Tissue-specific regulation of the WT1 locus.

The 11p13 Wilms' tumor locus consists of two coordinately regulated transcripts, WT1 and WIT-1. These genes are highly expressed in the developing urogenital system, beginning with the urogenital ridge at day 10.5, the metanephric blastema at day 11.5, and during glomerular formation at day 13.5, becoming ultimately restricted to the podocytes. Stromal cells of the gonad also show abundant expression. WT1 is expressed at lower levels in spleen, uterus, mesothelial linings of organs in the abdominal and thoracic cavities, and the ependymal layer of the ventral aspect of the spinal cord. WIT-1 mRNA is about 10-fold less abundant than WT1, but appears to be expressed in the same tissue-restricted manner. Expression of the WT1 protein is required for kidney development, although its physiological function remains to be determined. The function of WIT-1 is similarly unknown but one intriguing possibility is that it is an antisense regulator of WT1. An understanding of events controlling spatial and temporal regulation of these genes will greatly improve our ability to study the role of WT1 and WIT-1 in urogenital development. We have found that while chimeric reporter constructs containing 0.6-2.5 kb of the 5' region of the WT1 gene direct transcription in many different cell lines, we were unable to detect expression in 13.5-day mouse embryos. However, a cosmid containing about 42 kb encompassing this region was able to direct the expression of abundant levels of mRNA from the appropriate transcription initiation sites in both stable transfectants of mouse Leydig cells (TM3) or in transgenic embryos. We are currently localizing the DNA elements required for this expression.