Pathogenesis Of Acute Promyelocytic Leukemia Research Articles

History of Developing Acute Promyelocytic Leukemia Treatment and Role of Promyelocytic Leukemia Bodies.

The story of acute promyelocytic leukemia (APL) discovery, physiopathology, and treatment is a unique journey, transforming the most aggressive form of leukemia to the most curable. It followed an empirical route fueled by clinical breakthroughs driving major advances in biochemistry and cell biology, including the discovery of PML nuclear bodies (PML NBs) and their central role in APL physiopathology. Beyond APL, PML NBs have emerged as key players in a wide variety of biological functions, including tumor-suppression and SUMO-initiated protein degradation, underscoring their broad importance. The APL story is an example of how clinical observations led to the incremental development of the first targeted leukemia therapy. The understanding of APL pathogenesis and the basis for cure now opens new insights in the treatment of other diseases, especially other acute myeloid leukemias.

USP22 regulates APL differentiation via PML-RARα stabilization and IFN repression

Ubiquitin-specific peptidase 22 (USP22) is a deubiquitinating enzyme (DUB) that underlies tumorigenicity, proliferation, cell death and differentiation through deubiquitination of histone and non-histone targets. Ubiquitination determines stability, localization and functions of cell fate proteins and controls cell-protective signaling pathways to surveil cell cycle progression. In a variety of carcinomas, lymphomas and leukemias, ubiquitination regulates the tumor-suppressive functions of the promyelocytic leukemia protein (PML), but PML-specific DUBs, DUB-controlled PML ubiquitin sites and the functional consequences of PML (de)ubiquitination remain unclear. Here, we identify USP22 as regulator of PML and the oncogenic acute promyelocytic leukemia (APL) fusion PML-RARα protein stability and identify a destabilizing role of PML residue K394. Additionally, loss of USP22 upregulates interferon (IFN) and IFN-stimulated gene (ISG) expression in APL and induces PML-RARα stabilization and a potentiation of the cell-autonomous sensitivity towards all-trans retinoic acid (ATRA)-mediated differentiation. Our findings imply USP22-dependent surveillance of PML-RARα stability and IFN signaling as important regulator of APL pathogenesis, with implications for viral mimicry, differentiation and cell fate regulation in other leukemia subtypes.

Mutations at proximal cysteine residues in PML impair ATO binding by destabilizing the RBCC domain.

Acute promyelocytic leukemia (APL) is characterized by the fusion gene promyelocytic leukemia-retinoic acid receptor-alpha (PML-RARA) and is conventionally treated with arsenic trioxide (ATO). ATO binds directly to the RING finger, B-box, coiled-coil (RBCC) domain of PML and initiates degradation of the fusion oncoprotein PML-RARA. However, the mutational hotspot at C212-S220 disrupts ATO binding, leading to drug resistance in APL. Therefore, structural consequences of these point mutations in PML that remain uncertain require comprehensive analysis. In this study, we investigated the structure-based ensemble properties of the promyelocytic leukemia-RING-B-box-coiled-coil (PML-RBCC) domains and ATO-resistant mutations. Oligomeric studies reveal that PML-RBCC wild-type and mutants C212R, S214L, A216T, L217F, and S220G predominantly form tetramers, whereas mutants C213R, A216V, L218P, and D219H tend to form dimers. The stability of the dimeric mutants was lower, exhibiting a melting temperature (Tm) reduction of 30 °C compared with the tetrameric mutants and wild-type PML protein. Furthermore, the exposed surface of the C213R mutation rendered it more prone to protease digestion than that of the C212R mutation. The spectroscopic analysis highlighted ATO-induced structural alterations in S214L, A216V, and D219H mutants, in contrast to C213R, L217F, and L218P mutations. Moreover, the computational analysis revealed that the ATO-resistant mutations C213R, A216V, L217F, and L218P caused changes in the size, shape, and flexibility of the PML-RBCC wild-type protein. The mutations C213R, A216V, L217F, and L218P destabilize the wild-type protein structure due to the adaptation of distinct conformational changes. In addition, these mutations disrupt several hydrogen bonds, including interactions involving C212, C213, and C215, which are essential for ATO binding. The local and global structural features induced by these mutations provide mechanistic insight into ATO resistance and APL pathogenesis.

Research on Different Compound Combinations of Realgar-Indigo Naturalis Formula to Reverse APL Arsenic Resistance By Regulating Autophagy through mTOR Pathway

Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia, and 90% of APL patients carry positive fusion gene of the promyelocytic leukemia protein (PML)- retinoic acid receptor α (RARα), which drives the development of APL. All-trans retinoic acid and arsenic trioxide is able to cure 90% of APL patients by targeting PML-RARα. However, there are still 5%-10% of patients who will relapse due to drug resistance. Compound realgar-indigo naturalis formula (RIF) was developed based on the characteristics of APL pathogenesis. It contains four Chinese herbs: realgar, natural indigo, danshen root and heterophylly falsestarwort root. Clinical studies have confirmed that RIF has a certain efficacy on APL, which shown non-inferiority compared with arsenic trioxide. In addition, RIF combined with all-trans retinoic acid has become the first-line treatment regimen in APL guidelines in China. This study further clarified the efficacy of RIF on arsenic-resistant APL, explored the mechanism, and laid the foundation for further improving the efficacy of APL. O bjective: To investigate the mechanism of different combinations of the compound of RIF in reversing arsenic resistance in APL. M ethods: The arsenic-resistant cell line HL60-PML A216V-RARα, which could stably express PML A216V-RARα, was established by lentiviral transfection. Among compounds of RIF, the active compound of realgar is tetra-arsenic tetra-sulfide (As 4S 4, A), for natural indigo is indirubin (I), for danshen root is tanshinone IIa (T), and the for heterophylly falsestarwort root is total saponins of Radix Pseudostellariae (S). The four compounds were applied on the HL60-PML A216V-RARα cell line. Cell viability was detected by CCK-8 method. Hoechst staining was used to observe cell morphology, Annexin V/PI double staining was applied to detect cell apoptosis, JC-1 detection was used to detect mitochondrial membrane potential, flow cytometry was used to detect CD33 expression to evaluate cell differentiation, and Western blotting was used to observe the changes in the levels of PML A216V-RARα, apoptosis-related factors, PI3K/AKT/mTOR and autophagy in the control, A, ITS and AITS groups. The PI3K inhibitor LY294002 and the autophagy inhibitor Baf A1 were combined and applied to observe the changes in the levels of PML A216V-RARα, mTOR and p62 after blocking the PI3K or autophagy pathway. Results: The arsenic-resistant cell line HL60-PML A216V-RARα was successfully established. Compared with the control group, A, ITS and AITS groups all inhibited the growth of HL60-PML A216V-RARα cells in a time-dependent manner ( P < 0.0001). The IC50 value of AITS was significantly lower than the other groups ( P < 0.0001), and AITS group exhibited the strongest inhibitory effect, followed by ITS and A. Hoechst staining and JC-1 detection both showed that AITS had the strongest effect in inducing apoptosis. Annxin V/PI detection showed that the apoptosis rate of the control group was 10.04%, 15.19% for A group, 12.46% for ITS group, and 28.05% for AITS group. The differences between the groups were statistically significant ( P < 0.0001). Detection of CD33 expression showed: control group (79.72%), ITS group (79.19%), A group (28.60%), AITS group (24.46%), and there were significant differences between groups ( P < 0.001 or P < 0.0001). AITS could reduce PML A216V-RARα protein, caspase3, Bcl-2 levels more than ITS and A ( P < 0.05 or P < 0.0001), and ITS was better than AITS and A in increasing cleaved PARP-1 levels ( P < 0.0001). AITS decreased the levels of PI3K, mTOR, P-mTOR, AKT, p-AKT and p62 and increased the level of LC3B ( P < 0.001 or P < 0.0001). Electron microscopic detection of autophagy showed that AITS had the highest level of autophagy, AITS combined with LY294002 increased the level of p62 and decreased the level of mTOR ( P < 0.0001) compared to the AITS group; while when AITS was combined with Baf A1, the level of p62 increased ( P < 0.0001) and the level of mTOR also increased ( P < 0.001). Autophagy could induced by serum starvation. As the concentration of fetal bovine serum decreased, the effects of degrading PML A216V-RARα and p62, and LC3B level increasing were stronger. C onclusion: Tanshinone IIa, indirubin and total saponins of Radix Pseudostellariae could enhance the effect of tetra-arsenic tetra-sulfide down-regulating PML A216V-RARα, and the mechanism was suggested to be related to inhibiting mTOR pathway to activate autophagy.

Critical role of MXRA7 in differentiation blockade in human acute promyelocytic leukemia cells

The biology of the matrix remodeling associated 7 (MXRA7) gene had been ill-defined. Inspired by public datasets and domestic findings concerning MXRA7, we hypothesized that MXRA7 might be involved in the pathogenesis of leukemia. This study aimed to investigate the role and effect of MXRA7 expression in acute promyelocytic leukemia (APL) cells differentiation and behavior. Bioinformatics of public datasets revealed MXRA7 mRNA was expressed highly in acute myeloid leukemia (AML), especially in APL. High expression of MXRA7 was associated with poor overall survival of AML patients. We confirmed MXRA7 expression was upregulated in APL patients and cell line. Knockdown or overexpression of MXRA7 did not affect the proliferation of NB4 cells directly. Knockdown of MXRA7 in NB4 cells promoted drug induced cell apoptosis, while overexpression of MXRA7 had no obvious influence on the drug induced cell apoptosis. Lowering MXRA7 protein levels in NB4 cells promoted ATRA-induced cell differentiation possibly through decreasing PML-RARα level and increasing PML and RARα levels. Correspondingly, overexpression of MXRA7 showed the consistent results. We also demonstrated that MXRA7 altered the expression of genes involved in leukemic cell differentiation and growth. Knockdown of MXRA7 upregulated the expression levels of C/EBPB, C/EBPD and UBE2L6, and downregulated the expression levels of KDM5A, CCND2 and SPARC. Moreover, knockdown of MXRA7 inhibited the malignancy of NB4 cells in NOD-SCID mice model. In conclusion, this study demonstrated that MXRA7 influences the pathogenesis of APL via regulation of the cell differentiation. The novel findings about the role of MXRA7 in leukemia not only shed lights on the biology of this gene, but also proposed this gene as a new target for APL treatment.

The treatment of acute promyelocytic leukemia in 2023: Paradigm, advances, and future directions.

The transformation of acute promyelocytic leukemia (APL) from an often fatal to highly curable cancer with long-term survival exceeding 90% is one of the greatest and most inspiring successes in oncology. A deeper understanding of the pathogenesis of APL heralded the introduction of highly effective therapies targeting the mutant protein that drives the disease, leading to the chemotherapy-free approach to cure almost all patients. In this review, we discuss the paradigm of treatment of APL in 2023, reinforce the high risk of early death without prompt initiation of treatment at first clinical suspicion, and dedicate a special focus to novel agents and future directions to improve cure rates and quality of life in patients affected by APL.

In Silico Analysis of miR-223 and miR-181b and Their Target Genes in the Pathogenesis of Acute Promyelocytic Leukaemia

Acute promyelocytic leukaemia (APL) is the M3 subtype of acute myeloid leukaemia (AML). The hallmark for APL diagnosis is the reciprocal chromosomal translocation between chromosome 15 and 17 to produce the PML/RARα fusion gene [1]. The combination treatment of all-trans retinoic acid and arsenic trioxide (ATRA- ATO) is 97% effective in treating APL, however the molecular mechanism of the pathogenesis of APL is yet to be understood. The discovery of miRNAs and epigenetics as potential prognostic biomarkers and therapeutic targets in various cancers including APL must be further explored. miR-223 downregulation in APL was widely shown to be associated with worse prognosis in previous studies [2]; while miR-181b upregulation in APL was shown to be associated with worse prognosis in previous studies, although the findings varied based on the ethnic populations that were involved in the studies [3].
 
 In order to understand more about the roles of these miRNAs, this study was conducted with the objective to investigate the roles of targeted genes regulated by mir-223 and mir-181b in APL and other cancers, as well as the signaling pathways involving these miRNAs in the pathogenesis of APL and other cancers. In silico analysis was conducted using miRDB 6.0 database to identify the target genes of miR-223 and miR-181b and filter them to relevance and significant expression in APL cell lines NB4 and HL60. Only the target genes with a target score of 80 or higher, and target expression of 20 RPKM or higher, with confirmation by experimentally validated articles were included.
 
 In silico analysis of miR-223 revealed 162 target genes, while 612 target genes were regulated by miR-181b. The validated target genes were then obtained by comparing the predicted target genes to experimentally validated articles. 116 of the target genes of miR-223 were found to be relevant in cancers in experimentally validated articles, whereas 156 of the target genes of miR-181b were found to be relevant in cancers in experimentally validated articles. The 14 target genes of miR-223 relevant in APL cell lines with expression level more than 20 RPKM and supported by experimentally validated articles include NXT1, ACSL4, LRRFIP1, NOP2, DENR, AIMP1, DEK, LRRC59, YWHAQ, XRCC5, LAMP2, RAB14, ARF6, and ATP6V1B2. On the other hand, the 15 target genes of miR-181b relevant in APL cell lines with expression level more than 20 RPKM and supported by experimentally validated articles include DDX3X, TNPO1, DMXL2, ARF6, ZFP36L2, PTBP3, SCD, GIGYF1, SLC25A37, PNRC2, TCERG1, RBBP7, WDR82, PRKCD, and ACSL4.
 
 GeneCodis 4 database was used for pathway analysis, which identified 159 KEGG signaling pathways involving the target genes of miR-223 and 203 KEGG signaling pathways involving the target genes of miR-181b (Table 1 and Table 2). There were 13 KEGG signaling pathways involving the target genes of miR-223 which were most significant pertaining to APL and other cancers. Meanwhile, there were 14 KEGG signaling pathways involving the target genes of miR-181b which were most significant pertaining to APL and other cancers.
 It can be concluded from this study that miR-223 and miR-181b play important roles in the pathogenesis of APL through the regulation of gene expression and activation of signaling pathways. Nevertheless, the functional roles of miR-223 and miR-181b in APL need to be further investigated through experimental validation by in vitro study using APL cell lines to further aid in the discovery of novel prognostic biomarkers and potential therapeutic targets of APL.

Multimodal approach to characterize the tetrameric form of human PML-RBCC domain and ATO-mediated conformational changes

RING-B box-coiled coil (RBCC) domain of promyelocytic leukemia (PML) comprises a zinc finger motif that is targeted by arsenic trioxide (ATO) to treat acute promyelocytic leukemia (APL) pathogenesis. Preliminary evidence suggests that the PML-RBCC has different functional characteristics, but no structural details have been reported despite its importance in differential expression and cell-cycle regulation. Therefore, the recombinant h-PML-RBCC protein was purified to its homogeneity, and characterized for oligomeric behaviour which indicated that RBCC domain exists as a tetramer in solution. Furthermore, nano-DSF and circular-dichroism demonstrated that the tetrameric form preserves its native conformation along with thermal stability (Tm = 83.2 °C). In-silico-based PML-RBCC structure was used to perform the molecular dynamics simulation for 300 ns in the presence of zinc atoms, which demonstrated the differential dynamic of PML-RBCC tetrameric chains. MMPBSA analysis also indicated the role of hydrophobic interactions that favours stable tetrameric structure of PML-RBCC. ATO-induced secondary and tertiary structure changes were observed in PML-RBCC using circular dichroism and fluorescence spectroscopy. Dynamic light scattering and transmission electron microscopy revealed ATO-induced higher-order oligomerization and aggregation of PML-RBCC. The unique oligomeric nature of the h-PML-RBCC protein and its interactions with ATO will help to understand the mechanism of APL pathogenesis and drug resistance.

Downregulation of hsa-miR-4328 and target gene prediction in Acute Promyelocytic Leukemia

Abstract Introduction: Acute promyelocytic leukemia (APL) is defined by the PML-RARA fusion gene. APL treatment can have significant side effects, therefore the development of optimal therapeutic options is crucial. Although the study of miRNAs is still in its infancy, it has been shown that these molecules are involved in the pathogenesis of neoplasms by modulating the expression of target genes. miRNAs can be considered possible biomarkers in APL and can be used as therapeutic targets or as markers for the therapeutic response. Objectives: The purpose of this study was to determine whether differentially expressed putative miRNAs that have RARA as a target gene could be considered reliable biomarkers for APL. Methods: Using bioinformatics tools, a panel of 6 miRNAs with possible tropism for the RARA gene was selected from miRDB. We evaluated their expression levels in samples from patients with APL (n=20) or from healthy subjects without mutations in genes associated with leukemia or myeloproliferative diseases (n=21). Results: All 6 putative miRNAs were identified using electrophoresis (hsamir-4299, hsa-mir-4328, hsa-mir-7851-3p, hsa-mir-6827-5p, hsa-mir-6867-5p, hsa-mir-939-5p). Of the six miRNAs, hsa-mir-4328 is deeply downregulated in subjects diagnosed with APL compared to healthy subjects, whereas hsa-mir-4299 and hsa-mir-7851-3p show small differences in expression between the two study groups, but without statistical significance. Our results suggest that hsa-mir-4328 may have a role in the pathogenesis of APL and may represent a new biomarker for this type of leukemia. Key Words: miRNA, APL, leukemia, bioinformatics.

PRMT5-mediated RNF4 methylation promotes therapeutic resistance of APL cells to As2O3 by stabilizing oncoprotein PML-RARα.

Acute promyelocytic leukemia (APL) is a hematological malignancy driven by the oncoprotein PML-RARα, which can be treated with arsenic trioxide (As2O3) or/and all-trans retinoic acid. The protein arginine methyltransferase 5 (PRMT5) is involved in tumorigenesis. However, little is known about the biological function and therapeutic potential of PRMT5 in APL. Here, we show that PRMT5 is highly expressed in APL patients. PRMT5 promotes APL by interacting with PML-RARα and suppressing its ubiquitination and degradation. Mechanistically, PRMT5 attenuates the interaction between PML-RARα and its ubiquitin E3 ligase RNF4 by methylating RNF4 at Arg164. Notably, As2O3 treatment triggers the dissociation of PRMT5 from PML nuclear bodies, attenuating RNF4 methylation and promoting RNF4-mediated PML-RARα ubiquitination and degradation. Moreover, knockdown of PRMT5 and pharmacological inhibition of PRMT5 with the specific inhibitor EPZ015666 significantly inhibit APL cells growth. The combination of EPZ015666 with As2O3 shows synergistic effects on As2O3-induced differentiation of bone marrow cells from APL mice, as well as on apoptosis and differentiation of primary APL cells from APL patients. These findings provide mechanistic insight into the function of PRMT5 in APL pathogenesis and demonstrate that inhibition of PRMT5, alone or in combination with As2O3, might be a promising therapeutic strategy against APL.

Effects of arsenic on the topology and solubility of promyelocytic leukemia (PML)-nuclear bodies.

Promyelocytic leukemia (PML) proteins are involved in the pathogenesis of acute promyelocytic leukemia (APL). Trivalent arsenic (As3+) is known to cure APL by binding to cysteine residues of PML and enhance the degradation of PML-retinoic acid receptor α (RARα), a t(15;17) gene translocation product in APL cells, and restore PML-nuclear bodies (NBs). The size, number, and shape of PML-NBs vary among cell types and during cell division. However, topological changes of PML-NBs in As3+-exposed cells have not been well-documented. We report that As3+-induced solubility shift underlies rapid SUMOylation of PML and late agglomeration of PML-NBs. Most PML-NBs were toroidal and granular dot-like in GFPPML-transduced CHO-K1 and HEK293 cells, respectively. Exposure to As3+ and antimony (Sb3+) greatly reduced the solubility of PML and enhanced SUMOylation within 2 h in the absence of changes in the number and size of PML-NBs. However, the prolonged exposure to As3+ and Sb3+ resulted in agglomeration of PML-NBs. Exposure to bismuth (Bi3+), another Group 15 element, did not induce any of these changes. ML792, a SUMO activation inhibitor, reduced the number of PML-NBs and increased the size of the NBs, but had little effect on the As3+-induced solubility change of PML. These results warrant the importance of As3+- or Sb3+-induced solubility shift of PML for the regulation intranuclear dynamics of PML-NBs.

Deneddylation of PML/RARα reconstructs functional PML nuclear bodies via orchestrating phase separation to eradicate APL.

Acute promyelocytic leukemia (APL) is driven by the oncoprotein PML/RARα, which destroys the architecture of PML nuclear bodies (NBs). PML NBs are critical to tumor suppression, and their disruption mediated by PML/RARα accelerates APL pathogenesis. However, the mechanisms of PML NB disruption remain elusive. Here, we reveal that the failure of NB assembly in APL results from neddylation-induced aberrant phase separation of PML/RARα. Mechanistically, PML/RARα is neddylated in the RARα moiety, and this neddylation enhances its DNA-binding ability and further impedes the phase separation of the PML moiety, consequently disrupting PML NB construction. Accordingly, deneddylation of PML/RARα restores its phase separation process to reconstruct functional NBs and activates RARα signaling, thereby suppressing PML/RARα-driven leukemogenesis. Pharmacological inhibition of neddylation by MLN4924 eradicates APL cells both in vitro and in vivo. Our work elucidates the neddylation-destroyed phase separation mechanism for PML/RARα-driven NB disruption and highlights targeting neddylation for APL eradication.

Acute Promyelocytic Leukemia with t(2;3): An Unusual Additional Chromosomal Abnormality.

Acute promyelocytic leukemia is a distinct subset of acute myeloid leukemia with characteristic clinical, morphological and genetic features. The gene product PML-RAR alpha resulting from reciprocal t(15;17) translocation, plays a pivotal role in the pathogenesis of acute promyelocytic leukemia and classified as favorable cytogenetic features. We are describing an unusual additional chromosomal abnormality t(2;3) in APL patient.

PML-RARa of the Short but Not the Long/Variant Isoform Initiate from CD34+ TIM-3+ LSCs with Hierarchical Leukemic Organization

Acute promyelocytic leukemia (APL) is successfully treated with PML-RARa-targeted therapies. APL is divided into two groups with distinct PML-RARa subtypes: Long/variant (the L/V type) and short (the S type) isoforms. The L/V type PML-RARa is observed ~70% of APL patients, whereas the S type isoform, which lacks nucleus localization sequence located in exon6 of PML gene, is involved in ~30% of APL cases. PML-RARa contributes to leukemogenesis presumably by repressing transcriptional activities via its DNA binding properties in nucleus. However, recent studies have shown that PML-RARa mainly localizes in nucleus in the L/V type APL, whereas it dominantly distributes in cytoplasms in the S type APL. This study was planned to clarify difference in the role of PML-RARa in leukemogenesis in APL patients with the L/V or the S type.We performed the transcriptome analysis of APL blast cells from 4 cases each of the L/V type and the S type. Surprisingly, APL cells of the L/V and the S type showed significantly different gene expression pattern by gene set enrichment analysis. For example, pathways for apoptosis involving BCL-2, prostaglandin and T lymphocyte maturation were enriched only in the S type APL, whereas cytokine pathways were mainly enriched in the L/V type APL. More interestingly, the gene sets associated with functional leukemic stem cells (LSCs) (K.Eppert, et al. Nat Med, 2011) were enriched in the S type APL (NES 1.72, FDR q-value 0.083).In most AML of FAB types except for M3 (APL), self-renewing LSCs are concentrated in the CD34+CD38- population, and transplantation of CD34+CD38- LSCs of AML patients resulted in reconstitution of human AML in xenograft models. Based on this data, it is widely accepted that AML is hierarchically organized and is maintained by LSCs, like normal hematopoiesis is initiated from normal HSCs. However, it has been reported that reconstitution of human APL is usually difficult in xenograft models (D.Thomas and R.Majeti. Blood, 2017), although human APL was reconstituted in rare cases by a recent report (S.Patel, et al. Leukemia, 2012). We have reported that self-renewing AML LSCs express TIM-3, a signaling molecule activating the β-catenin pathway (Y.Kikushige, et al. Cell Stem Cell, 2015), but TIM-3 expression was negative in leukemic cells in most cases of APL. However, because the S type but not the L/V type APL has LSC signatures by transcriptome analysis, we revisited to test the LSC activity of the S type APL.By FACS analysis, CD34+CD38- cells were rarely found in the L/V type APL patients, and they largely lacked TIM-3 expression and did not have PML-RARa transcript (n=10), suggesting that these CD34+CD38-TIM-3- cells may be residual normal hematopoietic stem cells (HSCs). In contrast, CD34+CD38- cells from the S type APL strongly expressed TIM-3 and PML-RARa transcript (n=12), suggesting that they may be functional LSCs. We then tested whether CD34+ cells of the S type APL can reconstitute human APL in a xenograft model. Purified 0.4-3.0 x106 CD34+ APL cells were transplanted into sublethally irradiated NSG mice. Strikingly in the S type APL, CD34+ but not CD34- cells reconstituted human PML-RARa+ APL in NSG mice in 3 out of 5 patient tested, whereas in the L/V type APL, CD34+ cells were incapable of reconstitution in all 3 cases tested. The hCD45+CD34+TIM-3+ cells harvested from the primary NSG recipients again reconstituted the S type APL in serial xenotransplantion experiments. These results collectively suggest that the S type APL have LSCs capable of self-renewal within the CD34+ leukemic population, and may use TIM-3 signaling for self-renewal as in other FAB types of AML.Thus, APL with the S type, but not the L/V type PML-RARa have expression signature related to LSCs, and have functional LSCs with TIM-3 expression. Therefore, the leukemic hematopoiesis of APL with the S type is hierarchically organized as in case of AML of other FAB types. The mechanisms by which these 2 types of PML-RARa exert different functions to the LSC property should be critical to understand pathogenesis of APL. DisclosuresNo relevant conflicts of interest to declare.

A novel fusion protein TBLR1-RAR\u03b1 acts as an oncogene to induce murine promyelocytic leukemia: identification and treatment strategies

Acute promyelocytic leukemia (APL) is characterized by a specific chromosome translocation involving RARα and its fusion partners. For decades, the advent of all-trans retinoic acid (ATRA) synergized with arsenic trioxide (As2O3) has turned most APL from highly fatal to highly curable. TBLR1-RARα (TR) is the tenth fusion gene of APL identified in our previous study, with its oncogenic role in the pathogenesis of APL not wholly unraveled. In this study, we found the expression of TR in mouse hematopoietic progenitors induces blockade of differentiation with enhanced proliferative capacity in vitro. A novel murine transplantable leukemia model was then established by expressing TR fusion gene in lineage-negative bone marrow mononuclear cells. Characteristics of primary TR mice revealed a rapid onset of aggressive leukemia with bleeding diathesis, which recapitulates human APL more accurately than other models. Despite the in vitro sensitivity to ATRA-induced cell differentiation, neither ATRA monotherapy nor combination with As2O3 confers survival benefit to TR mice, consistent with poor clinical outcome of APL patients with TR fusion gene. Based on histone deacetylation phenotypes implied by bioinformatic analysis, HDAC inhibitors demonstrated significant survival superiority in the survival of TR mice, yielding insights into clinical efficacy against rare types of APL.

C/EBPα is indispensable for PML/RARα-mediated suppression of long non-coding RNA NEAT1 in acute promyelocytic leukemia cells.

Better understanding of the transcriptional regulatory network in acute promyelocytic leukemia (APL) cells is critical to illustrate the pathogenesis of other types of acute myeloid leukemia. Previous studies have primarily focused on the retinoic acid signaling pathway and how it is interfered with by promyelocytic leukemia/retinoic acid receptor-α (PML/RARα) fusion protein. However, this hardly explains how APL cells are blocked at the promyelocytic stage. Here, we demonstrated that C/EBPα bound and transactivated the promoter of long non-coding RNA NEAT1, an essential element for terminal differentiation of APL cells, through C/EBP binding sites. More importantly, PML/RARα repressed C/EBPα-mediated transactivation of NEAT1 through binding to NEAT1 promoter. Consistently, mutation of the C/EBP sites or deletion of retinoic acid responsive elements (RAREs) and RARE half motifs abrogated the PML/RARα-mediated repression. Moreover, silencing of C/EBPα attenuated ATRA-induced NEAT1 upregulation and APL cell differentiation. Finally, simultaneous knockdown of C/EBPα and C/EBPβ reduces ATRA-induced upregulation of C/EBPε and dramatically impaired NEAT1 activation and APL cell differentiation. In sum, C/EBPα binds and transactivates NEAT1 whereas PML/RARα represses this process. This study describes an essential role for C/EBPα in PML/RARα-mediated repression of NEAT1 and suggests that PML/RARα could contribute to the pathogenesis of APL through suppressing C/EBPα targets.

Suppression of proteasome induces apoptosis in APL cells and increases chemo-sensitivity to arsenic trioxide: Proposing a perception in APL treatment

In the last three decades, the pathogenesis of acute promyelocytic leukemia (APL) has been mostly studied with regard to the oncogenic role of PML/RAR fusion protein; however, the latest discoveries have stated that the concerns with the treatment of APL patients would not be resolved until the role of aberrant networks is overlooked. The present study was designed to evaluate the anti-cancer property of second-generation of the proteasome inhibitors carfilzomib (CFZ) on APL-derived NB4 cells. Our results showed that pharmacologic targeting of proteasome in NB4 reduced the proliferative rate of malignant cells through a c-Myc-mediated G2/M cell cycle arrest. Moreover, we found that the suppression of proteasome was coupled with the induction of apoptotic NB4 cell death, which is probably mediated through down-regulation of anti-apoptotic target genes. Interestingly, our results suggested that the suppression of the autophagy system using chloroquine could serve as a mechanism through which the cytotoxicity of CFZ in APL cells was ameliorated. Finally, and consistent with the favorable efficacy of single agent of CFZ, we also noted an intensifying effect of the inhibitor on the anti-leukemic activity of arsenic trioxide (ATO) when it was used in combination. Overall, this study suggests that pharmaceutical targeting of proteasome using CFZ, either as a single agent or in combination with ATO, could be a promising mechanism through which the obstacle on the way of APL would be tackled; however, further investigations are needed to determine the advantages of the inhibitor in clinical applications.

Predicting Resistance to the Combination of ATO and ATRA in APL Patients with PML-Rara Fusions, Using a Computational Biology Modeling Approach: Mycare-021-01

Background: Acute promyelocytic leukemia (APL) is a biologically and clinically distinct subtype of acute myeloid leukemia (AML) with unique molecular pathogenesis, clinical manifestations, and treatment. APL is cytogenetically characterized by a balanced translocation t(15;17) (q24;q21), which involves the retinoic acid receptor alpha (RARA) gene on chromosome 17 and the promyelocytic leukemia (PML) gene on chromosome 15 that results in a PML-RARA fusion gene (PMID: 30575821). The PML-RARA fusion gene is the most critical event involved in the pathogenesis of APL, reported in 99% of APL patients (PMID: 32182684). The fusion confers a selective sensitivity to the targeted drugs, arsenic trioxide (ATO) and all-trans-retinoic acid (ATRA), with response rates over 90% (PMID: 31635329). However, the mechanism of resistance in the minority of non-responders is not well understood. This study used the Cellworks Omics Biology Model (CBM) to predict response to the combination of ATO-ATRA in patients harboring the PML-RARA fusion and identify mechanisms of resistance. Methods: Outcomes of 30 APL patients treated with ATRA or ATRA plus ATO were compared with outcomes predicted by CBM (Table 1). Genomic data from 6 publications (Table 2) derived from whole exome sequencing (WES), targeted next-generation sequencing (NGS), copy number variation (CNV) and/or karyotype data were used. All data was anonymized, de-identified and exempt from IRB review. The available genomic data for each profile was entered into the CBM which generates a patient-specific disease protein network model using PubMed and other online resources. The CBM predicts the patient-specific biomarker and phenotype response of a personalized diseased cell to drug agents, radiation and cell signaling. Disease biomarkers that are unique to each patient were identified within the protein network models. ATO and ATRA were simulated on all 30 patient cases. The treatment impact was assessed by quantitatively measuring the drug's effect on a cell growth score which is a composite of the quantified values for cell proliferation, survival, and apoptosis, along with the simulated impact on each patient-specific disease biomarker score. Each patient-specific model was also digitally screened to identify response to ATO and ATRA. Results: The CBM correctly predicted the response to ATO and ATRA in 28 of 30 cases. The overall prediction accuracy was 93% with a PPV of 100%, NPV of 60%, sensitivity of 93%, and specificity of 100%. In 2 of 30 patients who did not respond to ATO and ATRA, the CBM identified clinically relevant deletions to EZH2, KMT2E, and HIPK2 genes. All three genes are located on chromosome 7 and these non-responders had monosomy 7. Conclusions: The Cellworks Omics Biology Model predicted response to ATO and ATRA in APL patients harboring PML-RARA fusions. Predicting non-response to ATO and ATRA in patients with PML-RARA fusion up-front could prevent ineffective treatment, avoid unnecessary adverse events and reduce treatment costs. Additionally, computational modeling can identify new mechanisms of resistance and suggest alternative regimens for non-responding patients by targeting the patient-specific disease biomarkers unique to each. Disclosures Howard: Servier: Consultancy, Other: Speaker; Boston Scientific: Consultancy; Sanofi: Consultancy, Other: Speaker; EUSA Pharma: Consultancy; Cellworks: Consultancy. Nair:Cellworks Research India Private Limited: Current Employment. Grover:Cellworks Research India Private Limited: Current Employment. Tyagi:Cellworks Research India Private Limited: Current Employment. Kumari:Cellworks Research India Private Limited: Current Employment. Prasad:Cellworks Research India Private Limited: Current Employment. Mitra:Cellworks Research India Private Limited: Current Employment. Lala:Cellworks Research India Private Limited: Current Employment. Azam:Cellworks Research India Private Limited: Current Employment. Gupta:Cellworks Research India Private Limited: Current Employment. Mohapatra:Cellworks Research India Private Limited: Current Employment. G:Cellworks Research India Private Limited: Current Employment. Mundkur:Cellworks Group Inc.: Current Employment. Macpherson:Cellworks Group Inc.: Current Employment. Kapoor:Cellworks Research India Private Limited: Current Employment.

SERS-Based Assessment of MRD in Acute Promyelocytic Leukemia?

Acute promyelocytic leukemia (APL) is characterized by a unique chromosome translocation t(15;17)(q24;q21), which leads to the PML/RARA gene fusion formation. However, it is acknowledged that this rearrangement alone is not able to induce the whole leukemic phenotype. In addition, epigenetic processes, such as DNA methylation, may play a crucial role in leukemia pathogenesis. DNA methylation, catalyzed by DNA methyltransferases (DNMTs), involves the covalent transfer of a methyl group (-CH3) to the fifth carbon of the cytosine ring in the CpG dinucleotide and results in the formation of 5-methylcytosine (5-mC). The aberrant gene promoter methylation can be an alternative mechanism of tumor suppressor gene inactivation. Understanding cancer epigenetics and its pivotal role in oncogenesis, can offer us not only attractive targets for epigenetic treatment but can also provide powerful tools in monitoring the disease and estimating the prognosis. Several genes of interest, such as RARA, RARB, p15, p16, have been studied in APL and their methylation status was correlated with potential diagnostic and prognostic significance. In the present manuscript we comprehensively examine the current knowledge regarding DNA methylation in APL pathogenesis. We also discuss the perspectives of using the DNA methylation patterns as reliable biomarkers for measurable residual disease (MRD) monitoring and as a predictor of relapse. This work also highlights the possibility of detecting aberrant methylation profiles of circulating tumor DNA (ctDNA) through liquid biopsies, using the conventional methods, such as methylation-specific polymerase chain reaction (MS-PCR), sequencing methods, but also revolutionary methods, such as surface-enhanced Raman spectroscopy (SERS).

Classic and Variants APLs, as Viewed from a Therapy Response.

Most acute promyelocytic leukemia (APL) are caused by PML-RARA, a translocation-driven fusion oncoprotein discovered three decades ago. Over the years, several other types of rare X-RARA fusions have been described, while recently, oncogenic fusion proteins involving other retinoic acid receptors (RARB or RARG) have been associated to very rare cases of acute promyelocytic leukemia. PML-RARA driven pathogenesis and the molecular basis for therapy response have been the focus of many studies, which have now converged into an integrated physio-pathological model. The latter is well supported by clinical and molecular studies on patients, making APL one of the rare hematological disorder cured by targeted therapies. Here we review recent data on APL-like diseases not driven by the PML-RARA fusion and discuss these in view of current understanding of “classic” APL pathogenesis and therapy response.