Prognostic significance of ras/p21 alterations in human ovarian cancer

Molecular pathogenesis and therapeutic targets in epithelial ovarian cancer

Ovarian cancer, the most aggressive gynecologic cancer, is the foremost cause of death from gynecologic malignancies in the developed world. Two primary reasons explain its aggressive behavior: most patients present with advanced disease at diagnosis, and die of recurrences from disease that has become resistant to conventional chemotherapies. In this paper on epithelial ovarian cancer (EOC), we will review molecular alterations associated with the few precursor lesions identified to date, followed by the more commonly recognized processes of de novo carcinogenesis, metastasis, and the development of chemoresistance. We will propose a unifying model of ovarian epithelial tumorigenesis that takes into account various hypotheses. We will also review novel approaches to overcome the major problem of chemoresistance in ovarian cancer. Finally, we will discuss advances and new challenges in the development of mouse model systems to investigate EOC precursor lesions, progression, metastasis, and chemoresistance.

Resistance to chemotherapy in ovarian carcinoma

Resistance to cytotoxic chemotherapy is the main cause of therapeutic failure and death in women suffering from ovarian carcinoma. The standard first-line chemotherapy of ovarian cancer consists of a combination of a taxane and a platinum-containing drug. Thus, the cellular and molecular mechanisms involved in resistance against these compounds are of vital importance in the context of chemotherapy of ovarian cancer. This review will discuss the current state of knowledge of drug resistance-associated factors and their impact on clinical chemotherapy response in ovarian carcinoma as well as different strategies for reversal of drug resistance.

Drug resistance reversal--are we getting closer?

Clinical drug resistance is a major barrier to overcome before chemotherapy can become curative for most patients presenting with metastatic cancer. Rational attempts to tackle clinical drug resistance need to be based on an understanding of the mechanisms involved; these are likely to be complex and multifactorial, and may be due to inadequate drug exposure or alterations in the cancer cell itself. This article reviews a number of strategies used to tackle drug resistance, focussing on work in our institution related to the treatment of ovarian cancer and resistance to platinum and taxane-based chemotherapy. Further progress towards drug resistance reversal will require a three-pronged approach, namely: the development of novel cytotoxics which exploit selectively expressed targets; modulation of resistance to conventional agents and, most importantly, a serious attempt to understand resistance mechanisms in tumour samples taken both pre- and post-chemotherapy.

Role of KRAS and BRAF gene mutations in mucinous ovarian carcinoma

OBJECTIVES

The goals of this study were to perform a comprehensive assessment of the prevalence of KRAS oncogene mutations in invasive epithelial ovarian carcinomas of various histologic subtypes, and for any subgroup(s) in which KRAS mutation was found to be common, to address the hypothesis that those tumors without KRAS mutation had sustained alternative activation of this signaling pathway through mutation of the BRAF oncogene.

METHODS

A total of 104 primary, invasive epithelial ovarian carcinomas from a 10-year period at this institution were selected for study based on histologic classification. The histologic cell type was serous in 21 cases, endometrioid in 30 cases, clear cell in 31 cases, and mucinous in 22 cases. Additional clinical and pathological information was abstracted from patient records, and pathology review was performed for all cases. Direct sequence analysis of exon 2 of the KRAS gene, containing codon 12, was performed using DNA isolated from all tumor specimens. Sequence analyses of exons 11 and 15 of the BRAF gene were performed for the 22 cases of mucinous ovarian carcinoma.

RESULTS

Activating KRAS mutations were more common in mucinous tumors (50%) than in all other histologic types combined (5%; P < 10(-7)). Mutation of KRAS was more common in stage I tumors than in advanced stage tumors (P = 0.0004). Of the 11 mucinous tumors with KRAS mutations, 6 were of Mullerian (endocervical) type and 5 were of gastrointestinal type. No mucinous tumor was found to harbor a BRAF mutation.

CONCLUSIONS

These data indicate that KRAS oncogene mutations exist in several histologic types of invasive epithelial ovarian carcinoma, especially stage I tumors, but are common only in tumors of mucinous histology. Mutations are equally prevalent in mucinous ovarian cancers of Müllerian and gastrointestinal types. In contrast to other solid tumor types frequently affected by KRAS mutation, mucinous ovarian cancers without a KRAS mutation have not sustained alternative activation of this signaling pathway through mutation of the BRAF oncogene.

K-ras alterations in Danish ovarian tumour patients. From the Danish "Malova" Ovarian Cancer study

OBJECTIVE

Activation of ras oncogenes has been demonstrated in ovarian tumours. All the reported studies are based on a relatively small number of patients and the results therefore remain a subject of debate.

METHODS

In this study, we analyzed the presence of mutations at codons 12 and 13 of the K-ras gene in 165 Danish women with ovarian tumours, including 138 invasive ovarian cancers and 27 borderline ovarian tumours, using a restriction fragment length polymorphism-polymerase chain reaction technique and evaluated whether such alterations were associated with the clinicopathological parameters of the patients and survival.

RESULTS

K-ras codon 12 gene mutations were found in 8.7% of ovarian cancer patients and in 14.8% of the borderline ovarian tumour patients. A K-ras codon 13 gene mutation was found in 1.5% of ovarian cancer patients. K-ras mutations were found with a significantly higher frequency in mucinous tumours compared to serous tumours (P = 0.011).

CONCLUSIONS

Mutation frequency was correlated with the histological type of tumour, but not with stage, radicality of operation, and age. Furthermore, no significant difference in survival was demonstrated between patients with or without K-ras mutation, neither in the univariate nor in the multivariate survival analyses.

p21 and p53 in ovarian carcinoma: their combined staining is more valuable than either alone

BACKGROUND

Mutations in p53 are the most common genetic alterations in human malignancies. Expression of its protein product has been linked to decreased survival rate in ovarian carcinoma. Less is known about the importance of p21 expression. The purpose of this study was to determine the value of the combination of p21 and p53 expression in patients with epithelial ovarian malignancies.

METHODS

One hundred three consecutive patients with epithelial ovarian carcinoma were studied using snap-frozen tissue specimens. Immunohistochemical staining utilizing the pAb1801 monoclonal antibody to p53 and NCL-WAF-1 monoclonal antibody to p21 was performed. Image analysis was used to determine whether nuclear staining for either antibody was present. In addition to p21 and p53, International Federation of Gynecology and Obstetrics stage, grade, histology, level of cytoreduction, and DNA index were analyzed as prognostic factors. Univariate and multivariate analyses was performed.

RESULTS

One hundred three patients were observed for more than 5 years. Immunohistochemical staining for p21 and p53 were significantly inversely related (P = 0.041). Among the patients whose tumors showed p21 staining but no p53 staining, there were no recurrences and all patients were alive at 5-year follow-up. The 5-year survival rate for these patients was significantly better than for the patients with other combinations of p21/p53 staining (P < 0.0001). The DNA index between these 2 groups was not significantly different (P = 0.057). Multivariate analysis shows the combination of p21 and p53 (P = 0.013) staining to be more valuable as a prognostic indicator than either p53 (P = 0.015) or p21 (P = 0.5) alone.

CONCLUSIONS

This study confirms the importance of the combination of p21 and p53 nuclear staining in patients with ovarian carcinoma. Cox regression analysis revealed combination of p21 positive and p53 negative to be a better independent indicator of prognosis and survival in patients with ovarian carcinoma than either p21 or p53 alone.

Are DNA mismatch repair deficiencies responsible for accumulation of genetic alterations in epithelial ovarian cancers?

To investigate the association of DNA mismatch repair deficiencies in the development and/or progression of epithelial ovarian cancers, the relationship between replication errors (RERs) and genetic alterations in three genes (p53, c-erbB2, K-ras) and loss of heterozygosity (LOH) on 6q27 was investigated in 70 patients with epithelial ovarian cancers. The presence of RERs was examined by PCR using five microsatellite markers. Mutations of p53 were analyzed by PCR-SSCP and sequencing. Amplification of c-erbB2 was analyzed by Southern blot hybridization. Point mutations of K-ras codon 12 were identified by PCR-PHFA, while 6q27LOH was examined by Southern blot hybridization. As a result, 18 of 70 patients with epithelial ovarian cancers (26%) were RER-positive and 52 patients (74%) were RER-negative. Tumors with two or three genetic alterations accounted for 28% and 33% of RER-positive tumors, respectively, and these were significantly more frequent than in the RER-negative tumors (17% and 6%, respectively)(P =.002). These results are consistent with mismatch repair deficiencies being involved in the development and/or progression of a proportion of epithelial ovarian cancers through accumulation of genetic alterations.

ras gene activation and infrequent mutation in papillary serous carcinoma of the peritoneum

OBJECTIVE

The ras genes are a well-studied family of proto-oncogenes whose involvement in many cancers has been delineated. However, K-ras mutations have not previously been examined in papillary serous carcinoma of the peritoneum (PSCP), a tumor which resembles serous epithelial ovarian carcinoma (SEOC) both in histology and epidemiology. In this study we examine the role of the K-ras oncogene in PSCP compared to SEOC.

METHODS

Using single-strand conformational polymorphism analysis and cycle sequencing protocols, we evaluated our collection of 51 cases of PSCP for K-ras mutations and compared these findings with the experience in SEOC. We then examined 5 cases of PSCP for activation of ras proteins and MAP kinase to evaluate the potential involvement of the ras pathway in PSCP tumorigenesis.

RESULTS

We found only one K-ras mutation in our 51 cases (2%) of PSCP compared to three mutations in 46 cases (6.5%) of high-grade, late-stage SEOC. This was not significantly different (P > 0.10). In the single PSCP case with a K-ras mutation, the mutation was found in only one of five tumor sites tested. All four mutations involved a single nucleotide alteration in codon 12 (GGT to GTT, Gly to Val). To evaluate the ras pathway in PSCP, we used the known activated ras binding domain on Raf-1 to perform an assay to test for activated ras. We identified ras activation in 4 of 5 PSCP cases tested and, to confirm that the activation was functional, we tested and found similar activation of MAP kinase, a downstream mediator for K-ras expression.

CONCLUSIONS

K-ras mutations occur at low rates in both PSCP and high-grade, late-stage SEOC, and therefore K-ras mutations are not involved in the development of these two diseases. Finding the mutation in only one of multiple tumor sites in the PSCP case supports growing evidence for a multifocal origin of PSCP. Our findings of ras activation in four of five cases of PSCP suggest that ras activation by mechanisms other than genetic mutation is important for PSCP tumorigenesis.