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Advances in Brief |
Department of Laboratory Medicine, University of California San Francisco Comprehensive Cancer Center, University of California, San Francisco, California 94143
| ABSTRACT |
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| Introduction |
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30 kb (2, 3, 4)
. However, assays of allelic
imbalance typically require allelic heterozygosity so that only a
subset of the loci will be informative in any individual
(5)
. This is particularly limiting in experimental murine
systems using inbred mouse strains that have lost heterozygosity.
Analyses of these mice require cross-breeding to reintroduce
heterozygosity. This is both time-consuming and experimentally
undesirable because the cross-breeding may introduce DNA sequence
variations that alter tumor phenotype. Here we describe a technique called QuMA that overcomes some of these limitations by allowing analysis of the relative DNA copy number at microsatellite markers distributed throughout the genomes of higher eukaryotes. This real-time, quantitative PCR technique is an alternative to analysis of allelic imbalance because most (but not all) allelic imbalance also results in a change in relative copy number. We demonstrate that QuMA is sufficiently sensitive to detect gain or loss of a single copy of a test locus in mouse and human. We show the biological utility of QuMA in localization of a putative tumor suppressor gene associated with radiation-induced murine myeloid leukemia on chromosome 2 and in establishing an association between increased copy number of the putative oncogene ZNF217 and reduced survival duration in human ovarian cancer.
| Materials and Methods |
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Ct [
Ct = Ct (test
locus) - Ct (pooled reference)].
Ct values
were measured for each unknown sample [
Ct (test DNA)] and for
samples from several unrelated, normal (calibrator) individuals
[average =
Ct (calibrator DNA)]. Relative copy
number at each locus in the test sample was then calculated as:
![]() | (1) |

Ct =
Ct (test
DNA) -
Ct (calibrator DNA), and
E = PCR efficiency. For simplicity, we used
primers with PCR efficiencies of >85% and calculated the relative DNA
copy number as 2-
Ct
(x 2 when known to be a diploid test sample, such as normal male DNA,
or mouse knockout DNA or with FISH analysis of mouse tumors).
To determine whether a QuMA measurement on a single sample was
significantly different from the mean of measurements made on samples
from a number of normal individuals, a TI was calculated using the mean
and SD of
Ct values from each marker and the pooled reference
according to the following formula:
![]() | (2) |
PCR
PCR was conducted in triplicate with 50-µl reaction volumes of
1x PCR buffer A (PE Biosystems), 2.5 mM
MgCl2, 0.4 µM each primer, 200
µM each deoxynucleotide triphosphate, 100 nM
probe, and 0.025 unit/µl Taq Gold (PE Biosystems) with 15 ng of
genomic DNA. A large master mix of the above-mentioned components was
made for each experiment and aliquoted into each optical reaction tube.
Each primer set (510 µl volume) was then added, and PCR was
conducted using the following cycle parameters: 1 cycle of 95°C for
12 min and 40 cycles of 95°C for 20 s, 55°C for 20 s,
72°C for 45 s. Analysis was carried out using the
sequence detection software supplied with the ABI 7700 (PE Biosystems).
The Ct values for each set of three reactions were averaged for all
subsequent calculations. Pooled variance for all sets of PCR
triplicates was 0.018 (n = 288), indicating
sufficient statistical power with this level of intra-assay variation
to detect a difference of 0.25 cycle between samples with greater than
95% confidence.
Oligonucleotides
PCR primer sequences for microsatellite loci were obtained
from either the Whitehead Institute Massachusetts Institute of
Technology Center for Genome
Research3
or Washington University (St. Louis,
MO).4
Primers were synthesized by Life Technologies, Inc. (Gaithersburg, MD)
or Integrated DNA Technologies (Coralville, IA). The reference
pools contained primer pairs for six or seven different loci. For human
analyses, loci were chosen in regions of the genome that usually did
not show alterations in human breast tumors (human pool 1,
D4S1605, D5S478, D11S913, D12S1699, D14S988, D21S1904, and
D22S922) or ovarian tumors (human pool 2, D1S2868, D2S385,
D4S1605, D5S643, D10S586, and D11S1315) when analyzed by CGH
(11)
. The murine reference loci were D1MIT64, D2MIT175,
D3MIT12, D12MIT10, D13MIT250, and D14MIT5. Primer sequences for the
test and reference loci analyzed in this study are listed in Table 1
. The TaqMan CA-repeat fluorogenic probe consisted of the following
sequence: 5'-FAM-TGTGTGTGTGTGTGTGTGTGT-6-carboxy tetramethyl
rhodamine-3'. ZNF217 primers and probe were as reported previously
(12)
. Both probes were purchased from PE Biosystems.
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Specimens
Murine.
Two inbred strains of mice were evaluated: (a) a FVB
knockout mouse (designated M11) that lacked a 700-kb segment of
chromosome 11 (kindly provided by Dr. E. Rubin; Lawrence Berkeley
National Laboratory, Berkeley, CA); tail DNA samples were obtained from
M11+/+, M11+/-, and
M11-/- FVB mice; and (b)
3-month-old SJL mice (Jackson Laboratories, Bar Harbor, ME) exposed to
3 Gy of ionizing radiation from a Cs137
irradiator. Spleen and marrow samples were collected for DNA isolation
after 79 months.
Human.
All specimens were collected and used with the approval of the
Committee on Human Research at the University of California, San
Francisco. These specimens included: (a) blood samples
collected from normal, healthy, male and female donors for analysis of
X chromosome copy number; (b) ovarian tumor samples
collected as described by Suzuki et al. (15)
; and
(c) human breast cancer cell lines HBL100,
MDA-MB-361, HS578T, SKBR3, and MCF-7 obtained from American Type
Culture Collection (Manassas, VA) and cultured as recommended by the
supplier.
DNA Isolation
DNA was extracted from either tail clippings or spleens of mice
and processed with a Puregene DNA extraction kit (Gentra Systems,
Minneapolis, MN) used according to the manufacturers purification
recommendations. DNA was extracted from human blood specimens and cell
lines using a Wizard Genomic DNA purification kit from Promega
(Madison, WI). Ovarian tumor DNA was isolated as described previously
(15)
.
CGH
CGH was performed on DNA from three SJL mice as described by
Kallioniemi et al. (16)
. Mouse metaphase
chromosome spreads were prepared from phytohemagglutinin-stimulated
murine peripheral blood lymphocytes of a normal C57BL mouse. Image
analysis was conducted as described by Piper et al.
(17)
.
| Results and Discussion |
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200-bp wide locus, whereas FISH measures copy
number using an
100-kb probe. The number of hybridization signals
generated using FISH will vary in an uncertain manner if the level of
amplification changes across the extent of the region covered by the
probe. In addition, copy number is difficult to measure in interphase
nuclei using FISH when the level of amplification is high. On the other
hand, FISH is less sensitive to the presence of admixed normal cells.
The level of aneusomy in these cell lines may also contribute to the
difference because the reference loci may be elevated in copy number,
resulting in underrepresentation of the absolute DNA copy number. Thus,
analysis using both methods may be appropriate in cases of high-level
amplification or aneusomy.
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To further define the sensitivity of QuMA, we mixed DNA from a
chromosome 11 knockout mouse with increasing amounts of DNA from normal
FVB mice. QuMA was performed on these DNA mixtures at D11MIT23,
D2MIT175, and D13MIT250. Fig. 2A
shows that QuMA was able to distinguish the reduced copy
number at D11MIT23 even in the presence of 30% of normal DNA.
Specifically, QuMA showed a 90% chance of detecting a difference
between one copy and two copies, with 95% confidence under these
conditions. Thus, QuMA should be able to detect the loss of one allele
in tumor samples contaminated by this amount of normal DNA.
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30-cM
region of chromosome 2E has been reported in tumors in
F1 crosses of these mice (19
, 20)
.
We used QuMA to assess deletions at 27 loci along chromosome 2 using
spleen DNA from eight irradiated SJL mice. Six of the eight spleens
were comprised mostly of leukemic cells. Fig. 2B
23-cM-wide region of common deletion
between D2MIT9 and D2MIT337. The QuMA results are consistent with those
obtained using FISH and CGH. CGH analysis of DNA from spleens from two
irradiated mice with leukemia (mice 2.4B and 11.2) and one irradiated
mouse without leukemia (mouse 2.1B) showed chromosome 2 copy number
losses in the two leukemic mice and no loss in the nonleukemic mouse.
FISH analyses of spleens from two other irradiated mice with leukemia
(mice 2.5 and 12.2) and one mouse without leukemia (mouse 8.5) were
performed using BAC clones mapped to chromosome regions 2A(BAC356),
2E(BAC62), and 2H(BAC415). For the nonleukemic mouse (mouse 8.5), the
2A:2E copy number ratio was 0.92, whereas in leukemic mice (mice 2.5
and 12.2), the ratios were 1.67 and 1.89, respectively. The 2A:2H
ratios for the same mice were both 1.02, indicating that these mice
have an interstitial deletion of in the region of 2E. These results
demonstrate the utility of QuMA for high-resolution mapping of the
extent of physical deletions in inbred mouse strains, obviating the
need for expensive and time-consuming backcrossing.
Reproducibility.
To assess the reproducibility of QuMA, 13 markers were measured three
times on four irradiated mice. The average relative copy number for the
disomic loci was 2.00 ± 0.26 (n = 123), with 5 of 123 measurements outside the TI (three above the
cutoff values and two below the cutoff values). The average relative
copy for monosomic loci was 0.94 ± 016
(n = 30), with all values outside the TI.
Assessment of Prognostic Markers in Human Tumors.
Amplifications of several regions of chromosome 20q occur frequently in
a broad range of human cancers including ovarian cancer
(11)
. Several putative oncogenes have been identified in
these regions including the putative zinc-finger transcription factor
ZNF217 (12)
. Amplification of this gene has
been associated with reduced survival duration in human breast cancer
(21)
. We used QuMA to test the association between
ZNF217 copy number increase and reduced survival for 60
ovarian cancer patients. The Kaplan-Meier survival curves for patients
with or without increased copy number of ZNF217 in Fig. 3
show that increased copy number of this gene was significantly
associated with reduced survival duration in late-stage tumors and all
tumors. This study demonstrates the utility of QuMA for rapid detection
of prognostic copy number changes in human cancers.
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| FOOTNOTES |
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1 To whom requests for reprints should be
addressed, at University of California San Francisco Comprehensive
Cancer Center, Cancer Genetics and Breast Oncology Program, 2340 Sutter
Street, Room S431, San Francisco, CA 94143. ![]()
2 The abbreviations used are: SSR, simple sequence
repeats; QuMA, quantitative microsatellite analysis; FAM,
6-carboxy fluorescein; TI, tolerance interval; CGH, comparative genomic
hybridization; FISH, fluorescence in situ hybridization;
BAC, bacterial artificial chromosome; Ct, threshold cycle; MDA 361,
MDA-MB-361. ![]()
3 World Wide Web address: www.genome.wi.mit.edu. ![]()
4 World Wide Web address: www.genlink.wustl.edu. ![]()
5 Collins, C., unpublished observations. ![]()
6 Rubin et al., unpublished
observations. ![]()
Received 5/ 5/00. Accepted 8/16/00.
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