Original articleCytogenetic analysis of carboplatin resistance in early-stage epithelial ovarian carcinoma
Introduction
Ovarian carcinoma is the leading cause of death among women with gynecological malignancies in western Europe and the fifth most common cause of cancer among women. The overall five-year survival rate is only around 40% [1], [2], [3]. The high mortality rate is largely due to drug resistance, as some primary tumors and most recurrent tumors develop chemotherapy resistance [4]. From a biologic standpoint, resistance to chemotherapy may be classified as intrinsic or acquired. Intrinsic resistance is present at tumor diagnosis, whereas tumors with acquired resistance often respond well initially to chemotherapy, but exhibit clinical chemotherapy resistance upon tumor recurrence [5]. Chemotherapy resistance may be clinically defined as the progression of disease during therapy, absence of regression during therapy, or recurrence within 6 months after completed treatment. Tumors are considered to be sensitive if they exhibit complete clinical response to therapy or if relapse occurs after clinical freedom from tumor is established and treatment has not been administered for more than 6 months [6].
After primary surgical cytoreduction, the standard treatment for patients with early-stage ovarian cancer is carboplatin, a cisplatin analogue with fewer side effects but similar response rates [7]. The mechanisms behind platinum resistance have been examined extensively but seem complex and are hitherto not fully understood. Several explanations have been suggested [4], [5], [8]: decreased net intracellular drug accumulation, activation of detoxification pathways, and enhanced DNA repair mechanisms. Cisplatin induces apoptosis [9], thus avoiding apoptosis is an important mechanism of resistance in tumors [10].
Hence, multiple factors seem to coexist in platinum-resistant ovarian cancer cells, and an increased understanding of these underlying mechanisms of drug resistance is of great importance for improved treatment and prognosis of women with ovarian cancer. One approach is comparing resistant and sensitive tumors by molecular genetic analysis. To perform such an analysis, we used comparative genomic hybridization (CGH), a cytogenetic technique for detecting and mapping DNA sequence copy number changes. Resistance to platinum agents has been studied previously using CGH, but the results have been without satisfying concordance, presumably due to the generally low number of items studied. Furthermore, the majority of the CGH studies have examined platinum resistance in vitro, using cultured tumor cells rather than human tumor tissue [11], [12], [13], [14], [15].
Epithelial ovarian cancer is heterogeneous, the most common histologic subtypes being serous, mucinous, endometrioid, and clear-cell carcinomas. Serous carcinoma is the most frequent, and mucinous and clear-cell carcinomas tend to be diagnosed in early stages [16]. All histologic subtypes are diagnosed, treated, and followed up according to the same principles. Prognosis and biologic behavior of the various subtypes differ in many respects, however, and it has been suggested that the different subtypes might have divergent molecular tumor progression pathways [17], [18], [19], [20].
The objective of our study was to explore cytogenetic alterations in ovarian carcinoma, comparing carboplatin-resistant and carboplatin-sensitive tumors. We used CGH as a tool for highlighting chromosomal regions of interest. We also wanted to investigate whether there are cytogenetic alterations that differ among the three histologic subtypes — serous, mucinous, and clear-cell carcinoma.
Section snippets
Materials and methods
The 63 epithelial ovarian tumors investigated were collected from patients diagnosed between 1993 and 1999 at Sahlgrenska University Hospital. The median age of the patients at initial diagnosis was 61 years (range 31–87 years) and median follow-up time was 8 years (range 5–11 years). Epithelial ovarian tumors were removed during surgery (hysterectomy with bilateral salpingooophorectomy and omentectomy) and stored in −80°C for analysis. None of the patients had been given preoperative
Results
Sixty-three epithelial ovarian carcinomas were analyzed by CGH. All patients were treated exclusively with carboplatin. Clinically, 10 of the patients were primary resistant (PR) to carboplatin, 46 were considered to be sensitive, and 7 patients were secondary resistant (SR; Table 1).
The majority of the tumors (57 of 63) displayed at least one cytogenetic alteration, and only six tumors did not contain any DNA copy number changes that could be detected by CGH. The average number of genetic
Discussion
Using CGH, we examined cytogenetic alterations in ovarian tumors and compared the carboplatin-resistant tumors with the carboplatin-sensitive tumors. Several chromosomal regions were more frequently altered in the resistant tumors than in the sensitive tumors, and some of these differences were statistically significant (Table 2; Fig. 1). Interestingly, the average number of genetic changes per tumor was virtually twice as high in the PR tumors as in the SR tumors. This much smaller number of
Acknowledgments
We thank Dr. Kerstin Wiklander and Jenny Andersson for help with statistical analysis, Elisabet Jansson for technical assistance, and Ghita Fallenius for cytologic evaluation. This work was supported by the King Gustav V Jubilee Clinic Cancer Research Foundation and the Health and Medical Care Executive Board of the Region Västra Götaland.
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