P455: BREAKING THE PUMP: TARGETING THE SODIUM-POTASSIUM PUMP AS A THERAPEUTIC STRATEGY IN ACUTE MYELOID LEUKEMIA

Abstract

Background: Acute myeloid leukemia (AML) is a heterogenous hematologic malignancy with poor overall survival despite intensive cytotoxic chemotherapy and newer targeted agents. With a 5-year survival rate of only 29% in adults with AML, new therapeutic strategies are needed. Aims: In order to identify novel, selective targets for AML, we used the Cancer Dependency Map, which provides genome-scale CRISPR/Cas9 depletion screen data in hundreds of cancer cell lines. Here, we identified ATP1B3 as a context-specific dependency that could be therapeutically exploited. ATP1B3 is the smaller glycoprotein subunit (beta) of the sodium-potassium pump (Na/K-ATP pump). Together with the catalytic alpha subunit ATP1A1, it forms a heterodimer located in the plasma membrane, regulating the electrochemical gradient through the transport of Na and K ions across the membrane. The Na/K-ATP pump is of vital importance in the maintenance of cellular homeostasis and membrane potential and because of its function as a receptor and signal transducer. While the ATP1A1 carries out ion transport and enzymatic activity, making it a common essential gene (a gene which ranks among the most depleted genes in at least 90% of screened cell lines), ATP1B3 is not essential in all cancer cells. This beta subunit seems to be crucial for intracellular transport and stabilization of the alpha subunit in the membrane. The beta subunit has 4 paralogs, showing similar expression patterns among tissues, with one exception, ATP1B1. Methods: We used a diverse panel of functional genomic based assays, validating our finding using CRISPR/Cas9 knockout and overexpression constructs for the Na/K-ATP pump beta subunit paralogs. In an approach to analyze protein-protein interactions we generated BioID constructs for a mass spectrometry-based analysis. For an in vivo study we used a bioluminescent imaging (BLI)-based orthotopic mouse model of AML, carrying non-targeting, ATP1B1 or ATP1B3 knockout guides. Results: By using CRISPR/Cas9 knockout we could validate ATP1B3 as a selective dependency and performing competitive growth assays we showed that loss of ATP1B3 in ATP1B1 low expressing AML cells leads to synthetic lethality. The absence of both paralogs of the beta subunit results in the loss of their common essential binding partner ATP1A1 in hematologic malignancies, while higher expression of ATP1B1 can stabilize ATP1A1 under the loss of ATP1B3 in solid tumors or in ATP1B1 overexpressing AML cells. Next, we validated our findings in vivo. In a BLI-based orthotopic mouse model of AML, we found that the AML model with ATP1B3 knockout showed lower leukemia burden and normal spleen weight in comparison to a non-targeting or ATP1B1 knockout control. To understand the specific role of ATP1B3 we produced BioID (proximity-dependent biotin identification) constructs for ATP1B1 and ATP1B3. In ongoing studies, through this mass spectrometry-based analysis of protein-protein interactions, we will gain a greater insight into the protein partners of the Na/K-ATP pump. Summary/Conclusion: Taken together, we identified ATP1B3 as a selective dependency in AML. We propose that the elimination of ATP1B3 leads to the destabilization of the sodium-potassium pump when ATP1B1 levels are low, making it a potential tumor-selective therapeutic target for AML and other hematologic malignancies with low expression of ATP1B1.