Cell types expressing the Wilms' tumour gene (WT1) in Wilms' tumours: implications for tumour histogenesis

Wilms' tumour (nephroblastoma), a childhood embryonal kidney tumour, is believed to arise from malignant transformation of abnormally persistent metanephric blastemal cells. At a histological level, tumours show a remarkable mimicry of the normal nephrogenic pathway. There is histological and epidemiological evidence for at least two pathogenetic groupings within Wilms' tumour which may reflect different timings of the tumorigenic insult in this pathway and/or involvement of different genes. Tumorigenesis is thought to result from loss of function of a so-called tumour-suppressor gene which has an essential role in control of normal genitourinary development. Such a candidate, Wilms' tumour gene (WT1) mapping to chromosome 11p13, has been isolated and is known to be mutated in some tumours. We have examined the cell types expressing this gene in 32 Wilms' tumours and in nephroblastomatosis by in situ mRNA hybridization. Our results show that WT1 is expressed only in neoplastic structures whose normal counterparts also express the gene and that abnormally persistent high levels of expression are common in both these lesions. Thus, WT1 expression is a good marker for tumour differentiation and reveals how the normal pattern of differentiation is disrupted in Wilms' tumours. We postulate that mutation of the WT1 gene at the 11p13 locus results in Wilms' tumours associated with intralobar nephrogenic rests, which frequently show stromal-predominant histology. We have used our results and ideas to reinterpret current theories on tumour histogenesis and propose a model which explains how patterns of epithelial differentiation are disrupted in Wilms' tumour and how malignant stroma can result from mutation in WT1.