A p53 score derived from TP53 CRISPR/Cas9 HMCLs predicts survival and reveals major role of BAX in BH3 mimetics response [2]

In order to establish a strict p53-dependent gene expression profile, p53 negative clones were derived from two TP53+/+ and two TP53-/mut t(4;14) human myeloma cell lines (HMCLs) using CRISPR/Cas9 technology. From the 17 dysregulated genes shared between the 6 clones from the two TP53+/+ myeloma cell lines, we established a 13-gene functional p53 score, involving genes downregulated on p53-silencing. This functional score allowed us to segregate clones, myeloma cell lines and patients’ samples according to the presence or absence of a single or double hit in TP53 gene i.e., chromosomal deletion and/or mutation. The score also distinguished 1,162 cell lines from hematological and solid cancers according to hits in TP53. Moreover, the 13-gene score was predictive of overall survival in two independent cohorts of patients. Among the 13 genes, we showed that p53-regulated BAX expression correlated to, and directly impacted, the MCL1 BH3 mimetic S63845 sensitivity of myeloma cells by modulating the amount of MCL1-BAX complexes. Resistance to cell death induced by p53-silencing was overcome by combining S63845 with venetoclax, although the synergy was significantly lower in p53-deficient versus -proficient clones and patients’ samples. Finally, by using scRNAseq analysis, we further showed in patients’ bone marrow samples that myeloma cells surviving to the BH3 mimetic combination had an about 3-fold lower p53 score. This data established a functional p53 score and showed that p53-mediating BAX expression greatly impacts mitochondria-mediated cell death in myeloma cells.