Immunohistochemical detection of human telomerase reverse transcriptase (hTERT) and c-kit in serous ovarian carcinoma: A clinicopathologic study

Abstract

Objective.

This study investigated the expression of human telomerase reverse transcriptase (hTERT) and c-kit in a cohort of serous ovarian carcinomas by immunohistochemistry with regard to outcome and clinicopathologic variables.

Methods.

Formalin-fixed, paraffin-embedded archival tissue sections of 10 benign serous cystadenomas, 10 serous neoplasms of low malignant potential (LMP), and 41 serous ovarian carcinomas were immunostained with antibodies to hTERT and c-kit. Immunostaining was scored with regard to quantity and intensity of positively stained cells as negative or weak, moderate, and strong. Mitotic activity was determined as mitotic figures per 10 high power fields.

Results.

hTERT expression was negative in serous cystadenomas; 70% of LMP showed strong nuclear immunoreactivity. In serous carcinomas, nuclear and sometimes cytoplasmic immunoreactivity was observed; 14% of cases were scored as negative, 42% as moderate, and 44% as strong. hTERT immunoreactivity increased with grade (P < 0.0192) and mitotic activity (P = 0.0018), but not with FIGO stage (P = 0.2752), and was related with outcome (P = 0.0477). No c-kit immunoreactivity was observed in serous cystadenomas and LMP; 27% of serous carcinomas were negative, 46% showed moderate, and 27% strong immunostaining. c-kit immunoreactivity was positively correlated with grade (P = 0.0008) and FIGO stage (P = 0.0247), but not with mitotic activity (P = 0.1433) and outcome (P = 0.1145); however, c-kit expression was positively related with poor outcome in FIGO II and III stages (P = 0.0105).

Conclusions.

hTERT and c-kit are frequently up-regulated in serous ovarian carcinomas; c-kit immunoexpression may serve as a marker of aggressive behavior in high stage tumors.

Introduction

Telomeres are specific DNA–protein complexes, located at the ends of eukaryotic chromosomes and essential for chromosome stability [1]. In human chromosomes, they consist of thousands of copies of the nucleotide sequence 5′-TTAGGG-3′ ranging from 5–20 kb in length. With each cell division, telomeres are progressively shortened because of the inability of the DNA polymerase complex to replicate the 5′ end of the lagging strand [2]. This shortening of telomeres may function as a mitotic clock, by which normal cells count their divisions and eventually signal their senescence [3]. Telomerase is a ribonuclein complex that extends and maintains telomeres. By activation of this enzyme, cells are able to overcome replicative senescence and to divide indefinitely [4]. Telomerase is frequently activated in many kinds of cancers or cancer cell lines but not in most normal tissues [5]. There are two major subunits of the telomerase enzyme complex that contribute to in vitro enzymatic activity. hTERC is an intrinsic RNA component that contains a template region and binds to TTAGGG repeats in telomeres. hTERT is a catalytic subunit with reverse transcriptase activity. hTERC is constitutively present in normal and cancer cells. Expression of hTERT is almost exclusively limited to cancer cells [4]. Experimentally, telomerase activity can be induced by introduction of the hTERT gene into telomerase-negative normal cells [6]. Telomerase activity and hTERT mRNA expression are closely associated in human cancers [4]. It has been discussed whether telomerase activity may merely mirror the fraction of proliferating cells or whether aggressively growing cells independently up-regulate their telomerase levels in the course of malignant transformation [7].

c-kit is a proto-oncogene that encodes a transmembrane-tyrosine-kinase receptor (CD117) and is related to the platelet-derived growth-factor receptor family [8]. Together with its ligand stem-cell factor, c-kit plays a role in the embryonic development of migratory cell lineages, including hematopoietic stem cells, primordial germ cells, and melanocytes. c-kit is expressed in various solid tumors, including ovarian carcinomas [9], [10]. It is related with cell proliferation, as described by a mitogenic effect in breast carcinomas and small cell carcinomas of the lung [11], [12]. c-kit represents a potential drug target for anticancer therapy. Imatinib mesylate, a selective inhibitor of c-kit tyrosine kinase activity, has been reported to have therapeutic effects in gastrointestinal stromal tumors (GIST), which express a constitutively activated mutant c-kit [13].

Ovarian carcinomas are the most frequently lethal gynecologic tumors and the fourth leading cause of cancer-related deaths in women in the United States. Serous ovarian carcinomas are the most common malignant ovarian tumors. More than 70% of women with ovarian cancer present at an advanced stage when prognosis is generally poor [14]. Since abnormalities in proliferation and oncogene expression may contribute to neoplastic transformation and tumor growth, genes or their protein products, respectively, involved in the former may have predictive and even therapeutic significance. The purpose of this paper was to investigate the relevance of hTERT and c-kit expression in serous ovarian carcinomas, with special regard to clinicopathologic variables and its relation to proliferation as determined by mitotic activity.