The utility of calretinin, inhibin, and WT1 immunohistochemical staining in the differential diagnosis of ovarian tumors

Summary

Calretinin has been proposed as a novel marker of ovarian sex cord-stromal tumors (SCST); this study aims to determine whether calretinin can complement or supplant the established utility of inhibin in the differential diagnosis of SCST. WT1 has been shown to be expressed in ovarian serous, but not mucinous neoplasms; its expression in a variety of ovarian tumors is also examined. Formalin-fixed, paraffin-embedded archival tissues from 111 primary ovarian tumors were analyzed with commercially available antibodies using semi-automated immunohistochemistry. Results were graded on a 4-tiered scale with staining of more than 0 but less than 5% of cells considered focal. Of 27 SCST, 56% were calretinin and 56% inhibin positive overall; 90% of granulosa cell tumors, 57% of Sertoli-Leydig cell tumors, 33% of thecomas, and 14% of fibromas were calretinin positive. Inhibin was expressed in 60% of granulosa cell tumors, 71% of Sertoli-Leydig cell tumors, 43% of fibromas, and 33% of thecomas. Of 35 surface epithelial tumors (SET), 8% of serous papillary tumors were calretinin positive, whereas 8% of serous papillary tumors and 13% of poorly differentiated carcinomas expressed inhibin. WT1 was expressed in 29% of all endometrioid carcinomas, 10% of borderline mucinous tumors, and no mucinous carcinomas; however, most of the other SETs were positive (77% serous papillary and 88% poorly differentiated carcinomas). Among the SCST, WT1 stained only granulosa cell tumors (75%), though often weakly or variably. Calretinin has only slightly greater sensitivity (76% versus 65%) and equal specificity to inhibin (92%) in the differential staining of granulosa or Sertoli-Leydig cell tumors, that is, nonstromal SCST. Hence, calretinin cannot replace but could complement inhibin as part of an immunohistochemical panel used for diagnostically challenging SCST. Although WT1 should be reliably positive in non-mucinous SET, staining of granulosa cell tumors and lack of expression in a sizable subset of endometrioid carcinomas may confound interpretation.

Introduction

Inhibin has been recommended as the best marker to help distinguish between sex cord-stromal tumors (SCST) of the ovary and their mimics [1], [2], [3], [4], [5], [6], [7], [8]. The gene for inhibin α is a member of the transforming growth factor β superfamily of genes. Inhibin is a peptide hormone secreted by ovarian granulosa cells and luteinized cells, which inhibits follicle-stimulating hormone release. Antibodies to the inhibin α subunit have proved especially useful in separating epithelial tumors with Sertoliform features from SCST. However, it has been shown that inhibin is neither completely specific nor sensitive for SCST, especially for fibrothecomatous tumors.

Calretinin, one of the E-F hand proteins, is a calcium-binding protein primarily expressed by selected neurons in the peripheral and central nervous systems. Its expression in mesotheliomas has made it the primary confirmatory antibody in standard mesothelioma immunohistochemical panels [9], [10], [11]. In the process of examining tumors that might metastasize to the pleura, calretinin positivity was in turn noted in granulosa cell tumors [10], leading other investigators to examine its utility to mark SCST [3], [12], [13], [14], [15]. These studies have reached variable conclusions with respect to the reliability of calretinin expression within various subtypes of SCST. For this reason, the current study was designed to determine whether calretinin might complement or supplant the use of inhibin in the differential diagnosis of SCST.

The Wilms' tumor gene, WT1, a tumor suppressor gene found on chromosome 11p13, is important in the development of the urogenital system; however, its expression has also been found in various human tumors, including mesotheliomas and surface epithelial nonmucinous ovarian tumors [16], [17], [18], [19]. Because surface epithelial tumors (SET) that can be confused with SCST were being examined in this study, WT1 was included to investigate whether its expression in non-mucinous SET could be confirmed.