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

Deciphering Acute Myeloid Leukemia Associated Transcription Factors in Human Primary CD34+ Hematopoietic Stem/Progenitor Cells

Hemato-oncological diseases account for nearly 10% of all malignancies and can be classified into leukemia, lymphoma, myeloproliferative diseases, and myelodysplastic syndromes. The causes and prognosis of these disease entities are highly variable. Most entities are not permanently controllable and ultimately lead to the patient's death. At the molecular level, recurrent mutations including chromosomal translocations initiate the transformation from normal stem-/progenitor cells into malignant blasts finally floating the patient's bone marrow and blood system. In acute myeloid leukemia (AML), the so-called master transcription factors such as RUNX1, KMT2A, and HOX are frequently disrupted by chromosomal translocations, resulting in neomorphic oncogenic fusion genes. Triggering ex vivo expansion of primary human CD34+ stem/progenitor cells represents a distinct characteristic of such chimeric AML transcription factors. Regarding oncogenic mechanisms of AML, most studies focus on murine models. However, due to biological differences between mice and humans, findings are only partly transferable. This review focuses on the genetic manipulation of human CD34+ primary hematopoietic stem/progenitor cells derived from healthy donors to model acute myeloid leukemia cell growth. Analysis of defined single- or multi-hit human cellular AML models will elucidate molecular mechanisms of the development, maintenance, and potential molecular intervention strategies to counteract malignant human AML blast cell growth.

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.

All-trans retinoic acid and arsenic trioxide fail to derepress the monocytic differentiation driver Irf8 in acute promyelocytic leukemia cells

All-trans retinoic acid (ATRA) and/or arsenic trioxide (ATO) administration leads to granulocytic maturation and/or apoptosis of acute promyelocytic leukemia (APL) cells mainly by targeting promyelocytic leukemia/retinoic acid receptor alpha (PML/RARα). Yet, ~10–15% of APL patients are not cured by ATRA- and ATO-based therapies, and a potential failure of ATRA and ATO in completely reversing PML/RARα-driven oncogenic alterations has not been comprehensively examined. Here we characterized the in vivo primary responses of dysregulated genes in APL cells treated with ATRA and ATO using a GFP-labeled APL model. Although induced granulocytic differentiation of APL cells was evident after ATRA or ATO administration, the expression of the majority of dysregulated genes in the c-Kit⁺ APL progenitors was not consistently corrected. Irf8, whose expression increased along with spontaneous differentiation of the APL progenitors in vivo, represented such a PML/RARα-dysregulated gene that was refractory to ATRA/ATO signaling. Interestingly, Irf8 induction, but not its knockdown, decreased APL leukemogenic potential through driving monocytic maturation. Thus, we reveal that certain PML/RARα-dysregulated genes that are refractory to ATRA/ATO signaling are potentially crucial regulators of the immature status and leukemogenic potential of APL cells, which can be exploited for the development of new therapeutic strategies for ATRA/ATO-resistant APL cases.

A humanized bone marrow ossicle xenotransplantation model enables improved engraftment of healthy and leukemic human hematopoietic cells

Xenotransplantation models represent powerful tools for the investigation of healthy and malignant human hematopoiesis. However, current models do not fully mimic the components of the human bone marrow (BM) microenvironment, and they enable only limited engraftment of samples from some human malignancies. Here we show that a xenotransplantation model bearing subcutaneous humanized ossicles with an accessible BM microenvironment, formed by in situ differentiation of human BM-derived mesenchymal stromal cells, enables the robust engraftment of healthy human hematopoietic stem and progenitor cells, as well as primary acute myeloid leukemia (AML) samples, at levels much greater than those in unmanipulated mice. Direct intraossicle transplantation accelerated engraftment and resulted in the detection of substantially higher leukemia-initiating cell (LIC) frequencies. We also observed robust engraftment of acute promyelocytic leukemia (APL) and myelofibrosis (MF) samples, and identified LICs in these malignancies. This humanized ossicle xenotransplantation approach provides a system for modeling a wide variety of human hematological diseases.

Transcription and methylation analyses of preleukemic promyelocytes indicate a dual role for PML/RARA in leukemia initiation

Acute promyelocytic leukemia is an aggressive malignancy characterized by the accumulation of promyelocytes in the bone marrow. PML/RARA is the primary abnormality implicated in this pathology, but the mechanisms by which this chimeric fusion protein initiates disease are incompletely understood. Identifying PML/RARA targets in vivo is critical for comprehending the road to pathogenesis. Utilizing a novel sorting strategy, we isolated highly purified promyelocyte populations from normal and young preleukemic animals, carried out microarray and methylation profiling analyses, and compared the results from the two groups of animals. Surprisingly, in the absence of secondary lesions, PML/RARA had an overall limited impact on both the transcriptome and methylome. Of interest, we did identify down-regulation of secondary and tertiary granule genes as the first step engaging the myeloid maturation block. Although initially not sufficient to arrest terminal granulopoiesis in vivo, such alterations set the stage for the later, complete differentiation block seen in leukemia. Further, gene set enrichment analysis revealed that PML/RARA promyelocytes exhibit a subtle increase in expression of cell cycle genes, and we show that this leads to both increased proliferation of these cells and expansion of the promyelocyte compartment. Importantly, this proliferation signature was absent from the poorly leukemogenic p50/RARA fusion model, implying a critical role for PML in the altered cell-cycle kinetics and ability to initiate leukemia. Thus, our findings challenge the predominant model in the field and we propose that PML/RARA initiates leukemia by subtly shifting cell fate decisions within the promyelocyte compartment.

Xenograft models for normal and malignant stem cells

The model systems available for studying human hematopoiesis, malignant hematopoiesis, and hematopoietic stem cell (HSC) function in vivo have improved dramatically over the last decade, primarily due to improvements in xenograft mouse strains. Several recent reviews have focused on the historic development of immunodeficient mice over the last 2 decades, as well as their use in understanding human HSC and leukemia stem cell (LSC) biology and function in the context of a humanized mouse. However, in the intervening time since these reviews, a number of new mouse models, technical approaches, and scientific advances have been made. In this review, we update the reader on the newest and best models and approaches available for studying human malignant and normal HSCs in immunodeficient mice, including newly developed mice for use in chemotherapy testing and improved techniques for humanizing mice without laborious purification of HSC. We also review some relevant scientific findings from xenograft studies and highlight the continued limitations that confront researchers working with human HSC and LSC in vivo.