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Int. J. Cancer: 80, 857–862 (1999)
r 1999 Wiley-Liss, Inc.
Publication of the International Union Against Cancer
Publication de l’Union Internationale Contre le Cancer
SPECTRUM OF TRANSFORMING SEQUENCES DETECTED BY TUMORIGENICITY
ASSAY IN A LARGE SERIES OF HUMAN NEOPLASMS
Johannes W.G. JANSSEN1,2*, Jürgen BRAUNGER1,2, Karin BALLAS2, Michael FAUST2, Ute SIEBERS2,
Ada C.M. STEENVOORDEN2 and Claus R. BARTRAM1,2
1Institut für Humangenetik, Ruprecht-Karls-Universität, Heidelberg, Germany
2Sektion Molekularbiologie, Abteilung Kinderheilkunde II, Klinikum der Universität, Ulm, Germany
We here summarize the analysis of 126 DNA samples from
patients with hematopoietic neoplasias and solid tumors and
from various tumor cell lines that were screened in the
tumorigenicity assay. Thirty-eight samples were able to induce tumors after transfection in NIH/3T3 cells and injection
into nude mice. Southern-blot analysis with a panel of oncogene probes revealed human ras genes in the vast majority of
cases (25 N-ras, 2 K-ras, 1 H-ras) but also activated FGF4, dbl,
ret and mas genes respectively. DNA samples from the 6
remaining transfectants were cloned into EMBL-3 phages and
screened with a human specific repetitive Alu probe. Direct
hybridization of a transfectant cDNA library allowed cloning
of the ufo oncogene. Application of the exon-trapping technique to alu-positive phage DNA from the other transfectants enabled us to isolate tre, cot, B-raf, p85␤/HUMORF8 and
a novel oncogene. Int. J. Cancer 80:857–862, 1999.
r 1999 Wiley-Liss, Inc.
Transfection of NIH/3T3 cells with genomic DNA from human
tumors or human tumor cell lines has allowed the detection and
cloning of a large number of oncogenes. (Wigler et al., 1979). The
various genes identified in this way play a pivotal role in cell
differentiation or cell proliferation and code for growth factors,
receptors and second messengers. In the initial phase of application
of the NIH/3T3 transformation strategy, the respective oncogenes
belonged mostly to the ras family (Barbacid, 1987). However,
improvements of the classical focus formation assay, such as the
tumorigenicity assay (Fasano et al., 1984), expression cDNA
cloning (Miki et al., 1991) and expression cloning by retroviral
transfer of cDNA libraries (Whitehead et al., 1995) led to the
discovery of various other transforming genes. The classical
NIH/3T3 transformation assay and its different variants have
already detected the following oncogenes: neu, met, ret, trk, raf,
dbl, FGF4 (formerly hst), mas, lca, B-raf, tre, vav, hhc, ufo/axl, ect,
mos, cot/tpl-2, tim, TC21, ost, lbc, FGF-8, lfc, lsc and NET1
(references in Janssen et al., 1998). Some of these oncogenes were
present in an activated state in the original tumor material, whereas
others became activated in vitro during the DNA-mediated gene
transfer process.
Here we summarize our experience in DNA transfection studies
aimed at detecting transforming genes in a large series of hematopoietic neoplasias, solid tumors and tumor cell lines.
MATERIAL AND METHODS
Patients and cell lines
Cells were studied from bone marrow or peripheral blood of
patients with overt acute non-lymphocytic leukemia, chronic
myeloproliferative syndromes and pre-leukemias. The samples
were generously provided by Dr. H. Heimpel (University of Ulm,
Germany). Samples obtained from patients with neuroblastomas
and stomach carcinomas were provided by participating investigators of the German Neuroblastoma Study Group and by Dr. K.
Grünewald (Department of Internal Medicine, University of Innsbrück, Austria) respectively.
The following human cell lines were used: U937 (monoblastic),
ML-1 (myelomonoblastic), Rc2a (myelomonocytic), HEL (erythroblastic), THP-1 (monoblastic), KG-1 (myeloblastic), CTV-2 (monoblastic), T24 (bladder carcinoma) and T47D (breast carcinoma).
Transfection assay
The tumorigenicity assay is based on the co-transfection of
mouse NIH/3T3 fibroblasts with human tumor DNA and a dominant drug-resistant selectable marker, pRSVneo, followed by G418
selection and injection of the resulting 3T3 colonies into nude
mice, as described (Janssen et al., 1987a).
Genomic cloning and exon-trapping analysis
DNA isolated from third-cycle tumor-induced mouse tumors
was partially digested with Sau3AI, ligated to EMBL-3 BamHI
arms, packaged, plated and screened with a human Alu probe
(Janssen et al., 1991). Genomic sub-fragments of alu-positive
phages were sub-cloned in both orientations into the SmaI site of
the exon-trapping vector pL53In and transiently transfected into
COS-7 cells (Auch and Reth, 1990). Total RNA (5 µg) was
transcribed into cDNA with random hexamers and superscript RT,
as recommended by the manufacturer (GIBCO BRL, Eggenstein,
Germany). An aliquot (2 of 20 µl) cDNA reaction mixture was used
in a 100-µl PCR reaction consisting of 20 mM Tris/HCl (pH 8.4),
50 mM KCl, 2.5 mM MgCl2, 0.001% gelatin (w/v), 200 µM of each
deoxynucleotide triphosphate, 40 pmoles primers and 1 U Taq
polymerase (Amplitaq DNA Polymerase, Perkin Elmer Applied
Biosystems, Weiterstadt, Germany). Amplification was performed
in an automated PCR processor (BioMed, Theres, Germany) as
follows: 35 cycles comprising de-naturation at 92°C for 30 sec,
annealing at 56°C for 60 sec, primer extension at 72°C for 90 sec,
with an initial de-naturating step at 92°C for 3 min and a final
extension step at 72°C for 10 min. PCR products were electrophoresed in a 2.5% agarose gel and stained with ethidium bromide.
Southern-blot analysis
Cellular DNA was digested with restriction endonucleases
(Boehringer, Mannheim, Germany), electrophoresed on a 0.6%
agarose gel and blotted onto Nytran 13N membranes (Schleicher
and Schuell, Dassel, Germany). Filters were hybridized in 3 ⫻ SSC
(0.45 M NaCl, 0.045 M sodium citrate), 5 ⫻ Denhardt’s, 200 µg/ml
de-natured salmon sperm DNA, 1% SDS and 10% dextran sulfate
at 63°C for 16 hr with random-prime-labeled probes (Pharmacia,
Freiburg, Germany) with 2 µg/ml pRSVneo DNA as competitor
DNA to avoid cross-hybridization with co-transfected pRSVneo
sequences. Subsequently the filters were extensively washed in 3 ⫻
SSC, 0.1% SDS at 63°C, followed by a wash at moderate or high
stringency. Filters were exposed to Kodak X-Omat DS film at
⫺70°C with Ilford intensifier screens. The following probes were
used for hybridization analyses: the 38 portion of FGF4 (formerly
Grant sponsor: Deutsche Krebshilfe; Grant number: 10-1253-JA-I; Grant
sponsor: Deutsche Forschungsgemeinschaft.
*Correspondence to: Institut für Humangenetik, Ruprecht-KarlsUniversität Heidelberg, Im Neuenheimer Feld 328, D-69120 Heidelberg,
Germany. Fax: (49)6221-565155.
E-mail: hans_janssen@ukl.uni-heidelberg.de
Received 24 September 1998; Revised 5 November 1998
JANSSEN ET AL.
858
HST1) pORF1 cDNA (position 588-872); a partial 1.0-kb dbl
cDNA clone (p3-7); a genomic ret fragment (pRet 3.3 and pRet
1.2); a 0.74-kb EcoRI-XhoI cDNA fragment of human B-raf.
Northern-blot analysis
Total RNA was isolated and purified by acid-guanidiniumisothiocyanate-phenol-chloroform extraction. Northern-blot analysis and stripping were performed as described (Janssen et al.,
1998). Briefly, 10 µg of total RNA or 2 micrograms of poly A⫹
RNA were loaded onto a de-naturating agarose gel, electrophoresed
in the presence of formaldehyde and transferred to Nytran 13 N
nylon membranes (Schleicher and Schuell). Hybridization and
washing were performed as described for Southern blotting.
RESULTS
DNA-transfection analyses of 126 tumor-DNA samples (46
acute myeloid leukemias, 33 chronic myeloproliferative syndromes, 7 myelodysplastic syndromes, 25 neuroblastomas, 12
stomach carcinomas, 2 breast-tumor cell lines and 1 bladdercarcinoma cell line) revealed that 38 samples were able to induce
tumors in nude mice. Hybridization analyses showed the presence
of an activated N-ras or K-ras gene in 25 and 2 hematopoietic
neoplasias, respectively, as well as the expected H-ras sequences in
the T24 bladder-carcinoma cell line. The results concerning the
ras-positive tumors have been reported (Bos et al., 1985; Janssen et
al., 1985, 1987a,b,c, 1988). DNA of the remaining 10 independent
ras-negative primary tumors was able to induce secondary and
TABLE I – DETECTION OF ACTIVATED GENES OTHER THAN ras FAMILY MEMBERS BY THE TUMORIGENICITY ASSAY
Designation of
primary tumor
Origin
CTV-2-T1
So-T3
SW-T1
U937-T2
HEL-T1
THP-1-T3
Gi-T1
Si-T2
Fr-T1
Ma-T1
monocytic cell line
myeloproliferative disorder
myeloproliferative disorder
monocytic cell line
erythroblastic cell line
monocytic cell line
neuroblastoma
chronic myelocytic leukemia
neuroblastoma (IV-N)
stomach carcinoma
Transforming
sequence
Mode of detection
Hybridization
with oncogene probes
Genomic cloning
and hybridization
Exon-trapping
strategy
mas
⫹ (Janssen et al., 1988)
ufo
⫹ (Janssen et al., 1991)
p85␤/HUMORF8
⫹ (Janssen et al., 1998)
ret
⫹
cot
⫹
tre
⫹
FGF4 (hst)
⫹
dbl
⫹
B-raf
⫹
novel
⫹
FIGURE 1 – (a) Northern-blot analysis of ret gene expression in a third-cycle U937-T2 mouse tumor (U937T2g2) and NIH/3T3 recipient cells.
poly A⫹ RNA (2 µg) isolated from NIH/3T3 cells and U937T2g2 tumor cells were submitted to Northern transfer. Filters were hybridized to a
32P-labelled genomic ret insert. 28S and 18S ribosomal RNA were used as molecular-weight markers. (b) Northern-blot analysis of FGF4 gene
expression in Gi-T1 second-cycle mouse tumors. Total RNA (10 µg) isolated from Gi-T1B and C tumors, murine NIH/3T3 cells and human CTV-2
cells were submitted to Northern transfer. Filters were hybridized to a 32P-labelled FGF4 cDNA insert. Human FGF4-specific transcripts are
indicated by an open arrow. The lower panel shows control hybridization with a 32P-labelled rat glyceraldehyde 3-phosphate dehydrogenase probe.
ONCOGENES DETECTED BY THE TUMORIGENICITY ASSAY
tertiary tumors. DNA of these secondary and tertiary tumors
contained human repetitive sequences, suggesting the involvement
of an activated human oncogene. The observation that all secondary and tertiary tumors that were derived from the same primary
tumor contained a common set of human repetitive sequences
supported this conclusion (Janssen et al., 1987c; and data not
shown). These tumors were screened with a panel of oncogene
probes, i.e., v-raf, c-fms, v-rel, v-src, v-fes, v-mos, p53, c-myc,
N-myc, rat erbB2 (formerly neu), met, mcf-2 (⫽ dbl), mcf-3, erbB,
c-mvb, PDGFB (formerly c-sis), c-abl, c-src, c-fos, c-ets1, mas,
dbl, lca, ret and FGF4 (formerly hst) (Janssen et al., 1987c).
DNA from 4 tumors contained the following oncogenes: mas
(CTV-2-T1 tumor, Janssen et al., 1988), ret (U937T2 tumor),
FGF4 (formerly hst) (GiT1 tumor) and dbl (SiT2 tumor) (Table I).
Northern-blot analysis using a human ret probe revealed the
common pattern of ret transcripts in RNA from the tertiary
U937T2g2 transfectant (Fig. 1a, lane 2) (Ishizaka et al., 1989).
RNA from normal NIH/3T3 cells did not show any ret transcripts
(lane 1). Sequence analysis of various ret cDNA clones isolated
from an U937T2g2 cDNA library confirmed the existence of 4
kinds of ret transcripts. These were generated by alternative
splicing and different polyadenylation and resulted in 2 types of ret
oncogene protein products (Ishizaka et al., 1989). Unfortunately,
we did not identify any cDNA clones containing the 58 end of the
ret transcripts and were thus not able to determine the possible
fusion partner. The Northern-blot data were supported by Southernblot analysis showing amplified human ret DNA exclusively in
secondary and tertiary U937T2 transfectants (data not shown). In
accordance with earlier studies, Northern-blot analysis of 2 secondary GiT1 transfectants, GiT1C and GiT1B (Fig. 1b, lanes 3 and 4)
showed one major FGF4 transcript of approximately 3.0 kb. and 2
minor transcripts (Yoshida et al., 1988), while RNA from a human
tumor cell line (CTV-2) and control murine NIH/3T3 cells revealed
no FGF4 transcripts (Fig. 1b, lanes 1 and 2). Southern-blot analysis
showed the presence of dbl sequences in one primary and one
secondary SiT2 transfectant (Fig. 2, lanes 1 and 2). Its hybridization pattern was similar to normal peripheral-blood-cell DNA (Fig.
2, lane 4). The DNA of NIH/3T3 cells did not show any
hybridization with the human dbl cDNA probe (Fig. 2, lane 3).
DNA derived from the remaining transfectants was cloned into
EMBL-3 phages and screened for human sequences with a human
repetitive Alu probe. Alu-positive phages were isolated and phage
DNA was used directly as a probe to hybridize RNA or a cDNA
library of the respective transfectant. In one case we were able to
isolate and characterize a tyrosine-kinase receptor, called ufo
(Janssen et al., 1991), but failed to identify any transcripts or
positive cDNA clones with alu-positive genomic phage clones of
the other transfectants. To circumvent possible problems related to
the direct hybridization of complete genomic phage DNA, we
decided to take advantage of the exon-trapping strategy. Genomic
sub-fragments of alu-positive phage clones derived from unrelated
tertiary transfectants were cloned in both orientations into the
polylinker site of the pL53In exon-trapping vector, residing in an
intron between 2 insulin exons (Auch and Reth, 1990). The
resulting constructs were transfected into COS-7 cells and transiently expressed. RNA was isolated from the transfected COS-7
cells, transcribed into cDNA, and PCR-amplified using 2 specific
exon-trap insulin primers. Various exon-trap constructs revealed a
larger PCR product than the intrinsic control insulin PCR product,
notably in one orientation (Fig. 3). The PCR products were cloned
into a T-vector and sequenced with primers derived from the
exon-trap vector. Sequence comparison of the PCR product of the
SWT1 transfectant, SWM4 (Fig. 3, lane 2), revealed highest
homology with the bovine p85␤ sub-unit of phosphatidylinositol
(PI) 3-kinase. As we have reported, fusion of the human p85␤
sub-unit of PI 3-kinase and HUMORF8, a putative de-ubiquitin-
859
FIGURE 2 – Southern-blot analysis of the dbl gene. DNA samples (10
µg) obtained from a second-cycle Si-T2 transfectant (Si-T2a), a
primary-cycle Si-T2 transfectant (Si-T2), control murine NIH/3T3
cells and normal peripheral blood were digested with EcoRI, electrophoresed in a 0.6% agarose gel and blotted onto nylon filter. The filter was
hybridized with a human dbl cDNA oncogene probe, washed and
exposed. Lambda-HindIII DNA fragments served as molecular-weight
markers.
ating enzyme, was generated during the DNA transfection process
(Janssen et al., 1998).
Sequence comparison of the PCR product of the HELT1
transfectant, HEL8 (Fig. 3, lane 4), revealed 100% homology with
exons 4, 5 and 6 of the human cot oncogene (nucleotides 496-1033;
Acc. Z14138). This transforming gene had been isolated earlier by
transfection analysis with DNA from a human thyroid-carcinoma
cell line using the SHOK cell line as a recipient, as well as by
expression cloning with DNA from a Ewing’s-sarcoma cell line
using NIH/3T3 cells as recipient cell line (Miyoshi et al., 1991;
Chan et al., 1993). Southern-blot analysis confirmed the exontrapping data by demonstrating that the HEL transfectants exhibited 2 amplified human genomic cot fragments of about 4.5 and 1.5
kb (Fig. 4, lanes 1, 2 and 4) in addition to murine genomic cot
fragments (Fig. 4, lane 3).
Sequence comparison of the PCR product of the THP-1-T3
transfectant, THP-1/56 (Fig. 3, lane 6), revealed 100% homology
with the human tre17 gene (nucleotides 4897-5838, clone 213;
Acc. X63547), an oncogene detected with genomic DNA of a
Ewing’s-sarcoma cell line via the tumorigenicity assay (Nakamura
et al., 1988). In line with our exon-trapping data, Southern-blot
analysis confirmed the presence of amplified human tre oncogene
sequences in THP-1-T3 transfectants (Fig. 5, lanes 1 and 2) but not
in NIH/3T3 cells (lane 3). Sequence comparison of the PCR
products of 3 different genomic subfragments of the Fr-T1
transfectant, Fr6A2, Fr6BE2 and Fr6BES2 (Fig. 3, lanes 10, 12 and
860
JANSSEN ET AL.
FIGURE 3 – RT-PCR analysis of exon-trap constructs from several different independent transfectants. Genomic sub-fragments of alu-positive
phages or cosmids isolated from independent tertiary transfectants were sub-cloned into the SmaI site of the pL53In exon-trapping vector in both
orientations and transiently transfected into COS-7 cells. RT-PCR products of the transfected COS-7 cells were loaded onto a 2.5% agarose gel.
The position of the insulin exon is marked by asterisks. Amplification products containing trapped exon sequences are indicated by arrows.
FIGURE 4 – Southern-blot analysis of the cot gene. DNA samples
obtained from one tertiary and one quarternary HEL-T1 transfectant
(HELT1b2 and HELT1b1b), NIH/3T3 cells and peripheral blood (BC)
were digested with EcoRI, electrophoresed, blotted and hybridized
with the HEL8 RT-PCR product obtained by exon trapping. LambdaHindIII DNA fragments are indicated.
FIGURE 5 – Southern-blot analysis of the tre gene. DNA samples
obtained from 2 secondary THP-1-T1 transfectants (THP-1T3b and
THP-1T3c), NIH/3T3 cells and normal blood (BC) were digested with
EcoRI, electrophoresed, blotted and hybridized with the THP-1/56
RT-PCR product obtained by exon trapping.
ONCOGENES DETECTED BY THE TUMORIGENICITY ASSAY
861
14), revealed identity with 3 independent successive exons of
the human B-raf oncogene (6BES with nucleotides 1235-1371;
6BE with nucleotides 1372-1489; 6A with nucleotide 1490-1574
of the human B-raf oncogene, Acc. M95712, M95720 and
X54072). Southern blotting confirmed the exon-trap data (not
shown). Northern-blot analysis showed an aberrant human
B-raf transcript in Fr-T1 transfectant RNA, indicating that the
human B-raf oncogene may have been activated through recombination, a common mode of raf activation (Fig. 6, lane 3).
Sequence comparison of the PCR product of the Ma-T1 transfectant, MA30 (Fig. 3, lane 8), did not reveal any homology with
known genes in the public database, suggesting that a novel
oncogene had been identified. Northern-blot analysis with RNA of
Ma transfectants and NIH/3T3 cells applying this Ma-exon-trap
PCR product identified Ma-specific transcripts (data not shown).
Identification of this novel oncogene is currently under investigation.
DISCUSSION
The application of the NIH/3T3 transformation assay has
allowed the detection of a large number of oncogenes exhibiting
diverse functions. In particular, the development of several variants
of the classical NIH/3T3 focus formation assay made major
contributions to this field. Except for ras genes, most genes became
activated during the transfection process, by (i) quantitative
alterations (over-expression) as a consequence of gene amplification, loss of genomic negative regulatory sequences, insertion of
promoter sequences from the dominant drug-resistant selectable
marker plasmid or the promoter sequences of the cloning vehicle
used or (ii) qualitative changes such as gene truncation, point
mutations or gene fusion. In other cases the oncogenes were
already activated in the original tumor prior to gene transfer, as,
e.g., activated ras genes in various neoplasms, the met gene in the
MNNG-HOS cell line (Park et al., 1986), the erbB2 (formerly neu)
gene in ENU-induced rat neuroblastomas (Shih et al., 1981) and
the trk oncogene in a human colon carcinoma (Martin-Zanca et al.,
1986).
We and others have reported on activation of the mas gene by
loss of negative regulatory genomic sequences (Young et al., 1986;
Janssen et al., 1988), the activation of ufo/axl through amplification
resulting in over-expression of this tyrosine-kinase receptor (Janssen et al., 1991), and the fusion of the human p85␤ sub-unit of PI
3-K with HUMORF8 sequences (Janssen et al., 1998). Our
Northern-blot data suggest that the FGF4 (formerly hst) gene was
activated due to the loss of negative regulatory sequences, which
resulted in over-expression of the gene, a process similar to that
described by Sakamoto et al. (1986). Our Northern analysis
showed an aberrant B-raf transcript suggesting that the B-raf gene
may have been activated by gene truncation. Northern analysis
revealed high tre expression in THP-1-T1 transfectants compared
with a moderate level of endogenous mouse tre expression in other
transfectants. This observation may imply that de-regulation and
over-expression of the human tre gene has led to transformation of
the NIH/3T3 cells. Since human and murine tre transcripts have
similar molecular weight, the exact mode of activation of tre in
THP-1 transfectants remains elusive.
Our data clearly demonstrate the usefulness of the exon-trapping
technique in identification of activated oncogenes. In the case of
the cot oncogene in HEL-T1 transfectants, exon trapping enabled
us to detect 3 consecutive cot exons on one genomic sub-fragment
of HELT1 tertiary transfectants and 3 consecutive B-raf exons on 3
different genomic fragments from Fr-T1 tertiary transfectants. In
addition, this strategy enabled us to identify (i) the known tre
oncogene, (ii) an oncogenic fusion product between the phosphatidylinositol 3-kinase p85␤ sub-unit and HUMORF8, and (iii) a
novel oncogene.
FIGURE 6 – Northern-blot analysis of B-raf gene expression in Fr-T1
second-cycle mouse tumors. Poly A⫹ RNA (2 µg) isolated from a
secondary Fr-T1 transfectant (FrT1a), murine NIH/3T3 cells and the
human CTV-2 cell line were submitted to Northern transfer. Filters
were hybridized to a 32P-labelled B-raf cDNA insert. Human B-rafspecific transcripts are marked with an open arrow at the left of the
upper panel. The aberrant B-raf transcript is indicated with an open
arrow at the right of the upper panel.
Our data support the view that the tumorigenicity assay may
have a bias for the detection of specific oncogenes, such as mas, tre,
B-raf, ret, cot, dbl and FGF4. The mode of oncogene activation
(e.g., over-expression, gene fusion or loss of specific coding
sequences) combined with inherent properties of the transfection
procedure (e.g., DNA breakage, ligation of DNA fragments and
amplification of DNA), its normal function, such as cellular
signaling and, last but not least, the ability of a particular gene to
transform NIH/3T3 cells, are factors that influence the detection
frequency of this assay. Variants of the classical NIH/3T3 transformation assay and methods for isolating exon sequences from
genomic sub-fragments continue to make possible the identification of even more genes involved in the complex process of
tumorigenesis.
ACKNOWLEDGEMENTS
The excellent technical assistance of Mrs. M. Schmidberger,
Ms A. Wunderlich and Ms U. Spadinger is gratefully acknowledged. We thank Mrs. A. Jordan for secretarial help with the
manuscript. We thank the following colleagues for providing us
with oncogene probes: Dr. H. Sakamoto (hst), Dr. A. Eva (dbl), Dr.
G.M. Cooper (ret), Dr. T. Yamamoto (B-raf ). We also thank Dr. M.
Reth for providing us with the pL53In exon-trapping vector. This
work was supported by grants from the Deutsche Forschungsgemeinschaft and Deutsche Krebshilfe (10-1253-Ja I) to C.R.B.
and J.W.G.J.
862
JANSSEN ET AL.
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