The PML hub: An emerging actor of leukemia therapies

Conventional chemotherapy: millions of cures, unresolved therapeutic index

In recent decades, millions of patients with cancer have been cured by chemotherapy alone. By 'cure', we mean that patients with cancers that would be fatal if left untreated receive a time-limited course of chemotherapy and their cancer disappears, never to return. In an era when hundreds of thousands of cancer genomes have been sequenced, a remarkable fact persists: in most patients who have been cured, we still do not fully understand the mechanisms underlying the therapeutic index by which the tumour cells are killed, but normal cells are somehow spared. In contrast, in more recent years, patients with cancer have benefited from targeted therapies that usually do not cure but whose mechanisms of therapeutic index are, at least superficially, understood. In this Perspective, we will explore the various and sometimes contradictory models that have attempted to explain why chemotherapy can cure some patients with cancer, and what gaps in our understanding of the therapeutic index of chemotherapy remain to be filled. We will summarize principles which have benefited curative conventional chemotherapy regimens in the past, principles which might be deployed in constructing combinations that include modern targeted therapies.

RPS 2.0: an updated database of RNAs involved in liquid-liquid phase separation

Liquid-liquid phase separation (LLPS) is a crucial process for the formation of biomolecular condensates such as coacervate droplets, P-bodies and stress granules, which play critical roles in many physiological and pathological processes. Increasing studies have shown that not only proteins but also RNAs play a critical role in LLPS. To host LLPS-associated RNAs, we previously developed a database named 'RPS' in 2021. In this study, we present an updated version RPS 2.0 (https://rps.renlab.cn/) to incorporate the newly generated data and to host new LLPS-associated RNAs driven by post-transcriptional regulatory mechanisms. Currently, RPS 2.0 hosts 171 301 entries of LLPS-associated RNAs in 24 different biomolecular condensates with four evidence types, including 'Reviewed', 'High-throughput (LLPS enrichment)', 'High-throughput (LLPS perturbation)' and 'Predicted', and five event types, including 'Expression', 'APA', 'AS', 'A-to-I' and 'Modification'. Additionally, extensive annotations of LLPS-associated RNAs are provided in RPS 2.0, including RNA sequence and structure features, RNA-protein/RNA-RNA interactions, RNA modifications, as well as diseases related annotations. We expect that RPS 2.0 will further promote research of LLPS-associated RNAs and deepen our understanding of the biological functions and regulatory mechanisms of LLPS.

PML Nuclear bodies: the cancer connection and beyond

Promyelocytic leukemia (PML) nuclear bodies, membrane-less organelles in the nucleus, play a crucial role in cellular homeostasis. These dynamic structures result from the assembly of scaffolding PML proteins and various partners. Recent crystal structure analyses revealed essential self-interacting domains, while liquid-liquid phase separation contributes to their formation. PML bodies orchestrate post-translational modifications, particularly stress-induced SUMOylation, impacting target protein functions. Serving as hubs in multiple signaling pathways, they influence cellular processes like senescence. Dysregulation of PML expression contributes to diseases, including cancer, highlighting their significance. Therapeutically, PML bodies are promising targets, exemplified by successful acute promyelocytic leukemia treatment with arsenic trioxide and retinoic acid restoring PML bodies. Understanding their functions illuminates both normal and pathological cellular physiology, guiding potential therapies. This review explores recent advancements in PML body biogenesis, biochemical activity, and their evolving biological roles.

Nuclear p62 condensates stabilize the promyelocytic leukemia nuclear bodies by sequestering their ubiquitin ligase RNF4

Liquid-liquid phase separation has emerged as a crucial mechanism driving the formation of membraneless biomolecular condensates, which play important roles in numerous cellular processes. These condensates, found both in the nucleus and cytoplasm, are formed through multivalent, low-affinity interactions between various molecules. P62-containing condensates serve, among other functions, as proteolytic hubs for the ubiquitin-proteasome system. In this study, we investigated the dynamic interplay between nuclear p62 condensates and promyelocytic nuclear bodies (PML-NBs). We show that p62 condensates stabilize PML-NBs under both basal conditions and following exposure to arsenic trioxide which stimulates their degradation. We further show that this effect on the stability of PML-NBs is due to sequestration of their ubiquitin E3 ligase RNF4 in the p62 condensates with subsequent rapid degradation of the ligase. The sequestration of the ligase is made possible by association between the proline-rich domain of the PML protein and the PB1 domain of p62, which results in the formation of a PML-NB shell around the p62 condensates. Importantly, these hybrid structures do not undergo fusion and mixing of their contents which leaves unsolved the mechanism of sequestration of RNF4 in the condensates. These findings suggest an additional possible mechanism of PML-NB as a tumor suppressor which is mediated via interactions between different biomolecular condensates.

Nuclear export of PML promotes p53-mediated apoptosis and ferroptosis

Promyelocytic leukemia protein (PML), a tumor suppressor protein, plays a key role in cell cycle regulation, apoptosis, senescence and cellular metabolism. Here, we report that PML promotes apoptosis and ferroptosis. Our data showed that PML over-expression inhibited cell proliferation and migration. PML over-expression increased apoptotic cells, nuclear condensation and the loss of mitochondrial membrane potential, accompanied by regulation of Bcl-2 family proteins and reactive oxygen species (ROS) level, suggesting that PML enhanced apoptosis. Meanwhile, PML over-expression not only increased lipid ROS accumulation and Malondialdehyde (MDA) content but also downregulated solute carrier family 7 member 11 (SLC7A11) and glutathione peroxidase 4 (GPX4) expression, indicating that PML enhanced ferroptosis. Additionally, knockdown of p53 attenuated the effect of PML on SLC7A11 and GPX4, and inhibited the increase of lipid ROS and ROS by PML over-expression. Moreover, translocation of PML from nucleus to cytoplasm not only promoted apoptosis and ferroptosis, but also inhibited cell proliferation. Taken together, PML promotes apoptosis and ferroptosis, in which the mediation of p53 and the nuclear export of PML play important roles.

Structural Basis of PML-RARA Oncoprotein Targeting by Arsenic Unravels a Cysteine Rheostat Controlling PML Body Assembly and Function

PML nuclear bodies (NB) are disrupted in PML-RARA-driven acute promyelocytic leukemia (APL). Arsenic trioxide (ATO) cures 70% of patients with APL, driving PML-RARA degradation and NB reformation. In non-APL cells, arsenic binding onto PML also amplifies NB formation. Yet, the actual molecular mechanism(s) involved remain(s) elusive. Here, we establish that PML NBs display some features of liquid-liquid phase separation and that ATO induces a gel-like transition. PML B-box-2 structure reveals an alpha helix driving B2 trimerization and positioning a cysteine trio to form an ideal arsenic-binding pocket. Altering either of the latter impedes ATO-driven NB assembly, PML sumoylation, and PML-RARA degradation, mechanistically explaining clinical ATO resistance. This B2 trimer and the C213 trio create an oxidation-sensitive rheostat that controls PML NB assembly dynamics and downstream signaling in both basal state and during stress response. These findings identify the structural basis for arsenic targeting of PML that could pave the way to novel cancer drugs.

SIGNIFICANCE

Arsenic curative effects in APL rely on PML targeting. We report a PML B-box-2 structure that drives trimer assembly, positioning a cysteine trio to form an arsenic-binding pocket, which is disrupted in resistant patients. Identification of this ROS-sensitive triad controlling PML dynamics and functions could yield novel drugs. See related commentary by Salomoni, p. 2505. This article is featured in Selected Articles from This Issue, p. 2489.

Pediatric glioma histone H3.3 K27M/G34R mutations drive abnormalities in PML nuclear bodies

BACKGROUND

Point mutations in histone variant H3.3 (H3.3K27M, H3.3G34R) and the H3.3-specific ATRX/DAXX chaperone complex are frequent events in pediatric gliomas. These H3.3 point mutations affect many chromatin modifications but the exact oncogenic mechanisms are currently unclear. Histone H3.3 is known to localize to nuclear compartments known as promyelocytic leukemia (PML) nuclear bodies, which are frequently mutated and confirmed as oncogenic drivers in acute promyelocytic leukemia.

RESULTS

We find that the pediatric glioma-associated H3.3 point mutations disrupt the formation of PML nuclear bodies and this prevents differentiation down glial lineages. Similar to leukemias driven by PML mutations, H3.3-mutated glioma cells are sensitive to drugs that target PML bodies. We also find that point mutations in IDH1/2-which are common events in adult gliomas and myeloid leukemias-also disrupt the formation of PML bodies.

CONCLUSIONS

We identify PML as a contributor to oncogenesis in a subset of gliomas and show that targeting PML bodies is effective in treating these H3.3-mutated pediatric gliomas.