Putting the STING back into BH3-mimetic drugs for TP53-mutant blood cancers

TP53-mutant blood cancers remain a clinical challenge. BH3-mimetic drugs inhibit BCL-2 pro-survival proteins, inducing cancer cell apoptosis. Despite acting downstream of p53, functional p53 is required for maximal cancer cell killing by BH3-mimetics through an unknown mechanism. Here, we report p53 is activated following BH3-mimetic induced mitochondrial outer membrane permeabilization, leading to BH3-only protein induction and thereby potentiating the pro-apoptotic signal. TP53-deficient lymphomas lack this feedforward loop, providing opportunities for survival and disease relapse after BH3-mimetic treatment. The therapeutic barrier imposed by defects in TP53 can be overcome by direct activation of the cGAS/STING pathway, which promotes apoptosis of blood cancer cells through p53-independent BH3-only protein upregulation. Combining clinically relevant STING agonists with BH3-mimetic drugs efficiently kills TRP53/TP53-mutant mouse B lymphoma, human NK/T lymphoma, and acute myeloid leukemia cells. This represents a promising therapy regime that can be fast-tracked to tackle TP53-mutant blood cancers in the clinic.

STAT5 activation promotes progression and chemotherapy resistance in early T-cell precursor acute lymphoblastic leukemia

Interleukin-7 (IL-7) supports the growth and chemoresistance of T-cell acute lymphoblastic leukemia (T-ALL), particularly the early T-cell precursor subtype (ETP-ALL), which frequently has activating mutations of IL-7 signaling. Signal transducer and activator of transcription (STAT5) is an attractive therapeutic target because it is almost universally activated in ETP-ALL, even in the absence of mutations of upstream activators such as the IL-7 receptor (IL-7R), Janus kinase, and Fms-like tyrosine kinase 3 (FLT3). To examine the role of activated STAT5 in ETP-ALL, we have used a Lmo2-transgenic (Lmo2Tg) mouse model in which we can monitor chemoresistant preleukemia stem cells (pre-LSCs) and leukemia stem cells (LSCs) that drive T-ALL development and relapse following chemotherapy. Using IL-7R-deficient Lmo2Tg mice, we show that IL-7 signaling was not required for the formation of pre-LSCs but essential for their expansion and clonal evolution into LSCs to generate T-ALL. Activated STAT5B was sufficient for the development of T-ALL in IL-7R-deficient Lmo2Tg mice, indicating that inhibition of STAT5 is required to block the supportive signals provided by IL-7. To further understand the role of activated STAT5 in LSCs of ETP-ALL, we developed a new transgenic mouse that enables T-cell specific and doxycycline-inducible expression of the constitutively activated STAT5B1∗6 mutant. Expression of STAT5B1∗6 in T cells had no effect alone but promoted expansion and chemoresistance of LSCs in Lmo2Tg mice. Pharmacologic inhibition of STAT5 with pimozide-induced differentiation and loss of LSCs, while enhancing response to chemotherapy. Furthermore, pimozide significantly reduced leukemia burden in vivo and overcame chemoresistance of patient-derived ETP-ALL xenografts. Overall, our results demonstrate that STAT5 is an attractive therapeutic target for eradicating LSCs in ETP-ALL.

Acquired mutations in BAX confer resistance to BH3-mimetic therapy in acute myeloid leukemia

Randomized trials in acute myeloid leukemia (AML) have demonstrated improved survival by the BCL-2 inhibitor venetoclax combined with azacitidine in older patients, and clinical trials are actively exploring the role of venetoclax in combination with intensive chemotherapy in fitter patients with AML. As most patients still develop recurrent disease, improved understanding of relapse mechanisms is needed. We find that 17% of patients relapsing after venetoclax-based therapy for AML have acquired inactivating missense or frameshift/nonsense mutations in the apoptosis effector gene BAX. In contrast, such variants were rare after genotoxic chemotherapy. BAX variants arose within either leukemic or preleukemic compartments, with multiple mutations observed in some patients. In vitro, AML cells with mutated BAX were competitively selected during prolonged exposure to BCL-2 antagonists. In model systems, AML cells rendered deficient for BAX, but not its close relative BAK, displayed resistance to BCL-2 targeting, whereas sensitivity to conventional chemotherapy was variable. Acquired mutations in BAX during venetoclax-based therapy represent a novel mechanism of resistance to BH3-mimetics and a potential barrier to the long-term efficacy of drugs targeting BCL-2 in AML.

Epigenetic Activation of Plasmacytoid DCs Drives IFNAR-Dependent Therapeutic Differentiation of AML

Pharmacologic inhibition of epigenetic enzymes can have therapeutic benefit against hematologic malignancies. In addition to affecting tumor cell growth and proliferation, these epigenetic agents may induce antitumor immunity. Here, we discovered a novel immunoregulatory mechanism through inhibition of histone deacetylases (HDAC). In models of acute myeloid leukemia (AML), leukemia cell differentiation and therapeutic benefit mediated by the HDAC inhibitor (HDACi) panobinostat required activation of the type I interferon (IFN) pathway. Plasmacytoid dendritic cells (pDC) produced type I IFN after panobinostat treatment, through transcriptional activation of IFN genes concomitant with increased H3K27 acetylation at these loci. Depletion of pDCs abrogated panobinostat-mediated induction of type I IFN signaling in leukemia cells and impaired therapeutic efficacy, whereas combined treatment with panobinostat and IFNα improved outcomes in preclinical models. These discoveries offer a new therapeutic approach for AML and demonstrate that epigenetic rewiring of pDCs enhances antitumor immunity, opening the possibility of exploiting this approach for immunotherapies.

SIGNIFICANCE

We demonstrate that HDACis induce terminal differentiation of AML through epigenetic remodeling of pDCs, resulting in production of type I IFN that is important for the therapeutic effects of HDACis. The study demonstrates the important functional interplay between the immune system and leukemias in response to HDAC inhibition. This article is highlighted in the In This Issue feature, p. 1397.

Small molecule inhibition of Dynamin-dependent endocytosis targets multiple niche signals and impairs leukemia stem cells

Intensive chemotherapy for acute leukemia can usually induce complete remission, but fails in many patients to eradicate the leukemia stem cells responsible for relapse. There is accumulating evidence that these relapse-inducing cells are maintained and protected by signals provided by the microenvironment. Thus, inhibition of niche signals is a proposed strategy to target leukemia stem cells but this requires knowledge of the critical signals and may be subject to compensatory mechanisms. Signals from the niche require receptor-mediated endocytosis, a generic process dependent on the Dynamin family of large GTPases. Here, we show that Dynole 34-2, a potent inhibitor of Dynamin GTPase activity, can block transduction of key signalling pathways and overcome chemoresistance of leukemia stem cells. Our results provide a significant conceptual advance in therapeutic strategies for acute leukemia that may be applicable to other malignancies in which signals from the niche are involved in disease progression and chemoresistance.

The tumour microenvironment provides signals to support leukaemic stem cells (LSC) maintenance and chemoresistance. Here, the authors show that disrupting niche-associated signalling by inhibiting receptor-mediated endocytosis with a dynamin GTPase inhibitor overcomes chemoresistance of LSC.