CSIG-01. GAIN-OF-FUNCTION MUTANT P53 REGULATES LONG-NONCODING RNA LINC00643 TO MODULATE HIF1Α IN GLIOBLASTOMA PROGRESSION

Abstract Gain-of-function mutations of p53 (GOF-MUT-p53) are oncogenic drivers in many human cancers, including glioblastoma (GBM), where they promote tumor progression through diverse mechanisms, including acting as transcription factors. To uncover the targets of the most common GOF-MUT-p53, we used ChIP-seq in wild-type (WT-p53) and GOF-MUT-p53 (R273H) GBM cells. We identified 3406 unique peaks in GOF-MUT-p53 cells, including 535 long non-coding RNAs (lncRNAs) with up to 145-fold enrichment compared to WT-p53. We investigated one of these lncRNAs, LINC00643, in depth. We found that GOF-MUT-p53 binds to the promoter of LINC00643 and represses its expression, whereas WT-p53 did not. LINC00643 was significantly downregulated in GBM and low-grade glioma (LGG) compared to normal brain, with its expression correlating with patient survival. Functional assays demonstrated that overexpression of LINC00643 inhibits GBM cell proliferation, migration, invasion, stem cell self-renewal and xenograft tumor growth. Notably, we showed that the third exon of LINC00643 (Exon3) is highly conserved across species and is essential for the LINC00643 tumor-suppressive functions. Ectopic expression of LINC00643 or Exon3 significantly reduced tumor growth and improved survival in GBM mouse models, including immunocompetent RCAS-Tva and immunodeficient orthotopic xenograft models. To elucidate the mechanism of action of LINC00643, we performed Chromatin Isolation by RNA Purification (ChIRP)-seq and identified HIF1α as a key binding target of LINC00643. We demonstrated that LINC00643 binds to the enhancer of HIF1α and prevents its activation. Under hypoxia conditions, LINC00643 overexpression significantly reduced HIF1α expression and its downstream gene targets. Furthermore, knockdown of GOF-MUT-p53 led to the upregulation of LINC00643 and subsequent reduction in HIF1α expression. Our findings reveal, for the first time, that GOF-MUT-p53 regulates LINC00643, which, in turn, mediates the oncogenic effects of MUT-p53 in GBM by modulating the HIF1α pathway.