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Short Communications |


From the Departments of Gynecology and
Obstetrics*
and Pathology,
University of Kiel, Kiel; and the Department of
Pathology,
University of Mainz,
Mainz, Germany
| Abstract |
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| Introduction |
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A series of genetic aberrations is required for tumor initiation and progression.4 The phenotypes of endometrial hyperplasia possess different malignant potential and can be defined as different histomorphological stages during the carcinogenesis of endometrioid adenocarcinomas of the endometrium. They are an appropriate model for studying the sequences of genetic alterations during tumorigenesis. So far, the genetic events involved in the multistep process in endometrial cancer are primarily unknown. Microsatellite instability (MI) is one of the genetic alterations observed in ~25% of sporadic endometrial cancers.5 Furthermore, mutations and inactivations of various oncogenes and tumor suppressor genes, eg, c-erb-2, c-myc, PTEN, and TP53 have been reported in endometrial cancer.6,7 However, with the exception of PTEN alterations, which occur in ~40% of sporadic endometrial carcinomas, the frequencies of these alterations were low, and it has been suggested that mutations in other genes may play key roles in endometrial cancer. DNA replication error-positive (RER+) phenotypes were also detected in a small subset of cases of atypical endometrial hyperplasia, which progress to RER+ endometrial carcinomas.8 Recently, Levine et al9 and Maxwell et al10 reported that somatic PTEN mutations occur in ~20 to 27% of cases of endometrial hyperplasia and might be an early event in endometrial carcinogenesis.
For the analysis of genetic imbalances within entire tumor genomes and the identification of gross genetic target regions, comparative genomic hybridization (CGH) is a powerful tool.11 With this method, information about gains and losses of DNA sequences throughout the genome is easily obtained without the need for mitotic cells and time-consuming tissue culture. It has also been successfully applied to microdissected archival paraffin-embedded tissue by several investigators.12
We applied CGH to 47 paraffin-embedded specimens of endometrial hyperplasia using the microdissection technique to establish a correlation between the microscopic phenotype and the genotype of defined stages of endometrial carcinogenesis. It was the aim of our study to gain insights into genetic changes in precursor lesions of endometrial adenocarcinomas to verify whether the multistep model of Fearon and Vogelstein13 is also applicable to adenocarcinomas of the endometrium.
| Materials and Methods |
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Forty-seven formalin-fixed, paraffin-embedded specimens of endometrial hyperplasia were obtained from the archives of the Departments of Pathology at the Universities of Kiel and Mainz. Ten serial 10-µm sections were cut from the tissue blocks. The first and the last section were stained with hematoxylin and eosin (H&E) for histological analysis and the endometrial lesions were classified using standard World Health Organization criteria.14 Areas of unequivocal endometrial hyperplasia were identified on the first section, which was stained with H&E. The corresponding areas were labeled on the subsequent serial sections for tissue microdissection and DNA extraction. Evaluation of the last section, which was again stained with H&E, assured that the correct area of the tissue section was indeed chosen for further analysis.
DNA Extraction
DNA was extracted as described by Speicher et al15 with minor modifications. Briefly, after dewaxing (3 x 10 minutes in xylol and 2 x 5 minutes in methanol), cells were dissected from labeled areas of the eight unstained serial sections and collected in sterile tubes. The tissues were incubated overnight in 450 µl of DNA extraction buffer (75 mmol/L NaCl, 2.5 mmol/L ethylenediaminetetraacetic acid, pH 8.0, 0.5% Tween 20, 0.1 µg/µl proteinase K) at 55°C. The mixture was boiled for 10 minutes to inactivate the proteinase K and chilled on ice. After incubation with RNase A (20 µg/ml) for one hour at 37°C, the DNA was extracted by phenol/chloroform and ethanol precipitation. There was enough DNA of sufficient quality so there was no need to perform degenerate oligonucleotide-priomed polymerase chain reaction amplification. Reference DNA was isolated from peripheral blood lymphocytes of normal males according to standard procedures.
Comparative Genomic Hybridization
CGH analysis was done as described by Arnold et al16 with minor modifications. Briefly, reference metaphase spreads were prepared following standard procedures from peripheral blood lymphocytes of a healthy male donor. Preparations were stored in 70% ethanol at 4°C until use. To minimize background signals and to increase accessibility of the DNA probes to the target chromosomes RNase A and pepsin treatment of the slides before denaturation was performed. Genomic DNA from a healthy male donor (reference DNA) was labeled in a standard nick translation assay with digoxigenin-11dUTP, and test DNA was labeled with biotin-16dUTP. Equal amounts of labeled reference and test DNA including 30 µg of Cot-1 DNA (Life Technologies, Inc., Rockville, MD) were hybridized to normal metaphase spreads. After incubation for 3 days at 37°C in a humidified chamber, the slides were washed two times in 1x phosphate buffered detergent (PBD; Appligene Oncor, Illkirch, France), once in 2x standard saline citrate (0.15 mol/L NaCl, 0.015 mol/L Na-citrate), pH 7.0, at 70°C without shaking and once in 1x PBD at room temperature for 5 minutes each. Slides were stained with 5 µg/ml of streptavidin-fluorescein isothiocyanate (FITC) and 1 µg/ml of antidigoxigenin rhodamine (both from Boehringer Mannheim, Mannheim, Germany) diluted in 1x PBD at 37°C for 45 minutes. Slides were then washed three times in 1x PBD at room temperature for 2 minutes and mounted in an antifade solution (Vectashield, Vector Laboratories, Burlingame, CA) containing 4',6-diamidino-2-phenylindole as a counterstain.
Digital Image Analysis
Images of the hybridized metaphases were obtained and evaluated with a digital imaging system (MetaSystems GmbH, Sandhausen, Germany), connected to a Zeiss fluorescence microscope (Jena, Germany). Images were captured using an integrated uncooled black and white charge-coupled device camera. Subsequently, the three color components, green (fluorescein isothiocyanate) for the tumor DNA, red (rhodamine) for the normal reference DNA, and blue (4',6-diamidino-2-phenylindole) for the DNA counterstain were digitized. The automatic exposure control of the imaging system made sure that the full dynamic range of the system was used for each color component. A real-color image was displayed on the monitor. A detailed description of the evaluation process is given by Arnold et al.16 The average number of copy alterations (ANCA) were calculated by dividing the total number of copy alterations per chromosome arm presented in a karyogram by the number of analyzed lesions.
| Results |
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| Discussion |
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To our knowledge, this is the first study indicating that endometrial hyperplasia reveals recurrent chromosomal changes. A total of 24 hyperplastic lesions (51%) had detectable DNA copy changes. The incidence of aberrant CGH profiles rose with increasing architectural complexity and cellular atypia. Only 2 out of 9 specimens of simple hyperplasia (22%) had detectable chromosomal imbalances, whereas specimens with complex hyperplasia revealed DNA copy number changes in 50% of cases (10 out of 20) and complex atypical lesions in 67% (12 out of 18). Our findings support the concept that accumulating genetic alterations increase genomic instability, which is phenotypically associated with an increasing risk of developing an invasive carcinoma. This idea is further underlined by the observation that the ANCA also increases with increasing cellular atypia. Simple hyperplasia showing a low risk for the development of endometrial cancer revealed an ANCA value of 0.3. Hyperplasia with architectural abnormalities, however, which is associated with a 3 to 17% risk of endometrial cancer,2,3 had an ANCA value of 1.85. The presence of atypical cells in complex atypical hyperplasia with a 29 to 45% risk of an invasive cancer,2,3 resulted in ANCA values of 2.55. ANCA values in endometrioid adenocarcinomas of the endometrium are even higher, rising to 4.6.17 Earlier CGH studies on precursor and invasive lesions of cervical18 and colon cancer19,20 are in line with our results, suggesting that the ANCA index correlates with disease progression and might be a valid indicator of genomic instability and malignant potential.
Among those complex lesions with DNA copy number changes
(n = 22), the most consistent alterations were
decreased fluorescence values at 1p, 20q, and 16p, which occurred in 23
to 32% (5 to 7 out of 22) of the samples. Increased DNA copy number
changes were less common. Mostly, they were observed on 4q in 23% (5
out of 22) of the samples with CGH imbalances. These consistent
aberrations were only observed in complex hyperplasia. Simple
hyperplasia showed only three random alterations in two cases. The
partial karyogram of common chromosomal gains and losses in endometrial
hyperplasia (Figure 2)
indicates the smallest regions of overlap
mapping the regions of interest to chromosome 1pterp36, 20q13.1q13.2,
and 16p13.1. These findings suggest that genes involved in the
initiation and progression of complex hyperplasia may be located within
these chromosomal regions. A possible candidate gene deleted at 1p36
might be the cell division cycle 2-like 1 protein (CDC2L1). This
well-conserved protein kinase p58 is supposed to be a negative
regulator of normal cell cycle progression.21
This cell
division control-related gene may also be implicated in the
pathogenesis of other tumors that have deletions in the region of 1p36,
such as ductal carcinoma of the breast, endocrine neoplasia, and
malignant melanoma.22
Recently, the epithelial membrane
protein 2 was mapped to 16p13.2 by Liehr et al.23
It has
been suggested that it is involved in cell proliferation and cell-cell
interactions.24
In addition, the deleted regions on 16p
and 20q contain two ubiquitin-conjugating enzymes (UBE2I and UBE2V1).
This family of proteins is involved in essential cellular processes
such as DNA repair, cell cycle control, and stress
responses.25,26
An additional gene of interest at 20q13 is
the cellular apoptosis susceptibility (CAS) gene, the human
homologue of the yeast chromosome segregation gene CSE1.27
The CAS protein, which has a dual function in mammalian cells,
may be involved in both apoptosis and cell proliferation processes.
Interestingly, CAS is associated with the mitotic spindle and has been
suggested to be part of the cell cycle checkpoint that assures correct
chromosome distribution to daughter cells. CAS depletion may lead to
aberrant chromosome segregation during mitosis in both yeast and
mammalian cells.27
Alterations of these candidate genes in
endometrial cells may provide growth advantages and increase genomic
instability.
The pattern of chromosomal changes in invasive endometrial cancer is totally different from that in endometrial hyperplasia. So far, CGH analysis of 47 endometrioid adenocarcinomas of the endometrium has been reported.18,28,29 Aberrant CGH profiles were found in 27 carcinomas, mainly comprising cases without MI. 1q and 8q overrepresentations were found to be the most consistent alterations, occurring in 20 out of 27 (74%) and 8 out of 27 (62%) of abnormal cases, respectively. In contrast to endometrial hyperplasia, decreased fluorescence intensities were less frequent in endometrial carcinomas. Moreover, 1q amplifications were less common in endometrial hyperplasia and were detected in only three of 47 hyperplastic lesions in the present study. 8q overrepresentations, the second most common alteration found in invasive endometrial cancer, were totally lacking in its precursor lesions.
The comparison of chromosomal gains and losses in simple, complex, and
complex atypical hyperplasia suggests a sequence of genetic events
(Figure 3
). Progression from simple to
complex hyperplasia involves loss of genetic material at 16p and 20q,
which did not occur in any of the specimens of simple hyperplasia, in
contrast to 23 to 27% of complex hyperplasia. Deletions in 1p seem to
be associated with the presence of atypical cells. They are not
detected in simple hyperplasia, are rare in complex lesions (2 out of
20; 10%), and occur in 42% (5 out of 12) of specimens of complex
atypical hyperplasia with aberrant CGH profiles. The combination of our
data with the results of CGH analysis of invasive endometrial cancer in
the literature18,28,29
suggests that the gain of
chromosome arms 1q and 8q, which is rare or lacking in the precursor
lesions, may define the transition from complex atypical hyperplasia to
invasive endometrioid adenocarcinoma of the endometrium.
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In summary, endometrial hyperplasia reveals consistent chromosomal abnormalities, including 16p, 20q, and 1p underrepresentations. Targeted molecular investigations of these regions may reveal genes that play an important role in the initiation and progression of endometrial tumors that are probably MI-negative. In the present study, aberrant CGH profiles tended to parallel cellular complexity and atypia and might be a useful marker for a phenotype with an increased malignant potential.
| Acknowledgements |
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| Footnotes |
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Supported by a grant from the Interdisciplinary Center of Clinical Research of the University of Kiel (IZKF).
The research was performed in the oncological laboratory of the Department of Gynecology and Obstetrics, University of Kiel Medical School, Michaelisstrasse 16, D-24105 Kiel, Germany.
Accepted for publication February 16, 2000.
| References |
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