Probes for WT1

The homeoboxB9 (HOXB9) gene is necessary for specification of the anterior-posterior body axis during embryonic development and expressed in various types of cancer. Here we show that the Wilms tumor transcription factor WT1 regulates the HOXB9 gene in a bidirectional manner. Silencing of WT1 activates HOXB9 in Wt1 expressing renal cell adenocarcinoma-derived 786-0 cells, mesonephric M15 cells and ex vivo cultured murine embryonic kidneys. In contrast, HOXB9 expression in U2OS osteosarcoma and human embryonic kidney (HEK) 293 cells, which lack endogenous WT1, is enhanced by overexpression of WT1. Consistently, Hoxb9 promoter activity is stimulated by WT1 in transiently transfected U2OS and HEK293 cells, but inhibited in M15 cells with CRISPR/Cas9-mediated Wt1 deletion. Electrophoretic mobility shift assay and chromatin immunoprecipitation demonstrate binding of WT1 to the HOXB9 promoter in WT1-overexpressing U2OS cells and M15 cells. BASP1, a transcriptional co-repressor of WT1, is associated with the HOXB9 promoter in the chromatin of these cell lines. Co-transfection of U2OS and HEK293 cells with BASP1 plus WT1 prevents the stimulatory effect of WT1 on the HOXB9 promoter. Our findings identify HOXB9 as a novel downstream target gene of WT1. Depending on the endogenous expression of WT1, forced changes in WT1 can either stimulate or repress HOXB9, and the inhibitory effect of WT1 on transcription of HOXB9 involves BASP1. Consistent with inhibition of Hoxb9 expression by WT1, both transcripts are distributed in an almost non-overlapping pattern in embryonic mouse kidneys. Regulation of HOXB9 expression by WT1 might become relevant during kidney development and cancer progression.

Mammalian nephrons arise from a population of nephron progenitor cells (NPCs) expressing the master transcription factor, WT1, which is crucial for NPC proliferation, migration, and differentiation. In humans, biallelic loss of WT1 precludes nephrogenesis and leads to formation of Wilms tumor precursor lesions. We hypothesize that WT1 normally primes the NPC for nephrogenesis by inducing expression of NPC-specific DNA-repair genes that protect the genome. We analyzed transcript levels for a panel of DNA-repair genes in E17.5 vs adult mouse kidneys and noted seven that were increased >20-fold. We then isolated d1(+) NPCs from E17.5 kidneys and found that only one, Neil3, was enriched. RNAscope ISH of E17.5 mouse kidneys showed increased Neil3 expression in the nephrogenic zone vs mature nephron structures. To determine whether Neil3-expression is WT1-dependent, we knocked down Wt1 in d1(+) NPCs (60% knockdown efficiency) and noted a 58% reduction in Neil3 transcript levels. We showed that WT1 directly binds to the Neil3 promoter and that activity of a Neil3 promoter-reporter vector was increased two-fold in WT1(+) vs WT1(-) cells. We propose that Neil3 is a WT1-dependent DNA-repair gene, expressed at high levels in d1(+) NPCs where it repairs mutational injury to the genome during nephrogenesis. NEIL3 is likely just one of many such lineage-specific repair mechanisms that respond to genomic injury during kidney development.

Pancreatic development requires spatially and temporally controlled expression of growth factors derived from mesenchyme. Here, we report that in mice the secreted factor Fgf9 is expressed principally by mesenchyme and then mesothelium during early development, then subsequently by both mesothelium and rare epithelial cells by E12.5 and onwards. Global knockout of the Fgf9 gene resulted in the reduction of pancreas and stomach size, as well as complete asplenia. The number of early Pdx1+ pancreatic progenitors was reduced at E10.5, as was proliferation of mesenchyme at E11.5. Although loss of Fgf9 did not interfere with differentiation of later epithelial lineages, single-cell RNA-Sequencing identified transcriptional programs perturbed upon loss of Fgf9 during pancreatic development, including loss of the transcription factor Barx1. Lastly, we identified conserved expression patterns of FGF9 and receptors in human fetal pancreas, suggesting that FGF9 expressed by pancreatic mesenchyme may similarly affect the development of the human pancreas.