Suppression of HSF1 activity by wildtype p53 creates the driving force for p53 loss-of-heterozygosity, enabling mutant p53 stabilization and invasion

A prerequisite for gain-of-function (GOF) p53 missense mutants (mutp53) is protein stabilization. Moreover, a prerequisite for mutp53 stabilization is loss of the remaining wildtype (WT) p53 allele (loss -of-heterozygosity, p53LOH) in mutp53/+ tumors. Thus, GOF, mutp53 stabilization and p53LOH are strictly linked. However, the driving force for p53LOH is unknown. Typically, heterozygous tumors are an instable transition state. Here we identify the repressive WTp53-HSF1 axis as the driver of p53LOH. We find that the WTp53 allele in AOM/DSS-induced colorectal tumors (CRC) of p53R248Q/+ mice retains its haploid transcriptional activity. Notably, WTp53 represses heat-shock factor 1 (HSF1) activity, the master transcription factor of the proteotoxic stress defense response (HSR) that is ubiquitously and constitutively activated in cancer tissues. HSR is critical for stabilizing oncogenic proteins including mutp53. WTp53-retaining murine CRC tumors and tumor-derived organoids and human CRC cells all suppress the tumor-promoting HSF1 transcriptional program. Mechanistically, the retained WTp53 allele activates CDKN1A/p21, leading to cell cycle inhibition and suppression of the E2F target gene MLK3. MLK3 links cell cycle to the MAPK stress pathway to activate the HSR response. We show that in p53R248Q/+ tumors WTp53 activation by constitutive stress (emanating from proliferative/metabolic stresses and genomic instability) represses MLK3, consequently inactivating the MAPK-HSF1 response necessary to ensure tumor survival. This creates strong selection pressure for p53LOH which eliminates the repressive WTp53-HSF1 axis and unleashes the tumor-promoting HSF1 functions, inducing mutp53 stabilization and enabling invasion. HIGHLIGHTS: - heterozygous p53R248Q/+ tumors retain p53 transcriptional activity in a mouse model of colorectal cancer (CRC). - wildtype p53 actively represses the tumor-promoting HSF1-regulated chaperone system and proteotoxic stress response. - the repressive WTp53-HSF1 axis creates a selective pressure for WTp53 loss-of-heterozygosity in CRC tumors. - p53 loss-of-heterozygosity enables stabilization of the gain-of-function p53R248Q mutant protein which in turn enables CRC invasion