WT1

(Wilms tumour-1 protein)
Characteristics
The WT1 gene located at chromosome 11p13 codes for a transcription factor, a DNA-binding nucleoprotein, 52-62 kDa, that plays a role primarily in the development of genitourinary organs. There are at least eight isoforms ranging between 52 and 62 kDa produced by a combination of alternative splicing and RNA editing. WT1 is synthesized and reside in the cytoplasm in an inactive form. When activated through phosphorylation it is translocated to the nucleus. WT1 influences cell proliferation by suppressing bcl-2 and regulating cadherin and p53. In normal epithelia, nuclear WT1 expression is largely restricted to ovary (surface epithelium and inclusion cysts) and fallopian tube, while WT1 is not found in endometrial or cervical epithelium. As regards nonepithelial cells, nuclear WT1 is found in mesothelium and some submesothelial stromal cells, stromal cells of the female genital tract, testicular non-germinal cells, and kidney (podocytes), CD34+ bone marrow stem cells, and some splenic cells. Cytoplasmic staining, which is seen in may cell types, is probably due to Ab cross reaction with an unrelated epitope.
Neoplasms
Although originally identified as a tumour suppressor gene, a variety neoplasms are associated with WT1 over expression. Over expression of both wild-type and mutant WT1 has been reported. Some cases show WT1 gene mutation which lead to loss of its suppressor activity. In other cases it appears that wild-type WT1 is accumulated due to mutations in downstream pathways. Among epithelial tumours, nuclear WT1 is strongly expressed in ovarian serous carcinoma (97% of the tumours, usually a widespread reaction), peritoneal serous carcinoma, ovarian transitional carcinoma, and about half of ovarian endometrioid carcinoma (grade 2 and 3 but not grade 1). Also metanephric adenoma is positive. Limited nuclear WT1 expression has been documented in a small percentage of various others carcinomas such as ovarian clear cell carcinoma, uterine papillary serous carcinoma, uterine endometrioid carcinoma, renal cell carcinoma (chromophobic and papillary), breast carcinoma, lung carcinoma, and pancreaticobiliary carcinoma. Among nonepithelial tumours, nuclear WT1 is strongly expressed in the large majority of malignant mesothelioma and sex cord-stromal tumours. Nuclear WT1 has moreover been demonstrated in Wilms' tumour (about 50% of the cases, involving epithelial, stromal and blastemal elements), malignant rhabdoid tumour, adenomatoid tumour, endometrial stromal sarcoma, uterine leiomyosarcoma, mixed mullerian tumour, as well as in some malignant lymphomas (lymphoblastic and Burkitt's lymphoma), and most cases of acute leukaemia. In desmoplastic small round cell tumour (DSRCT), nuclear WT1 is expressed in the large majority of cases due to a specific chromosomal abnormality, t(11;22)(p13;q12) that fuses EWS with WT1 leading to production of a chimeric protein with transcriptional regulatory activity. However, in the chimeric protein the C-terminus is lost so that expression may not be detected with the N-terminal directed mAb clone 6F-H2. In rhabdomyosarcoma, rhabdomyoblastic differentiation in Wilms' tumour, and neuroblastoma, a cytoplasmic (but not nuclear) staining for WT1 may be seen. WT1 is not demonstrated in Ewing's sarcoma/peripheral primitive neuroectodermal tumour.
Application
WT1 is particularly used for distinguishing malignant mesothelioma and ovarian serous carcinoma from nonserous carcinomas. As for malignant mesothelioma, calretinin and WT1 are superior to cytokeratin 5/6, N-cadherin and thrombomodulin. WT1 is also applicable for the differential diagnostic of small cell childhood tumours.
Controls
Fallopian tube and kidney are both recommendable as positive tissue controls when mmAb WT49 and rmAb EP122 are used for WT1 detection. In Fallopian tube, the protocol must be calibrated to provide a strong, distinct nuclear staining in virtually all epithelial and smooth muscle cells. In kidney, optimally calibrated protocols must show a strong, distinct nuclear staining in podocytes and parietal epithelial cells of Bowman’s capsule. Epithelial cells of the tubules should not display any nuclear or cytoplasmic staining. When mAb clone 6F-H2 is used with HIER as sole retrieval method, the resulting cytoplasmic staining reaction in endothelial and muscle cells makes kidney less useful as positive tissue. On the other hand, if mAb clone 6F-H2 is used with combined HIER-Protease retrieval, the cytoplasmic staining reaction is virtually eliminated and consequently kidney can be very useful as positive tissue control.
Selected references
Carpentieri DF, Nichols K, Chou PM, Matthews M, Pawel B, Huff D. The expression of WT1 in the differentiation of rhabdomyosarcoma from other pediatric small round blue cell tumors. Mod Pathol. 2002 Oct;15(10):1080-6. Goldstein NS, Uzieblo A. WT1 immunoreactivity in uterine papillary serous carcinomas is different from ovarian serous carcinomas. Am J Clin Pathol. 2002 Apr;117(4):541-5. Goldstein NS, Bassi D, Uzieblo A. WT1 is an integral component of an antibody panel to distinguish pancreaticobiliary and some ovarian epithelial neoplasms. Am J Clin Pathol. 2001 Aug;116(2):246-52. Hill DA, Pfeifer JD, Marley EF, Dehner LP, Humphrey PA, Zhu X, Swanson PE. WT1 staining reliably differentiates desmoplastic small round cell tumor from Ewing sarcoma/primitive neuroectodermal tumor. An immunohistochemical and molecular diagnostic study. Am J Clin Pathol. 2000 Sep;114(3):345-53. O'neill CJ, Deavers MT, Malpica A, Foster H, McCluggage WG. An Immunohistochemical Comparison Between Low-Grade and High-Grade Ovarian Serous Carcinomas: Significantly Higher Expression of p53, MIB1, BCL2, HER-2/neu, and C-KIT in High-Grade Neoplasms. Am J Surg Pathol. 2005 Aug;29(8):1034-1041. Ordonez NG. The diagnostic utility of immunohistochemistry in distinguishing between mesothelioma and renal cell carcinoma: a comparative study. Hum Pathol. 2004 Jun;35(6):697-710. Ordonez NG. The immunohistochemical diagnosis of mesothelioma: a comparative study of epithelioid mesothelioma and lung adenocarcinoma. Am J Surg Pathol. 2003 Aug;27(8):1031-51. Waldstrom M, Grove A. Immunohistochemical expression of Wilm's tumor gene protein in different histologic subtypes of ovarian carcinomas. Arch Pathol Lab Med. 2005 Jan;129(1):85-8.
22.04.15 - MV/LE