Fine-tuning PU.1

The synergistic effect of c-Myb hyperactivation and Pu.1 deficiency induces Pelger-Huët anomaly and promotes sAML

Approximately 30% of patients with myelodysplastic syndrome (MDS) progress to secondary acute myeloid leukemia (sAML) via accumulating gene mutations. Genomic analyses reveal a complex interplay among mutant genes, with co-occurring and mutually exclusive patterns. Hyperactivation of c-MYB and deficiency of PU.1 have been linked to myeloid disorders. We report a case of AML with concurrent PU.1 and c-MYB mutations, exhibiting early onset, high blast count, chemo-resistance, indicating high-risk features, along with elevated Pelger-Huët anomaly (PHA). However, the synergistic mechanism of c-MYB and PU.1 in sAML remains unclear. Using c-Myb-hyperactivation and Pu.1-deficient double-strain (c-mybhyper;pu.1G242D/G242D) zebrafish, we investigated MDS/sAML progression. Surprisingly, the double mutant exhibited a distinct type of neutrophil resembling clinical PHA cells and demonstrated a higher rate of MDS/sAML transformation. Further expression analysis revealed reduced lmnb1 expression in double-mutant zebrafish. Knockdown of lmnb1 resulted in PHA and increased blast cells, while overexpression of lmnb1 in c-mybhyper;pu.1G242D/G242D reduced PHA cell level. This suggests that c-Myb hyperactivation and Pu.1 deficiency synergistically reduce lmnb1 expression, inducing the development of PHA-like neutrophils and promoting MDS/sAML progression in zebrafish. Moreover, coadministration of cell cycle inhibitor cytarabine (Ara-C) and the differential inducer all-trans retinoic acid (ATRA) could effectively relieve the neutrophil expansion and PHA symptoms in c-mybhyper;pu.1G242D/G242D zebrafish. Our findings revealed that c-Myb hyperactivation and Pu.1 deficiency played a synergistic role in sAML development and suggests a phenotypic association between the emergence of PH-like cells and the transformation to sAML. Furthermore, c-mybhyper;pu.1G242D/G242D zebrafish might serve as a suitable sAML model for drug screening.

Modulating DNA by polyamides to regulate transcription factor PU.1-DNA binding interactions

Hairpin polyamides are synthetic small molecules that bind DNA minor groove sequence-selectively and, in many sequences, induce widening of the minor groove and compression of the major groove. The structural distortion of DNA caused by polyamides has enhanced our understanding of the regulation of DNA-binding proteins via polyamides. Polyamides have DNA binding affinities that are comparable to those proteins, therefore, can potentially be used as therapeutic agents to treat diseases caused by aberrant gene expression. In fact, many diseases are characterized by over- or under-expressed genes. PU.1 is a transcription factor that regulates many immune system genes. Aberrant expression of PU.1 has been associated with the development of acute myeloid leukemia (AML). We have, therefore, designed and synthesized ten hairpin polyamides to investigate their capacity in controlling the PU.1-DNA interaction. Our results showed that nine of the polyamides disrupt PU.1-DNA binding and the inhibition capacity strongly correlates with binding affinity. One molecule, FH1024, was observed forming a FH1024-PU.1-DNA ternary complex instead of inhibiting PU.1-DNA binding. This is the first report of a small molecule that is potentially a weak agonist that recruits PU.1 to DNA. This finding sheds light on the design of polyamides that exhibit novel regulatory mechanisms on protein-DNA binding.

PU.1 is a major transcriptional activator of the tumour suppressor gene LIMD1

LIMD1 is a tumour suppressor gene (TSG) down regulated in ∼80% of lung cancers with loss also demonstrated in breast and head and neck squamous cell carcinomas. LIMD1 is also a candidate TSG in childhood acute lymphoblastic leukaemia. Mechanistically, LIMD1 interacts with pRB, repressing E2F-driven transcription as well as being a critical component of microRNA-mediated gene silencing. In this study we show a CpG island within the LIMD1 promoter contains a conserved binding motif for the transcription factor PU.1. Mutation of the PU.1 consensus reduced promoter driven transcription by 90%. ChIP and EMSA analysis demonstrated that PU.1 specifically binds to the LIMD1 promoter. siRNA depletion of PU.1 significantly reduced endogenous LIMD1 expression, demonstrating that PU.1 is a major transcriptional activator of LIMD1.

A two-mutation model of radiation-induced acute myeloid leukemia using historical mouse data

From studies of the atomic bomb survivors, it is well known that ionizing radiation causes several forms of leukemia. However, since the specific mechanism behind this process remains largely unknown, it is difficult to extrapolate carcinogenic effects at acute high-dose exposures to risk estimates for the chronic low-dose exposures that are important for radiation protection purposes. Recently, it has become clear that the induction of acute myeloid leukemia (AML) in CBA/H mice takes place through two key steps, both involving the Sfpi1 gene. A similar mechanism may play a role in human radiation-induced AML. In the present paper, a two-mutation carcinogenesis model is applied to model AML in several data sets of X-ray- and neutron-exposed CBA/H mice. The models obtained provide good fits to the data. A comparison between the predictions for neutron-induced and X-ray-induced AML yields an RBE for neutrons of approximately 3. The model used is considered to be a first step toward a model for human radiation-induced AML, which could be used to estimate risks of exposure to low doses.

Hematopoietic stem cells and retroviral infection

Retroviral induced malignancies serve as ideal models to help us better understand the molecular mechanisms associated with the initiation and progression of leukemogenesis. Numerous retroviruses including AEV, FLV, M-MuLV and HTLV-1 have the ability to infect hematopoietic stem and progenitor cells, resulting in the deregulation of normal hematopoiesis and the development of leukemia/lymphoma. Research over the last few decades has elucidated similarities between retroviral-induced leukemogenesis, initiated by deregulation of innate hematopoietic stem cell traits, and the cancer stem cell hypothesis. Ongoing research in some of these models may provide a better understanding of the processes of normal hematopoiesis and cancer stem cells. Research on retroviral induced leukemias and lymphomas may identify the molecular events which trigger the initial cellular transformation and subsequent maintenance of hematologic malignancies, including the generation of cancer stem cells. This review focuses on the role of retroviral infection in hematopoietic stem cells and the initiation, maintenance and progression of hematological malignancies.

Isoform-specific potentiation of stem and progenitor cell engraftment by AML1/RUNX1

BACKGROUND

AML1/RUNX1 is the most frequently mutated gene in leukaemia and is central to the normal biology of hematopoietic stem and progenitor cells. However, the role of different AML1 isoforms within these primitive compartments is unclear. Here we investigate whether altering relative expression of AML1 isoforms impacts the balance between cell self-renewal and differentiation in vitro and in vivo.

METHODS AND FINDINGS

The human AML1a isoform encodes a truncated molecule with DNA-binding but no transactivation capacity. We used a retrovirus-based approach to transduce AML1a into primitive haematopoietic cells isolated from the mouse. We observed that enforced AML1a expression increased the competitive engraftment potential of murine long-term reconstituting stem cells with the proportion of AML1a-expressing cells increasing over time in both primary and secondary recipients. Furthermore, AML1a expression dramatically increased primitive and committed progenitor activity in engrafted animals as assessed by long-term culture, cobblestone formation, and colony assays. In contrast, expression of the full-length isoform AML1b abrogated engraftment potential. In vitro, AML1b promoted differentiation while AML1a promoted proliferation of progenitors capable of short-term lymphomyeloid engraftment. Consistent with these findings, the relative abundance of AML1a was highest in the primitive stem/progenitor compartment of human cord blood, and forced expression of AML1a in these cells enhanced maintenance of primitive potential both in vitro and in vivo.

CONCLUSIONS

These data demonstrate that the "a" isoform of AML1 has the capacity to potentiate stem and progenitor cell engraftment, both of which are required for successful clinical transplantation. This activity is consistent with its expression pattern in both normal and leukaemic cells. Manipulating the balance of AML1 isoform expression may offer novel therapeutic strategies, exploitable in the contexts of leukaemia and also in cord blood transplantation in adults, in whom stem and progenitor cell numbers are often limiting.

Preanalytical mRNA stabilization of whole bone marrow samples

BACKGROUND

Gene expression profiling is a useful tool for cancer diagnosis and basic research. A major limitation is that, even during short-term storage of native specimens of peripheral blood or bone marrow (BM) and/or RNA isolation, significant changes of gene expression pattern can occur because of gene induction, repression, and RNA degradation.

METHODS

We investigated the effectiveness of a newly developed RNA stabilization and preparation system for BM specimens (PAXgene Bone Marrow RNA System) over time. We analyzed 256 RNA samples, processed from 64 BM specimens.

RESULTS

Although the overall RNA yield (normalized to 1 x 10(7) leukocytes) was not different, the RNA preparation using unstabilized reference samples had an approximately 3 times higher failure rate. With the PAXgene system, we observed significantly higher RNA integrity compared with the reference RNA preparation system (P <0.01). In the stabilized samples, we found very high pairwise correlation in gene expression (DeltaDeltaC(T) 0.16-0.53) for the analyzed genes (GATA1, RUNX1, NCAM1, and SPI1) after 48-h storage compared with immediate preparation of RNA (2 h after BM collection). However, we found major differences in half of the analyzed genes using the reference RNA isolation procedure (DeltaDeltaC(T) 1.07 and 1.32).

CONCLUSIONS

The PAXgene system is able to stabilize RNA from clinical BM samples and is suitable to isolate high-quality and -quantity RNA.

PU.1: An ETS Family Transcription Factor That Regulates Leukemogenesis Besides Normal Hematopoiesis

The hematopoietic transcription factor PU.1, which is required for lymphomyeloid differentiation of stem cells, was originally identified as an oncogene. In erythroid progenitors, the integration of spleen focus-forming virus (SFFV) into the PU.1 locus causes its overexpression, which blocks their terminal differentiation into erythrocytes and ultimately leads to the development of erythroleukemia. However, in myeloid lineages, PU.1 promotes granulocytic and monocytic differentiation, and graded reduction in its expression blocks their differentiation or maturation and thereby causes myelogenous leukemia. Thus, in addition to normal hematopoietic regulation, PU.1 plays a significant role in leukemogenesis. In the following review, we have consolidated our understanding of the role of transcription factor PU.1 in the development of erythroid as well myeloid leukemia.