Synthetic retinoids activate RARA- or PML/RARA-dependent transcription, but fail to degrade RARA or PML/RARA protein, which is insufficient for eradication of acute promyelocytic leukemia.
In PML/RARA-driven acute promyelocytic leukemia (APL), retinoic acid (RA) induces leukemia cell differentiation and transiently clears the disease. Molecularly, RA activates PML/RARA-dependent transcription and also initiates its proteasome-mediated degradation. In contrast, arsenic, the other potent anti-APL therapy, only induces PML/RARA degradation by specifically targeting its PML moiety. The respective contributions of RA-triggered transcriptional activation and proteolysis to clinical response remain disputed. Here, we identify synthetic retinoids that potently activate RARA- or PML/RARA-dependent transcription, but fail to down-regulate RARA or PML/RARA protein levels. Similar to RA, these uncoupled retinoids elicit terminal differentiation, but unexpectedly fail to impair leukemia-initiating activity of PML/RARA-transformed cells ex vivo or in vivo. Accordingly, the survival benefit conferred by uncoupled retinoids in APL mice is dramatically lower than the one provided by RA. Differentiated APL blasts sorted from uncoupled retinoid–treated mice retain PML/RARA expression and reinitiate APL in secondary transplants. Thus, differentiation is insufficient for APL eradication, whereas PML/RARA loss is essential. These observations unify the modes of action of RA and arsenic and shed light on the potency of their combination in mice or patients.
Because PML-RARA-induced acute promyelocytic leukemia (APL) is a morphologically differentiated leukemia, many groups have speculated about whether its leukemic cell of origin is a committed myeloid precursor (e.g. a promyelocyte) versus an hematopoietic stem/progenitor cell (HSPC). We originally targeted PML-RARA expression with CTSG regulatory elements, based on the early observation that this gene was maximally expressed in cells with promyelocyte morphology. Here, we show that both Ctsg, and PML-RARA targeted to the Ctsg locus (in Ctsg-PML-RARA mice), are expressed in the purified KLS cells of these mice (KLS = Kit+Lin−Sca+, which are highly enriched for HSPCs), and this expression results in biological effects in multi-lineage competitive repopulation assays. Further, we demonstrate the transcriptional consequences of PML-RARA expression in Ctsg-PML-RARA mice in early myeloid development in other myeloid progenitor compartments [common myeloid progenitors (CMPs) and granulocyte/monocyte progenitors (GMPs)], which have a distinct gene expression signature compared to wild-type (WT) mice. Although PML-RARA is indeed expressed at high levels in the promyelocytes of Ctsg-PML-RARA mice and alters the transcriptional signature of these cells, it does not induce their self-renewal. In sum, these results demonstrate that in the Ctsg-PML-RARA mouse model of APL, PML-RARA is expressed in and affects the function of multipotent progenitor cells. Finally, since PML/Pml is normally expressed in the HSPCs of both humans and mice, and since some human APL samples contain TCR rearrangements and express T lineage genes, we suggest that the very early hematopoietic expression of PML-RARA in this mouse model may closely mimic the physiologic expression pattern of PML-RARA in human APL patients.
The PML protein and PML nuclear bodies (PML-NB) are implicated in multiple cellular functions relevant to tumor suppression, including DNA damage response. In most cases of acute promyelocytic leukemia, the PML and retinoic acid receptor alpha (RARA) genes are translocated, resulting in expression of oncogenic PML-RARα fusion proteins. PML-NB fail to form normally, and promyelocytes remain in an undifferentiated, abnormally proliferative state. We examined the involvement of PML protein and PML-NB in homologous recombinational repair (HRR) of chromosomal DNA double-strand breaks. Transient overexpression of wild-type PML protein isoforms produced hugely enlarged or aggregated PML-NB and reduced HRR by ~2 fold, suggesting that HRR depends to some extent upon normal PML-NB structure. Knockdown of PML by RNA interference sharply attenuated formation of PML-NB and reduced HRR by up to 20 fold. However, PML-knockdown cells showed apparently normal induction of H2AX phosphorylation and RAD51 foci after DNA damage by ionizing radiation. These findings indicate that early steps in HRR, including recognition of DNA double-strand breaks, initial processing of ends, and assembly of single-stranded DNA/RAD51 nucleoprotein filaments, do not depend upon PML-NB. The HRR deficit in PML-depleted cells thus reflects inhibition of later steps in the repair pathway. Expression of PML-RARα fusion proteins disrupted PML-NB structure and reduced HRR by up to 10 fold, raising the possibility that defective HRR and resulting genomic instability may figure in the pathogenesis, progression and relapse of acute promyelocytic leukemia.
Acute promyelocytic leukemia; PML nuclear bodies; DNA repair; homologous recombination; all-trans retinoic acid; arsenic trioxide
Pandolfi et al. provide an in-depth discussion on the synergism between all-trans-retinoic acid and arsenic trioxide treatment and their mechanisms of action on acute promyelocytic leukemia.
Acute promyelocytic leukemia (APL) is a hematological malignancy driven by a chimeric oncoprotein containing the C terminus of the retinoic acid receptor-a (RARa) fused to an N-terminal partner, most commonly promyelocytic leukemia protein (PML). Mechanistically, PML-RARa acts as a transcriptional repressor of RARa and non-RARa target genes and antagonizes the formation and function of PML nuclear bodies that regulate numerous signaling pathways. The empirical discoveries that PML-RARa–associated APL is sensitive to both all-trans-retinoic acid (ATRA) and arsenic trioxide (ATO), and the subsequent understanding of the mechanisms of action of these drugs, have led to efforts to understand the contribution of molecular events to APL cell differentiation, leukemia-initiating cell (LIC) clearance, and disease eradication in vitro and in vivo. Critically, the mechanistic insights gleaned from these studies have resulted not only in a better understanding of APL itself, but also carry valuable lessons for other malignancies.
Acute promyelocytic leukemia (APL) is initiated by the PML-RARA fusion oncogene and has a characteristic expression profile that includes high levels of the Notch ligand JAG1. In this study, we used a series of bioinformatic, in vitro, and in vivo assays to assess the role of Notch signaling in human APL samples, and in a PML-RARA knockin mouse model of APL (Ctsg-PML-RARA). We identified a Notch expression signature in both human primary APL cells and in Kit+Lin−Sca1+ (KLS) cells from pre-leukemic Ctsg-PML-RARA mice. Both genetic and pharmacologic inhibition of Notch signaling abrogated the enhanced self-renewal seen in hematopoietic stem/progenitor cells (HSPCs) from pre-leukemic Ctsg-PML-RARA mice, but had no influence on cells from age-matched wildtype mice. In addition, 6 of 9 murine APL tumors tested displayed diminished growth in vitro when Notch signaling was inhibited pharmacologically. Finally, we found that genetic inhibition of Notch signaling with a dominant negative MAML protein reduced APL growth in vivo in a subset of tumors. These findings expand the role of Notch signaling in hematopoietic diseases, and further define the mechanistic events important for PML-RARA-mediated leukemogenesis.
Notch; Acute Promyelocytic Leukemia; Self-renewal
Acute promyelocytic leukemia (APL) is characterized by a reciprocal translocation t(15;17)(q22;q21) leading to the disruption of Promyelocytic leukemia (PML) and Retionic Acid Receptor Alpha (RARA) followed by reciprocal PML–RARA fusion in 90% of the cases. Fluorescence in situ hybridization (FISH) has overcome the hurdles of unavailability of abnormal and/or lack of metaphase cells, and detection of cryptic, submicroscopic rearrangements. In the present study, besides diagnostic approach we sought to analyze these cases for identification and characterization of cryptic rearrangements, deletion variants and unknown RARA translocation variants by application of D-FISH and RARA break-apart probe strategy on interphase and metaphase cells in a large series of 200 cases of APL. Forty cases (20%) had atypical PML–RARA and/or RARA variants. D-FISH with PML/RARA probe helped identification of RARA insertion to PML. By application of D-FISH on metaphase cells, we documented that translocation of 15 to 17 leads to 17q deletion which results in loss of reciprocal fusion and/or residual RARA on der(17). Among the complex variants of t(15;17), PML–RARA fusion followed by residual RARA insertion closed to PML–RARA on der(15) was unique and unusual. FISH with break-apart RARA probe on metaphase cells was found to be a very efficient strategy to detect unknown RARA variant translocations like t(11;17)(q23;q21), t(11;17)(q13;q21) and t(2;17)(p21;q21). These findings proved that D-FISH and break-apart probe strategy has potential to detect primary as well as secondary additional aberrations of PML, RARA and other additional loci. The long-term clinical follow-up is essential to evaluate the clinical importance of these findings.
PML-RARA; RARA variant; D-FISH; APL; 17q deletion
PML–RARA was proposed to initiate acute promyelocytic leukemia (APL) through PML–RARA homodimer–triggered repression. Here, we examined the nature of the PML–RARA protein complex and of its DNA targets in APL cells. Using a selection/amplification approach, we demonstrate that PML–RARA targets consist of two AGGTCA elements in an astonishing variety of orientations and spacings, pointing to highly relaxed structural constrains for DNA binding and identifying a major gain of function of this oncogene. PML–RARA-specific response elements were identified, which all conveyed a major transcriptional response to RA only in APL cells. In these cells, we demonstrate that PML–RARA oligomers are complexed to RXR. Directly probing PML–RARA function in APL cells, we found that the differentiation enhancer cyclic AMP (cAMP) boosted transcriptional activation by RA. cAMP also reversed the normal silencing (subordination) of the transactivating function of RXR when bound to RARA or PML–RARA, demonstrating that the alternate rexinoid/cAMP-triggered APL differentiation pathway also activates PML–RARA targets. Finally, cAMP restored both RA-triggered differentiation and PML–RARA transcriptional activation in mutant RA-resistant APL cells. Collectively, our findings directly demonstrate that APL cell differentiation parallels transcriptional activation through PML–RARA-RXR oligomers and that those are functionally targeted by cAMP, identifying this agent as another oncogene-targeted therapy.
therapy; leukemia; selex; transcription; oncogene
The PLZF/RARA fusion protein generated by the t(11;17)(q23;q21) translocation in acute promyelocytic leukaemia (APL) is believed to act as an oncogenic transcriptional regulator recruiting epigenetic factors to genes important for its transforming potential. However, molecular mechanisms associated with PLZF/RARA-dependent leukaemogenesis still remain unclear.
We searched for specific PLZF/RARA target genes by ChIP-on-chip in the haematopoietic cell line U937 conditionally expressing PLZF/RARA. By comparing bound regions found in U937 cells expressing endogenous PLZF with PLZF/RARA-induced U937 cells, we isolated specific PLZF/RARA target gene promoters. We next analysed gene expression profiles of our identified target genes in PLZF/RARA APL patients and analysed DNA sequences and epigenetic modification at PLZF/RARA binding sites. We identify 413 specific PLZF/RARA target genes including a number encoding transcription factors involved in the regulation of haematopoiesis. Among these genes, 22 were significantly down regulated in primary PLZF/RARA APL cells. In addition, repressed PLZF/RARA target genes were associated with increased levels of H3K27me3 and decreased levels of H3K9K14ac. Finally, sequence analysis of PLZF/RARA bound sequences reveals the presence of both consensus and degenerated RAREs as well as enrichment for tissue-specific transcription factor motifs, highlighting the complexity of targeting fusion protein to chromatin. Our study suggests that PLZF/RARA directly targets genes important for haematopoietic development and supports the notion that PLZF/RARA acts mainly as an epigenetic regulator of its direct target genes.
Acute promyelocytic leukemia (APL) is a malignancy of the bone marrow, in which there is a deficiency of myeloid cells and an excess of immature cells called promyelocytes. APL is most commonly caused by a translocation (15:17) and expression of the promyelocytic leukemia and the retinoic receptor α (PML-RARA) fusion product; however, the events that cooperate with PML-RARA in APL pathogenesis are not well understood. In this issue of the JCI, Wartman and colleagues use an innovative approach to find other relevant mutations in APL. They performed whole genome sequencing and copy number analysis of a well-characterized APL mouse model to uncover somatic mutations in Jak1 and lysine (K)-specific demethylase 6A (Kdm6a, also known as Utx) in mice with APL and validated the ability of Jak1 mutations to cooperate with PML-RARA in APL. The findings implicate the JAK/STAT pathway in the pathogenesis of APL and illustrate the power of whole genome sequencing to identify novel disease alleles in murine models of disease.
Acute promyelocytic leukemia (APL) is characterized by the t(15;17) translocation
that generates the fusion protein promyelocytic leukemia–retinoic acid
receptor α (PML-RARA) in nearly all cases. Multiple prior mouse models of
APL constitutively express PML-RARA from a variety of
non-Pml loci. Typically, all animals develop a myeloproliferative
disease, followed by leukemia in a subset of animals after a long latent period. In
contrast, human APL is not associated with an antecedent stage of myeloproliferation.
To address this discrepancy, we have generated a system whereby
PML-RARA expression is somatically acquired from the mouse
Pml locus in the context of Pml
haploinsufficiency. We found that physiologic PML-RARA expression
was sufficient to direct a hematopoietic progenitor self-renewal program in vitro and
in vivo. However, this expansion was not associated with evidence of
myeloproliferation, more accurately reflecting the clinical presentation of human
APL. Thus, at physiologic doses, PML-RARA primarily acts to increase
hematopoietic progenitor self-renewal, expanding a population of cells that are
susceptible to acquiring secondary mutations that cause progression to leukemia. This
mouse model provides a platform for more accurately dissecting the early events in
The leukemogenic effects of Myc drive recurrent trisomy in a mouse model of acute myeloid leukemia.
Gain of chromosome 8 is the most common chromosomal gain in human acute myeloid leukemia (AML). It has been hypothesized that gain of the MYC protooncogene is of central importance in trisomy 8, but the experimental data to support this are limited and controversial. In a mouse model of promyelocytic leukemia in which the MRP8 promoter drives expression of the PML-RARA fusion gene in myeloid cells, a Myc allele is gained in approximately two-thirds of cases as a result of trisomy for mouse chromosome 15. We used this model to test the idea that MYC underlies acquisition of trisomy in AML. We used a retroviral vector to drive expression of wild-type, hypermorphic, or hypomorphic MYC in bone marrow that expressed the PML-RARA transgene. MYC retroviruses cooperated in myeloid leukemogenesis and suppressed gain of chromosome 15. When the PML-RARA transgene was expressed in a Myc haploinsufficient background, we observed selection for increased copies of the wild-type Myc allele concomitant with leukemic transformation. In addition, we found that human myeloid leukemias with trisomy 8 have increased MYC. These data show that gain of MYC can contribute to the pathogenic effect of the most common trisomy of human AML.
Acute promyelocytic leukemia (APL) is characterized by the t(15;17) chromosomal translocation, which results in fusion of the retinoic acid receptor α (RARA) gene to another gene, most commonly promyelocytic leukemia (PML). The resulting fusion protein, PML-RARA, initiates APL, which is a subtype (M3) of acute myeloid leukemia (AML). In this report, we identify a gene expression signature that is specific to M3 samples; it was not found in other AML subtypes and did not simply represent the normal gene expression pattern of primary promyelocytes. To validate this signature for a large number of genes, we tested a recently developed high throughput digital technology (NanoString nCounter). Nearly all of the genes tested demonstrated highly significant concordance with our microarray data (P < 0.05). The validated gene signature reliably identified M3 samples in 2 other AML datasets, and the validated genes were substantially enriched in our mouse model of APL, but not in a cell line that inducibly expressed PML-RARA. These results demonstrate that nCounter is a highly reproducible, customizable system for mRNA quantification using limited amounts of clinical material, which provides a valuable tool for biomarker measurement in low-abundance patient samples.
The PML–RARA fusion protein is found in approximately 97% of patients with acute promyelocytic leukemia (APL). APL can be associated with life-threatening bleeding complications when undiagnosed and not treated expeditiously. The PML–RARA fusion protein arrests maturation of myeloid cells at the promyelocytic stage, leading to the accumulation of neoplastic promyelocytes. Complete remission can be obtained by treatment with all-trans-retinoic acid (ATRA) in combination with chemotherapy. Diagnosis of APL is based on the detection of t(15;17) by karyotyping, fluorescence in situ hybridization or PCR. These techniques are laborious and demand specialized laboratories. We developed a fast (performed within 4–5 h) and sensitive (detection of at least 10% malignant cells in normal background) flow cytometric immunobead assay for the detection of PML–RARA fusion proteins in cell lysates using a bead-bound anti-RARA capture antibody and a phycoerythrin-conjugated anti-PML detection antibody. Testing of 163 newly diagnosed patients (including 46 APL cases) with the PML–RARA immunobead assay showed full concordance with the PML–RARA PCR results. As the applied antibodies recognize outer domains of the fusion protein, the assay appeared to work independently of the PML gene break point region. Importantly, the assay can be used in parallel with routine immunophenotyping for fast and easy diagnosis of APL.
PML–RARA protein; t(15;17); APL; immunobead; flow cytometry
We utilized a mouse model of acute promyelocytic leukemia (APL) to investigate how aberrant activation of cytokine signaling pathways interacts with chimeric transcription factors to generate acute myeloid leukemia. Expression in mice of the APL-associated fusion, PML-RARA, initially has only modest effects on myelopoiesis. Whereas treatment of control animals with interleukin-3 (IL-3) resulted in expanded myelopoiesis without a block in differentiation, PML-RARA abrogated differentiation that normally characterizes the response to IL-3. Retroviral transduction of bone marrow with an IL-3-expressing retrovirus revealed that IL-3 and promyelocytic leukemia-retinoic acid receptor alpha (PML-RARα) combined to generate a lethal leukemia-like syndrome in <21 days. We also observed that a constitutively activated mutant IL-3 receptor, βcV449E, cooperated with PML-RARα in leukemogenesis, whereas a different activated mutant, βcI374N, did not. Analysis of additional mutations introduced into βcV449E showed that, although tyrosine phosphorylation of βc is necessary for cooperation, the Src homology 2 domain-containing transforming protein binding site is dispensable. Our results indicate that chimeric transcription factors can block the differentiative effects of growth factors. This combination can be potently leukemogenic, but the particular manner in which these types of mutations interact determines the ability of such combinations to generate acute myeloid leukemia.
Promyelocytic leukemia (PML) is a cell-growth suppressor, and PML-retinoic acid receptor α (PML-RARα) is known as a fusion gene of acute promyelocytic leukemia (APL). Studies have reported that neutrophil elastase(NE) cleaved bcr-1-derived PML-RARα in early myeloid cells leading to the removal of nuclear localization signal (NLS) from PML. The resultant PML without NLS named PML(NLS-). PML(NLS-) mainly locates and functions in the cytoplasm. PML(NLS-) may act in different ways from PML, but its biological characteristics have not been reported. In this study, the PML (NLS-) was silenced with shRNA [HL-60/pPML(NLS-)-shRNA] and over-expressed by preparation of recombinant adenovirus [HL-60/pAd-PML(NLS-)]. The mRNA and protein expression of PML(NLS-) were detected by RT-PCR and Western blot respectively. Cell proliferation in vitro was assessed by MTT assay. Flow cytometry (FCM) was used to detect apoptotic cells. The transcription of BCL-2, BAX and C-MYC was detected in HL-60/pAd-PML(NLS-) cells. Our results showed that compared to the control group, the expression of PML(NLS-) was significantly reduced in the HL-60/pPML(NLS-)-shRNA cells, and increased significantly in the HL-60/pAd-PML(NLS-) cells. The proliferation was significantly inhibited in the HL-60/pPML(NLS-)-shRNA cells in a time-dependent manner, but markedly promoted in the HL-60/pAd-PML(NLS-) cells treated with 60 μmol/L emodin. FCM revealed the apoptosis increased in HL-60/pPML(NLS-)-shRNA cells, and decreased in the HL-60/pAd-PML(NLS-) cells. The expression of BAX decreased significantly, while that of BCL-2 and C-MYC increased significantly in HL-60/ pAd-PML(NLS-) cells. Down-regulation of PML(NLS-) expression inhibits the proliferation and induces the apoptosis of HL-60 cells. On the contrary, over-expression of PML(NLS-) promotes the proliferation and reduce the emodin-induced apoptosis of HL-60 cells.
PML(NLS-); shRNA; over-expression; proliferation; apoptosis.
Molecular detection of minimal residual disease (MRD) has become established to assess remission status and guide therapy in patients with ProMyelocytic Leukemia–RARA+ acute promyelocytic leukemia (APL). However, there are few data on tracking disease response in patients with rarer retinoid resistant subtypes of APL, characterized by PLZF–RARA and STAT5b–RARA. Despite their rarity (<1% of APL) we identified 6 cases (PLZF–RARA, n = 5; STAT5b–RARA, n = 1), established the respective breakpoint junction regions and designed reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) assays to detect leukemic transcripts. The relative level of fusion gene expression in diagnostic samples was comparable to that observed in t(15;17) – associated APL, affording assay sensitivities of ∼1 in 104−105. Serial samples were available from two PLZF–RARA APL patients. One showed persistent polymerase chain reaction positivity, predicting subsequent relapse, and remains in CR2, ∼11 years post-autograft. The other, achieved molecular remission (CRm) with combination chemotherapy, remaining in CR1 at 6 years. The STAT5b–RARA patient failed to achieve CRm following frontline combination chemotherapy and ultimately proceeded to allogeneic transplant on the basis of a steadily rising fusion transcript level. These data highlight the potential of RT-qPCR detection of MRD to facilitate development of more individualized approaches to the management of rarer molecularly defined subsets of acute leukemia.
minimal residual disease; acute myeloid leukemia
Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia (AML). It is characterized by the t(15;17)(q22;q11.2) chromosomal translocation that creates the promyelocytic leukemia–retinoic acid receptor α (PML-RARA) fusion oncogene. Although this fusion oncogene is known to initiate APL in mice, other cooperating mutations, as yet ill defined, are important for disease pathogenesis. To identify these, we used a mouse model of APL, whereby PML-RARA expressed in myeloid cells leads to a myeloproliferative disease that ultimately evolves into APL. Sequencing of a mouse APL genome revealed 3 somatic, nonsynonymous mutations relevant to APL pathogenesis, of which 1 (Jak1 V657F) was found to be recurrent in other affected mice. This mutation was identical to the JAK1 V658F mutation previously found in human APL and acute lymphoblastic leukemia samples. Further analysis showed that JAK1 V658F cooperated in vivo with PML-RARA, causing a rapidly fatal leukemia in mice. We also discovered a somatic 150-kb deletion involving the lysine (K)-specific demethylase 6A (Kdm6a, also known as Utx) gene, in the mouse APL genome. Similar deletions were observed in 3 out of 14 additional mouse APL samples and 1 out of 150 human AML samples. In conclusion, whole genome sequencing of mouse cancer genomes can provide an unbiased and comprehensive approach for discovering functionally relevant mutations that are also present in human leukemias.
Acute promyelocytic leukemia (APL) is driven by a chromosomal translocation whose product, the PML/retinoic acid (RA) receptor α (RARA) fusion protein, affects both nuclear receptor signaling and PML body assembly. Dissection of APL pathogenesis has led to the rediscovery of PML bodies and revealed their role in cell senescence, disease pathogenesis, and responsiveness to treatment. APL is remarkable because of the fortuitous identification of two clinically effective therapies, RA and arsenic, both of which degrade PML/RARA oncoprotein and, together, cure APL. Analysis of arsenic-induced PML or PML/RARA degradation has implicated oxidative stress in the biogenesis of nuclear bodies and SUMO in their degradation.
Promyelocytic leukemia protein (PML) is a major component of PML nuclear bodies (PML NBs). Fusion of promyelocytic leukemia alpha gene (PML) with retinoic acid receptor gene with the t (15;17) translocation causes disassembly of PML NBs, leading to development of acute promyelocytic leukemia. In contrast, PML overexpression as well as different morphological changes of PML NBs were described in a few solid tumors. In this study, the expression of PML through the multistep hepatocarcinogenesis was analyzed in 95 cases of human hepatocellular carcinomas (HCCs) for comparison along with dysplastic nodules (DNs) and background liver cirrhosis (LC) or chronic hepatitis by immunohistochemistry and immunoblot. In addition, cases of HCCs were further evaluated according to their histologic grade and etiology. The amount of PML as well as the number and size of PML NBs increased gradually through the progression from LC, DNs to HCCs. The overexpression of PML in HCCs was much more closely associated with HBV infection than HCV infection or alcoholic liver disease. The PML expression, however, was not correlated with histologic grade of HCCs. These results suggest that PML is involved in the early stage of multistep hepatocarcinogenesis, and HBV infection may be associated with the overexpression of PML and the morphological alteration of PML NBs.
Different fusion oncogenes in acute myeloid leukemia (AML) have distinct clinical and laboratory features suggesting different modes of malignant transformation. Here we compare the in vitro effects of representatives of 4 major groups of AML fusion oncogenes on primary human CD34+ cells. As expected from their clinical similarities, MLL-AF9 and NUP98-HOXA9 had very similar effects in vitro. They both caused erythroid hyperplasia and a clear block in erythroid and myeloid maturation. On the other hand, AML1-ETO and PML-RARA had only modest effects on myeloid and erythroid differentiation. All oncogenes except PML-RARA caused a dramatic increase in long-term proliferation and self-renewal. Gene expression profiling revealed two distinct temporal patterns of gene deregulation. Gene deregulation by MLL-AF9 and NUP98-HOXA9 peaked 3 days after transduction. In contrast, the vast majority of gene deregulation by AML1-ETO and PML-RARA occurred within 6 hours, followed by a dramatic drop in the numbers of deregulated genes. Interestingly, the p53 inhibitor MDM2 was upregulated by AML1-ETO at 6 hours. Nutlin-3, an inhibitor of the interaction between MDM2 and p53, specifically inhibited the proliferation and self-renewal of primary human CD34+ cells transduced with AML1-ETO, suggesting that MDM2 upregulation plays a role in cell transformation by AML1-ETO. These data show that differences among AML fusion oncogenes can be recapitulated in vitro using primary human CD34+ cells and that early gene expression profiling in these cells can reveal potential drug targets in AML.
Promyelocytic leukemia (PML) protein binds to and stabilizes CIITA at PML nuclear bodies, which promotes expression of the MHC class II gene locus in response to interferon-γ exposure.
Promyelocytic leukemia (PML) nuclear bodies selectively associate with transcriptionally active genomic regions, including the gene-rich major histocompatibility (MHC) locus. In this paper, we have explored potential links between PML and interferon (IFN)-γ–induced MHC class II expression. IFN-γ induced a substantial increase in the spatial proximity between PML bodies and the MHC class II gene cluster in different human cell types. Knockdown experiments show that PML is required for efficient IFN-γ–induced MHC II gene transcription through regulation of the class II transactivator (CIITA). PML mediates this function through protection of CIITA from proteasomal degradation. We also show that PML isoform II specifically forms a stable complex with CIITA at PML bodies. These observations establish PML as a coregulator of IFN-γ–induced MHC class II expression.
The growth suppressor promyelocytic leukemia protein (PML) is disrupted by the chromosomal translocation t(15;17) in acute promyelocytic leukemia (APL). PML plays a key role in multiple pathways of apoptosis and regulates cell cycle progression. The present study demonstrates that PML represses transcription by functionally and physically interacting with histone deacetylase (HDAC). Transcriptional repression mediated by PML can be inhibited by trichostatin A, a specific inhibitor of HDAC. PML coimmunoprecipitates a significant level of HDAC activity in several cell lines. PML is associated with HDAC in vivo and directly interacts with HDAC in vitro. The fusion protein PML-RARα encoded by the t(15;17) breakpoint interacts with HDAC poorly. PML interacts with all three isoforms of HDAC through specific domains, and its expression deacetylates histone H3 in vivo. Together, the results of our study show that PML modulates histone deacetylation and that loss of this function in APL alters chromatin remodeling and gene expression. This event may contribute to the development of leukemia.
Promyelocytic leukemia (PML) protein is the organizer of nuclear matrix-associated nuclear bodies (NBs), and its conjugation to the small ubiquitin-like modifier (SUMO) is required for the formation of these structures. Several alternatively spliced PML transcripts from a single PML gene lead to the production of seven PML isoforms (PML isoform I [PMLI] to VII [PMLVII]), which all share a N-terminal region that includes the RBCC (RING, B boxes, and a α-helical coiled-coil) motif but differ in the C-terminal region. This diversity of PML isoforms determines the specific functions of each isoform. There is increasing evidence implicating PML in host antiviral defense and suggesting various strategies involving PML to counteract viral production. We reported that mouse embryonic fibroblasts derived from PML knockout mice are more sensitive than wild-type cells to infection with encephalomyocarditis virus (EMCV). Here, we show that stable expression of PMLIV or PMLIVa inhibited viral replication and protein synthesis, leading to a substantial reduction of EMCV multiplication. This protective effect required PMLIV SUMOylation and was not observed with other nuclear PML isoforms (I, II, III, V, and VI) or with the cytoplasmic PMLVII. We demonstrated that only PMLIV interacted with EMCV 3D polymerase (3Dpol) and sequestered it within PML NBs. The C-terminal region specific to PMLIV was required for both interaction with 3Dpol and the antiviral properties. Also, depletion of PMLIV by RNA interference significantly boosted EMCV production in interferon-treated cells. These findings indicate the mechanism by which PML confers resistance to EMCV. They also reveal a new pathway mediating the antiviral activity of interferon against EMCV.
The hematopoietically expressed GATA family of transcription factors function as key regulators of blood cell fate. Among these, GATA-2 is implicated in the survival and growth of multipotential progenitors. Here we report that the promyelocytic leukemia protein (PML) can complex with GATA-2 and potentiate its transactivation capacity. The binding is mediated through interaction of the zinc finger region of GATA-2 and the B-box domain of PML. The B-box region of PML is retained in the PML-RARα (retinoic acid receptor alpha) fusion protein generated by the t(15;17) translocation characteristic of acute promyelocytic leukemia (APL). Consistent with this, we provide evidence that GATA-2 can physically associate with PML-RARα. Functional experiments further demonstrated that this interaction has the capacity to render GATA-dependent transcription inducible by retinoic acid, raising the possibility that GATA target genes may be involved in the molecular pathogenesis of APL.
A key oncogenic force in acute promyelocytic leukemia (APL) is the ability of the promyelocytic leukemia–retinoic acid receptor α (PML-RARA) oncoprotein to recruit transcriptional repressors and DNA methyltransferases at retinoic acid–responsive elements. Pharmacological doses of retinoic acid relieve transcriptional repression inducing terminal differentiation/apoptosis of the leukemic blasts. APL blasts often harbor additional recurrent chromosomal abnormalities, and significantly, APL prevalence is increased in Latino populations. These observations suggest that multiple genetic and environmental/dietary factors are likely implicated in APL. We tested whether dietary or targeted chemopreventive strategies relieving PML-RARA transcriptional repression would be effective in a transgenic mouse model. Surprisingly, we found that 1) treatment with a demethylating agent, 5-azacytidine, results in a striking acceleration of APL; 2) a high fat, low folate/choline–containing diet resulted in a substantial but nonsignificant APL acceleration; and 3) all-trans retinoic acid (ATRA) is ineffective in preventing leukemia and results in ATRA-resistant APL. Our findings have important clinical implications because ATRA is a drug of choice for APL treatment and indicate that global demethylation, whether through dietary manipulations or through the use of a pharmacologic agent such as 5-azacytidine, may have unintended and detrimental consequences in chemopreventive regimens.
APL; 5-azacytidine; ATRA; Western diet; chemoprevention