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From the Institute of Pathology, Würzburg University, Würzburg, Germany
| Abstract |
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| Introduction |
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The molecular basis underlying the pathogenesis of these lymphomas and the progression from low- to high-grade disease has not been elucidated yet, although some frequently occurring abnormalities, features of genomic instability, have been studied and well documented. Genomic instability is a basic property of tumor cells, it generates the diversity necessary for a cancer cell to escape from inherent restraints on growth. One form of genomic instability is the result of inactivation of tumor suppressor genes, which is the hallmark of the tumor suppressor pathway of oncogenesis. The other form results from the malfunction of the DNA mismatch repair system and leads to replication error (RER) phenotype characteristic of the mutator pathway of oncogenesis.4,5 Several recent findings suggest that mismatch repair system defects might be involved in the pathogenesis of extranodal MALT lymphomas. However, the range of MALT lymphoma patients found to manifest microsatellite instability (MSI) at two or more repeat loci starts at 0%6 and ends with >50%.7 This contrasts with other non-Hodgkins lymphoma types showing frequently karyotypic but rarely MSI.8 Because of such conflicting results on MSI frequency in MALT lymphoma, there is no consensus regarding the role of MSI in MALT lymphoma pathogenesis and no universal MSI screening panel for lymphomas is available.
Both forms of genomic instability can be assayed for by one method, microsatellite analysis. Microsatellite markers can be used to identify genetic loci that have been lost to detect genomic alterations (losses of heterozygosity and homozygous deletions) in neoplasms. Analysis with the same markers reveals also any MSI and any novel additional alleles of different length present in the tumor. To evaluate the contribution of the mutator pathway to the gastric lymphoma pathogenesis, we analyzed 25 gastric low-grade marginal-zone B-cell lymphomas of MALT-type and 31 gastric high-grade DLBCLs, with 29 and 118 microsatellite markers, respectively, including markers from a MSI screening panel for colorectal carcinoma.9 We report the results of these analyses and provide an assessment of the mutator pathway role in lymphomagenesis.
| Materials and Methods |
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Thirty-one consecutive extranodal gastric high-grade DLBCL patients and 25 gastric low-grade marginal-zone B-cell lymphoma of MALT-type cases from the lymph node registry at the Institute of Pathology in Wuerzburg on whom fresh frozen tissue (all high-grade and 15 low-grade lymphomas) or formalin-fixed/paraffin-embedded tissue (10 low-grade lymphomas) was available were selected for the study. The diagnosis was established according to the criteria of the Revised European-American Lymphoma Classification10 and Chan et al11 by morphological and immunophenotypic analyses of paraffin-embedded and fresh-frozen tissue sections using standard staining methods as described recently.3 The high-grade lymphoma patients presented in various stages of disease (four patients were in stage EI1, 13 in stage EI2, nine in stage EII1, four in stage EII2, and one in stage EIII).12 The tumors were localized at presentation with no clinical evidence of generalized disease. The high-grade lymphoma patient population showed an age distribution from 31 to 79 years of age with a mean of 58. The male-to-female ratio was 2.44, there were 22 males and nine females included in the study. From the low-grade MALT-lymphoma patients, five patients were in stage EI1, five in stage EI2, 10 in stage EII1, two in stage EII2, one in stage EIII, there were no staging data available on the remaining two cases. The low-grade patient population showed an age distribution from 32 to 75 years of age with a mean of 54. The male-to-female ratio was 2.13, there were 17 males and eight females included.
Microdissection and DNA Extraction
In each case, 16 serial 10-µm-thick tissue sections were cut. The first and last cuts were stained with hematoxylin and eosin (H&E) to assure high tumor content and as guidance for the following dissection. The fresh-frozen tissue sections were visualized under microscope and, after removal of surrounding normal tissue, an area showing high- or low-grade lymphoma was scraped using a blade. In a similar way, control genomic DNA was derived from separate tissue blocks not involved by the tumor. Paraffin-embedded/formalin-fixed tissue sections were additionally stained by Nuclear-Fast Red to precisely delineate tumor-containing areas and the collected tissue deparaffinized with xylene before digestion. DNA extraction was performed using proteinase K and phenol-chloroform according to routine molecular biology protocols.13 To exclude the possibility of tumor infiltrating the tissue used as a normal control, all low-grade lymphoma control tissue samples were confirmed by polymerase chain reaction (PCR)-based IgH clonality analysis as not being contaminated by lymphoma cells.14
Microsatellite Analysis
Microsatellite primer panels ABI PRISM LMS-MD10 for chromosomes 5
and 6 were bought from Perkin-Elmer-Cetus (Foster City, CA). Primer
sequences for the amplification of the remaining microsatellite repeats
listed in Table 1
were retrieved from
Genome Database (http://gdbwww.gdb.org). PCR primers were synthesized
at MWG Biotech (Munich, Germany) and one oligonucleotide of each primer
pair labeled with fluorescent dye phosphoramidites FAM, TAMRA, or HEX.
Paired normal and tumor DNA samples from each patient were amplified
with PE AmpliTaq Gold enzyme (Perkin-Elmer-Cetus) in multiplex PCR
reactions using 50 ng of genomic DNA as template under conditions
specified by the Genome Database. Thirty cycles were performed in a
PE-2400 thermal cycler (Perkin-Elmer-Cetus) in a total volume of 20
µl. Aliquots of the PCR reactions were then mixed with size standard
and formamide, denatured, and subjected to electrophoresis on a 373 DNA
Sequencer (ABI, Foster City, CA). The automatically collected data were
analyzed using GENESCAN software as described in the manufacturers
manual. Patients heterozygous and homozygous at a given locus
were regarded to be informative for MSI; MSI was defined as a change of
allele length caused by of either insertion or deletion of repeating
units.
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Immunohistochemical staining for MSH2 (hybridoma clone FE11; Calbiochem, Darmstadt, Germany) and MLH1 (hybridoma clone G16815; Pharmingen, Hamburg, Germany) was performed on pressure-cooker pretreated formalin-fixed/paraffin-embedded tissue sections and visualized using standard immunoperoxidase technique. Normal tissue of the same patient served as a control. A case was considered positive for each antigen, when >80% of the tumor cells showed strong nuclear staining as compared to normal cells on the same slide.
| Results |
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We assumed that if MSI were to play a role in the pathogenesis of
gastric extranodal lymphomas, then it would be more pronounced in the
high-grade lymphomas than in their low-grade counterparts. We therefore
screened 31 gastric high-grade DLBCLs with 118 highly polymorphic,
mostly dinucleotide microsatellite markers first (Table 1)
and
established on that material a panel of markers with which then the
low-grade disease was investigated. The 118 markers were chosen to
cover chromosomal regions shown to harbor gross chromosomal aberrations
detected in a previous study using a part of the same patient
material,15
we also included markers showing high-level
MSI in a work published previously,7
markers from a
colorectal cancer MSI screening panel,9
and additionally,
screening markers from the ABI PRISM LMS-MD10 panels for chromosomes 5
and 6. The overall level of MSI as detected with the 118-marker panel
was low, with a mean of 2.6% MSI-positive markers and SD of 1.85%.
Ninety-six (2.7%) of 3,568 genotypes revealed MSI, the majority (71%)
of the novel alleles showed only one repeat difference to the original
allele. Eighty-two percent were additions and 18% were deletions of
one repeat from the original allele. All of the MSI cases were type II
mutations (only one novel allele occurred per marker); three (10%) of
31 patients did not show any MSI and were thus microsatellite stable.
None of the tumors displayed 40% or more MSI-positive markers, a level
established as a cut-off for high-frequency MSI (MSI-H) in colorectal
cancer.9
All these lymphomas showing MSI are thus MSI-L
tumors. With this corresponds also our finding of sufficient levels of
the MLH1 and MSH2 mismatch repair proteins in the lymphoma cells. All
tumors showed strong nuclear signal when immunostained with
-MLH1 and
-MSH2 antibodies (Figure 1)
. The lymphomas were further analyzed
for mutations at the polydeoxyadenine tract of the transforming growth
factor-ß type II receptor gene and polydeoxyguanine tracts of the
insulin-like growth factor II receptor and BAX genes or the AGC repeat
in the coding region of the E2F-4 gene, mutations that are
characteristically associated with the MSI-H phenotype.16
However, none of the 31 analyzed patients revealed any MSI at these
repeats.
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When the frequency of MSI in the individual patients was plotted
in a diagram (Figure 2)
, it became
obvious that the occurrence of MSI at low-frequency levels is very
common in these lymphomas (90% of the tumors are MSI-positive).
However, there is a considerable difference in MSI frequency as
demonstrated by individual cases. One extreme was the three patients
not showing any MSI at all, the other extreme the patients, case 8 and
case 16, showing 10 (8.5%), and eight (6.8%) MSI-positive markers,
respectively. The former patient seems to be an outlier, the latter
with 6.8% MSI-positive markers is very close to the border of the ±2
standard deviations confidence interval (0 to 6.3% MSI-positive
markers). Because the MSI frequency of almost all evaluated DLBCLs lies
within 2 standard deviations from the mean of the population, it seems
that we are dealing with a biological process generally present within
the studied material and the differences in the level of MSI in
individual cases are either random or depend on additional factors. A
search for factors having an influence on MSI levels revealed that
older age of patients seems to associate with higher MSI frequency. MSI
showed a positive correlation with age (r =
0.46; Figure 2
); indeed, the tendency to increase with age was
statistically significant (Jonckheere-Terpstra test, P
= 0.012), as was increasing MSI variability with age
(F-test, P = 0.02). The MSI frequency
difference between younger and older patients was further confirmed by
comparison of two groups of patients; those
50 years of age and those
60 years revealing a significant result (Mann-Whitney
U-Test, P = 0.009).
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To compare MSI frequency in extranodal lymphomas and colorectal
carcinomas, we used an established panel of five microsatellite markers
considered to be extremely sensitive for the detection of MSI in
colorectal carcinoma17
to screen for MSI in 31 gastric
high-grade DLBCLs. The panel consisted of mononucleotide repeats BAT-26
and BAT-40, and dinucleotide repeats D2S123, D5S346, and
D17S250.9
Comparison of the tumor and normal tissue
electropherograms revealed novel-length alleles characteristic of MSI
in seven (5%) genotypes in six (19%) patients (Figures 1 and 3A)
. Only patient 11, that is one (3%)
of 31 patients showed MSI with two markers, fulfilling in this way the
criteria for MSI-H as defined by this marker panel.18
In
contrast, the same patient had only 3.4% MSI-positive markers when
investigated with the 118-marker panel and, therefore, at the end had
only MSI-L, not MSI-H.9
Patient 3 showed MSI at the BAT-40
locus (Figure 1)
, which is frequently mutated in HNPCC, however, that
was the only MSI detected with the poly(A) markers BAT-26 and BAT-40 in
this study.
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Search for MSI at individual loci of the 118-marker panel revealed
62 (52.5%) repeats to be negative for MSI in all DLBCL tumors
analyzed, 30 (25.4%) markers showing only one MSI event, 18 (15.2%)
with two MSI events, five (4.2%) with three MSI events, and one
(0.8%) of each with either four, five, or six MSI events (Figure 4)
. Microsatellites showing frequent MSI
were not evenly spaced across the genome, unusually many markers
showing higher number of MSI events were concentrated on chromosomes 3,
5, and 18 (Figure 4)
. Obviously, frequently MSI-positive
microsatellites should be used for screening in lymphoma. We therefore
designed a new MSI screening panel for lymphoma using the following
microsatellites: D3S1261, D3S1530, D5S346, D17S250, D18S474, and DCC
(Figure 3B)
. Such a panel would detect five patients, including cases 8
and 16, as having an increased MSI frequency and is thus more sensitive
in detection of possible RER+ phenotype.
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Twenty-nine markers (Table 1
, loci marked by an asterisk), which
showed any consistent MSI or allelic imbalance in the high-grade part
of the study, were then used to screen 25 low-grade gastric
marginal-zone B-cell lymphomas of MALT-type. Of 659 genotypes (some
formalin-fixed tissue derived DNA did not give any amplificate even
after repeated PCRs), only four (0.6%) showed MSI (raw data not
shown). These MSI events seemed to be randomly dispersed, as they
occurred at loci not showing very frequent MSI in the high-grade
disease and all four markers were affected only once. Twenty-two
patients did not show any MSI at all, two patients had one MSI-positive
marker, one patient displayed two MSI-affected markers. The overall MSI
frequency in this group (LG29) was very low with mean of 0.55%
MSI-positive marker and standard deviation of 1.63% (Figure 5)
. To perform MSI frequency comparison
of the low- and high-grade lymphomas with exactly the same markers, we
reevaluated the high-grade lymphomas for the same 29-marker panel the
low-grade cases were screened with. With these markers, the DLBCLs
showed a mean of 4.1% MSI-positive repeats with standard deviation of
5% (HG29; Figure 5
). That somewhat differs from the MSI frequency
established for the DLBCL group by the use of the 118-marker panel;
however, the difference is primarily because of the selection of
repeats for the 29-marker panel (underrepresentation of repeats not
showing any MSI). Statistical analysis of the MSI frequency data
collected for the LG29 and HG29 groups confirmed a significant
difference in MSI frequency between the low-grade gastric marginal-zone
B-cell lymphoma of MALT type and gastric high-grade DLBCL (Mann-Whitney
U-Test, P = 0.009).
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| Discussion |
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To determine whether MSI characterizes a subset of sporadic extranodal
gastric low- and high-grade lymphomas and to investigate the
contribution of the mutator pathway to lymphomagenesis, we evaluated 31
gastric high-grade DLBCLs and 25 gastric low-grade marginal-zone B-cell
lymphomas of MALT-type for MSI. A thorough analysis with a panel of 118
microsatellite markers showed a low background MSI with a mean of 2.6%
MSI-positive microsatellites in the high-grade lymphoma patients. Three
patients were microsatellite stable and two patients showed somewhat
more frequent MSI with 6.8% and 8.5% of the markers used,
respectively. A direct comparison of the low- and high-grade lymphomas
with a panel of 29 frequently mutated markers showed a mean of 4.1%
MSI-positive markers per patient in the former group and 0.55% in the
latter group. However, compared to epithelial cancers and, especially,
colorectal cancer, this level of MSI is still too low to consider these
tumors to be MSI-H. Only cases with 40% or more of all markers
unstable are diagnosed as MSI-H or having the
RER+ phenotype in sporadic colorectal carcinoma.
A panel of five markers has been proposed for screening purposes and
defining the MSI-H colorectal cancer group.9
Using this
reference panel on the studied high-grade lymphomas, one of our
patients (case 11; Figure 3A
) would be defined as MSI-H (the same
patient showed only 3.4% unstable microsatellite markers with the
118-marker panel and was thus definitely MSI-L not MSI-H). However,
patients displaying the most MSI in our study (cases 8 and 16) would
not be detected by this MSI screening panel as having increased MSI at
all. Moreover, the markers comprising this reference panel were only
partly among those showing frequent MSI in the studied high-grade
DLBCLs. Mononucleotide poly(A) repeats characteristically showing
frequent instability in HNPCC and MSI-H colorectal
cancer17,22
proved to be unstable in only one of the
high-grade DLBCL patients. From the dinucleotide-repeat markers, only
D5S346 and D17S250 revealed frequent MSI also in the lymphomas.
Interestingly, of the evaluated 118-repeat panel, microsatellites
showing most frequently MSI seem to be predominantly concentrated on
chromosomes 3, 5, and 18 (Figure 4)
. This could be partly a result of
clonal aberrations, because several of the patients showed trisomy 3
and there were also several trisomies 18 present as revealed by
cytogenetic analysis of the material (results not shown). Nevertheless,
microsatellites showing most frequently MSI thus might be
tumor-specific, meaning that different markers are appropriate for MSI
screening in different types of cancer and the optimal set of loci to
diagnose MSI in lymphoma is different from that one for colorectal
carcinoma. A MSI screening panel for lymphomas should therefore consist
of different markers, microsatellites showing frequent MSI in lymphoma
as opposed to a marker set appropriate for colorectal cancer MSI
screening. We therefore introduce here a new lymphoma MSI screening
panel composed of markers showing frequent MSI in the studied
high-grade lymphomas (Figure 3B)
. Such a panel consisting of six
markers (D3S1261, D3S1530, D5S346, D17S250, D18S474, and DCC) would
detect DLBCL patients having increased level of MSI, including cases 8
and 16 with the highest MSI frequency detected in this study.
A small fraction of many tumor types, in addition to those of the
colon, display some level of MSI.23
In most of these
tumors, the instability is considerably less pronounced than that
observed in colon tumors and it is questionable if it is because of
mismatch repair gene defects. There are several lines of evidence
against a substantial role of mismatch repair genes and MSI in the
pathogenesis of extranodal high-grade lymphomas. These tumors show
widespread karyotypic instability and are mostly aneuploid, as revealed
by recent cytogenetic15,24
and comparative genomic
hybridization (CGH)25
studies. All MSI-positive
analyses in this work showed only type II mutations with a single
slightly size-changed novel allele which is not typical for the
RER+ phenotype and that can be also detected in
RER-negative tumors.26
Our current search for mutations
characteristically associated with the RER+
phenotype,27-32
like MSI at the polydeoxyadenine tract of
the transforming growth factor-ß type II receptor gene and
polydeoxyguanine tracts of insulin-like growth factor II
receptor and BAX genes or the AGC repeat in the coding region of
the E2F-4 gene did not reveal any instability at these repeats. Several
studies reported close correlation between tumors displaying MSI-H and
the absence of protein expression for either MSH2 or MLH1 in colorectal
carcinoma.17,33
However, staining with
-MLH1 and
-MSH2 antibodies confirmed the presence of these mismatch repair
proteins in all DLBCL patients studied. These features considered
together, widespread genomic alterations in the form of losses of
heterozygosity and amplifications of genetic material we saw
previously on the same DLBCL material,34
and the
low-frequency of MSI detected in the presented study are a rather
substantial evidence for the mutator pathway having only a minor role
in the pathogenesis of this disease.
Nevertheless, the approximately sevenfold higher MSI frequency in the high-grade lymphomas when compared to their low-grade counterparts (P = 0.009) and the increase of MSI with age within the high-grade lymphoma group (P = 0.01) show that there is a selection pressure for MSI to increase during the transition from low- to high-grade disease on one side and with older age on the other side. Dysfunction of the mismatch repair mechanism is a relatively late phenomenon during lymphomagenesis and its role in the pathogenesis of the disease seems to be contributory, but not initiating. These lymphomas do not achieve that degree of MSI characteristic of HNPCC or MSI-H-positive colorectal carcinoma meaning that the mismatch repair system is still to some degree relatively preserved as confirmed by the detection of the MLH1 and MSH2 proteins in all DLBLs investigated. How exactly this MSI-L contributes to lymphomagenesis is not clear. Using enough microsatellite markers for analysis, probably all tumors can be made to show MSI with one of the repeats used. The cause and effect relationships between MSI-H and mutations in mononucleotide repeats of genes like transforming growth factor-ß type II receptor gene27 or BAX30 are well described, however, the role of MSI-L in cancerogenesis has not been established yet. To determine how it exactly contributes to the development of lymphomas will require an agreement on definition of MSI-L in lymphomas and a thorough search for target genes disabled by MSI-L. To facilitate such studies, we propose to use a marker panel sensitive enough to detect MSI-L. Our findings that different microsatellite repeats exhibit frequent MSI when DLBCLs are compared to MSI-H colorectal cancer, most striking being the minimal involvement of the mononucleotide poly(A) repeats prompted us to define a MSI screening panel for lymphomas. This panel consisting of dinucleotide repeats D3S1261, D3S1530, D5S346, D17S250, D18S474, and DCC is more sensitive in MSI detection than a previously established panel for colorectal carcinoma. Only using a sensitive common marker panel for detection of MSI in lymphomas, the controversial issues of MSI frequency and its role in the development of extranodal lymphoma can be resolved.
| Acknowledgements |
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| Footnotes |
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Supported by grants from the Interdisziplinäres Zentrum für Klinische Forschung (B3) and the Sonderforschungsbereich 172, B13 of the Deutsche Forschungsgemeinschaft.
Accepted for publication June 14, 2000.
| References |
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