## WT1 Immunohistochemical Expression in Human Embryonal/Fetal and Neoplastic Tissues
### WT1 Expression in Malignant Peripheral Nerve Sheath Tumors  
Malignant peripheral nerve sheath tumors (MPNSTs) are an aggressive and rare type of sarcoma, usually arising from peripheral nerves. They can occur sporadically or more frequently (up to 50% of cases) from pre-existing neurofibromas in the context of neurofibromatosisis type 1 (NF1) [127]. Most MPNSTs are deeply localized, and often they are greater than 10 cm in maximum diameter by the time of presentation. Histologically, they show a spindle-cell fascicular appearance with abrupt alternations between cellular and more myxoid areas, suggesting neural differentiation. Some cases have a uniformly cellular and fascicular pattern reminiscent of a monophasic synovial sarcoma. In other cases, an abundant myxoid stroma can be observed, configuring the diagnosis of myxoid MPNST. Cells have a pale, poorly defined cytoplasm and narrow nuclei, often with a wavy or buckled configuration. In addition, the nuclei tend to be hyperchromatic, and at least focally pleomorphic with inconspicuous nucleoli. Mitoses are generally frequent. In about 10-15% of MPNSTs, especially in those arising in patients with NF1, heterologous differentiation can be present [128]. The most frequent divergent differentiation is the rhabdomysocromatous component, configuring the so-called malignant Triton tumor, which is associated with a poor prognosis. Less frequently, osteosarcoma  
Figure 6: Glioblastoma multiforme showing pseudopalisading necrosis (**A**). Neoplastic cells showing strong and diffuse cytoplasmic expression of WT1 (_N-terminus_) (**B**).  
or chondrosarcomatus differentiation can be observed. Immunohistochemistryistochemically, the S-100 protein is positive in about 50% of cases of MPNST [129; 130].  
Early studies on WT1 expression reported the positivity in Schwann cells of normal nerves. In peripheral nerve sheath tumors, including MPNSTs, neurofibromas and schwannomas, neoplastic cells expressed WT1 protein at the cytoplasmic level (Figure 7) [44]. A recent study by [PERSON] et al. [131] evaluated WT1 expression in 87 cases of soft tissue sarcoma, including liposome, MFH, rhabdomyosarcoma, leiomyosarcoma, MPNST, synovial sarcoma, fibrosarcoma and others. The authors observed cytoplasmic expression of WT1 (6F-H2) in 71% of the cases of malignant peripheral nerve sheath tumors, revealing no association between WT1 expression and overall survival or disease-free survival. Conversely, [PERSON] et al. [132] reported that the WT1 gene is frequently overexpressed in various types of soft tissue sarcoma [132], and that WT1 mRNA overexpression is significantly associated with a poor prognosis. This result was proved only in 4 out of 52 and 3 out of 36 samples, by immunoblotting [121] and immunohistochemistry. Therefore, further studies on the correlation between the protein and mRNA of the WT1 gene in larger cohorts are required, together with survival analysis, to validate the WT1 expression level as a prognostic factor. [PERSON] et al. [133] investigated WT1 expression in the MPNST SNP6.2 cell line, showing a strong WT1 staining in the nuclear and perinuclear areas of neoplastic cells. In addition, they studied the effects of silencing WT1 by RNA interference through Western Blot analysis and proliferation assays. The result was a reduction of cell growth in a time- and dose-dependent manner, suggesting that WT1 is involved in the development and progression of MPNSTs, and that it could be a potential therapeutic target for MPNSTs.