The genetic analysis of ovarian cancer

Biological characterization of human epithelial ovarian carcinoma cells in primary culture: the insulin-like growth factor system

Little is known about the factors regulating epithelial ovarian cancer cell growth. This is due, in large part, to the difficulty in obtaining and culturing human ovarian cells for relevant in vitro studies. We recently developed a method for culturing epithelial carcinoma cells derived from fresh, untreated epithelial ovarian cancer specimens. The cell populations are free of fibroblasts and reflect the primary tumor as determined by chromosomal analysis. In this study we report on the cells' growth in serum-free medium and their secretion of CA-125, a glycoprotein marker for ovarian cancer. Furthermore we characterize the insulin-like growth factor (IGF) system in these primary ovarian carcinoma cell cultures. The cells secrete IGF peptides and IGF-binding proteins, possess specific type I IGF receptors, and respond to exogenous IGFs. The culture system reported here provides the basis for further study and manipulation of the IGF system as well as other regulators of epithelial ovarian cancer. Greater understanding of the cellular and molecular mediators of primary human ovarian cancer cell growth may translate into relevant clinical interventions.

Expression of E-cadherin, alpha-catenin and beta-catenin in normal ovarian surface epithelium and epithelial ovarian cancers

AIMS

To study the expression of the epithelial adhesion molecule E-cadherin and its associated proteins alpha and beta catenin in paraffin sections of normal ovaries, benign cystadenomas and ovarian carcinomas, and in immortalized normal ovarian surface epithelial cells and ovarian carcinoma cells in culture.

METHODS AND RESULTS

Immunocytochemistry was used to study expression of the proteins in paraffin sections and western blotting was used to determine levels of expression of the proteins in cell extracts. E-cadherin expression was found to be absent in ovarian surface epithelial cells in culture and infrequently expressed in normal ovarian surface epithelial cells in vivo, although apical punctate staining was occasionally seen. Seven of nine benign cystadenomas and 29/34 epithelial ovarian carcinomas showed some expression of E-cadherin, but expression was absent in poorly differentiated tumours. Expression of alpha and beta catenin was consistently detected on the lateral membranes of normal ovarian epithelium and benign cystadenomas. alpha and beta catenin expression was lost in 18% and 21% of ovarian carcinomas, respectively: other ovarian carcinomas expressed these proteins at a reduced level. A small number of these tumours showed a diffuse cytoplasmic rather than membranous staining. Reduced staining for alpha and beta catenin appeared to correlate with a more spindly, less adhesive morphology and increased invasive potential in matrigel.

CONCLUSIONS

The results suggest that E-cadherin expression is generally induced in well differentiated ovarian cancers. In contrast, alpha and beta catenins are consistently expressed in the normal ovarian surface epithelium and benign tumours, but are sometimes reduced or absent in ovarian carcinomas. It is likely that the catenins associate with membrane proteins other than E-cadherin in ovarian epithelium, and they may possibly function as tumour suppressors in this epithelium.

Phenotypic analysis of ovarian carcinoma: polypeptide expression in benign, borderline and malignant tumors

Studies of multiple markers in tumors are required for adequate biological characterization. We have characterized the expression of multiple proteins in human ovarian tumors using the technique of 2-dimensional gel electrophoresis (2-DE/PDQUEST). Tumor cells were prepared from the tissue of 22 ovarian tumors. Large variations were observed between tumors in the expression of various polypeptides, indicating heterogeneity in gene expression. An increase in the spot density of 2 cell-cycle-related proteins, PCNA and OP18/stathmin, was observed in carcinomas. Borderline tumors expressed low levels of these proteins. Significant increases in the levels of nm23, GST-pi, elongation factor 2 and triose phosphate isomerase were recorded in ovarian carcinomas. Furthermore, decreases in the levels of tropomyosin-2 and lamin C were observed in malignant as compared with benign tumors. The pattern of expression of 9 protein markers was examined in individual tumors. All malignant tumors showed simultaneous alterations in the expression of 5 or more of these proteins, whereas no benign tumor showed alterations in the expression of more than 3 polypeptides. Borderline tumors showed alterations in 0 to 6 markers. We conclude that the simultaneous analysis of multiple polypeptides, which can be achieved by 2-DE, is useful for characterization of gene expression and diagnostic studies in ovarian tumors.

Loss of heterozygosity in human ovarian cancer on chromosome 19q

Abnormalities in the function of oncogenes and tumor suppressor genes have been associated with many human malignancies. The recognition of sites of loss of heterozygosity (LOH) has led to the identification of such genes. We previously reported that abnormalities of mRNA expression of ERCC1 and ERCC2 may be characteristic of epithelial ovarian carcinoma and brain tumors. This led to an investigation of chromosome 19q13.2-q13.4 which contains these DNA repair genes. A 7-Mb region was analyzed using six microsatellite repeats. Loss of heterozygosity has been identified in 53% (8/15) of cases at marker D19S246 which lies in a 2-Mb segment between HRC and KLK1. The genetic material both centromeric and telomeric to the region of loss was conserved. This area is telomeric to three DNA repair genes where LIG1 is 1-Mb centromeric and ERCC1 and ERCC2 are 3.5- and 4.0-Mb centromeric, respectively. These findings represent the first report of a biologically significant rate of LOH on chromosome 19q13.2-q13.4 in human ovarian carcinoma.

Rare mutations and no hypermethylation at the CDKN2A locus in epithelial ovarian tumours

The tumour-suppressor gene CDKN2A (p16, MTS1, CDK4I) encodes a cell cycle-regulatory protein and is located on chromosome 9p21, a region deleted in a wide variety of human cancers. To determine the role of the CDKN2A gene in the development of ovarian adenocarcinomas, we examined a large series of benign, low malignant potential (LMP) and invasive ovarian neoplasms for evidence of loss of heterozygosity (LOH), homozygous deletions, point mutations and hypermethylation of the CDKN2A locus. We have previously reported LOH on 9p in 45% of malignant ovarian neoplasms and a smaller percentage of benign and LMP tumours. In the current study, 6 malignant tumours were identified with partial deletions of 9p21. In 5 of these, the CDKN2A gene lays within the minimal deleted region. Homozygous deletions of CDKN2A were observed in only 2/88 invasive ovarian tumours and in 5/11 ovarian cancer cell lines. Of 15 primary ovarian tumours analyzed, one nonsense mutation was identified in a mucinous LMP tumour. No evidence of hypermethylation of the CDKN2A gene was found in 50 primary ovarian adenocarcinomas nor in 3 ovarian cancer cell lines. In conclusion, homozygous deletions, mutations and the de novo methylation of 5' CpG island are not frequent modes of inactivation of the CDKN2A gene in ovarian cancer. The target of 9p LOH in ovarian adenocarcinomas is therefore unknown.

Analysis of loss of heterozygosity and KRAS2 mutations in ovarian neoplasms: clinicopathological correlations

The molecular events that give rise to ovarian epithelial neoplasms are not well understood. In particular, it is not known whether adenocarcinomas arise from benign or low malignant potential (LMP) precursors. We have examined a large series of benign (25) and LMP (31) ovarian tumors for loss of heterozygosity (LOH) at multiple loci on 17 chromosomes. LOH was observed in benign tumors on chromosomes 6 (14%) and 9 (5%) and on the X chromosome (33%) only. LOH on these chromosomes was also detected in a small number of LMP neoplasms, suggesting that these may derive sometimes from benign precursors. In addition, we examined LOH in 93 adenocarcinomas. Analysis of associations between LOH events showed that LOH on chromosomes 5 and 17 (P = 0.0002) and on chromosomes 17 and 18 (P = 0.00007) were associated significantly with each other, which suggests that these may represent cooperative, progressive events. No novel significant associations were identified between LOH events and stage, grade, or histology, which would indicate the existence of genetic heterogeneity in ovarian neoplasms. KRAS2 mutations were detected more often in LMP neoplasms than in malignant tumors (P = 0.004) and were detected more often in Stage I/II malignant tumors than in Stage III/IV malignant tumors (P = 0.033), suggesting that LMP tumors with KRAS2 mutations are unlikely to progress to frank malignancy. Univariate (but not multivariate) survival analysis showed that LOH of chromosomes 11 (P = 0.039) and 17 (P = 0.04) was associated with a significantly worse prognosis. Replication of these novel findings is necessary, and the identification, isolation, and characterization of the critical genes affected by LOH will determine their importance in the pathogenesis of ovarian malignancies.

Sporadic CDKN2 (MTS1/p16ink4) gene alterations in human ovarian tumours

The cell cycle regulatory proteins p16 and p21 cause cell cycle arrest at the G1 checkpoint by inhibiting activity of cyclin D-CDK4 complexes. The TP53 gene, regulating the p21 protein, is mutated at high frequency in ovarian cancer. The CDKN2 gene, encoding the p16 protein, has been mapped to chromosome 9p21 and encompasses three exons. To establish the frequency of CDKN2 gene abnormalities in ovarian tumour specimens, we have studied this gene in five ovarian cancer cell lines and in 32 primary and five metastatic ovarian adenocarcinomas. Using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) and sequencing techniques both exon 1 and 2 of the CDKN2 gene, encompassing 97% of the coding sequence, were analysed. In addition, the TP53 gene was studied for the presence of mutations. The cell line HOC-7 showed a 16 bp deletion in exon 2 of the CDKN2 gene, resulting in a stop codon, whereas in cell line SK-OV-3 this gene was found to be homozygously deleted. Nine primary tumour specimens showed a migration shift on SSCP. Sequencing revealed a common polymorphism (Ala148Thr) in seven of these ovarian tumour specimens. The two other tumour samples were found to contain silent mutations, one at codon 23 (GGT-->GGA) and the other at codon 67 (GGC-->GGT). Mutations in the TP53 gene were observed in 46% of the ovarian tumour specimens. We conclude that CDKN2 gene alterations are rare events in human ovarian cancer. The low prevalence of these alterations do not allow for analysis of an association of this gene with prognosis.

Allele loss on chromosome arm 6q and fine mapping of the region at 6q27 in epithelial ovarian cancer

Genetic changes have been shown to be important in determining the multistep progression of cancer. Allele loss studies and karyotypic analysis of epithelial ovarian tumours have indicated the presence of putative tumour suppressor genes on chromosomes 6, 11, 13, 17, 18, 22, and X. We have focused on chromosome arm 6q to identify the minimal region that may contain a putative tumour suppressor gene. Nineteen polymorphic microsatellite markers from 6q and one centromeric marker (D6S294) have been used to detect allele loss in 68 ovarian tumours (six benign, six borderline, and 56 with malignant histology). Allele loss was evaluated by separation of fluorescence labelled polymerase chain reaction-amplified products. Forty-six of fifty-six (82%) malignant tumours showed allele loss on 6q, whereas only four of 56 had lost all the markers tested. Forty-one of fifty-six (73%) malignant tumours showed allele loss at 6q26-27. The minimal region of allele loss was between markers D6S264 and D6S297 (3 cM), with maximal allele loss of 62% at D6S193 and 52% at D6S297. Three tumours showed loss of D6S193 only, while retaining flanking informative markers. Allele loss around 6q26-27 was observed in all histological types of epithelial ovarian cancer and did not correlate with any clinical factors. In addition, there was allele loss at ESR (56%) and D6S286 (47%), though a minimal region was not defined. Allele loss at 6q12-25 correlated significantly with endometrioid and mucinous ovarian malignant tumours (P = 0.01). The physical mapping of the region between D6S297 and D6S264 will allow the eventual identification of the putative tumour suppressor gene.

Identification of exons in a region of human chromosome 6q known to contain tumour suppressor genes

An important objective in human genomic mapping is the isolation of expressed sequences from defined chromosomal regions. The 6q27 region of the long arm of chromosome 6 is an important target in this endeavour because a number of tumour suppressor genes have been mapped to this region. We have used exon trapping and amplification in conjunction with a chromosome specific genomic library to generate ESTs in 6q27. Nine novel expressed sequences have been identified and these provide additional markers and potential candidate genes for the tumour suppressor genes known to be in 6q27.