Persistent SIX2 expression as a first hit mechanism in Wilms tumorigenesis

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DESCRIPTION (provided by applicant): Wilms tumor (WT) is a pediatric renal malignancy thought to arise from persistence and aberrant differentiation of embryonic kidney stem cells. Precisely how nephron progenitors escape pathways of epithelial commitment and conversion, mechanisms that potentially represent "first hits" in Wilms tumorigenesis, have not been clarified. WT blastema, the putative malignant analogue of these nephron progenitors, retains expression of the transcriptional regulator, SIX2, which in mouse development promotes self-renewal of the cap mesenchyme (CM) and prevents premature epithelial differentiation. Our previous work has shown that SIX2, normally absent in the adult kidney, is persistently expressed across a broad spectrum of human WT. Putting together these observations of SIX2 activity in development and disease, the fundamental question arises whether this gene provides a mechanism for the CM and its progeny to self-perpetuate in the WT sequence. The purpose of developing this mouse model is to test the hypothesis that persistent expression of SIX2 in the CM impairs epithelial differentiation, and promotes retention of this progenitor population and development of nephrogenic rests, the putative precursor lesion of WT. We have two aims: 1) To generate and validate a tetracycline-regulated transgenic mouse line that allows temporal control of SIX2 expression in the embryonic kidney, and 2) To characterize the phenotype of persistent SIX2 expression in the embryonic and adult kidney. A founder transgenic mouse line will be engineered to express SIX2 persistently under the control of a tetracycline response element (TRE-RFP-2A-SIX2). Once validated, this mouse will be crossed with the ROSA26-M2-rtTA line, which provides global and constitutive expression of rtTA (Tet-On). Doxycycline will be administered to dams at e15.5 to activate SIX2 transgene expression, and its effects on CM differentiation and persistence will be evaluated. Persistence of CM cells within the post-natal kidney resembling nephrogenic rests is the anticipated phenotype. These pilot studies will lay the foundation for future, more definitive investigation targeting persisten SIX2 expression to the CM only. These future studies will use the CM-specific CITED1-Cre-ERT2 mouse engineered previously in our laboratory.