) Transgenic
GFP-mTRF2 is expressed in the hematopoietic system of recipient C57BL/6J
mice. Indirect immunofluorescence of bone marrow (top) and spleen (bottom)
isolated from a secondary recipient of GFP-mTRF2 expressing bone marrow.
GFP-mTRF2 was visualized by the GFP-tag, TRF1 was detected by a mTRF1
specific antibody. Chromatin was counterstained with DAPI. White arrows
indicate co-localization of endogenous TRF1 with recombinant GFP-mTRF2.
Development
of T-cell lymphoma without TRF2 overexpression
A modest increase in the development of
T-cell lymphoma was observed in secondary recipient mice that were transduced
with TRF2 expressing cells within 12-month post transplantations, as opposed to
GFP control cells. 8 out of 54 mice (14.8%) that tested positive for mTRF2
transgenes succumbed to visible tumors within 52 weeks of transplantation,
whereas none of the 30 control mice that carried the GFP transgene displayed
visible tumors within the same timeframe. Tissue samples from a secondary
recipient mouse suggested development of a large cell blastic T-cell lymphoma
(Figure 3A, lower two panels). Spleen, liver and thymus were infiltrated and
the organ cells replaced by a diffuse monotonous neoplastic infiltrate composed
of cells with large oval nuclei with a delicate chromatin pattern and spare
cytoplasm. The cells had a high mitotic index, but also displayed the foci
characteristic for apoptosis. The renal glomeruli displayed thickening of the
basement membrane and some were hyalinized. The upper panels show control
tissue from a healthy C57BL/6J mouse.
A
hallmark of tumors and telomere-dysfunction derived tumors is genome instability,
resulting from chromosomal breakage fusion cycles. To analyze the T-cell
lymphomas for fused chromosomes as indicators of genome instability we screened
pathological samples for fused chromosomes, visible as anaphase bridges. Spleen
and liver samples readily displayed anaphases where the sets of daughter
chromosomes were connected by DNA bridges, suggesting that breakage fusions
cycles occur, and the genome in the tumors is unstable (Figure 3B).
Analysis
of the thymomas by flow cytometry with the markers CD4 and CD8 suggested the
presence of a donor derived CD4/CD8+/+ T cell lymphoma. However, even when the
donor cells were derived from mice that had been transduced with GFP-mTRF2
expressing bone marrow, less than 1% of total thymocytes were positive for GFP
(data not shown). These experiments raised the possibility that the observed
CD4/CD8+/+ T cell lymphomas originated from the CD45.1 donor population
expressing the GFP-mTRF2 transgene, but at the time of analysis most tumor
cells did not overexpress mTRF2 anymore, raising the possibility that TRF2
overexpression is a cancer initiating, but not a cancer maintaining event.
Figure 3. Development of genetically unstable T cell lymphoma in TRF2 overexpressing mice. (A) H&E staining of
tumor tissue. Tissue samples from a secondary recipient mouse, which
developed a blastic T cell lymphoma, involving liver, spleen and thymus.
10x and 40x magnifications are shown. The upper panels show sections of
control tissue from a healthy C57BL/6J mouse. (B) Anaphase bridges
in H&E stained liver and spleen sections from a secondary recipient
mouse carrying a CD4/CD8+/+ T cell lymphoma. The arrows point to the
chromatin bridges between the separating chromosomes. A dashed line
outlines normal anaphases displayed in the images to the very left.
TRF2
has been proposed to directly interact and suppress ATM activation. The
underlying hypothesis of this study was to test whether TRF2 dependent ATM
suppression can lead to tumorigenesis. Therefore we tested whether ATM
auto-phosphorylation was compromised in the tumors resulting from TRF2
expression in the hematopoietic system. Splenocytes from GFP control mice, as
well as cells from enlarged spleens in GFP-mTRF2 expressing mice were isolated,
cultivated, and subjected to ionizing irradiation. Then ATM activation was tested by
immunofluorescence with antibodies specific for the ATM-S1981
autophosphorylation event. No difference in ATM autophosphorylation could be
observed between the samples, suggesting that ATM activation is not compromised
in tumors resulting from overexpression of TRF2 in hematopoietic precursors
(Figure 4A).
Finally we investigated by western
analysis whether TRF2 was still overexpressed in the tumors, and we tested
GFP-mTRF2 expression in splenocytes isolated from a mouse affected by a T cell
lymphoma. No band could be observed in western analysis with an anti-GFP antibody
(Figure 4B, upper panel) that readily recognizes the GFP-mTRF2 fusion expressed
in HeLa 1.2.11 cells (upper panel, right lane). However, endogenous TRF2
levels, normalized to the g-tubulin loading control, were equal. Our data
therefore suggest that mTRF2 is not overexpressed anymore in the lymphomas
observed in recipient mice.
Figure 4. The ATM dependent damage response is not compromised in tumor samples. (A) Immunofluorescence
of ATM autophosphorylation after ionizing irradiation. Splenocytes were
irradiated with 5 Gy and microtome sections were prepared as described in
the "Materials and Methods" section. ATM activation was measured by
immunofluorescence with an antibody specific for the phosphorylation event
at serine 1981. The left panels represent cells from a control animal, the
right panel from an animal expressing the GFP-mTRF2 fusion. The upper
panels are before, the lower panels after irradiation. (B) Western
analysis of spleen from a secondary recipient mouse, which expressed
GFP-mTRF2 and died from a CD4/CD8+/+ T cell lymphoma. Protein samples were
probed with antibodies against GFP and mTRF2. g-Tubulin was included as a
loading control. As a positive control for GFP expression protein extract
from HeLa 1.2.11 cells expressing GFP-mTRF2 was loaded.
In
summary, moderate overexpression of TRF2 in hematopoietic precursors in mice
leads to an increase of tumor incident in affected animals. Tumors were
characterized as CD4/CD8+/+ T cell lymphomas, and they exhibited anaphase
bridges, strongly suggesting the possibility of genome instability. The modest increase
in cancer formation is contrary to the
strong increase in tumor numbers observed upon overexpression of TRF2 in basal
and stem cells of the epidermis [31,32], which
led to XPF dependent increased skin cancer levels in the animals
[31],
suggesting a less severe impact of increased TRF2 levels in the hematopietic
system than in the epidermis. Furthermore, we observed that the tumors
resulting from transduction with TRF2 overexpressing cells do not exhibit TRF2
over-expression in most of their cells, raising the possibility that increased
TRF2 expression is a driving event for cancer formation, but not required for
tumor maintenance.
Materials
and methods
Constructs
and virus production.
Lentiviral
constructs were generated using standard cloning procedures. The viral
backbones p156RRLsinPPTCAG-EGFP-PRE and p156RRLsinPPTmCMV-GFP-PRE [35] were kindly
provided by the Verma laboratory.
Lentivirus
production.
293T cells were plated on one 15 cm plate and grown in
1x DMEM supplemented with 10% (v/v) FBS, non-essential amino acids (0.1 mM),
Penicillin (100 units/ml) and Streptomycin (0.1 mg/ml) and grown to confluence.
Cells were trypsinized and split into twelve 15 cm plates coated with
poly-L-lysine (Sigma). When cells reached about 70% confluence 95 μg of pVSVG, 68 μg of pREV, 176 μg pMDL, and 270 μg transgene containing lentiviral vector were mixed. Under swirling
CaCl2 solution was added to a final concentration of 0.25 M.
Subsequently an equal volume of 2x BBS solution to the calcium-DNA mixture was
added. The mixture was incubated for 10 minutes at room temperature, added drop
wise at a volume of 2.25 ml per 15 cm plate and cells were incubated at 3% CO2at 37oC. The medium was exchanged 12 to 16 hours post transfection
and virus-containing media was harvested at 24 h intervals twice, beginning 24
hours after changing the medium. Every sample was immediately filtered through
a 0.22 μm cellulose acetate filter and stored at 4oC.
The collected medium was loaded into ultracentrifuge tubes and spun in a SW28
rotor for 2 hours at 19400 rpm in a L8-80M ultracentrifuge (Beckman). The
supernatant was poured off and remaining medium drops were aspirated from the
tubes. The pellet was resuspended in 1 ml HBSS, all pellets from two
collections were pooled and loaded on top of 1.5 ml of phosphate-buffered 20%
(w/v) sucrose in small ultracentrifuge tubes. Tubes were then spun in the SW55
rotor for 2 hours at 21000 rpm in a L8-80M ultracentrifuge (Beckman).
Supernatant was removed and pellet resuspended in 200 μl in HBSS. The virus suspension was vortexed for 1 to 2 hours at low
speed at room temperature, quick-spun in microcentrifuge for 2 seconds, the
supernatant aliquoted in 20 μl aliquots and stored at -80oC.
Virus titer was determined by the p24 ELISA kit (PerkinElmer) according to the
manufacturer.
Mouse
strains.
B6.SJL-PtprcaPep3b/BoyJ mice (The Jackson Laboratory) were used to isolate bone
marrow positive for CD45.1, TgN(beta-act-EGFP) mice [36] (a gift
from the Verma lab, The Salk Institute) were used to isolate GFP-positive donor
bone marrow. As recipient mice, C57BL/6J (The Jackson Laboratory) were chosen.
Isolation
of bone marrow.
For the generation of
the "Set 1" cohort bone marrow was isolated from 15 male B6.SJL-PtprcaPep3b/BoyJ mice (CD45.1 donor, The Jackson Laboratory) and 5 male
TgN(beta-act-EGFP) mice (GFP donor, a gift from the Verma lab, The Salk
Institute). For the generation of the "Set 2" population done bone marrow was
isolated from 20 male B6.SJL-Ptprca Pep3b/BoyJ animals
(CD45.1 donor, The Jackson Laboratory). The mice were sacrificed by cerebral
dislocation and femur and tibia were placed into 1x PBS/2 % (v/v) BIT9500
(StemCell Technologies). To isolate the bone marrow, femur and tibia were
mortared, the suspension filtered through a Cell Strainer (BD Falcon) and
centrifuged for 10 minutes at 700 x g. The pellet was resuspended in 1x PBS and
cell numbers were determined by counting a 1:20 dilution of the suspension
using a Coulter Counter. Suspensions were diluted to 5x107 cells/ml.
To enrich hematopoietic stem cells, cell suspensions were separated using the
StemStepTM cell separation system (StemCell Tech-nologies) as
directed. The cell numbers of the enriched hematopoietic stem cells were
determined and resuspended in Myelocult M5300 (StemCell Tech-nologies).
Infection
of bone marrow.
Sorted bone marrow
cells were diluted to approximately 1.2x107 to 1.4x107cells/ml in Myelocult M5300 medium and 200 ml virus was added to the cells. The
suspension was incubated at 37oC o/n and then the suspension was
washed once with 1x HBSS, centrifuged for 5 minutes at 400 x g, and resuspended
in 1x HBSS.
Transplantation of bone marrow.
Prior to transplantation
recipient C57BL/6J mice were irradiated with 11Gy and subsequently deeply
anesthetized. Each mouse received lateral tail vein injections of 100.000 to
200.000 cells diluted in 300 ml 1x HBSS. During the first two weeks post
transplantation all mice were maintained on Baytril water (Bayer Health Care).
All mice were stored in the Biohazard suite at the Salk Institute's Animal
Facility throughout the course of the experiment.
Genotyping
of primary and secondary recipient mice.
Genomic DNA from blood and tissue samples of C57BL/6J mice as well as HeLa
1.2.11 expressing GFP-mTRF2 cells was isolated using the DNeasy tissue kit
(Qiagen). Nested PCR was performed with the outer primer pair (mTRF2 Outer F1:
5'-GCA GAT TGC TGT TGG AGG AGG-3'; WPRE R1: 5'-gcc aca act cct cat aaa gag aca g-3') generating a 626 bp
PCR-product, followed by PCR with the inner pair (mTRF2 Inner F1: 5'-ATG TCA
GCA TCC AAG CCC AGA G-3'; mTRF2 Inner R1: 5'-CCA GTT TCC TTC CCC GTA TTT G-3')
generating a 252 bp PCR-product. Integration of the transgene into the hemato-poietic
system of primary recipient mice was verified by nested PCR and the PCR-product
was separated on a 1.3% (w/v) Agarose gel. The gel was blotted onto a Hybond-N+
nitrocellulose membrane, (Amersham) following standard Southern analysis
procedures, using the mTRF2 cDNA as probe.
Protein
isolation.
Primary cells were washed
with 1x PBS on the plate and trypsinized using 2.5% (v/v) Trypsin/EDTA. Cell
numbers were determined with a Coulter Counter. Cells were spun for 5 minutes
at 1000 rpm and washed twice in 1x PBS and the cell pellet was resuspended at a
dilution of 10000 cells/ml in 4x NuPAGE LDS sample buffer (Invitrogen).
Tissue
samples were mashed through a 70 mm Cell Strainer (BD Falcon) with the rubber
end of a syringe in the presence of 1x PBS/2% (v/v) FCS. Cell numbers were
determined with a Coulter Counter. The cell suspension was then spun for 5
minutes at 1000 rpm and washed twice in 1x PBS. The cell pellet was resuspended
at a dilution of 10000 cells/μl in 4x NuPAGE LDS sample buffer
(Invitrogen).
Western blotting.
Whole cell extracts of primary cells or protein extracts
isolated from tissue samples in 4x NuPAGE LDS sample buffer were separated on
3-8% (w/v) Tris-Acetat gradient gels (Invitrogen) and transferred to
nitrocellulose. Blocking and incubation with primary and secondary antibodies
was performed in 5% (w/v) milk and 0.1% (v/v) Tween in 1x PBS. Antibodies:
rabbit-anti-mTRF2 #6889 (1/1000, Karlseder lab), mouse-anti-g-Tubulin GTU-88
(1/10000, Sigma), mouse-anti-GFP (1/200, Chemicon International). After
incubation with secondary antibodies (1/5000, Amersham), all blots were
developed using the ECL kit (Amersham).
Immunofluorescence on cultured cells, bone marrow and spleen.
Immunofluorescence on cultured cells was per-formed as described [21,37]. Bone marrow and spleen suspension
from C57BL/6J mice were attached to microscope slides by loading 200 μl of
cell suspension into cytofunnels (Thermo Electron Corporation) and
centrifugation in a Shandon Cytospin 4
Cytocentrifuge (Thermo Scientific) for 10
minutes at 800 rpm. Primary antibodies: rabbit-anti-mTRF1 #6888 (1/500,
Karlseder lab), rabbit-anti-mTRF2 #6889 (1/500, Karlseder lab), mouse-anti-TRF2
(1/500, upstate biotechnology). Secondary antibodies: donkey-anti-rabbit-FITC
(1/200, Jackson), donkey-anti-mouse-FITC (1/200, Jackson),
donkey-anti-rabbit-TRITC (1/200, Jackson). Pictures were taken on an Axioplan2
Zeiss microscope with a Hamamatsu digital camera supported by OpenLab software.
Immunofluorescence
on microtome sections.
Tissue sections of mice were isolated and fixed in
phosphate-buffered 4% (v/v) formaldehyde and transferred to phosphate-buffered
30% (w/v) sucrose after one day. Sections were blocked with 3% (v/v) FCS in TBS
with 0.25% (v/v) Triton-X 100 for 1 hour and incubated o/n at 4oC.
Primary antibody: rabbit-anti-ATM pS1981 (1/500, Rockland). Then the sections
were rinsed in 3% (v/v) FCS in TBS with 0.25% (v/v) Triton-X 100. The sections
were incubated with the second antibody in 3% (v/v) FCS in TBS with 0.25% (v/v)
Triton-X 100 for 1 to 2 hours, followed by three washing steps in TBS.
Secondary antibody: goat-anti-rabbit-TRITC (1/200, Jackson). To stain DNA the
sections were incubated in a 1/30000 dilution of 4',
6'-diamidino-2-phenylindole (DAPI) in TBS for 5 minutes. Sections were mounted
on coverslip using Dabco/PVA, dried over night at 4 oC in the dark,
and sealed with nail polish. Pictures were taken on a Leica TCS SP2 AOBS and analyzed by LCS Lite software.
Flow
cytometry
. Aliquots of bonemarrow cells and
thymocytes were stained with anti-mouse CD45.1 antibody (A20)conjugated
to R-Phycoerythrin (R-PE) to detect CD45.1 donor bone marrow. If mice were
transplanted with GFP-donor bone marrow, the presence of the GFP signal was
used to evaluate the presence of donor bone marrow. Lineage analysis was
performed by double staining using anti-mouseCD45.1
R-PE antibody with each of the following antibodies: CD4 (RM4-5.B) and CD8 (53-6.7).
Primary antibodies were purchased from BD Biosciences, the secondary antibody
was purchased from Molecular Probes. Flow cytometric analysis was performed on
a LSR I 3-laser 6-color analytical flow
cytometer (Becton-Dickinson) and data were analyzed using the CellQuest
software (Becton-Dickinson).
Pathology.
Tissue samples of mice were fixed in 4% (v/v)
p-formaldehyde, transferred to phosphate-buffered 30% (w/v) sucrose and stored
at 4oC in the dark. Pathological studies were carried out at the
Department of Pathology, UC Davis.
Acknowledgments
We
thank Nien Hoong und Nina Tonnu for help with lentivirus production, Niels
Bjarne Woods and the members of the Verma laboratory for fruitful discussion,
and Dr. Robert Cardiff (U. C. Davis) for mouse pathology.
Conflicts of Interest
The authors of this manuscript have no conflict of interests to declare.
References
-
1.
de Lange
T
Protection of mammalian telomeres.
Oncogene.
2002;
21(4):
532
-540.
[PubMed]
.
-
2.
de Lange
T
Shelterin: the protein complex that shapes and safeguards human telomeres.
Genes Dev.
2005;
19(18):
2100
-2110.
[PubMed]
.
-
3.
Blasco
MA
, Lee
HW
, Hande
MP
, Samper
E
, Lansdorp
PM
, DePinho
RA
and Greider
CW.
Telomere shortening and tumor formation by mouse cells lacking telomerase RNA.
Cell.
1997;
91(1):
25
-34.
[PubMed]
.
-
4.
Liu
Y
, Snow
BE
, Hande
MP
, Yeung
D
, Erdmann
NJ
, Wakeham
A
, Itie
A
, Siderovski
DP
, Lansdorp
PM
, Robinson
MO
and Harrington
L.
The telomerase reverse transcriptase is limiting and necessary for telomerase function in vivo.
Curr Biol.
2000;
10(22):
1459
-1462.
[PubMed]
.
-
5.
Verdun
RE
and Karlseder
J.
Replication and protection of telomeres.
Nature.
2007;
447(7147):
924
-931.
[PubMed]
.
-
6.
Smogorzewska
A
, Karlseder
J
, Holtgreve-Grez
H
, Jauch
A
and de Lange
T.
DNA ligase IV-dependent NHEJ of deprotected mammalian telomeres in G1 and G2.
Curr Biol.
2002;
12:
1635
-1644.
[PubMed]
.
-
7.
Artandi
SE
, Chang
S
, Lee
SL
, Alson
S
, Gottlieb
GJ
, Chin
L
and DePinho
RA.
Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice.
Nature.
2000;
406(6796):
641
-645.
[PubMed]
.
-
8.
Artandi
SE
and DePinho
RA.
A critical role for telomeres in suppressing and facilitating carcinogenesis.
Curr Opin Genet Dev.
2000;
10(1):
39
-46.
[PubMed]
.
-
9.
Chiang
YJ
, Kim
SH
, Tessarollo
L
, Campisi
J
and Hodes
RJ.
Telomere-associated protein TIN2 is essential for early embryonic development through a telomerase-independent pathway.
Mol Cell Biol.
2004;
24(15):
6631
-6634.
[PubMed]
.
-
10.
Karlseder
J
, Kachatrian
L
, Takai
H
, Mercer
K
, Hingorani
S
, Jacks
T
and de Lange
T.
Targeted deletion reveals an essential function for the telomere length regulator Trf1.
Mol Cell Biol.
2003;
23(18):
6533
-6541.
[PubMed]
.
-
11.
Kim
SH
, Beausejour
C
, Davalos
AR
, Kaminker
P
, Heo
SJ
and Campisi
J.
TIN2 mediates functions of TRF2 at human telomeres.
J Biol Chem.
2004;
279(42):
43799
-43804.
[PubMed]
.
-
12.
Ye
JZ
, Donigian
JR
, van
Overbeek M
, Loayza
D
, Luo
Y
, Krutchinsky
AN
, Chait
BT
and de Lange
T.
TIN2 binds TRF1 and TRF2 simultaneously and stabilizes the TRF2 complex on telomeres.
J Biol Chem.
2004;
279(45):
47264
-47271.
[PubMed]
.
-
13.
Houghtaling
BR
, Cuttonaro
L
, Chang
W
and Smith
S.
A dynamic molecular link between the telomere length regulator TRF1 and the chromosome end protector TRF2.
Curr Biol.
2004;
14(18):
1621
-1631.
[PubMed]
.
-
14.
Hockemeyer
D
, Sfeir
AJ
, Shay
JW
, Wright
WE
and de Lange
T.
POT1 protects telomeres from a transient DNA damage response and determines how human chromosomes end.
Embo J.
2005;
24(14):
2667
-2678.
[PubMed]
.
-
15.
Hockemeyer
D
, Palm
W
, Else
T
, Daniels
JP
, Takai
KK
, Ye
JZ
, Keegan
CE
, de Lange
T
and Hammer
GD.
Telomere protection by mammalian Pot1 requires interaction with Tpp1.
Nat Struct Mol Biol.
2007;
14:
754
-61.
[PubMed]
.
-
16.
Denchi
EL
and de Lange
T.
Protection of telomeres through independent control of ATM and ATR by TRF2 and POT1.
Nature.
2007;
448(7157):
1068
-1071.
[PubMed]
.
-
17.
van
Steensel B
, Smogorzewska
A
and de Lange
T.
TRF2 protects human telomeres from end-to-end fusions.
Cell.
1998;
92:
401
-13.
[PubMed]
.
-
18.
Celli
GB
and de Lange
T.
DNA processing is not required for ATM-mediated telomere damage response after TRF2 deletion.
Nat Cell Biol.
2005;
7(7):
712
-718.
[PubMed]
.
-
19.
Celli
GB
, Denchi
EL
and de Lange
T.
Ku70 stimulates fusion of dysfunctional telomeres yet protects chromosome ends from homologous recombination.
Nat Cell Biol.
2006;
8(8):
855
-890.
[PubMed]
.
-
20.
Lazzerini
Denchi E
, Celli
G
and de Lange
T.
Hepatocytes with extensive telomere deprotection and fusion remain viable and regenerate liver mass through endoreduplication.
Genes Dev.
2006;
20(19):
2648
-2653.
[PubMed]
.
-
21.
Karlseder
J
, Hoke
K
, Mirzoeva
OK
, Bakkenist
C
, Kastan
MB
, Petrini
JH
and de Lange
T.
The telomeric protein TRF2 binds the ATM kinase and can inhibit the ATM-dependent DNA damage response.
PLoS Biol.
2004;
2(8):
E240
[PubMed]
.
-
22.
Matsutani
N
, Yokozaki
H
, Tahara
E
, Tahara
H
, Kuniyasu
H
, Haruma
K
, Chayama
K
, Yasui
W
and Tahara
E.
Expression of telomeric repeat binding factor 1 and 2 and TRF1-interacting nuclear protein 2 in human gastric carcinomas.
Int J Oncol.
2001;
19(3):
507
-512.
[PubMed]
.
-
23.
Oh
BK
, Kim
YJ
, Park
C
and Park
YN.
Up-regulation of telomere-binding proteins, TRF1, TRF2, and TIN2 is related to telomere shortening during human multistep hepatocarcinogenesis.
Am J Pathol.
2005;
166(1):
73
-80.
[PubMed]
.
-
24.
Savage
SA
, Giri
N
, Baerlocher
GM
, Orr
N
, Lansdorp
PM
and Alter
BP.
TINF2, a component of the shelterin telomere protection complex, is mutated in dyskeratosis congenita.
American journal of human genetics.
2008;
82(2):
501
-509.
[PubMed]
.
-
25.
Tsangaris
E
, Adams
SL
, Yoon
G
, Chitayat
D
, Lansdorp
P
, Dokal
I
and Dror
Y.
Ataxia and pancytopenia caused by a mutation in TINF2.
Human genetics.
2008;
124(5):
507
-513.
[PubMed]
.
-
26.
Walne
AJ
, Vulliamy
T
, Beswick
R
, Kirwan
M
and Dokal
I.
TINF2 mutations result in very short telomeres: analysis of a large cohort of patients with dyskeratosis congenita and related bone marrow failure syndromes.
Blood.
2008;
112(9):
3594
-3600.
[PubMed]
.
-
27.
Zhu
XD
, Kuster
B
, Mann
M
, Petrini
JH
and Lange
T.
Cell-cycle-regulated association of RAD50/MRE11/NBS1 with TRF2 and human telomeres.
Nat Genet.
2000;
25(3):
347
-352.
[PubMed]
.
-
28.
Opresko
PL
, Von
Kobbe C
, Laine
JP
, Harrigan
J
, Hickson
ID
and Bohr
VA.
Telomere binding protein TRF2 binds to and stimulates the Werner and Bloom syndrome helicases.
J Biol Chem.
2002;
277:
41110
-9.
[PubMed]
.
-
29.
Zhu
XD
, Niedernhofer
L
, Kuster
B
, Mann
M
, Hoeijmakers
JH
and de Lange
T.
ERCC1/XPF removes the 3' overhang from uncapped telomeres and represses formation of telomeric DNA-containing double minute chromosomes.
Mol Cell.
2003;
12(6):
1489
-1498.
[PubMed]
.
-
30.
Dantzer
F
, Giraud-Panis
MJ
, Jaco
I
, Ame
JC
, Schultz
I
, Blasco
M
, Koering
CE
, Gilson
E
, Menissier-de
Murcia J
, de Murcia
G
and Schreiber
V.
Functional interaction between poly(ADP-Ribose) polymerase 2 (PARP-2) and TRF2: PARP activity negatively regulates TRF2.
Mol Cell Biol.
2004;
24(4):
1595
-1607.
[PubMed]
.
-
31.
Munoz
P
, Blanco
R
, Flores
JM
and Blasco
MA.
XPF nuclease-dependent telomere loss and increased DNA damage in mice overexpressing TRF2 result in premature aging and cancer.
Nat Genet.
2005;
37(10):
1063
-1071.
[PubMed]
.
-
32.
Blanco
R
, Munoz
P
, Flores
JM
, Klatt
P
and Blasco
MA.
Telomerase abrogation dramatically accelerates TRF2-induced epithelial carcinogenesis.
Genes Dev.
2007;
21(2):
206
-220.
[PubMed]
.
-
33.
Bradshaw
PS
, Stavropoulos
DJ
and Meyn
MS.
Human telomeric protein TRF2 associates with genomic double-strand breaks as an early response to DNA damage.
Nat Genet.
2005;
37(2):
193
-197.
[PubMed]
.
-
34.
Barlow
C
, Hirotsune
S
, Paylor
R
, Liyanage
M
, Eckhaus
M
, Collins
F
, Shiloh
Y
, Crawley
JN
, Ried
T
, Tagle
D
and Wynshaw-Boris
A.
Atm-deficient mice: a paradigm of ataxia telangiectasia.
Cell.
1996;
86(1):
159
-171.
[PubMed]
.
-
35.
Follenzi
A
, Ailles
LE
, Bakovic
S
, Geuna
M
and Naldini
L.
Gene transfer by lentiviral vectors is limited by nuclear translocation and rescued by HIV-1 pol sequences.
Nat Genet.
2000;
25(2):
217
-222.
[PubMed]
.
-
36.
Okabe
M
, Ikawa
M
, Kominami
K
, Nakanishi
T
and Nishimune
Y.
'Green mice' as a source of ubiquitous green cells.
FEBS Lett.
1997;
407(3):
313
-319.
[PubMed]
.
-
37.
van
Steensel B
and de Lange
T.
Control of telomere length by the human telomeric protein TRF1.
Nature.
1997;
385(6618):
740
-743.
[PubMed]
.
-
38.
Niwa
H
, Yamamura
K
and Miyazaki
J.
Efficient selection for high-expression transfectants with a novel eukaryotic vector.
Gene.
1991;
108(2):
193
-199.
[PubMed]
.