Abstract 6622: Identification of early oncogenic lesions following concomitant Vhl and Pbrm1 loss in the murine kidney

Abstract Characterizing the early events preceding tumorigenesis is a major challenge in understanding cancer. To address this, we are investigating the paradigm of clear cell renal cell carcinoma (ccRCC) that is driven by the biallelic loss of the von Hippel Lindau (VHL) tumor suppressor gene. Though VHL loss is required, additional mutations have been found in the evolution of the cancer, most commonly in Polybromo-1 (PBRM1), a subunit of the PBAF SWI/SNF chromatin remodeling complex. Concomitant Vhl and Pbrm1 deletion, but not Vhl deletion alone, in the renal tubular epithelium (RTE) has been reported to be sufficient to drive the formation of renal tumors in mice. However, the early in vivo consequences of Vhl loss, and the exact requirement for Pbrm1 loss to tumorigenesis remain poorly studied. We have previously reported the development of a novel lineage-tracing model of Vhl deletion that directly couples loss of a conditional Vhl allele to the expression of a tdTomato reporter and allows accurate identification and retrieval of marked Vhl-null cells. In the work reported here, we have combined this system with conditional Pbrm1 deletion using an RTE-restricted Pax8-CreERT2 to precisely identify and assay Vhl/Pbrm1-null cells. We describe the histological abnormalities specifically comprising Vhl/Pbrm1-null cells observed at early (1-3 weeks), intermediate (10 months), and late (17 months) intervals post recombination. We found that Vhl/Pbrm1-null cells formed extensive ‘tumorlets’, disorganized, multilayered tubules, and cystic dilated tubules specifically in the renal cortex at 17 months post recombination. Immunohistochemistry confirmed that all epithelial cells in the lesions were Vhl/Pbrm1 null and expressed HIF1 and HIF2. These lesions contained cycling cells as identified by dual tdTomato/Ki67 staining, foci of immune cells and were strikingly surrounded by cells expressing high levels of p-ERK. We then traced these lesions to earlier timepoints, and observed Vhl/Pbrm1-null lesions resembling ‘tumorlet’ precursors and disorganized tubules at 10 months post recombination specifically in the renal cortex. Interestingly, these precursors were encompassed by macrophages and could be discerned in situ by a ring of p-ERK-expressing cells surrounding these lesions. No similar lesions were detected at 3 weeks after recombination. Interestingly, the presence of cortical lesions was associated with a region-specific expansion of the total Vhl/Pbrm1-null population over time. Our data suggest that morphological abnormalities arise after a general proliferation of Vhl/Pbrm1-null cells in the renal cortex and from a subset of cells expressing specific markers. We are now comparing the differential gene expression patterns of Vhl-null cells and Vhl/Pbrm1-null normal and transformed cells at single cell resolution to expand on these observations. Citation Format: Joanna Lima, Samvid Kurlekar, Norma Masson, Ayslan B. Barros, Maria Fernanda Soares, Christopher W. Pugh, Julie Adam, Peter J. Ratcliffe. Identification of early oncogenic lesions following concomitant Vhl and Pbrm1 loss in the murine kidney abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6622.