Altered Myeloid Development and Acute Leukemia in Transgenic Mice Expressing PML-RARα Under Control of Cathepsin G Regulatory Sequences

Distinct leukemogenic mechanism of acute promyelocytic leukemia based on genomic structure of PML::RARα

Leukemic stem cells (LSCs) of acute myeloid leukemia (AML) can be enriched in the CD34+CD38- fraction and reconstitute human AML in vivo. However, in acute promyelocytic leukemia (APL), which constitutes 10% of all AML cases and is driven by promyelocytic leukemia-retinoic acid receptor alpha (PML::RARα) fusion genes, the presence of LSCs has long been unidentified because of the difficulty in efficient reconstitution of human APL in vivo. Herein, we show that LSCs of the short-type isoform APL, a subtype of APL defined by different breakpoints of the PML gene, concentrate in the CD34+CD38- fraction and express T cell immunoglobulin mucin-3 (TIM-3). Short-type APL cells exhibited distinct gene expression signatures, including LSC-related genes, compared to the other types of APL. Moreover, CD34+CD38-TIM-3+ short-type APL cells efficiently reconstituted human APL in xenograft models with high penetration, whereas CD34- differentiated APL cells did not. Furthermore, CD34+CD38-TIM-3+ short-type APL cells reconstituted leukemia cells after serial transplantation. Thus, short-type APL was hierarchically organized by self-renewing APL-LSCs. The identification of LSCs in a subset of APL and establishment of an efficient patient-derived xenograft model may contribute to further understanding the APL leukemogenesis and devise individual treatments for the eradication of APL LSCs.

PML::RARA and GATA2 proteins interact via DNA templates to induce aberrant self-renewal in mouse and human hematopoietic cells

The underlying mechanism(s) by which the PML::RARA fusion protein initiates acute promyelocytic leukemia is not yet clear. We defined the genomic binding sites of PML::RARA in primary mouse and human hematopoietic progenitor cells with V5-tagged PML::RARA, using anti-V5-PML::RARA chromatin immunoprecipitation sequencing and CUT&RUN approaches. Most genomic PML::RARA binding sites were found in regions that were already chromatin-accessible (defined by ATAC-seq) in unmanipulated, wild-type promyelocytes, suggesting that these regions are "open" prior to PML::RARA expression. We found that GATA binding motifs, and the direct binding of the chromatin "pioneering factor" GATA2, were significantly enriched near PML::RARA binding sites. Proximity labeling studies revealed that PML::RARA interacts with ~250 proteins in primary mouse hematopoietic cells; GATA2 and 33 others require PML::RARA binding to DNA for the interaction to occur, suggesting that binding to their cognate DNA target motifs may stabilize their interactions. In the absence of PML::RARA, Gata2 overexpression induces many of the same epigenetic and transcriptional changes as PML::RARA. These findings suggested that PML::RARA may indirectly initiate its transcriptional program by activating Gata2 expression: Indeed, we demonstrated that inactivation of Gata2 prior to PML::RARA expression prevented its ability to induce self-renewal. These data suggested that GATA2 binding creates accessible chromatin regions enriched for both GATA and Retinoic Acid Receptor Element motifs, where GATA2 and PML::RARA can potentially bind and interact with each other. In turn, PML::RARA binding to DNA promotes a feed-forward transcriptional program by positively regulating Gata2 expression. Gata2 may therefore be required for PML::RARA to establish its transcriptional program.

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.

Caloric restriction leads to druggable LSD1-dependent cancer stem cells expansion

Caloric Restriction (CR) has established anti-cancer effects, but its clinical relevance and molecular mechanism remain largely undefined. Here, we investigate CR's impact on several mouse models of Acute Myeloid Leukemias, including Acute Promyelocytic Leukemia, a subtype strongly affected by obesity. After an initial marked anti-tumor effect, lethal disease invariably re-emerges. Initially, CR leads to cell-cycle restriction, apoptosis, and inhibition of TOR and insulin/IGF1 signaling. The relapse, instead, is associated with the non-genetic selection of Leukemia Initiating Cells and the downregulation of double-stranded RNA (dsRNA) sensing and Interferon (IFN) signaling genes. The CR-induced adaptive phenotype is highly sensitive to pharmacological or genetic ablation of LSD1, a lysine demethylase regulating both stem cells and dsRNA/ IFN signaling. CR + LSD1 inhibition leads to the re-activation of dsRNA/IFN signaling, massive RNASEL-dependent apoptosis, and complete leukemia eradication in ~90% of mice. Importantly, CR-LSD1 interaction can be modeled in vivo and in vitro by combining LSD1 ablation with pharmacological inhibitors of insulin/IGF1 or dual PI3K/MEK blockade. Mechanistically, insulin/IGF1 inhibition sensitizes blasts to LSD1-induced death by inhibiting the anti-apoptotic factor CFLAR. CR and LSD1 inhibition also synergize in patient-derived AML and triple-negative breast cancer xenografts. Our data provide a rationale for epi-metabolic pharmacologic combinations across multiple tumors.

Function of PML-RARA in Acute Promyelocytic Leukemia

The transformation of acute promyelocytic leukemia (APL) from the most fatal to the most curable subtype of acute myeloid leukemia (AML), with long-term survival exceeding 90%, has represented one of the most exciting successes in hematology and in oncology. APL is a paradigm for oncoprotein-targeted cure.APL is caused by a 15/17 chromosomal translocation which generates the PML-RARA fusion protein and can be cured by the chemotherapy-free approach based on the combination of two therapies targeting PML-RARA: retinoic acid (RA) and arsenic. PML-RARA is the key driver of APL and acts by deregulating transcriptional control, particularly RAR targets involved in self-renewal or myeloid differentiation, also disrupting PML nuclear bodies. PML-RARA mainly acts as a modulator of the expression of specific target genes: genes whose regulatory elements recruit PML-RARA are not uniformly repressed but also may be upregulated or remain unchanged. RA and arsenic trioxide directly target PML-RARA-mediated transcriptional deregulation and protein stability, removing the differentiation block at promyelocytic stage and inducing clinical remission of APL patients.

PML/RARa leukemia induced murine model for immunotherapy evaluation

Even though leukemia murine models are valuable tools for new drug therapy studies, most of these models consist of immunocompromised mice, which do not exhibit immune responses. In order to obtain an adequate leukemia model, we established an acute promyelocytic leukemia transplantation-based model (PML/RARa) in immunocompetent BALB/c mice, thus making it possible to study drug-induced cellular immune responses in leukemia. The development of PML/RARa leukemia was confirmed by leukocytosis (76.27 ± 21.8 vs. 3.40 ± 1.06; P < 0.0001), anemia (7.46 ± 1.86 vs. 15.10 ± 0.96; P < 0.0001), and thrombocytopenia (131.85 ± 39.32 vs. 839.50 ± 171.20; P < 0.0001), and the presence of blasts in the peripheral blood of mice (approximately 50% blasts; P < 0.0001), 15 days after the transplants. These findings were corroborated through differential counts, flow cytometry, and in vivo imaging, which indicated increased number of immature cells in the bone marrow (15.75 ± 3.30 vs 6.69 ± 0.55; P < 0.001), peripheral blood (7.88 ± 2.67 vs 1.22 ± 0.89; P < 0.001), and spleen (35.21 ± 4.12 vs 1.35 ± 0.86; P < 0.0001), as well as promyelocytes in the bone marrow (41.23 ± 4.80 vs 5.73 ± 1.50; P < 0.0001), peripheral blood (46.08 ± 7.52 vs 1.10 ± 0.59; P < 0.0001) and spleen (35.31 ± 8.26 vs 2.49 ± 0.29; P < 0.0001) of PML/RARa mice. Compared to basal conditions of untransplanted mice, the PML/RARa mice exhibited frequencies of T lymphocytes CD4 helper = 14.85 ± 2.91 vs 20.77 ± 2.9 in the peripheral blood (P < 0.05); 12.75 ± 1.33 vs 45.90 ± 2.02 in the spleen (P < 0.0001); CD8 cytotoxic = 11.27 ± 3.44 vs 11.05 ± 1.22 in the peripheral blood (P > 0.05); 10.48 ± 1.16 vs 30.02 ± 1.80 in the spleen (P < 0.0001); natural killer (NK) cells = 3.68 ± 1.35 vs 6.84 ± 0.52 in the peripheral blood (P < 0.001); 4.43 ± 0.57 vs 6.40 ± 1.14 in the spleen (P < 0.05); B cells 2.50 ± 0.60 vs 15.20 ± 5.34 in the peripheral blood (P < 0.001); 17.77 ± 4.39 vs 46.90 ± 5.92 in the spleen (P < 0.0001); neutrophils = 5.97% ± 1.88 vs 31.57 ± 9.14 (P < 0.0001); and monocytes = 6.45 ± 2.97 vs 15.85 ± 2.57 (P < 0.001), selected as classical (3.33 ± 3.40 vs 57.80 ± 16.51, P < 0.0001), intermediate (57.42 ± 10.61 vs 21.75 ± 5.90, P < 0.0001), and non-classical monocytes (37.51 ± 10.85 vs 18.08 ± 7.13, P < 0.05) in the peripheral blood; and as classically activated (M1) within in the bone marrow (3.70 ± 0.94 vs 1.88 ± 0.39, P < 0.05) and spleen 15.19 ± 3.32 vs 9.47 ± 1.61, P < 0.05), in addition to alternatively activated (M2) macrophages within the bone marrow (23.06 ± 5.25 vs 1.76 ± 0.74, P < 0.0001) and spleen (46.51 ± 11.18 vs 30.58 ± 2.64, P < 0.05) compartments. All-trans retinoic acid (ATRA) treatment of PML/RARa mice reduced blast (immature cells) in the bone marrow (8.62 ± 1.81 vs 15.76 ± 1.25; P < 0.05) and spleen (8.75 ± 1.31 vs 35.21 ± 1.55; P < 0.0001) with no changes in the peripheral blood (10.13 ± 3.33 vs 7.88 ± 1.01; P > 0.05), as well as reduced promyelocytes in the bone marrow (19.79 ± 4.84 vs 41.23 ± 1.81; P < 0.05), peripheral blood (31.65 ± 3.92 vs 46.09 ± 2.84; P < 0.05) and spleen (24.84 ± 2.03 vs 41.46 ± 2.39; P < 0.001), and increased neutrophils of the peripheral blood (35.48 ± 7.24 vs 7.83 ± 1.40; P < 0.05) which was corroborated by reducing of immature cells and increase of neutrophil in the stained smears from PML/RARa mice, thus confirming that this model can be used in drug development studies. Our results show the effective induction of PML/RARa leukemia in BALB/c mice, thus producing a low-priced and reliable tool for investigating cellular immune responses in leukemia.

Identifying STRN3-RARA as a new fusion gene for acute promyelocytic leukemia

Here we report a new fusion gene, STRN3-RARA, in acute promyelocytic leukemia (APL). It cooperates with UTX deficiency to drive full-blown APL in mice. Although STRN3-RARA leukemia quickly relapses after all-trans retinoic acid treatment, it can be restrained by cepharanthine.

Application of DNA Nanotweezers in biosensing: Nanoarchitectonics and advanced challenges

Deoxyribonucleic acid (DNA) is a carrier of genetic information. DNA hybridization is characterized by predictability, diversity, and specificity owing to the strict complementary base-pairing assembly mode, which stimulates the use of DNA to build a variety of nanomachines, including DNA tweezers, motors, walkers, and robots. DNA nanomachines have become prevalent for signal amplification and transformation in the field of biosensing, providing a new method for constructing highly sensitive sensing analysis strategies. DNA tweezers have exhibited unique advantages in biosensing applications owing to their simple structures and fast responses. The two-state conformation of DNA tweezers, the open and closed states, enable them to open and close autonomously after stimulation, thus facilitating the quick detection of corresponding signal changes of different targets. This review discusses the recent progress in the application of DNA nanotweezers in the field of biosensing, and the trends in their development for application in the field of biosensing are summarized.

Arsenic in medicine: past, present and future

Arsenicals are one of the oldest treatments for a variety of human disorders. Although infamous for its toxicity, arsenic is paradoxically a therapeutic agent that has been used since ancient times for the treatment of multiple diseases. The use of most arsenic-based drugs was abandoned with the discovery of antibiotics in the 1940s, but a few remained in use such as those for the treatment of trypanosomiasis. In the 1970s, arsenic trioxide, the active ingredient in a traditional Chinese medicine, was shown to produce dramatic remission of acute promyelocytic leukemia similar to the effect of all-trans retinoic acid. Since then, there has been a renewed interest in the clinical use of arsenicals. Here the ancient and modern medicinal uses of inorganic and organic arsenicals are reviewed. Included are antimicrobial, antiviral, antiparasitic and anticancer applications. In the face of increasing antibiotic resistance and the emergence of deadly pathogens such as the severe acute respiratory syndrome coronavirus 2, we propose revisiting arsenicals with proven efficacy to combat emerging pathogens. Current advances in science and technology can be employed to design newer arsenical drugs with high therapeutic index. These novel arsenicals can be used in combination with existing drugs or serve as valuable alternatives in the fight against cancer and emerging pathogens. The discovery of the pentavalent arsenic-containing antibiotic arsinothricin, which is effective against multidrug-resistant pathogens, illustrates the future potential of this new class of organoarsenical antibiotics.

Preleukemic Fusion Genes Induced via Ionizing Radiation

Although the prevalence of leukemia is increasing, the agents responsible for this increase are not definitely known. While ionizing radiation (IR) was classified as a group one carcinogen by the IARC, the IR-induced cancers, including leukemia, are indistinguishable from those that are caused by other factors, so the risk estimation relies on epidemiological data. Several epidemiological studies on atomic bomb survivors and persons undergoing IR exposure during medical investigations or radiotherapy showed an association between radiation and leukemia. IR is also known to induce chromosomal translocations. Specific chromosomal translocations resulting in preleukemic fusion genes (PFGs) are generally accepted to be the first hit in the onset of many leukemias. Several studies indicated that incidence of PFGs in healthy newborns is up to 100-times higher than childhood leukemia with the same chromosomal aberrations. Because of this fact, it has been suggested that PFGs are not able to induce leukemia alone, but secondary mutations are necessary. PFGs also have to occur in specific cell populations of hematopoetic stem cells with higher leukemogenic potential. In this review, we describe the connection between IR, PFGs, and cancer, focusing on recurrent PFGs where an association with IR has been established.

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.

A Pathogenic NRAS c.38 G>A (p.G13D) Mutation in RARA Translocation-negative Acute Promyelocytic-like Leukemia with Concomitant Myelodysplastic Syndrome

An acute promyelocytic leukemia (APL) patient not demonstrating the retinoic acid receptor α (RARA) translocation is rare. A 76-year-old man was diagnosed with myelodysplastic syndrome (MDS). After a year, abnormal promyelocytes were detected with pancytopenia and disseminated intravascular coagulopathy. Morphologically, the patient was diagnosed with APL; however, a genetic examination failed to detect RARA translocation. Thereafter, whole-genome sequencing revealed an NRAS missense mutation [c.38 G>A (p.G13D)]. This mutation was not detected in posttreatment bone marrow aspirate, despite residual MDS. Few reports are available on similar cases. Furthermore, the NRAS c.38 G>A mutation may be a novel pathogenic variant exacerbating RARA translocation-negative acute promyelocytic-like leukemia.

Recurrent noncoding somatic and germline WT1 variants converge to disrupt MYB binding in acute promyelocytic leukemia

Genetic alternations can occur at noncoding regions, but how they contribute to cancer pathogenesis is poorly understood. Here, we established a mutational landscape of cis-regulatory regions (CREs) in acute promyelocytic leukemia (APL) based on whole-genome sequencing analysis of paired tumor and germline samples from 24 patients and epigenetic profiling of 16 patients. Mutations occurring in CREs occur preferentially in active enhancers bound by the complex of master transcription factors in APL. Among significantly enriched mutated CREs, we found a recurrently mutated region located within the third intron of WT1, an essential regulator of normal and malignant hematopoiesis. Focusing on noncoding mutations within this WT1 intron, an analysis on 169 APL patients revealed that somatic mutations were clustered into a focal hotspot region, including one site identified as a germline polymorphism contributing to APL risk. Significantly decreased WT1 expression was observed in APL patients bearing somatic and/or germline noncoding WT1 variants. Furthermore, biallelic WT1 inactivation was recurrently found in APL patients with noncoding WT1 variants, which resulted in the complete loss of WT1. The high incidence of biallelic inactivation suggested the tumor suppressor activity of WT1 in APL. Mechanistically, noncoding WT1 variants disrupted MYB binding on chromatin and suppressed the enhancer activity and WT1 expression through destroying the chromatin looping formation. Our study highlights the important role of noncoding variants in the leukemogenesis of APL.

Blockade of deubiquitinase YOD1 degrades oncogenic PML/RARα and eradicates acute promyelocytic leukemia cells

In most acute promyelocytic leukemia (APL) cells, promyelocytic leukemia (PML) fuses to retinoic acid receptor α (RARα) due to chromosomal translocation, thus generating PML/RARα oncoprotein, which is a relatively stable oncoprotein for degradation in APL. Elucidating the mechanism regulating the stability of PML/RARα may help to degrade PML/RARα and eradicate APL cells. Here, we describe a deubiquitinase (DUB)-involved regulatory mechanism for the maintenance of PML/RARα stability and develop a novel pharmacological approach to degrading PML/RARα by inhibiting DUB. We utilized a DUB siRNA library to identify the ovarian tumor protease (OTU) family member deubiquitinase YOD1 as a critical DUB of PML/RARα. Suppression of YOD1 promoted the degradation of PML/RARα, thus inhibiting APL cells and prolonging the survival time of APL cell-bearing mice. Subsequent phenotypic screening of small molecules allowed us to identify ubiquitin isopeptidase inhibitor I (G5) as the first YOD1 pharmacological inhibitor. As expected, G5 notably degraded PML/RARα protein and eradicated APL, particularly drug-resistant APL cells. Importantly, G5 also showed a strong killing effect on primary patient-derived APL blasts. Overall, our study not only reveals the DUB-involved regulatory mechanism on PML/RARα stability and validates YOD1 as a potential therapeutic target for APL, but also identifies G5 as a YOD1 inhibitor and a promising candidate for APL, particularly drug-resistant APL treatment.

In vivo temporal resolution of acute promyelocytic leukemia progression reveals a role of Klf4 in suppressing early leukemic transformation

In this study, Mas et al. used primary hematopoietic stem and progenitor cells (HSPCs) and leukemic blasts that express the fusion protein PML-RARα as a paradigm to temporally dissect the dynamic changes in the epigenome, transcriptome, and genome architecture induced during oncogenic transformation. Their multiomics-integrated analysis identified Klf4 as an early down-regulated gene in PML-RARα-driven leukemogenesis, and they characterized the dynamic alterations in the Klf4 cis-regulatory network during APL progression and demonstrated that ectopic Klf4 overexpression can suppress self-renewal and reverse the differentiation block induced by PML-RARα.

Genome organization plays a pivotal role in transcription, but how transcription factors (TFs) rewire the structure of the genome to initiate and maintain the programs that lead to oncogenic transformation remains poorly understood. Acute promyelocytic leukemia (APL) is a fatal subtype of leukemia driven by a chromosomal translocation between the promyelocytic leukemia (PML) and retinoic acid receptor α (RARα) genes. We used primary hematopoietic stem and progenitor cells (HSPCs) and leukemic blasts that express the fusion protein PML-RARα as a paradigm to temporally dissect the dynamic changes in the epigenome, transcriptome, and genome architecture induced during oncogenic transformation. We found that PML-RARα initiates a continuum of topologic alterations, including switches from A to B compartments, transcriptional repression, loss of active histone marks, and gain of repressive histone marks. Our multiomics-integrated analysis identifies Klf4 as an early down-regulated gene in PML-RARα-driven leukemogenesis. Furthermore, we characterized the dynamic alterations in the Klf4 cis-regulatory network during APL progression and demonstrated that ectopic Klf4 overexpression can suppress self-renewal and reverse the differentiation block induced by PML-RARα. Our study provides a comprehensive in vivo temporal dissection of the epigenomic and topological reprogramming induced by an oncogenic TF and illustrates how topological architecture can be used to identify new drivers of malignant transformation.

Murine Models of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a phenotypically and genetically heterogeneous hematologic malignancy. Extensive sequencing efforts have mapped the genomic landscape of adult and pediatric AML revealing a number of biologically and prognostically relevant driver lesions. Beyond identifying recurrent genetic aberrations, it is of critical importance to fully delineate the complex mechanisms by which they contribute to the initiation and evolution of disease to ultimately facilitate the development of targeted therapies. Towards these aims, murine models of AML are indispensable research tools. The rapid evolution of genetic engineering techniques over the past 20 years has greatly advanced the use of murine models to mirror specific genetic subtypes of human AML, define cell-intrinsic and extrinsic disease mechanisms, study the interaction between co-occurring genetic lesions, and test novel therapeutic approaches. This review summarizes the mouse model systems that have been developed to recapitulate the most common genomic subtypes of AML. We will discuss the strengths and weaknesses of varying modeling strategies, highlight major discoveries emanating from these model systems, and outline future opportunities to leverage emerging technologies for mechanistic and preclinical investigations.

Deconvolution of Bulk Gene Expression Profiles with Single-Cell Transcriptomics to Develop a Cell Type Composition-Based Prognostic Model for Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is one of the malignant hematologic cancers with rapid progress and poor prognosis. Most AML prognostic stratifications focused on genetic abnormalities. However, none of them was established based on the cell type compositions (CTCs) of peripheral blood or bone marrow aspirates from patients at diagnosis. Here we sought to develop a novel prognostic model for AML in adults based on the CTCs. First, we applied the CIBERSORT algorithm to estimate the CTCs for patients from two public datasets (GSE6891 and TCGA-LAML) using a custom gene expression signature reference constructed by an AML single-cell RNA sequencing dataset (GSE116256). Then, a CTC-based prognostic model was established using least absolute shrinkage and selection operator Cox regression, termed CTC score. The constructed prognostic model CTC score comprised 3 cell types, GMP-like, HSC-like, and T. Compared with the low-CTC-score group, the high-CTC-score group showed a 1.57-fold [95% confidence interval (CI), 1.23 to 2.00; p = 0.0002] and a 2.32-fold (95% CI, 1.53 to 3.51; p < 0.0001) higher overall mortality risk in the training set (GSE6891) and validation set (TCGA-LAML), respectively. When adjusting for age at diagnosis, cytogenetic risk, and karyotype, the CTC score remained statistically significant in both the training set [hazard ratio (HR) = 2.25; 95% CI, 1.20 to 4.24; p = 0.0119] and the validation set (HR = 7.97; 95% CI, 2.95 to 21.56; p < 0.0001]. We further compared the performance of the CTC score with two gene expression-based prognostic scores: the 17-gene leukemic stem cell score (LSC17 score) and the AML prognostic score (APS). It turned out that the CTC score achieved comparable performance at 1-, 2-, 3-, and 5-years timepoints and provided independent and additional prognostic information different from the LSC17 score and APS. In conclusion, the CTC score could serve as a powerful prognostic marker for AML and has great potential to assist clinicians to formulate individualized treatment plans.

Drug Repurposing in Oncology, an Attractive Opportunity for Novel Combinatorial Regimens

The costs of developing, validating and buying new drugs are dramatically increasing. On the other hand, sobering economies have difficulties in sustaining their healthcare systems, particularly in countries with an elderly population requiring increasing welfare. This conundrum requires immediate action, and a possible option is to study the large, already present arsenal of drugs approved and to use them for innovative therapies. This possibility is particularly interesting in oncology, where the complexity of the cancer genome dictates in most patients a multistep therapeutic approach. In this review, we discuss a) Computational approaches; b) preclinical models; c) currently ongoing or already published clinical trials in the drug repurposing field in oncology; and d) drug repurposing to overcome resistance to previous therapies.

Descriptive and Functional Genomics in Acute Myeloid Leukemia (AML): Paving the Road for a Cure

Over the past decades, genetic advances have allowed a more precise molecular characterization of AML with the identification of novel oncogenes and tumor suppressors as part of a comprehensive AML molecular landscape. Recent advances in genetic sequencing tools also enabled a better understanding of AML leukemogenesis from the preleukemic state to posttherapy relapse. These advances resulted in direct clinical implications with the definition of molecular prognosis classifications, the development of treatment recommendations based on minimal residual disease (MRD) measurement and the discovery of novel targeted therapies, ultimately improving AML patients' overall survival. The more recent development of functional genomic studies, pushed by novel molecular biology technologies (short hairpin RNA (shRNA) and CRISPR-Cas9) and bioinformatics tools design on one hand, along with the engineering of humanized physiologically relevant animal models on the other hand, have opened a new genomics era resulting in a greater knowledge of AML physiopathology. Combining descriptive and functional genomics will undoubtedly open the road for an AML cure within the next decades.

NLS-RARα blocks cell differentiation by inhibiting the retinoic acid signalling pathway

A majority of acute promyelocytic leukaemia (APL) cases are characterized by the PML-RARα fusion gene. Previous studies have shown that neutrophil elastase (NE) can cleave PML-RARα and is important for the development of APL. Here, we demonstrate that one of the cleavage products of PML-RARα, NLS-RARα, can block cell differentiation by repressing the expression of the target genes within the retinoic acid signalling pathway. The results of reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis showed that NLS-RARα depressed the expression of the cell differentiation marker protein, CD11b and CEBPβ, as well as the retinoic acid signalling pathway target genes, RARβ and CEBPε. Studies have shown that NLS-RARα forms heterodimers with retinoid X receptor α(RXRα) and interacts with SMRT. When treated with all-trans retinoic acid (ATRA), NLS-RARα exhibits diminished transcriptional activity compared to RARα. Moreover, in the presence of high doses of ATRA, NLS-RARα could be degraded along with the consequent transactivation of retinoic acid signalling pathway target genes and cell differentiation induction in a dose- and time-dependent manner. Together, these results indicate that NLS-RARα blocks cell differentiation by inhibiting the retinoic acid signalling pathway.

Learning from mouse models of MLL fusion gene-driven acute leukemia

5-10% of human acute leukemias carry chromosomal translocations involving the mixed lineage leukemia (MLL) gene that result in the expression of chimeric protein fusing MLL to >80 different partners of which AF4, ENL and AF9 are the most prevalent. In contrast to many other leukemia-associated mutations, several MLL-fusions are powerful oncogenes that transform hematopoietic stem cells but also more committed progenitor cells. Here, I review different approaches that were used to express MLL fusions in the murine hematopoietic system which often, but not always, resulted in highly penetrant and transplantable leukemias that closely phenocopied the human disease. Due to its simple and reliable nature, reconstitution of irradiated mice with bone marrow cells retrovirally expressing the MLL-AF9 fusion became the most frequently in vivo model to study the biology of acute myeloid leukemia (AML). I review some of the most influential studies that used this model to dissect critical protein interactions, the impact of epigenetic regulators, microRNAs and microenvironment-dependent signals for MLL fusion-driven leukemia. In addition, I highlight studies that used this model for shRNA- or genome editing-based screens for cellular vulnerabilities that allowed to identify novel therapeutic targets of which some entered clinical trials. Finally, I discuss some inherent characteristics of the widely used mouse model based on retroviral expression of the MLL-AF9 fusion that can limit general conclusions for the biology of AML. This article is part of a Special Issue entitled: The MLL family of proteins in normal development and disease edited by Thomas A Milne.

Acute Promyelocytic Leukemia: A Constellation of Molecular Events around a Single PML-RARA Fusion Gene

Although acute promyelocytic leukemia (APL) is one of the most characterized forms of acute myeloid leukemia (AML), the molecular mechanisms involved in the development and progression of this disease are still a matter of study. APL is defined by the PML-RARA rearrangement as a consequence of the translocation t(15;17)(q24;q21). However, this abnormality alone is not able to trigger the whole leukemic phenotype and secondary cooperating events might contribute to APL pathogenesis. Additional somatic mutations are known to occur recurrently in several genes, such as FLT3, WT1, NRAS and KRAS, whereas mutations in other common AML genes are rarely detected, resulting in a different molecular profile compared to other AML subtypes. How this mutational spectrum, including point mutations in the PML-RARA fusion gene, could contribute to the 10%–15% of relapsed or resistant APL patients is still unknown. Moreover, due to the uncertain impact of additional mutations on prognosis, the identification of the APL-specific genetic lesion is still the only method recommended in the routine evaluation/screening at diagnosis and for minimal residual disease (MRD) assessment. However, the gene expression profile of genes, such as ID1, BAALC, ERG, and KMT2E, once combined with the molecular events, might improve future prognostic models, allowing us to predict clinical outcomes and to categorize APL patients in different risk subsets, as recently reported. In this review, we will focus on the molecular characterization of APL patients at diagnosis, relapse and resistance, in both children and adults. We will also describe different standardized molecular approaches to study MRD, including those recently developed. Finally, we will discuss how novel molecular findings can improve the management of this disease.

A Critical Review of Animal Models Used in Acute Myeloid Leukemia Pathophysiology

Acute myeloid leukemia (AML) is one of the most frequent, complex, and heterogeneous hematological malignancies. AML prognosis largely depends on acquired cytogenetic, epigenetic, and molecular abnormalities. Despite the improvement in understanding the biology of AML, survival rates remain quite low. Animal models offer a valuable tool to recapitulate different AML subtypes, and to assess the potential role of novel and known mutations in disease progression. This review provides a comprehensive and critical overview of select available AML animal models. These include the non-mammalian Zebrafish and Drosophila models as well as the mammalian rodent systems, comprising rats and mice. The suitability of each animal model, its contribution to the advancement of knowledge in AML pathophysiology and treatment, as well as its advantages and limitations are discussed. Despite some limitations, animal models represent a powerful approach to assess toxicity, and permit the design of new therapeutic strategies.

The Impact of the Cellular Origin in Acute Myeloid Leukemia: Learning From Mouse Models

Acute myeloid leukemia (AML) is a genetically heterogeneous disease driven by a limited number of cooperating mutations. There is a long-standing debate as to whether AML driver mutations occur in hematopoietic stem or in more committed progenitor cells. Here, we review how different mouse models, despite their inherent limitations, have functionally demonstrated that cellular origin plays a critical role in the biology of the disease, influencing clinical outcome. AML driven by potent oncogenes such as mixed lineage leukemia fusions often seem to emerge from committed myeloid progenitors whereas AML without any major cytogenetic abnormalities seem to develop from a combination of preleukemic initiating events arising in the hematopoietic stem cell pool. More refined mouse models may serve as experimental platforms to identify and validate novel targeted therapeutic strategies.

FLT3-ITD impedes retinoic acid, but not arsenic, responses in murine acute promyelocytic leukemias

Acute promyelocytic leukemia (APL) is often associated with activating FLT3 signaling mutations. These are highly related to hyperleukocytosis, a major adverse risk factor with chemotherapy-based regimens. APL is a model for oncogene-targeted therapies: all-trans retinoic acid (ATRA) and arsenic both target and degrade its ProMyelocytic Leukemia/Retinoic Acid Receptor α (PML/RARA) driver. The combined ATRA/arsenic regimen now cures virtually all patients with standard-risk APL. Although FLT3-internal tandem duplication (ITD) was an adverse risk factor for historical ATRA/chemotherapy regimens, the molecular bases for this effect remain unknown. Using mouse APL models, we unexpectedly demonstrate that FLT3-ITD severely blunts ATRA response. Remarkably, although the transcriptional output of initial ATRA response is unaffected, ATRA-induced PML/RARA degradation is blunted, as is PML nuclear body reformation and activation of P53 signaling. Critically, the combination of ATRA and arsenic fully rescues therapeutic response in FLT3-ITD APLs, restoring PML/RARA degradation, PML nuclear body reformation, P53 activation, and APL eradication. Moreover, arsenic targeting of normal PML also contributes to APL response in vivo. These unexpected results explain the less favorable outcome of FLT3-ITD APLs with ATRA-based regimens, and stress the key role of PML nuclear bodies in APL eradication by the ATRA/arsenic combination.

Murine Models of Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a rare but severe form of human cancer that results from a limited number of functionally cooperating genetic abnormalities leading to uncontrolled proliferation and impaired differentiation of hematopoietic stem and progenitor cells. Before the identification of genetic driver lesions, chemically, irradiation or viral infection-induced mouse leukaemia models provided platforms to test novel chemotherapeutics. Later, transgenic mouse models were established to test the in vivo transforming potential of newly cloned fusion genes and genetic aberrations detected in patients' genomes. Hereby researchers constitutively or conditionally expressed the respective gene in the germline of the mouse or reconstituted the hematopoietic system of lethally irradiated mice with bone marrow virally expressing the mutation of interest. More recently, immune deficient mice have been explored to study patient-derived human AML cells in vivo. Unfortunately, although complementary to each other, none of the currently available strategies faithfully model the initiation and progression of the human disease. Nevertheless, fast advances in the fields of next generation sequencing, molecular technology and bioengineering are continuously contributing to the generation of better mouse models. Here we review the most important AML mouse models of each category, briefly describe their advantages and limitations and show how they have contributed to our understanding of the biology and to the development of novel therapies.

PML-RARα interferes with erythropoiesis by repressing LMO2 in acute promyelocytic leukaemia

The PML‐RARα fusion gene, generated by the t(15;17) chromosome translocation, is regarded as the initiating factor of acute promyelocytic leukaemia (APL). In addition to the well‐known effects on blocking myeloid differentiation at the promyelocytic stage, promyelocytic leukaemia‐retinoic acid receptor α (PML‐RARα) has also been reported to interfere with multiple differentiation processes, including erythroid differentiation. However, the detailed molecular mechanism by which PML‐RARα impairs erythropoiesis has not yet been fully addressed. By chromatin immunoprecipitation‐PCR assay, we found that PML‐RARα bound to the distal promoter region of LMO2 (LIM‐only protein 2), a critical erythroid‐specific transcription factor. Luciferase reporter assays and qRT‐PCR results demonstrated that PML‐RARα down‐regulated the expression of the LMO2 distal transcript through transrepressing its promoter activity. Analysis of gene expression profiling data from large cohorts of acute myeloid leukaemia (AML) patients confirmed that LMO2 expressed at a markedly lower level in APL patients in comparison to non‐APL AML patients. Further flow cytometry analysis demonstrated that PML‐RARα inhibited erythropoietin‐induced erythroid differentiation by down‐regulating LMO2 expression. Our findings reveal a previously unidentified mechanism, by which PML‐RARα interferes with erythropoiesis through directly targeting and transrepressing LMO2 expression in the development of APL.

Next-Generation in vivo Modeling of Human Cancers

Animal models of human cancers played a major role in our current understanding of tumor biology. In pre-clinical oncology, animal models empowered drug target and biomarker discovery and validation. In turn, this resulted in improved care for cancer patients. In the quest for understanding and treating a diverse spectrum of cancer types, technological breakthroughs in genetic engineering and single cell "omics" offer tremendous potential to enhance the informative value of pre-clinical models. Here, I review the state-of-the-art in modeling human cancers with focus on animal models for human malignant gliomas. The review highlights the use of glioma models in dissecting mechanisms of tumor initiation, in the retrospective identification of tumor cell-of-origin, in understanding tumor heterogeneity and in testing the potential of immuno-oncology. I build on the deep review of glioma models as a basis for a more general discussion of the potential ways in which transformative technologies may shape the next-generation of pre-clinical models. I argue that refining animal models along the proposed lines will benefit the success rate of translation for pre-clinical research in oncology.

Donor cell-derived acute promyelocytic leukemia after allogeneic hematopoietic stem cell transplantation

Donor cell leukemia (DCL) is an infrequent complication after allogeneic hematopoietic stem cell transplantation (HSCT). Its true incidence is difficult to assess, although improvements in chimerism studies contributed to a better diagnosis of DCL. We report two rare cases of donor cell-derived acute promyelocytic leukemia (APL). To our knowledge, only two cases have been described in the literature. Here, we report one male and one female patients with acute myeloid leukemia (AML), who developed an APL in donor cells after HSCT. The latency between HSCT and DCL was 279 and 43 months, respectively. Fluorescent in situ hybridation and chimerism monitoring analysis proved the donor origin of APL. Surprisingly, donor lymphocyte infusion provided a hematological response during 19 months in the female patient. The mechanisms associated with pathogenesis of DCL are unclear and seem to be multifactorial. Increasing worldwide allogeneic hematopoietic stem cell transplantation activity and potentially the age of donor could explain the increasing incidence of DCL in the future. It is highlighted that long-term follow up of recipients will allow to report all cases of DCL, to clarify the genetic landscape and factors which contribute to DCL, to understand the response to DLI.

ΔNp73 overexpression promotes resistance to apoptosis but does not cooperate with PML/RARA in the induction of an APL-leukemic phenotype

Here, we evaluated whether the overexpression of transcriptionally inactive ΔNp73 cooperates with PML/RARA fusion protein in the induction of an APL-leukemic phenotype, as well as its role in vitro in proliferation, myeloid differentiation, and drug-induced apoptosis. Using lentiviral gene transfer, we showed in vitro that ΔNp73 overexpression resulted in increased proliferation in murine bone marrow (BM) cells from hCG-PML/RARA transgenic mice and their wild-type (WT) counterpart, with no accumulation of cells at G2/M or S phases; instead, ΔNp73-expressing cells had a lower rate of induced apoptosis. Next, we evaluated the effect of ΔNp73 on stem-cell self-renewal and myeloid differentiation. Primary BM cells lentivirally infected with human ΔNp73 were not immortalized in culture and did not present significant changes in the percentage of CD11b. Finally, we assessed the impact of ΔNp73 on leukemogenesis or its possible cooperation with PML/RARA fusion protein in the induction of an APL-leukemic phenotype. After 120 days of follow-up, all transplanted mice were clinically healthy and, no evidence of leukemia/myelodysplasia was apparent. Taken together, our data suggest that ΔNp73 had no leukemic transformation capacity by itself and apparently did not cooperate with the PML/RARA fusion protein to induce a leukemic phenotype in a murine BM transplantation model. In addition, the forced expression of ΔNp73 in murine BM progenitors did not alter the ATRA-induced differentiation rate in vitro or induce aberrant cell proliferation, but exerted an important role in cell survival, providing resistance to drug-induced apoptosis.

PML/RARa inhibits PTEN expression in hematopoietic cells by competing with PU.1 transcriptional activity

Acute promyelocitic leukemia (APL) is characterized by the pathognomonic presence in leukemic blasts of the hybrid protein PML/RARA, that acts as a transcriptional repressor impairing the expression of genes that are critical to myeloid differentiation. Here, we show that primary blasts from APL patients express lower levels of the oncosuppressor protein PTEN, as compared to blast cells from other AML subtypes or normal bone marrow, and demonstrate that PML-RARA directly inhibits PTEN expression. We show that All-Trans Retinoic Acid (ATRA) triggers in APL cells an active chromatin status at the core regulatory region of the PTEN promoter, that allows the binding of the myeloid-regulating transcription factor PU.1, and, in turn, the transcriptional induction of PTEN. ATRA, via PML/RARA degradation, also promotes PTEN nuclear re-localization and decreases expression of the PTEN target Aurora A kinase. In conclusion, our findings support the notion that PTEN is one of the primary targets of PML/RARA in APL

Pml nuclear body disruption cooperates in APL pathogenesis and impairs DNA damage repair pathways in mice

A hallmark of acute promyelocytic leukemia (APL) is altered nuclear architecture, with disruption of promyelocytic leukemia (PML) nuclear bodies (NBs) mediated by the PML-retinoic acid receptor α (RARα) oncoprotein. To address whether this phenomenon plays a role in disease pathogenesis, we generated a knock-in mouse model with NB disruption mediated by 2 point mutations (C62A/C65A) in the Pml RING domain. Although no leukemias developed in PmlC62A/C65A mice, these transgenic mice also expressing RARα linked to a dimerization domain (p50-RARα model) exhibited a doubling in the rate of leukemia, with a reduced latency period. Additionally, we found that response to targeted therapy with all-trans retinoic acid in vivo was dependent on NB integrity. PML-RARα is recognized to be insufficient for development of APL, requiring acquisition of cooperating mutations. We therefore investigated whether NB disruption might be mutagenic. Compared with wild-type cells, primary PmlC62A/C65A cells exhibited increased sister-chromatid exchange and chromosome abnormalities. Moreover, functional assays showed impaired homologous recombination (HR) and nonhomologous end-joining (NHEJ) repair pathways, with defective localization of Brca1 and Rad51 to sites of DNA damage. These data directly demonstrate that Pml NBs are critical for DNA damage responses, and suggest that Pml NB disruption is a central contributor to APL pathogenesis.

PML(NLS¯) protein: A novel marker for the early diagnosis of acute promyelocytic leukemia

Promyelocyte leukemia-retinoic acid receptor α (PML-RARα) is known as a fusion gene of acute promyelocytic leukemia (APL). Previous studies have reported that neutrophil elastase (NE) cleaves PML-RARα in early myeloid cells, which leads to the removal of the nuclear localization signal (NLS) in PML and increases the incidence of APL. The resultant PML without the NLS is termed PML(NLS⁻). The aim of the present study was to verify the existence and location of the PML(NLS⁻) protein in NB4 cells. NB4 cells underwent electroporation with the pCMV-HA-NE plasmid to form NB4-HA-NE cells, which were then transplanted to produce tumors in nude mice and samples were collected from patients with APL. Western blot analysis, an immunofluorescence assay, confocal laser microscopy and immunohistochemistry were performed to detect the expression and localization of the PML(NLS⁻) protein. The findings demonstrated that PML(NLS⁻) was detectable in the cytoplasm of NB4-HA-NE cells, the tumors in nude mice and in neutrophils from patients with APL. This indicated that PML(NLS⁻) may be an effective and novel target for the diagnosis of APL.

Validation of a Molecular Risk Score for Prognosis of Patients With Acute Promyelocytic Leukemia Treated With All-trans Retinoic Acid and Chemotherapy-containing Regimens

INTRODUCTION

Although treatment of acute promyelocytic leukemia (APL) has evolved dramatically during the past decades, especially with the introduction of all-trans retinoic acid, risk stratification remains an important issue. To date, relapse risk can be predicted by leukocyte and platelet counts only. In the present report, we present a validation study on 3 candidate genes and a newly developed molecular risk score for APL in 2 independent patient cohorts.

PATIENTS AND METHODS

An integrative risk score combining the expression levels of BAALC, ERG, and WT1 was calculated for 79 de novo APL patients from the original cohort and 76 de novo APL patients from a validation cohort. Gene expression analysis was executed the same for both cohorts, and the results regarding the effect on patient outcomes were compared.

RESULTS

The expression levels of BAALC, ERG, and WT1 were similar in both cohorts compared with the healthy controls. The relapse and survival rates were not different between the low- and high-risk patients according to the Sanz score. However, application of the molecular risk score on the validation cohort distinctly discriminated patients according to their risk of relapse and death just as in the original APL cohort, although single gene analyses could not reproduce the negative prognostic impact.

CONCLUSION

The analysis clearly validated the prognostic effect of the integrative risk score on the outcome in APL patients. The value was further empowered because the single gene analyses did not show similar results. Whether the integrative risk score retains its prognostic power in the chemotherapy-free setting should be investigated further.

Expression of the promyelocytic leukemia protein without the nuclear localization signal as a novel diagnostic marker for acute promyelocytic leukemia

Promyelocytic leukemia-retinoic acid receptor α (PML-RARα) is a fusion protein generated by the t(15;17)(q22;q12) translocation associated with acute promyelocytic leukemia (APL). PML-RARα is cleaved by neutrophil elastase, an early myeloid-specific serine protease, leading to translocation of the nuclear localization signal (NLS) of the PML protein to the N-terminal of RARα, and the mutational product PML(NLS-). The present study was designed to analyze the role of the NLS in mediating PML transport into the nucleus and to evaluate the value of measuring NLS translocation in the early diagnosis of APL. PML and PML(NLS-) localization was examined by immunofluorescence (IF). The interaction between PML/PML(NLS-) and importin α was detected by an in vivo binding assay using co-immunoprecipitation and double IF labeling. Twenty-seven untreated APL patients with PML-RARα and 22 non-APL controls were evaluated. PML(NLS-) was detected in primary APL, but not non-APL cells. IF showed that PML was localized to the nucleus, interacted with importin α in vivo, and co-localized in the PML nuclear bodies. PML(NLS-) was primarily localized in the cytoplasm and the interaction with importin α was lost. IF had a sensitivity and specificity of 92.6 and 77.3%, respectively, for diagnosing APL. These data suggest that PML(NLS-) may be a novel diagnostic biomarker for APL.

The lncRNA HOTAIRM1 regulates the degradation of PML-RARA oncoprotein and myeloid cell differentiation by enhancing the autophagy pathway

Increasing evidence has indicated that long noncoding RNAs (lncRNAs) are of great importance in different cell contexts. However, only a very small number of lncRNAs have been experimentally validated and functionally annotated during human hematopoiesis. Here, we report an lncRNA, HOTAIRM1, which is associated with myeloid differentiation and has pivotal roles in the degradation of oncoprotein PML-RARA and in myeloid cell differentiation by regulating autophagy pathways. We first revealed that HOTAIRM1 has different variants that are expressed at different levels in cells and that the expression pattern of HOTAIRM1 is closely related to that of the PML-RARA oncoprotein in acute promyelocytic leukemia (APL) patients. We further revealed that the downregulation of HOTAIRM1 could inhibit all-trans retinoic acid (ATRA) -induced degradation of PML-RARA in APL cells and repress the process of differentiation from promyelocytic to granulocytic cells. More importantly, we found that HOTAIRM1 regulates autophagy and that autophagosome formation was inhibited when HOTAIRM1 expression was reduced in the cells. Finally, through the use of a dual luciferase activity assay, AGO2 RNA immunoprecipitation and RNA pull-down, HOTAIRM1 was revealed to act as a microRNA sponge in a pathway that included miR-20a/106b, miR-125b and their targets ULK1, E2F1 and DRAM2. We constructed a human APL-ascites SCID mouse model to validate the function of HOTAIRM1 and its regulatory pathway in vivo. This is the first report showing that a lncRNAs regulates autophagy and the degradation of the PML-RARA oncoprotein during the process of myeloid cell differentiation blockade, suggesting that lncRNAs may be the potential therapeutic targets for leukemia.

Three-dimensional ex vivo co-culture models of the leukaemic bone marrow niche for functional drug testing

Acute myeloid leukaemia (AML) is a hierarchically structured malignancy in which aberrant leukemic stem cells drive the production of leukaemic blast cell clones. AML cells strictly depend on the bone marrow microenvironment (BMM) in which they reside. Classical AML cell cultures fail to mimic the BMM and, therefore, drug discovery studies are dominated by in vivo models. However, animal models are time consuming, labour intensive, provide limited mechanistic insight, and are unsuited for high-throughput studies, necessitating the development of novel AML models. The evolving ex vivo BMM-mimicking culture systems aim to fill this gap, with increasing success. Here, we discuss how AML-microenvironment co-culture models advance our understanding of this disease, and highlight their future potential for translational AML research.

Comprehensive mutational analysis of primary and relapse acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a subtype of myeloid leukemia characterized by differentiation block at the promyelocyte stage. Besides the presence of chromosomal rearrangement t(15;17), leading to the formation of PML-RARA (promyelocytic leukemia-retinoic acid receptor alpha) fusion, other genetic alterations have also been implicated in APL. Here, we performed comprehensive mutational analysis of primary and relapse APL to identify somatic alterations, which cooperate with PML-RARA in the pathogenesis of APL. We explored the mutational landscape using whole-exome (n=12) and subsequent targeted sequencing of 398 genes in 153 primary and 69 relapse APL. Both primary and relapse APL harbored an average of eight non-silent somatic mutations per exome. We observed recurrent alterations of FLT3, WT1, NRAS and KRAS in the newly diagnosed APL, whereas mutations in other genes commonly mutated in myeloid leukemia were rarely detected. The molecular signature of APL relapse was characterized by emergence of frequent mutations in PML and RARA genes. Our sequencing data also demonstrates incidence of loss-of-function mutations in previously unidentified genes, ARID1B and ARID1A, both of which encode for key components of the SWI/SNF complex. We show that knockdown of ARID1B in APL cell line, NB4, results in large-scale activation of gene expression and reduced in vitro differentiation potential.

PML-RARA requires DNA methyltransferase 3A to initiate acute promyelocytic leukemia

The DNA methyltransferases DNMT3A and DNMT3B are primarily responsible for de novo methylation of specific cytosine residues in CpG dinucleotides during mammalian development. While loss-of-function mutations in DNMT3A are highly recurrent in acute myeloid leukemia (AML), DNMT3A mutations are almost never found in AML patients with translocations that create oncogenic fusion genes such as PML-RARA, RUNX1-RUNX1T1, and MLL-AF9. Here, we explored how DNMT3A is involved in the function of these fusion genes. We used retroviral vectors to express PML-RARA, RUNX1-RUNX1T1, or MLL-AF9 in bone marrow cells derived from WT or DNMT3A-deficient mice. Additionally, we examined the phenotypes of hematopoietic cells from Ctsg-PML-RARA mice, which express PML-RARA in early hematopoietic progenitors and myeloid precursors, with or without DNMT3A. We determined that the methyltransferase activity of DNMT3A, but not DNMT3B, is required for aberrant PML-RARA-driven self-renewal ex vivo and that DNMT3A is dispensable for RUNX1-RUNX1T1- and MLL-AF9-driven self-renewal. Furthermore, both the PML-RARA-driven competitive transplantation advantage and development of acute promyelocytic leukemia (APL) required DNMT3A. Together, these findings suggest that PML-RARA requires DNMT3A to initiate APL in mice.

Novel working hypothesis for pathogenesis of hematological malignancies: combination of mutations-induced cellular phenotypes determines the disease (cMIP-DD)

Recent progress in high-speed sequencing technology has revealed that tumors harbor novel mutations in a variety of genes including those for molecules involved in epigenetics and splicing, some of which were not categorized to previously thought malignancy-related genes. However, despite thorough identification of mutations in solid tumors and hematological malignancies, how these mutations induce cell transformation still remains elusive. In addition, each tumor usually contains multiple mutations or sometimes consists of multiple clones, which makes functional analysis difficult. Fifteen years ago, it was proposed that combination of two types of mutations induce acute leukemia; Class I mutations induce cell growth or inhibit apoptosis while class II mutations block differentiation, co-operating in inducing acute leukemia. This notion has been proven using a variety of mouse models, however most of recently found mutations are not typical class I/II mutations. Although some novel mutations have been found to functionally work as class I or II mutation in leukemogenesis, the classical class I/II theory seems to be too simple to explain the whole story. We here overview the molecular basis of hematological malignancies based on clinical and experimental results, and propose a new working hypothesis for leukemogenesis.

STAT activation status differentiates leukemogenic from non-leukemogenic stem cells in AML and is suppressed by arsenic in t(6;9)-positive AML

Acute myeloid leukemia (AML) is characterized by an aberrant self-renewal of hematopoietic stem cells (HSC) and a block in differentiation. The major therapeutic challenge is the characterization of the leukemic stem cell as a target for the eradication of the disease. Until now the biology of AML-associated fusion proteins (AAFPs), such as the t(15;17)-PML/RARα, t(8;21)-RUNX1/RUNX1T1 and t(6;9)-DEK/NUP214, all able to induce AML in mice, was investigated in different models and genetic backgrounds, not directly comparable to each other. To avoid the bias of different techniques and models we expressed these three AML-inducing oncogenes in an identical genetic background and compared their influence on the HSC compartment in vitro and in vivo. These AAFPs exerted differential effects on HSCs and PML/RARα, similar to DEK/NUP214, induced a leukemic phenotype from a small subpopulation of HSCs with a surface marker pattern of long-term HSC and characterized by activated STAT3 and 5. In contrast the established AML occurred from mature populations in the bone marrow. The activation of STAT5 by PML/RARα and DEK/NUP214 was confirmed in t(15;17)(PML/RARα) and t(6;9)(DEK/NUP214)-positive patients as compared to normal CD34+ cells. The activation of STAT5 was reduced upon the exposure to Arsenic which was accompanied by apoptosis in both PML/RARα- and DEK/NUP214-positive leukemic cells. These findings indicate that in AML the activation of STATs plays a decisive role in the biology of the leukemic stem cell. Furthermore we establish exposure to arsenic as a novel concept for the treatment of this high risk t(6;9)-positive AML.

Establishment of a humanized APL model via the transplantation of PML-RARA-transduced human common myeloid progenitors into immunodeficient mice

Recent advances in cancer biology have revealed that many malignancies possess a hierarchal system, and leukemic stem cells (LSC) or leukemia-initiating cells (LIC) appear to be obligatory for disease progression. Acute promyelocytic leukemia (APL), a subtype of acute myeloid leukemia characterized by the formation of a PML-RARα fusion protein, leads to the accumulation of abnormal promyelocytes. In order to understand the precise mechanisms involved in human APL leukemogenesis, we established a humanized in vivo APL model involving retroviral transduction of PML-RARA into CD34⁺ hematopoietic cells from human cord blood and transplantation of these cells into immunodeficient mice. The leukemia well recapitulated human APL, consisting of leukemic cells with abundant azurophilic abnormal granules in the cytoplasm, which expressed CD13, CD33 and CD117, but not HLA-DR and CD34, were clustered in the same category as human APL samples in the gene expression analysis, and demonstrated sensitivity to ATRA. As seen in human APL, the induced APL cells showed a low transplantation efficiency in the secondary recipients, which was also exhibited in the transplantations that were carried out using the sorted CD34⁻ fraction. In order to analyze the mechanisms underlying APL initiation and development, fractionated human cord blood was transduced with PML-RARA. Common myeloid progenitors (CMP) from CD34⁺/CD38⁺ cells developed APL. These findings demonstrate that CMP are a target fraction for PML-RARA in APL, whereas the resultant CD34⁻ APL cells may share the ability to maintain the tumor.

Retinoic acid signaling in cancer: The parable of acute promyelocytic leukemia

Inevitably fatal some 40 years, acute promyelocytic leukemia (APL) can now be cured in more than 95% of cases. This clinical success story is tightly linked to tremendous progress in our understanding of retinoic acid (RA) signaling. The discovery of retinoic acid receptor alpha (RARA) was followed by the cloning of the chromosomal translocations driving APL, all of which involve RARA. Since then, new findings on the biology of nuclear receptors have progressively enlightened the basis for the clinical efficacy of RA in APL. Reciprocally, the disease offered a range of angles to approach the cellular and molecular mechanisms of RA action. This virtuous circle contributed to make APL one of the best-understood cancers from both clinical and biological standpoints. Yet, some important questions remain unanswered including how lessons learnt from RA-triggered APL cure can help design new therapies for other malignancies.

New insights in AML biology from genomic analysis

Advancements in sequencing techniques have led to the discovery of numerous genes not previously implicated in acute myeloid leukemia (AML) biology. Further in vivo studies are necessary to discern the biological impact of these mutations. Murine models, the most commonly used in vivo system, provide a physiologic context for the study of specific genes. These systems have provided deep insights into the role of genetic translocations, mutations, and dysregulated gene expression on leukemia pathogenesis. This review focuses on the phenotype of newly identified genes, including NPM1, IDH1/2, TET2, MLL, DNMT3A, EZH2, EED, and ASXL1, in mouse models and the implications on AML biology.

Associations between age, cytogenetics, FLT3-ITD, and marrow leukemia cells identified by flow cytometry

OBJECTIVES

To explore the relationships between age, cytogenetic subgroups, molecular markers, and cells with leukemic aberrant immunophenotype in patients with acute myeloid leukemia (AML).

METHODS

In this study, we evaluated the correlations between age, cytogenetic subgroups (normal, balanced and unbalance karyotype), molecular mutations (NPM1, FLT3-ITD, and CEBPA mutations) and marrow leukemia cells (LC) identified by flow cytometry in 256 patients with de novo AML.

RESULTS

From age group 10-19 years to age group ≥ 60 years, the percentage of LC decreased from 67.0 ± 18.4% to 49.0 ± 25.1% (F = 2.353, P = 0.041). LC percentage was higher in patients with balanced karyotypes (65.7 ± 22.4%), than those with unbalanced karyotypes (46.0 ± 26.6%) (u = 3.444, P = 0.001) or a normal karyotype (49.9 ± 22.1%) (u = 5.093, P < 0.001). Patients with FLT3-ITD (64.3 ± 19.5%) had higher LC percentages compared with those without (54.2 ± 24.3%) (u = 2.794, P = 0.007).

CONCLUSIONS

Associations between age, cytogenetics, molecular markers, and marrow leukemia cells may offer beneficial information to understand the biology and pathogenesis of AML.