Interrogating Wilms tumor transcriptional regulatory programs and the embryonic origin of the mammalian kidney

While the branching architecture and arrayed assembly of the mammalian kidney and nephrons are well-conserved between mouse and human, distinctions exist between the two species in the repertoire of transcription factors (TFs) within the nephron progenitor cells (NPCs), the multipotent progenitor population that gives rise to all segments of the nephrons. In human NPCs, the TF SIX1 is present throughout later stages of development compared to the mouse, in which Six1 is present only transiently before the initiating event of kidney morphogenesis. Given this early and fleeting nature, the regulatory role of Six1 in this population is incompletely understood. In human NPCs, SIX1 has been reported to act similarly to the closely related TF SIX2/Six2 which is required for maintenance of the proliferative NPCs in mouse. However, the regulatory function of SIX1 in establishing the early human NPC population also remains obscure. Nonetheless, the persistence of SIX1 in human NPCs coupled with the existence of a recurring mutation of SIX1 (SIX1-Q177R) in Wilms tumor, a childhood tumor thought to arise from aberrant differentiation of NPCs, indicate SIX1 might have regulatory functions in human NPCs not required in the mouse which could be exploited in the context of oncogenesis. Overall, the goal of the research outlined in this dissertation is to contribute to our understanding of the transcriptional control of cell fate in the development of the normal and diseased mammalian kidney, with particular focus on SIX1/Six1. Chapter II describes the investigation of transcriptional regulatory programs associated with SIX1-Q177R through integrated analyses of genomic datasets derived from primary Wilms tumor tissue and Chapter III describes the generation of an in vitro model of the SIX1-Q177R mutation in kidney organoids. Finally, Chapter IV describes preliminary evidence for unconventional mesodermal origins of the mammalian kidney mesenchyme linked to an alternative Six1-positive mesodermal lineage using light sheet fluorescence microscopy, challenging the long-standing paradigm of metanephric kidney ontogeny with potential implications for our understanding and interpretations of mechanisms underlying human developmental kidney diseases including Wilms tumor, as well as for the utilization of kidney organoids in vitro as accurate models of normal human kidney development.