3048 – DELETION OF BAX: A NOVEL MECHANISM OF RESISTANCE TO BCL2 PATHWAY INHIBITORS IN AML CELL LINES

Abstract

Acute myeloid leukemia (AML) is disease with poor outcomes, especially in older patients. Inhibition of apoptosis is a key mechanism by which leukemic cells proliferate and resist chemotherapy. Venetoclax (ABT199) is a selective inhibitor of the anti-apoptotic protein BCL2 that has been approved for use in combination with hypomethylating agents or with low dose cytarabine in elderly or unfit AML patients. The acquisition of resistance to ABT199 has been a cause of concern in venetoclax-based treatment approaches. Two major mechanisms of ABT199 resistance have been reported, namely: BCL2 mutations that affect drug binding (G101V, D103Y, etc.,) or over-expression of pro-survival proteins such as MCL-1 and BCL-xL. In order to study the mechanisms of acquired ABT199 resistance in AML, we created ABT199 resistant MV4-11 cells by treating the MV4-11 cells with incremental doses of ABT199. Five clones of ABT199 resistant cells, picked from semisolid media, were studied in detail. The resistant clones demonstrated a 160-350-fold higher resistance to ABT199 as compared to the parental MV4-11 cells. The resistant clones also demonstrated co-resistance to ABT-737 (BCL-2 and BCL-XL inhibitor), S63845 (MCL-1 inhibitor), and S55746 (BCL-2 inhibitor). There was no change in MCL-1 and BCL-XL at the protein level, however, there was a loss of BAX expression at RNA and protein levels in all resistant clones. Cytoscan HD arrays revealed a micro-deletion in the promoter region and the first 3 exons of the BAX gene in an ABT199 resistant clone. These deletions were further confirmed by performing genomic DNA PCR in all ABT199 resistant clones. In summary, we are reporting a novel mechanism of ABT199 resistance by genomic deletion of BAX in an AML cell line model. Loss of BAX renders the inhibitors of the BCL-2 pathway ineffective in inducing apoptosis. Alternative mechanisms of apoptosis induction need to be explored to overcome BAX deletion-induced resistance. BAX expression level/genomic status will be relevant in AML patients undergoing Venetoclax treatment.