Synthetic lethality in chronic lymphocytic leukaemia with DNA damage response defects by targeting the ATR pathway

Abstract Background DNA damage response (DDR) defects, particularly TP53 and biallelic ataxia telangiectasia mutated ( ATM ) aberrations, are associated with genomic instability, clonal evolution, and chemoresistance in chronic lymphocytic leukaemia (CLL). Therapies capable of providing long-term disease control in CLL patients with DDR defects are lacking. Using AZD6738, a novel ATR inhibitor, we investigated ATR pathway inhibition as a synthetically lethal strategy for targeting CLL cells with these defects. Methods The effect of AZD6738 was assessed by western blotting and immunofluorescence of key DDR proteins. Cytotoxicity was assessed by CellTiter-Gloluminescence assay (Promega, Madison, WI, USA) and by propidium iodide exclusion. Primary CLL cells with biallelic TP53 or ATM inactivation were xenotransplanted into NOD/Shi-scid/IL-2Rγ mice. After treatment with AZD6738 or vehicle, tumour load was measured by flow cytometric analysis of infiltrated spleens, and subclonal composition by fluorescence in-situ hybridisation for 17p( TP53 ) or 11q( ATM ) deletion. Findings AZD6738 provided potent and specific inhibition of ATR signalling with compensatory activation of ATM/p53 pathway in cycling CLL cells in the presence of genotoxic stress. In p53 or ATM defective cells, AZD6738 treatment resulted in replication fork stalls and accumulation of unrepaired DNA damage, as evidenced by γH2AX and 53BP1 foci formation, which was carried through into mitosis, resulting in cell death by mitotic catastrophe. AZD6738 displayed selective cytotoxicity towards ATM or p53 deficient CLL cells, and was highly synergistic in combination with cytotoxic chemotherapy. This finding was confirmed in primary xenograft models of DDR-defective CLL, where treatment with AZD6738 resulted in decreased tumour load and selective reduction of CLL subclones with ATM or TP53 alterations. Interpretation We have provided mechanistic insight and demonstrated in-vitro and in-vivo efficacy of a novel therapeutic approach that specifically targets p53-null or ATM-null CLL cells. Such an approach can potentially help to avert clonal evolution, a major cause of therapeutic resistance and disease relapse. Funding Leukaemia & Lymphoma Research.