Association of acute myeloid leukemia's most immature phenotype with risk groups and outcomes

MRD in Acute Leukemias: Lessons Learned from Acute Promyelocytic Leukemia

Introduction: Advances in molecular biology, polymerase chain reaction (PCR), and next-generation sequencing (NGS) have transformed the concept of minimal residual disease (MRD) from a philosophical idea into a measurable reality. Current Treatment Paradigms and Lessons Learned from APL: Acute promyelocytic leukemia (APL) leads the way in this transformation, initially using PCR to detect MRD in patients in remission, and more recently, aiming to eliminate it entirely with modern treatment strategies. Along the way, we have gained valuable insights that, when applied to other forms of acute leukemia, hold the potential to significantly improve the outcomes of these challenging diseases. Does the BM Microenvironment Play a Role in MRD?: In this review, we explore the current use of MRD in the management of acute leukemia and delve into the biological processes that contribute to MRD persistence, including its overlap with leukemia stem cells and the role of the bone marrow microenvironment.

The role of the atypical chemokine receptor CCRL2 in myelodysplastic syndrome and secondary acute myeloid leukemia

The identification of new pathways supporting the myelodysplastic syndrome (MDS) primitive cells growth is required to develop targeted therapies. Within myeloid malignancies, men have worse outcomes than women, suggesting male sex hormone-driven effects in malignant hematopoiesis. Androgen receptor promotes the expression of five granulocyte colony-stimulating factor receptor-regulated genes. Among them, CCRL2 encodes an atypical chemokine receptor regulating cytokine signaling in granulocytes, but its role in myeloid malignancies is unknown. Our study revealed that CCRL2 is up-regulated in primitive cells from patients with MDS and secondary acute myeloid leukemia (sAML). CCRL2 knockdown suppressed MDS92 and MDS-L cell growth and clonogenicity in vitro and in vivo and decreased JAK2/STAT3/STAT5 phosphorylation. CCRL2 coprecipitated with JAK2 and potentiated JAK2-STAT interaction. Erythroleukemia cells expressing JAK2V617F showed less effect of CCRL2 knockdown, whereas fedratinib potentiated the CCRL2 knockdown effect. Conclusively, our results implicate CCRL2 as an MDS/sAML cell growth mediator, partially through JAK2/STAT signaling.

p65/RelA NF-κB fragments generated by RIPK3 activity regulate tumorigenicity, cell metabolism, and stemness characteristics

Receptor‐interacting protein kinase 3 (RIPK3) can induce necroptosis, apoptosis, or cell proliferation and is silenced in several hematological malignancies. We previously reported that RIPK3 activity independent of its kinase domain induces caspase‐mediated p65/RelA cleavage, resulting in N‐terminal 1‐361 and C‐terminal 362‐549 fragments. We show here that a noncleavable p65/RelA D361E mutant expressed in DA1‐3b leukemia cells decreases mouse survival times and that coexpression of p65/RelA fragments increases the tumorigenicity of B16F1 melanoma cells. This aggressiveness in vivo did not correlate with NF‐κB activity measured in vitro. The fragments and p65/RelA D361E mutant induced different expression profiles in DA1‐3b and B16F1 cells. Stemness markers were affected: p65/RelA D361E increased ALDH activity in DA1‐3b cells, and fragment expression increased melanoma sphere formation in B16/F1 cells. p65/RelA fragments and the D361E noncleavable mutant decreased oxidative or glycolytic cell metabolism, with differences observed between models. Thus, p65/RelA cleavage initiated by kinase‐independent RIPK3 activity in cancer cells is not neutral and induces pleiotropic effects in vitro and in vivo that may vary across tumor types.

Cancer stem cells: a major culprit of intra-tumor heterogeneity

Cancer is recognized as a preeminent factor of the world's mortality. Although various modalities have been designed to cure this life-threatening ailment, a significant impediment in the effective output of cancer treatment is heterogeneity. Cancer is characterized as a heterogeneous health disorder that comprises a distinct group of transformed cells to assist anomalous proliferation of affected cells. Cancer stem cells (CSCs) are a leading cause of cancer heterogeneity that is continually transformed by cellular extrinsic and intrinsic factors. They intensify neoplastic cells aggressiveness by strengthening their dissemination, relapse and therapy resistance. Considering this viewpoint, in this review article we have discussed some intrinsic (transcription factors, cell signaling pathways, genetic alterations, epigenetic modifications, non-coding RNAs (ncRNAs) and epitranscriptomics) and extrinsic factors (tumor microenvironment (TME)) that contribute to CSC heterogeneity and plasticity, which may help scientists to meddle these processes and eventually improve cancer research and management. Besides, the potential role of CSCs heterogeneity in establishing metastasis and therapy resistance has been articulated which signifies the importance of developing novel anticancer therapies to target CSCs along with targeting bulk tumor mass to achieve an effective output.

A phase II study of azacitidine in combination with granulocyte-macrophage colony-stimulating factor as maintenance treatment, after allogeneic blood or marrow transplantation in patients with poor-risk acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS)

Relapse is the most common cause of treatment failure following allogeneic blood or marrow transplantation (alloBMT) for AML or MDS. Post-transplant maintenance therapies may prevent relapse. We conducted a phase II trial combining azacitidine (AZA) with GM-CSF in non-relapsed, post-transplant patients with AML or MDS. Patients received escalating doses of AZA to a maximum of 75 mg/m2 for 5 days per cycle for up to 12 cycles. GM-CSF was given on days 1-10 of each cycle. Eighteen patients were treated following non-myeloablative (17) and myeloablative (1) alloBMT for AML (61.1%), MDS (27.7%), or therapy-related myeloid neoplasm (11.1%). The majority of patients (72%) received their graft from an HLA-haploidentical donor. The treatment was well-tolerated with rare grade 3-4 hematologic toxicities. One patient suffered an exacerbation of GVHD. The 24-month relapse-free and overall survivals were 47 and 57%, respectively, with a median of 18.6 and 29 months.

Subgrouping by gene expression profiles to improve relapse risk prediction in paediatric B-precursor acute lymphoblastic leukaemia

Relapsed acute lymphoblastic leukaemia (ALL) remains a prevalent paediatric cancer and one of the most common causes of mortality from malignancy in children. Tailoring the intensity of therapy according to early stratification is a promising strategy but remains a major challenge due to heterogeneity and subtyping difficulty. In this study, we subgroup B-precursor ALL patients by gene expression profiles, using non-negative matrix factorization and minimum description length which unsupervisedly determines the number of subgroups. Within each of the four subgroups, logistic and Cox regression with elastic net regularization are used to build models predicting minimal residual disease (MRD) and relapse-free survival (RFS) respectively. Measured by area under the receiver operating characteristic curve (AUC), subgrouping improves prediction of MRD in one subgroup which mostly overlaps with subtype TCF3-PBX1 (AUC = 0·986 in the training set and 1·0 in the test set), compared to a global model published previously. The models predicting RFS displayed acceptable concordance in training set and discriminate high-relapse-risk patients in three subgroups of the test set (Wilcoxon test p = 0·048, 0·036, and 0·016). Genes playing roles in the models are specific to different subgroups. The improvement of subgrouped MRD prediction and the differences of genes in prediction models of subgroups suggest that the heterogeneity of B-precursor ALL can be handled by subgrouping according to gene expression profiles to improve the prediction accuracy.

Expression of putative leukemia stem cell targets in genetically-defined acute myeloid leukemia subtypes

Although most acute myeloid leukemia (AML) patients achieve complete remissions, the majority still eventually relapse and die of their disease. Rare primitive leukemia cells, so-called leukemia stem cells (LSCs), represent one potential type of resistant cell subpopulation responsible for this dissociation between response and cure. Several LSC targets have been described, but there is limited evidence about their relative utility or that targeting any can prevent relapse. LSCs not only appear to be biologically heterogeneous, but the classic immunocompromised mouse transplantation model also has serious shortcomings as an LSC assay. Out data suggest that the most immature cell phenotype that can be identified within a patient's leukemia may be clinically relevant and represent the de facto LSC. Moreover, although phenotypically heterogeneous, these putative LSCs show consistent phenotypes within individual genetically defined groups. Using this LSC definition, we studied several previously described putative LSC targets, CD25, CD26, CD47, CD96, CD123, and CLL-1, and all were expressed across heterogeneous LSC phenotypes. In addition, with the exception of CD47, there was at most low expression of these targets on normal hematopoietic stem cells (HSCs). CD123 and CLL-1 demonstrated the greatest expression differences between putative LSCs and normal HSCs. Importantly, CD123 monoclonal antibodies were cytotoxic in vitro to putative LSCs from all AML subtypes, while showing limited to no toxicity against normal HSCs and hematopoietic progenitors. Since minimal residual disease appears to be a more homogeneous population of cells responsible for relapse, targeting CD123 in this setting may be most effective.

High aldehyde dehydrogenase activity at diagnosis predicts relapse in patients with t(8;21) acute myeloid leukemia

Acute myeloid leukemia (AML) with t(8;21) is a heterogeneous disease. Although the detection of minimal residual disease (MRD), which is indicated by RUNX1‐RUNX1T1 transcript levels, plays a key role in directing treatment, risk stratification needs to be improved, and other markers need to be assessed. A total of 66 t(8;21) AML patients were tested for aldehyde dehydrogenase (ALDH) activity by flow cytometry at diagnosis, and 52 patients were followed up for a median of 20 (1‐34) months. The median percentage of CD34+ALDH+, CD34+CD38‐ALDH+, and CD34+CD38+ALDH+ cells among nucleated cells were 0.028%, 0.012%, and 0.0070%, respectively. The CD34+ALDH+‐H, CD34+CD38‐ALDH+‐H, and CD34+CD38+ALDH+‐H statuses (the percentage of cells that were higher than the individual cutoffs) were all significantly associated with a lower 2‐year relapse‐free survival (RFS) rate in both the whole cohort and adult patients (P = .015, .016, and .049; P = .014, .018, and .032). Patients with < 3‐log reduction in the RUNX1‐RUNX1T1 transcript level after the second consolidation therapy (defined as MRD‐H) had a significantly lower 2‐year RFS rate than patients with ≥ 3‐log reduction (MRD‐L) (P = .017). The CD34+ALDH+ status at diagnosis was then combined with the MRD status. CD34+ALDH+‐L/MRD‐H patients had similar 2‐year RFS rates to both CD34+ALDH+‐L/MRD‐L and CD34+ALDH+‐H/MRD‐L patients (P = .50 and 1.0); and CD34+ALDH+‐H/MRD‐H patients had significantly lower 2‐year RFS rate compared with CD34+ALDH+‐L and/or MRD‐L patients (P < .0001). Multivariate analysis showed that CD34+ALDH+‐H/MRD‐H was an independent adverse prognostic factor for relapse. In conclusion, ALDH status at diagnosis may improve MRD‐based risk stratification in t(8;21) AML, and concurrent high levels of CD34+ALDH+ at diagnosis and MRD predict relapse.

Relationship between CD34/CD38 and side population (SP) defined leukemia stem cell compartments in acute myeloid leukemia

Leukemic stem cells (LSCs), defined by CD34/CD38 expression, are believed to be essential for leukemia initiation and therapy resistance in acute myeloid leukemia. In addition, the side population (SP), characterized by high Hoechst 33342 efflux, reflecting therapy resistance, has leukemia initiating ability. The purpose of this study is, in both CD34-positive and CD34-negative AML, to integrate both types of LSC compartment into a new more restricted definition. Different CD34/CD38/SP defined putative LSC and normal hematopoietic compartments, with neoplastic or normal nature, respectively, were thus identified after cell sorting, and confirmed by FISH/PCR. Stem cell activity was assessed in the long-term liquid culture stem cell assay. SP fractions harbored the strongest functional stem cell activity in both normal and neoplastic cells in both CD34-positive and CD34-negative AML. Overall, inclusion of SP fraction decreased the size of the putative CD34/CD38 defined LSC compartment by a factor >500. For example, for the important CD34+CD38- LSC compartment, the median SP/CD34+CD38- frequency was 5.1 per million WBC (CD34-positive AML), and median SP/CD34-CD38+ frequency (CD34-negative AML) was 1796 per million WBC. Improved detection of LSC may enable identification of therapy resistant clones, and thereby identification of novel LSC specific, HSC sparing, therapies.

ALDHs in normal and malignant hematopoietic cells: Potential new avenues for treatment of AML and other blood cancers

Multiple studies have demonstrated that ALDH1A1 is elevated in hematopoietic stem cells (HSCs). As a means to better characterize such cells, we previously developed the fluorescent ALDH1A1 substrate Aldefluor to facilitate HSC identification and isolation. This has proven useful for counting and isolating HSCs from human bone marrow, peripheral blood and cord blood as well as stem cells in other tissues and organisms. Given the high level expression of ALDH1A1, we explored its biology and that of other ALDHs in HSCs and found that ALDH1A1 and ALDH3A1 were important in metabolizing reactive aldehydes (RAlds) and reactive oxygen species (ROS). In murine models, loss of these two isoforms resulted in a variety of effects on HSC biology, increased DNA damage and predisposition to leukemia formation when combined with a genetic driver of HSC proliferation and self-renewal. Loss of ALDH activity may also predispose to marrow failure and AML in Fanconi's anemia (FA). ALDHs also have importance in mediating drug resistance in AML, may be useful in the identification of leukemia stem cells (LSCs) and ALDH activity levels may have prognostic significance. Together these findings suggest that further studying ALDH biology in AML and other blood cancers may provide important insights into malignant transformation and may point the way to the development of novel diagnostics and therapies.

Prognostic impact of the CD34+/CD38- cell burden in patients with acute myeloid leukemia receiving allogeneic stem cell transplantation

In acute myeloid leukemia (AML), leukemia-initiating cells exist within the CD34+/CD38- cell compartment. They are assumed to be more resistant to chemotherapy, enriched in minimal residual disease cell populations, and responsible for relapse. Here we evaluated clinical and biological associations and the prognostic impact of a high diagnostic CD34+/CD38- cell burden in 169 AML patients receiving an allogeneic stem cell transplantation in complete remission. Here, the therapeutic approach is mainly based on immunological graft-versus-leukemia effects. Percentage of bone marrow CD34+/CD38- cell burden at diagnosis was measured using flow cytometry and was highly variable (median 0.5%, range 0%-89% of all mononuclear cells). A high CD34+/CD38- cell burden at diagnosis associated with worse genetic risk and secondary AML. Patients with a high CD34+/CD38- cell burden had shorter relapse-free and overall survival which may be mediated by residual leukemia-initiating cells in the CD34+/CD38- cell population, escaping the graft-versus-leukemia effect after allogeneic transplantation. Evaluating the CD34+/CD38- cell burden at diagnosis may help to identify patients at high risk of relapse after allogeneic transplantation. Further studies to understand leukemia-initiating cell biology and develop targeting therapies to improve outcomes of AML patients are needed.

Translating leukemia stem cells into the clinical setting: Harmonizing the heterogeneity

Considerable evidence suggests that rare leukemia cells with stem cell features, including self-renewal capacity and drug resistance, are primarily responsible for both disease maintenance and relapses. Traditionally, these so-called leukemia stem cells (LSCs) have been identified in the laboratory by their ability to engraft acute myeloid leukemia (AML) into immunocompromised mice. For many years, only those rare AML cells characterized by a hematopoietic stem cell (HSC) CD34+CD38- phenotype were believed capable of generating leukemia in immunocompromised mice. However, more recently, significant heterogeneity in the phenotypes of those AML cells that can engraft immunocompromised mice has been demonstrated. AML cells that engraft immunocompromised mice have also been shown to not necessarily represent either the founder clone or those cells responsible for relapse. A recent study found that the most immature phenotype present in an AML correlated with genetically defined risk groups and outcomes, but was heterogeneous. Patients with AML cells expressing a primitive HSC phenotype (CD34+CD38- with high aldehyde dehydrogenase activity) manifested significantly lower complete remission rates, as well as poorer event-free and overall survivals. Leukemias in which the most primitive cells displayed more mature phenotypes were associated with better outcomes. The strong clinical correlations suggest that the most immature phenotype detectable within a patient's AML might serve as a biomarker for "clinically relevant" LSCs.