In recent years, several studies have investigated the value of WT1 immunohistochemical staining in various aspects of gynecological pathology. Three such studies are published in this issue of the Journal (1–3). The WT1 gene is a tumor suppressor gene located on the short arm of chromosome 11 at p13. It was first reported as a candidate for the main gene implicated in Wilms’ tumor development (4,5). In normal human tissues, WT1 immunohistochemical expression is largely restricted to the kidney, ovary, testis, spleen, and mesothelium (6,7). WT1 expression also has been documented in a number of human malignancies, including malignant mesothelioma, leukemia, intraabdominal desmoplastic small round cell tumor, and breast carcinoma (8–16).
One of the first studies to investigate WT1 staining in the female genital reproductive system looked at epithelial ovarian tumors (17). Nuclear WT1 expression was present in normal ovarian surface epithelium, the epithelium of ovarian inclusion cysts, and tubal epithelium but not endometrial or cervical epithelium (17). In epithelial ovarian carcinomas, serous neoplasms exhibited nuclear WT1 expression at a significantly higher level than other histologic subtypes of ovarian carcinoma. This finding was reinforced in another study, which confirmed that in ovarian epithelial carcinomas, WT1 expression is much more common in serous than other morphologic subtypes including endometrioid carcinoma (18). Because ovarian serous carcinoma (OSC), especially when poorly differentiated, may be confused with an endometrioid carcinoma, widespread WT1 staining may assist in confirming a serous phenotype. It is possible that the different WT1 staining patterns between OSCs and other ovarian carcinomas suggest a different underlying pathogenesis. OSCs in most cases probably arise directly from the ovarian surface epithelium or from the epithelium of cortical inclusion cysts. WT1 immunoreactivity in OSCs is expected, because ovarian surface epithelium is a specialized form of mesothelium that is WT1-positive. Interestingly, a recent study has shown diffuse nuclear WT1 staining in most ovarian transitional cell carcinomas (19), perhaps suggesting that they are pathogenetically related to OSC.
Much of the recent interest in WT1 staining in gynecological pathology emanates from a study by Goldstein and Uzieblo (20). In that study, nuclear WT1 staining was present in 97% of OSCs, but there was no staining of uterine serous carcinomas (USCs). As well as suggesting a different underlying pathogenesis, this study suggested that WT1 staining is of value in the distinction between OSC and USC. On occasions this may be a problem, especially when there is disseminated disease involving more than one organ. USC commonly gives rise to extensive extrauterine disease and conversely OSC often spreads to involve the uterus. Additionally, with a small USC and coexistent ovarian, omental, or peritoneal disease, the question arises as to whether these represent synchronous neoplasms or whether one tumor is meta-static from the other. It may be important to distinguish between disseminated USC and OSC because, although similar chemotherapy is used for both, the former is thought to exhibit a poorer response to platinum-based compounds (21). Two other studies have reinforced the findings of Goldstein and Uzieblo, showing that WT1 is much more likely to be expressed in OSCs than USCs (18,22), although in one of these studies there was WT1 reactivity in 20% of USCs, albeit usually focal and much more restricted in nature than the diffuse positivity commonly found in OSCs (18). In these studies, there was positive WT1 staining of primary peritoneal and tubal serous carcinomas with a similar pattern to that found in OSCs.
With many aspects of immunohistochemistry, as additional studies are performed, initial encouraging results regarding the specificity of an antibody often are not borne out and confusion arises. However, although two of the studies in the present issue seem to reach different conclusions, a broad picture is emerging regarding WT1 staining in epithelial malignancies of the female genital tract. One of the studies concluded that WT1 does not seem to be a useful antibody for distinguishing between a USC and a uterine endometrioid carcinoma because most tumors are negative (1). In this study, two of 31 (6.5%) USCs were positive with WT1 and all endometrioid carcinomas were negative. Therefore, the results of this study support the premise that most USCs are WT1 negative. The second study examined WT1 expression in endometrial, ovarian, tubal, and peritoneal serous carcinomas (2). As expected, WT1 expression was present in most ovarian, tubal, and peritoneal serous carcinomas. In addition, immunoreactivity was seen in 10 of 16 USCs, casting doubt on the findings of the studies described previously and suggesting that the utility of WT1 in distinguishing between serous carcinomas of different origin may not be as useful as previously suggested. However, cases of USC were less positive than serous carcinomas arising elsewhere and staining in USC was usually focal and of weak to moderate intensity, in contrast to the diffuse strong nuclear positivity present in most ovarian, tubal, and peritoneal serous carcinomas. There was a statistically significant difference between USCs and serous carcinomas arising elsewhere both in the frequency of WT1-positive cases and the level of WT1 expression.
It is theoretically possible that some of the differing results are a consequence of the use of different antibodies, different methods of immunostaining, and/or different methods of scoring of immunohistochemical staining. However, most studies have used the Dako antibody 6F-H2 and the staining methods have been broadly similar. A small number of studies have used a Santa Cruz antibody.
As discussed, a broad consensus is emerging, although it is stressed that the results of WT1 staining should not be interpreted in isolation but should always be integrated with the clinical, radiologic, and pathologic features. It can be stated that in an ovarian carcinoma in which there is morphologic confusion between a serous and an endometrioid neoplasm, WT1 may be of value, because this is diffusely positive in almost all OSCs but negative in most endometrioid carcinomas. Ovarian transitional cell carcinomas also are often positive (19). With a disseminated serous carcinoma, diffuse strong nuclear WT1 positivity favors an ovarian, tubal, or peritoneal primary. Some USCs will be positive, usually focally so, but in cases of disseminated serous carcinoma diffuse positive WT1 staining is against a uterine primary, although it cannot definitively be excluded. It may be that as experience with this antibody increases, a threshold for WT1 positivity can be established with regards to intensity and distribution of staining whereby positively staining serous carcinomas of the endometrium and ovary can be distinguished. Negative WT1 staining also is of value, because in a serous carcinoma this should cast doubt on the likelihood of an ovarian primary, although a small number of OSCs will be negative. WT1 is of little or no value in distinguishing between a USC and a uterine endometrioid adenocarcinoma (both are usually negative), although positive staining is a weak pointer in favor of a serous neoplasm, because in the studies to date all uterine endometrioid adenocarcinomas have been negative. Of course, other antibodies such as estrogen receptor and p53 may be of value in the distinction between a USC and an endometrioid carcinoma. Primary peritoneal and tubal serous carcinomas are WT1-positive, and this antibody is of no value in distinguishing these from an OSC. Another study showed WT1 to be of value in distinguishing disseminated OSC involving the omentum or peritoneum from pancreatic carcinoma. In this study, all pancreatic carcinomas were negative with WT1 (23).
Similar WT1 staining patterns may indicate a common pathogenesis of ovarian, tubal, and peritoneal serous carcinomas or may simply be a reflection of the origin of these neoplasms from the ovarian surface epithelium, tubal epithelium, and peritoneal mesothelium, respectively. All of these cell types are, of course, normally immunoreactive with WT1.
Finally, the third study in this issue found positive WT1 staining of almost all adenomatoid tumors of the female and male genital tracts (3). Because adenomatoid tumors are of mesothelial derivation, this is to be expected and illustrates another potential usage of WT1 in a diagnostic setting. It is probable that further studies will unearth additional applications of WT1 staining in gynecological pathology. For example, staining of uterine smooth muscle and endometrial stroma with WT1 has been noted (3,18), and we have demonstrated WT1 staining in uterine smooth muscle and endometrial stromal neoplasms (submitted for publication). Additionally, WT1 positivity has recently been demonstrated in ovarian sex cord-stromal neoplasms (24), expanding the range of tumors that are known to be positive with this antibody.
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