Targeting ADSS2 Enhances BH3 Mimetics Treatment Induced Mitochondrial Apoptosis in AML Cells

Recently, the BH3 mimetic-Bcl2 inhibitor venetoclax (VEN), has been approved by FDA in 2018 for the treatment of patients with AML in combination with a hypomethylating agent (HMA) such as azacytidine (AZA). This synergy is likely due to HMA-induced mitochondrial vulnerability associated with cytoplasmic RNA metabolism. Nonetheless, most AML patients treated with the combination eventually relapse, possibly due to persistence of tumor cells not dependent on antiapoptotic proteins (Bcl2, Mcl1). There is a pressing need to develop targeted therapeutics sensitizing AML cells to mitochondrial apoptosis, particularly for those patients failed from VEN associated treatment.

To achieve the goal, we performed a CRISPR/Cas9 Knockout (KO) screen (using Bim peptide-based dynamic ΔBH3 profiling as read-out) targeting genes (n=534) involved in cytosolic nucleotides metabolism pathways to increase mitochondrial vulnerability in a BH3 mimetic-refractory AML cell line (HEL). That search identified ADSS2, which catalyzes the first committed/rate-limiting step of AMP biosynthesis from IMP, emerged as the top hit, exhibiting highest ΔBH3 priming. Subsequent BH3 priming assays validated that ADSS2 KO significantly enhanced sensitivity to BH3 mimetics across various AML cell lines, including MOLM13, SKM1, OCI-AML3, and HEL cells. ADSS2 KO also led to a noteworthy reduction in the IC50 values of both VEN and the Mcl1 inhibitor S63845 in these cell lines. Additionally, the synergistic effect between sgRNA targeting ADSS2 and VEN or S63845 was also confirmed in Cas9 expressing cells from two AML patient-derived-xenograft (PDX) samples. We next transplanted AML cells (form one PDX samples) with or without ADSS2-KO into NGS mice, then treated mice with VEN. Notably, VEN treatment (100 mg/kg, QDx14, PO) significantly reduced tumor burden in ADSS2-KO AML mice compared to WT AML mice. Additionally, ADSS2 KO combined with VEN treatment group displayed a survival advantage compared to single VEN treatment group.

Further mechanistic investigations revealed that ADSS2 KO led to a remarkable depletion of AMP rather than ATP, and a robust repression of AMPK activity in HEL cells. To assess whether AMPK inhibition mimics the effect of ADSS2 KO, AMPKα-deficient (Mx1-Cre/Prkaa1^fl/fl/ Prkaa2^fl/fl) MLL-AF9 (MA9) transformed murine AML cells were assessed. Indeed, AMPKα depletion substantially enhanced VEN treatment efficiency ex-vivo. Notably, enforced overexpression of a constitutively active AMPKa2 (T172D) construct in a VEN sensitive cell line SKM1, promoted cell resistance to mitochondrial apoptosis by VEN or S63845. Moreover, T172D expression can also abolish ADSS2-KO mediated mitochondrial vulnerability to VEN or S63845 treatment, indicating the pivotal role of AMPK signaling in mediating ADSS2 function.

There are no reported ADSS2 inhibitors. To explore ADSS inhibitor, we conducted a fragment-based virtual screen of 8,000,000 compounds (MolPort) library and requested the top-ranking hits for ADSS2 catalysis evaluation by ELISA. Notably, the top two hits AS104 and AS71 exhibited the most potent inhibitory effects (AS104 IC50=2.43µM, AS71 IC50=3.85µM). NMR-STD analysis was conducted in a cell-free system, which unequivocally confirmed the physical binding of inhibitors and the ADSS2 protein. To further investigate the interactions between AS71 and AS104 with ADSS2 in a cellular context, both compounds were subjected to a cellular thermal shift assay (CETSA). Notably, treatment with AS71 and AS104 led to substantial shifts in ADSS2 protein thermal stability, providing evidence of their interactions with ADSS2 within the cellular environment. In both HEL cells and a primary VEN-resistant AML samples, treatment with AS71 and AS104 significantly increased ΔBH3 priming.

Collectively, our observation revealed that targeting ADSS2 is critical for sensitizing AML cells to BH3 mimetics. We are now conducting preclinical assessments of ADSS2 inhibitors.