PAX8 Expression in Uterine Adenocarcinomas and Mesonephric Proliferations :

Immunohistochemical staining for p53 is used as a surrogate for mutational analysis in the diagnostic workup of carcinomas of multiple sites including ovarian cancers. Strong and diffuse immunoexpression of p53 is generally interpreted as likely indicating a TP53 gene mutation. The immunoprofile that correlates with wild-type TP53, however, is not as clear. In particular, the significance of completely negative immunostaining is controversial. The aim of this study was to clarify the relationship of the immunohistochemical expression of p53 with the mutational status of the TP53 gene in ovarian cancer. A total of 57 ovarian carcinomas (43 high-grade serous ovarian/peritoneal carcinomas, 2 malignant mesodermal mixed tumors (carcinosarcomas), 2 low-grade serous carcinomas, 4 clear cell carcinomas, 1 well-differentiated endometrioid carcinoma, and 5 carcinomas with mixed epithelial differentiation) were analyzed for TP53 mutations by nucleotide sequencing (exons 4-9), and subjected to immunohistochemical analysis of p53 expression. Thirty six tumors contained functional mutations and 13 had wild type TP53. Five tumors were found to harbor known TP53 polymorphism and changes in the intron region were detected in three. Tumors with wild-type TP53 displayed a wide range of immunolabeling patterns, with the most common pattern showing ≤10% of positive cells in 6 cases (46%). Mutant TP53 was associated with 60-100% positive cells in 23 cases (64% of cases). This pattern of staining was also seen in three cases with wild-type TP53. Tumors that were completely negative (0% cells staining) had a mutation of TP53 in 65% of cases and wild-type TP53 in 11%. Combining two immunohistochemical labeling patterns associated with TP53 mutations (0% and 60-100% positive cells), correctly identified a mutation in 94% of cases (P<0.001). Immunohistochemical analysis can be used as a robust method for inferring the presence of a TP53 mutation in ovarian carcinomas. In addition to a strong and diffuse pattern of p53 expression (in greater than 60% of cells), complete absence of p53 immunoexpression is commonly associated with a TP53 mutation. Accordingly, this latter pattern, unlike low-level expression (10-50% cells), should not be construed as indicative of wild-type TP53.

PAX8 is a paired-box gene important in embryogenesis of the thyroid, Müllerian, and renal/upper urinary tracts, and expression of PAX8 has been previously described in carcinomas from each of these sites. However, a large study including a wide variety of epithelial neoplasms from multiple organ sites other than the thyroid, kidney, or Müllerian system has not been performed. The goal of this study was to evaluate the utility of PAX8 immunostaining based on the evaluation of a wide range of epithelial tumors. PAX8 immunohistochemistry was performed on 1357 tumors (486 tumors in whole-tissue sections and 871 tumors in tissue microarrays, predominantly epithelial) from multiple organs. Only nuclear staining was scored as positive, and tumors were evaluated for the extent and intensity of staining. Western blot analysis with PAX8 was also performed on multiple tumor cell lines. Nuclear PAX8 staining was present in 91% (60 of 66) of thyroid tumors, 90% (158 of 176) of renal cell carcinomas (RCCs), 81% (13 of 16) of renal oncocytomas, 99% (164 of 165) of high-grade ovarian serous carcinomas, 71% (32 of 49) of nonserous ovarian epithelial neoplasms, 91% (10 of 11) of cervical epithelial lesions, and 98% (152 of 155) of endometrial adenocarcinomas. Of the remaining 719 evaluated tumors, only 30 cases (4%), including 12 thymic neoplasms, 3 bladder urothelial carcinomas, 4 lung squamous cell carcinomas, 2 esophageal adenocarcinomas, 1 pancreatic adenocarcinoma, 2 cholangiocarcinomas, 1 ovarian Sertoli-Leydig cell tumor, 1 ovarian sex cord stromal tumor, 3 testicular mixed germ cell tumors, and 1 acinic cell carcinoma, showed at least weak or focal PAX8 positivity. The unexpected finding was diffuse, moderate staining of PAX8 in a subset of thymomas and thymic carcinomas. The 689 remaining tumors, including but not limited to those from the prostate, colon, stomach, liver, adrenal gland, and head and neck, and small cell carcinomas from the lung, cervix, and ovary, were PAX8 negative. PAX8 specificity was confirmed by Western blot analysis, as expression was detected only in ovarian and RCC cell lines. These results show that PAX8 is a highly sensitive marker for thyroid, renal, Müllerian, and thymic tumors. Importantly, all lung adenocarcinomas, breast and adrenal neoplasms, and the majority of gastrointestinal tumors were negative for PAX8. Therefore, PAX8 is an excellent marker for confirming primary tumor site. In a subset of cases, additional markers, including but not limited to thyroid transcription factor-1, RCC, and Wilms tumor-1, may be needed to distinguish between the 3 most common PAX8-positive tumors.

The ovary is a common site of involvement for metastasis and the breast is one of the most common sources. Metastatic breast carcinoma can mimic a primary ovarian carcinoma. Pax8 is a crucial transcription factor for organogenesis of the thyroid gland, kidney, and Müllerian system, and it also regulates Wilms tumor suppressor gene (WT1) expression. A total of 124 cases of ovarian carcinomas (84 serous papillary, 18 endometrioid, 12 mucinous, 10 clear cell) and 243 cases of invasive breast carcinomas (178 ductal, 65 lobular) were immunostained with Pax8 and WT1 by tissue microarrays to see the differential expression. Pax8 reaction was found in 108 of 124 ovarian carcinomas (87.1%) generally in diffuse staining, including 81 of 84 serous papillary carcinomas (96.4%), 16 of 18 endometrioid carcinomas (88.9%), 10 of 10 clear cell carcinomas (100%), and 1 of 12 mucinous carcinomas (8.3%), whereas WT1 expression was seen in 78 of 124 ovarian carcinomas (62.9%), including 73 of 84 serous papillary carcinomas (86.9%), and 5 of 18 endometrioid carcinomas (27.8%), with no expression in all 10 clear cell carcinomas and 12 mucinous carcinomas. All the mammary carcinomas were completely negative for Pax8, but WT1 expression was seen in 5 of 243 cases (2.1%). Pax8 is a useful marker in the differential diagnosis of ovarian and breast carcinomas, and it seems to be superior to WT1 for the diagnosis of all types of nonmucinous ovarian carcinomas, notably clear cell and endometrioid types where WT1 expression is generally negative or only focal.