Targeting Interferon Pathway in TBLR1-RARa-Driven Promyelocytic Leukemia in the Context of ATRA and ATO Unresponsiveness

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia (AML) mostly driven by the t(15;17) translocation that yields the PML-RARα (PR) fusion protein. Apart from PML, other partner genes have been uncovered to fuse with RARα forming rare X-RARα fusion genes. Among them, TBLR1-RARα (TR) is a novel oncogene to induce APL identified in our previous studies, which elicited differed response and worse prognosis in contrast to the canonical PR fusion gene. Despite the textbook rationale of all-trans retinoid acid (ATRA) and arsenic trioxide (ATO) turning most APL patients with PR fusion gene from highly fatal to highly curable, TR-driven APL did not yield long-term remission both in murine models and patients, highlighting the necessity to dissect the mechanism of drug unresponsiveness and explore novel targeted therapies. In this study, we first established two doxycycline-inducible leukemia cell lines to directly initiate the expression of TR and PR fusion protein in U937 cells, respectively. Transcriptome sequencing analysis showed that interferon (IFN) pathways were significantly suppressed in TR rather than PR-induced leukemia cells. Then we administered IFNs as well as the standard regimen of ATRA and ATO in TR- and PR-induced APL, comparing therapy responses and biological phenotypes with in vitro and in vivo studies. Main results are summarized as follows concerning TR- in comparison to PR-driven APL. Firstly, ATO was unresponsive in TR-induced leukemia cells, which failed to elicit oncoprotein degradation, apoptosis and loss of self-renewal. Secondly, lower doses of ATRA were inadequate. Although a lower dose of ATRA was sufficient to trigger differentiation, increasing doses of ATRA further promoted the degradation of oncoprotein and reduced the colony formation capacity of TR leukemic cells, which ultimately conferred survival benefits in TR mice. Thirdly, type I IFNs was promising, which induced apoptosis, cooperated with ATRA to boost differentiation, and exhibited potential to reduce self-renewal, reflecting anti-leukemia efficacy. Finally, type I IFNs combining with ATRA displayed survival advantage to TR mice, and murine STING agonist DMXAA, which acted upstream to produce type I IFNs, significantly endowed TR mice with extended survival time. Based on our results, we further propose mechanistic hypotheses of TR-APL in comparison with the classic PR-APL. In TR-driven APL, despite the unresponsiveness of ATO, we may utilize STING-IFN pathway agonist to upregulate PML expression, which may obliterate self-renewal independent of ATO response triggered by initial PML nuclear body disruption observed in PR-driven APL, providing therapeutic implications for APL driven by rare RARα fusion genes. Our study aims to gain further understanding of TR-driven APL and integrate insights into principles underlying leukemogenesis mediated by rare RARα fusion genes, which may benefit such a therapy-resistant population and enable APL to be a bona fide curable leukemia.