Background: Wilms tumors (WTs) are embryonic neoplasms of the kidney that are believed to arise from primitive metanephrogenic blastema. Our previous reports and those of others indicate that WTs show an increased expression of insulin-like growth factor II (IGF-II) mRNA. However, the precise role of IGF-II on Wilms tumorigenesis is not known. A central question is to determine whether the increased IGF-II expression in WTs simply reflects the fetal nature of WTs (effect), or is induced by specific changes in gene expression (cause).
Experimental design: This study included 31 sporadic WTs, 7 fetal and 3 adult kidneys and 1 yolk sac tumor. Clinical and histologic summaries of WT cases are shown in Table 1. In WTs, the relative area of blastemal, epithelial, poorly differentiated spindle cell and heterologous cell components were assessed. Dot and Northern blot hybridization, using cDNA probes, were done to assess the level of IGF-II mRNA expression. In situ RNA hybridization was employed to localize IGF-II transcripts. Immunohistochemistry was applied to frozen sections to demonstrate cytokeratin and type-IV collagen. These results were then correlated with the histology of WTs and their precursor lesions, i.e., nephrogenic rests (NRs).
Results: Dot blot hybridization indicated that IGF-II transcripts were 32- to 64-fold more abundant in WTs than in the adjacent uninvolved kidneys. In situ hybridization showed that WTs, NRs, and fetal kidney shared a common feature in which IGF-II transcripts were predominantly associated with blastema. However, WTs and NRs differed from fetal kidney in that occasional epithelial structures and dense blastema showed aberrant, sustained IGF-II expression.
Conclusions: The data indicate two points. 1) There is an inverse correlation between nephroblastic differentiation and IGF-II expression in developing fetal kidney. 2) The IGF-II expression in WTs and NRs does not simply reflect the embryonal nature of the tumor but is rather significantly altered, suggesting a role as a transforming growth factor in Wilms tumorigenesis.