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Tumor Biology |
Gynaecology Cancer Research Unit, Department of Gynaecological Oncology, St. Bartholomews Hospital and The Royal London Hospital School of Medicine and Dentistry, London EC1A 7BE, United Kingdom [N. D. M., A. R., I. J. J.], and Department of Gynecology and Obstetrics [H. B. S., O. E. I.] and Department of Pathology, The Gade Institute [H. B. S., L. A. A.], Haukeland University Hospital, N-5021 Bergen, Norway
| ABSTRACT |
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| INTRODUCTION |
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In sporadic endometrial cancers, reported rates of MSI have varied between 9% and 43% (7, 8, 9, 10, 11, 12, 13, 14, 15, 16) . Comparison of results is difficult because the number of markers used, the type of repeated sequences analyzed, and the percentage of loci showing instability required to classify a tumor as RER+ differ between studies. Three studies have reported a correlation between high histological grade and the RER phenotype (7 , 12 , 14) ; however, other studies have not confirmed this finding (8 , 9 , 13) . Conflicting effects on prognosis have also been reported, but only small numbers of cases have been analyzed or relatively short periods of follow-up have been documented (7 , 9) . Insufficient evidence is available in the current literature to reach a conclusion regarding the impact of MSI as a prognostic factor in endometrial cancer.
This study was performed according to the criteria published by Bocker et al. (17) and Dietmaier et al. (18) for reliability and quality control in MSI analysis. We analyzed a large population-based series of endometrial carcinomas collected from Hordaland County in Norway with the intention of definitively establishing the rate of MSI in endometrial carcinoma. To avoid selection bias and to reduce problems with external validity, we examined this 10-year population-based series with complete follow-up. Because we also aimed to determine the prognostic effect of MSI, we calculated that with an overall 5-year survival rate of 70% for endometrial cancer, a sample size of 300 tumors would give us 90% power to detect a 20% change in mortality in the RER+ group if 30% of tumors exhibited the RER phenotype.
| MATERIALS AND METHODS |
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Follow-up.
The median follow-up period for the survivors was 9 years (range, 415
years). None of the patients was lost due to insufficient follow-up
data. Information about survival was obtained from the medical records
and correspondence with the primary physician. The data were
cross-checked with information from the Cancer Registry of Norway,
which is matched against the Register of Deaths of Statistics
Norway.
Tumor Specimens.
A total of 316 women were diagnosed with primary endometrial carcinoma
between 1981 and 1990 in Hordaland County. Twelve cases were excluded
due to a change in diagnosis at reclassification. In five cases, the
diagnosis was based on cytological examination only, with no
histological specimens available. From the remaining 299 patients,
paraffin blocks from the tumor specimen were available for further
investigation in 286 of the cases (96%). Both tumor tissue and
corresponding normal tissue were available from 259 of these patients
(91%). Normal tissue and tumor tissue were microdissected using
separate 5-µm, formalin-fixed, paraffin-embedded slides from each
case. Tissue was digested overnight at 56°C in a solution containing
10 µl of 1 M Tris, 0.4 µl of 0.5 M EDTA, 2
µl of 20 mg/ml proteinase K, and 87.6 µl of distilled water.
Proteinase K was inactivated by boiling for 8 min. Extracted DNA was
stored at 4°C.
MSI Analysis.
Tumor and corresponding normal DNA from each case were analyzed using a
panel of five markers for mononucleotide and dinucleotide repeat
sequences (CA repeats, D10S187, D18S55, and
D18S58; poly(A) repeats, BAT 26 and
BAT 40). The markers on chromosome 18 and the mononucleotide
markers were chosen because they had previously been analyzed in
studies of MSI in colorectal or endometrial carcinomas. BAT
26 lies within the hMSH2 gene on chromosome 2.
D10S187 was analyzed because it lies within the area of the
PTEN gene that has been shown to be mutated in a high
proportion of microsatellite unstable endometrial cancers.
PCR reactions were carried out in a 20-µl reaction containing 1.5 pmol of forward primer; 2 pmol of reverse primer; 0.5 pmol of 32P-labeled forward primer; 0.2 mM each of dATP, dGTP, dTTP, and dCTP; 0.25 unit of Taq Supreme enzyme (Helena Biosciences); and 1x PCR buffer. Thermal cycling was performed on an Omnigene thermal cycler (Hybaid) using the following conditions: (a) 97°C (5 min); (b) five cycles of 95°C (45 s), annealing temperature (60 s),and 68°C (60 s); (c) 30 cycles of 95°C (45 s) and annealing temperature (45 s); and (d) a final extension step at 68°C (10 min). The PCR enzyme was added after the initial denaturation step. PCR products were separated on a 5% denaturing polyacrylamide gel (Sequagel, National Diagnostics/Flowgen) and visualized using autoradiography. MSI at any locus was determined by the presence of new alleles in the tumor as compared with the normal tissue. Tumors were defined as RER+ if instability was demonstrated in at least two of the five markers.
Statistical Analysis.
The age at primary treatment, FIGO stage, histological type, and
grade were compared between RER+ and RER- cases using the
2 test. Differences were considered
significant for Ps < 0.05. These traditional
clinicopathological prognostic variables were also analyzed in relation
to MSI at individual marker loci and the number of unstable loci
overall. Univariate analyses of time to recurrence (recurrence-free
survival) or time to death due to endometrial cancer (cause-specific
death) were performed using the product-limit procedure (Kaplan-Meier
method), with the time of primary operation as the entry date. Only
those patients initially cured surgically were included for the
estimation of recurrence-free survival; the occurrence of recurrent
disease was defined as the event, whereas the patients not developing
recurrent disease were censored at the date of the latest clinical
examination. For the estimation of survival, death due to endometrial
carcinoma was defined as the event, whereas the survivors were censored
at the date of the latest clinical examination. Patients who died due
to other causes were censored at the time of death. The Mantel-Cox test
was used to compare the survival curves for different patient groups
defined by categories of each variable. Data were analyzed using the
SPSS software package.
Before commencing the analysis, it was calculated that with a reported overall 5-year survival for endometrial cancer of 70%, 300 tumors would need to be analyzed to achieve 90% power to detect a 20% change in survival due to the RER phenotype if 30% of the population were RER+.
| RESULTS |
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| DISCUSSION |
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Because this is the first reported population-based series of endometrial carcinomas to be analyzed using widely accepted criteria for determining RER status, the higher frequency of the RER phenotype demonstrated is likely to represent the true prevalence of the RER phenotype in endometrial carcinomas.
The lack of prognostic influence of RER status in our study conflicts with the findings of one smaller previous study (7) in which patients with RER+ tumors had a higher mortality than those with RER- tumors. One other study has reported a better prognosis for RER+ endometrial cancers, but the survival difference was not statistically significant (9) . The studies are difficult to compare, however, due to differences in patient selection, follow-up, and statistical methods applied. The absence of prognostic influence of RER status in our study of endometrial carcinomas also contrasts with evidence that RER+ colorectal carcinomas have a good prognosis (1) . This may be explained by the fact that somatic mutations that commonly accumulate in RER+ colorectal cancers, such as transforming growth factor ßRII, insulin-like growth factor IIR, and BAX mutations, do not occur at such high frequencies in RER+ endometrial cancers (5 , 16) ; therefore, the inhibitory and apoptotic pathways may behave differently in these cancers. Alternatively, these genes may not play such a large role in the tumorigenesis of endometrial cancers as they do in colorectal cancers, so that mutations within them have less effect on tumor behavior and patient outcome. It is of interest that parallel studies of hMLH1 and PTEN methylation performed in our population of endometrial cancer cases suggest that although these events are positively associated with the RER phenotype, neither event is significantly associated with prognosis. The RER phenotype, hMLH1 methylation, and abnormal hMLH1 expression were more common in diploid tumors and in those with normal p53 expression, whereas PTEN methylation correlated with metastatic disease.4,5These findings are consistent with the data presented here, which suggest that although MSI does not determine prognosis in endometrial cancer, distinct pathways of carcinogenesis exist between RER+ and RER- endometrial cancers.
In conclusion, the RER phenotype is common in endometrial carcinomas but is not significantly associated with either traditional clinicopathological variables or prognosis in our large population-based series with long and complete follow-up.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by The Norwegian Cancer Society
(D96032/D94070) and St. Bartholomews Hospital Cancer Research
Committee. N. D. M. and H. B. S. contributed equally to this work.
This research has been approved by the Norwegian Data
Inspec-torate. ![]()
2 To whom requests for reprints should be
addressed, at Gynaecological Cancer Research Unit, St. Bartholomews
Hospital, West Smithfield, London EC1A 7BE, United Kingdom. Phone:
0171-601-8261; Fax: 0171-601-7652; E-mail: n.d.macdonald{at}mds ![]()
3 The abbreviations used are: MSI, microsatellite
instability; RER, replication error repair; FIGO, Federation of
Gynecology and Obstetrics; HNPCC, hereditary nonpolyposis colorectal
cancer. ![]()
4 H. Salvesen. Methylation of hMLH1
indicates a distinct pathway in endometrial carcinogenesis,
submitted for publication. ![]()
5 H. Salvesen. PTEN methylation is
associated with advanced stage and MSI in endometrial cancer, submitted
for publication. ![]()
Received 8/23/99. Accepted 1/19/00.
| REFERENCES |
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