| Summary: | Mutant p53 impacts the expression of numerous genes at the level of transcription to mediate oncogenesis. To investigate how mutant p53 impacts transcription, we studied how mutant p53 regulates vascular endothelial growth factor receptor 2 (VEGFR2), one of its strongest target genes that we identified through global gene expression profiling in mutant p53 expressing MDA 468 breast cancer cells. VEGFR2, the primary functional VEGF receptor and clinical target of bevacizumab, mediates endothelial cell neovascularization by promoting increased cellular proliferation, migration, and pro-survival signaling. In breast tumors, VEGFR2 is often aberrantly expressed on the breast tumor epithelia,which correlates with worse overall survival.
We identify VEGFR2 as a mutant p53 transcriptional target in multiple breast cancer cell lines. Mutant p53 mediated upregulation of VEGFR2 mediates mutant 53 gain of function including increased cellular growth and migration. In humans, breast tumors with TP53 hotspot mutants have elevated VEGFR2 levels compared to tumors with loss of function mutations. The same class of tumors has significantly upregulated HIF1A and VEGFA compared to TP53 wild type tumors, indicating that mutant p53 containing breast tumors express a neoangiogenic gene signature that may intensify VEGFR2 autocrine signaling. A clinical trial suggests that TP53 mutated breast tumors may specifically respond to anti VEGF therapy, while TP53 wild type tumors may not respond. We suggest that mutant p53 containing breast tumors may be distinctively vulnerable to anti VEGF ntherapies.
We investigated how mutant p53 impacts transcription of VEGFR2 using multiple techniques including scanning ChIP, micrococcal nuclease PCR, and in vivo DNase I footprinting by ligation mediated PCR. Mutant p53 was found to bind near the VEGFR2 transcriptional start site, causing the promoter to adopt a transcriptionally active conformation. Using SILAC mass spectrometry, we identified subunits of the SWI/SNF chromatin remodeling complex as mutant p53 interactors. Importantly, re ChIP and immunodepletion ChIP demonstrate that mutant p53 and SWI/SNF co-occupy the VEGFR2 promoter. Depletion of multiple SWI/SNF subunits reduced VEGFR2 RNA expression, and SWI/SNF is required for maximal mutant p53 promoter occupancy.
Using RNA sequencing, we report that approximately half of all mutant p53 gene alteration impacts transcription of VEGFR2 as well as myriad other target genes by promoter remodeling through interaction with the SWI/SNF chromatin remodeling complex. Therefore, not only might mutant p53 expressing tumors be uniquely susceptible to anti VEGF therapies, but restoration of SWI/SNF tumor suppressor function by targeting mutant p53 may have therapeutic potential. Mutant p53 interaction with the SWI/SNF complex may explain how mutant p53 modulates the expression of such a diverse set of genes.
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