Activation of the PP2A-B56α heterocomplex synergizes with venetoclax therapies in AML through BCL2 and MCL1 modulation.

Venetoclax combination therapies are becoming the standard of care in acute myeloid leukemia (AML). However, the therapeutic benefit of these drugs in older/unfit patients is limited to only a few months, highlighting the need for more effective therapies. Protein phosphatase 2A (PP2A) is a tumor suppressor phosphatase with pleiotropic functions that becomes inactivated in ∼70% of AML cases. PP2A promotes cancer cell death by modulating the phosphorylation state in a variety of proteins along the mitochondrial apoptotic pathway. We therefore hypothesized that pharmacological PP2A reactivation could increase BCL2 dependency in AML cells and, thus, potentiate venetoclax-induced cell death. Here, by using 3 structurally distinct PP2A-activating drugs, we show that PP2A reactivation synergistically enhances venetoclax activity in AML cell lines, primary cells, and xenograft models. Through the use of gene editing tools and pharmacological approaches, we demonstrate that the observed therapeutic synergy relies on PP2A complexes containing the B56α regulatory subunit, of which expression dictates response to the combination therapy. Mechanistically, PP2A reactivation enhances venetoclax-driven apoptosis through simultaneous inhibition of antiapoptotic BCL2 and extracellular signal-regulated kinase signaling, with the latter decreasing MCL1 protein stability. Finally, PP2A targeting increases the efficacy of the clinically approved venetoclax and azacitidine combination in vitro, in primary cells, and in an AML patient-derived xenograft model. These preclinical results provide a scientific rationale for testing PP2A-activating drugs with venetoclax combinations in AML.

A new regulatory mechanism of protein phosphatase 2A activity via SET in acute myeloid leukemia.

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy. Although novel emerging drugs are available, the overall prognosis remains poor and new therapeutic approaches are required. PP2A phosphatase is a key regulator of cell homeostasis and is recurrently inactivated in AML. The anticancer activity of several PP2A-activating drugs (e.g., FTY720) depends on their interaction with the SET oncoprotein, an endogenous PP2A inhibitor that is overexpressed in 30% of AML cases. Elucidation of SET regulatory mechanisms may therefore provide novel targeted therapies for SET-overexpressing AMLs. Here, we show that upregulation of protein kinase p38ß is a common event in AML. We provide evidence that p38ß potentiates SET-mediated PP2A inactivation by two mechanisms: facilitating SET cytoplasmic translocation through CK2 phosphorylation, and directly binding to and stabilizing the SET protein. We demonstrate the importance of this new regulatory mechanism in primary AML cells from patients and in zebrafish xenograft models. Accordingly, combination of the CK2 inhibitor CX-4945, which retains SET in the nucleus, and FTY720, which disrupts the SET-PP2A binding in the cytoplasm, significantly reduces the viability and migration of AML cells. In conclusion, we show that the p38ß/CK2/SET axis represents a new potential therapeutic pathway in AML patients with SET-dependent PP2A inactivation.

A novel FTY720 analogue targets SET-PP2A interaction and inhibits growth of acute myeloid leukemia cells without inducing cardiac toxicity.

Acute myeloid leukemia (AML) is an aggressive disease associated with very poor prognosis. Most patients are older than 60 years, and in this group only 5-15% of cases survive over 5 years. Therefore, it is urgent to develop more effective targeted therapies. Inactivation of protein phosphatase 2 A (PP2A) is a recurrent event in AML, and overexpression of its endogenous inhibitor SET is detected in ~30% of patients. The PP2A activating drug FTY720 has potent anti-leukemic effects; nevertheless, FTY720 induces cardiotoxicity at the anti-neoplastic dose. Here, we have developed a series of non-phosphorylable FTY720 analogues as a new therapeutic strategy for AML. Our results show that the lead compound CM-1231 re-activates PP2A by targeting SET-PP2A interaction, inhibiting cell proliferation and promoting apoptosis in AML cell lines and primary patient samples. Notably, CM-1231 did not induce cardiac toxicity, unlike FTY720, in zebrafish models, and reduced the invasion and aggressiveness of AML cells more than FTY720 in zebrafish xenograft models. In conclusion, CM-1231 is safer and more effective than FTY720; therefore, this compound could represent a novel and promising approach for treating AML patients with SET overexpression.

NUP98 is fused to HOXA9 in a variant complex t(7;11;13;17) in a patient with AML-M2.

The t(7;11)(p15;p15.4) has been reported to fuse the NUP98 gene (11p15), a component of the nuclear pore complex, with the class-1 homeobox gene HOXA9 at 7p15. This translocation has been associated with myeloid leukemias, predominantly acute myeloid leukemia (AML) M2 subtype with trilineage myelodysplastic features, and with a poor prognosis. The derived fusion protein retains the FG repeat motif of NUP98 N-terminus and the homeodomain shared by the HOX genes, acting as an oncogenic transcription factor critical for leukemogenesis. We report here a new complex t(7;11)-variant, i.e., t(7;11;13;17)(p15;p15;p?;p1?2) in a patient with AML-M2 and poor prognosis. The NUP98-HOXA9 fusion transcript was detected by RT-PCR, suggesting its role in the malignant transformation as it has been postulated for other t(7;11)-associated leukemias. No other fusion transcripts involving the NUP98 or HOXA9 genes were present, although other mechanisms involving several genes on chromosomes 13 and 17 may also be involved. To our knowledge, this is the first t(7;11) variant involving NUP98 described in hematological malignancies.