Expression of WT1 in pediatric small cell tumors: Report of two cases witha possible mesothelial origin

The WT1 gene is normally expressed in fetal kidney and mesothelium, and its expression has been suggested as a marker for Wilms tumor and mesothelioma . We examined WT1 expression levels by reverse-transcriptase polymerase cha in reaction (RT-PCR) in 38 childhood small-cell tumors including Wilms tumo r, embryonal and alveolar rhabdomyosarcoma, Ewing sarcoma, lymphoma, desmop lastic small round-cell tumor (DSRCT), synovial sarcoma, extrarenal rhabdoi d tumor, and two tumors that were atypical for this group of tumors. WT1 ex pression was only detected in Wilms tumor, rhabdoid tumor, and in these two cases of uncertain histogenesis. Both arose in the peritoneal cavity and b y immunohistochemistry were diffusely positive for vimentin, keratin, and d esmin. Tonofilaments were identified by electron microscopy in one of the c ases. RT-PCR failed to detect the t(11;22) translocation associated with DS RCT in either case. Our results suggest that WT1 expression is an unusual f eature of childhood non-Wilms tumors and, in the right setting, it may indi cate a mesothelial origin. The expression of WT1 may play a role in mesoder mal cells acquiring epithelial characteristics, a concept supported by the mixed epithelial. and mesenchymal phenotype of these two cases.