Aberrant ATM signaling and homology-directed DNA repair as a vulnerability of p53-mutant GBM to AZD1390-mediated radiosensitization.

Abstract

ATM is a key mediator of radiation response, and pharmacological inhibition of ATM is a rational strategy to radiosensitize tumors. AZD1390 is a brain-penetrant ATM inhibitor and a potent radiosensitizer. This study evaluated the spectrum of radiosensitizing effects and the impact of <i>TP53</i> mutation status in a panel of <i>IDH1</i> wild-type (WT) glioblastoma (GBM) patient-derived xenografts (PDXs). AZD1390 suppressed radiation-induced ATM signaling, abrogated G₀-G₁ arrest, and promoted a proapoptotic response specifically in p53-mutant GBM in vitro. In a preclinical trial using 10 orthotopic GBM models, AZD1390/RT afforded benefit in a cohort of <i>TP53</i>-mutant tumors but not in <i>TP53</i>-WT PDXs. In mechanistic studies, increased endogenous DNA damage and constitutive ATM signaling were observed in <i>TP53</i>-mutant, but not in <i>TP53</i>-WT, PDXs. In plasmid-based reporter assays, GBM43 (<i>TP53</i>-mutant) showed elevated DNA repair capacity compared with that in GBM14 (p53-WT), whereas treatment with AZD1390 specifically suppressed homologous recombination (HR) efficiency, in part, by stalling RAD51 unloading. Furthermore, overexpression of a dominant-negative <i>TP53</i> (p53DD) construct resulted in enhanced basal ATM signaling, HR activity, and AZD1390-mediated radiosensitization in GBM14. Analyzing RNA-seq data from TCGA showed up-regulation of HR pathway genes in <i>TP53</i>-mutant human GBM. Together, our results imply that increased basal ATM signaling and enhanced dependence on HR represent a unique susceptibility of <i>TP53</i>-mutant cells to ATM inhibitor-mediated radiosensitization.