Every hematopoietic malignancy involves the BM microenvironment in some fashion including initiation, progression, resistance to chemotherapy and the suppression of normal hematopoiesis, so changes in the BM environment occur with the hematological malignancies and then their development, leading to HSC niche dysfunction and thereby ,result in the normal hematopoiesis loss which may eventually cause pancytopenia, hemorrhage and infections leading to much of the morbidity and symptoms associated with leukemias. Hence, signals from leukemic cells lead to BM microenvironment remodeling and the creation of a self-reinforcing leukemic niche that impairs normal hematopoiesis, favor leukemic stem cells (LSCs)function and contributes to bone marrow disruption and disorganized structure as well through positive feedback signaling loop.

Normal hematopoiesis follows a well-organized hierarchy through the hematopoietic stem cells (HSCs) differentiation. These cells reside in the bone marrow(BM), a highly orchestrated architecture. The stem cell is defined primarily by its function, but a number of markers have been found which can be used to enrich cell isolates for stem cells in function assays that compare stem cells with their progeny. In such a case, different types of mediators such as cytokines/chemokines, reactive oxygen species and exosomes play a pivotal role in the regulating the function of hematopoietic cells. [1-2]

Cancer stem cells (CSCs): Recent studies indicate that CSCs can arise from normal stem cells or progenitor cells. These CSCs, possess stem-ness properties similar to normal stem cells including self-renewal and the potential of differentiation and proliferation, namely, the term stem cell defines a specific cell type that possesses the main properties of self-renewal, multi-lineage differentiation and proliferation as well. In this regard, the term stem-ness refers to the degree to which a cell possesses these functional properties. In all, stem-ness is an elusive term. Stem cells are defined by the degree to which they possess stem-ness characteristics (particularly self-renewal and multi-lineage differentiation). From this definition, one could argue that stem cells may possess varying levels of stem-ness and may also exhibit different properties. In short, multiple studies have shown heterogeneity in the CSC population. Also these cells can display cellular plasticity that similar to normal counterparts which means they can alternate between epithelial and mesenchymal-like stem cell states. [1-5,10,12]

Figure 1 : Three fields ( cancer genetics- epigenetics and microenvironment) are coming together to provide increasing clarity to the processes which determine stem-ness and as a result influence clinical outcomes. In fact, malignant cells develop all aspects of stem-ness, so stem cells and cancer cells provide the building blocks for cancer maintenance and survival from self-renewal, differentiation potential to the organization of stem-ness supporting microenvironments. [1,20]

Meanwhile, the two basic properties of stem cells are self-renewal and differentiation into multiple lineages. Some of them, exhibit high proliferative potential. The capability to self-renew gives these cells the ability to maintain themselves and the capacity for tissue regeneration. As stem cells differentiate, they begin to lose their stem-ness. In other words, several stem-ness pathways are known to be dysregulated in cancers, and when they are activated in CSCs, they can contribute to their innate resistance. For instance, STAT3 is a transcription factor that is activated in many malignant diseases constitutively and it plays a pivotal role in the patients. These changes establish a hierarchy of cell populations that underlie organogenesis. Actually, tumors similar to normal organs which can imitate their behavior but tumors are structurally and functionally abnormal that can be compared to normal organs. That’s why, CSCs are considered as tumor- initiating cells (TICs) because they exhibit self-renewal, treatment resistance, metastasis and tumor formation capabilities. So CSCs and normal stem cells have many properties in common and targeting CSCs may adversely affect normal stem cells leading to untoward toxicity. But how?

How leukemia starts from bone marrow changes?

I- Pre-leukemia: Recent studies indicate two important points includes: 1) In pre-leukemic case, the HSC seems to retain multi-lineage differentiation potential with both myeloid and lymphoid lineages, as the majority of people with CHIP remain free of hematological diseases. As we know CHIP is the presence of a clonally expanded HSC caused by a leukemogenic mutation in individuals without evidence of hematologic malignancy, dysplasia or cytopenia which means it’s not abnormal. In this regard, pre-leukemic HSC may become cell- of- origin for hematologic neoplasm because the cell already harbors some of leukemia-specific mutation which is high risk point in CHIP people. 2) Dysfunction BM microenvironment (tumor microenvironment) contribute to the myeloid malignancies development and that, in turn leukemic hematopoiesis can create dysfunction BM microenvironments which means leukemic hematopoiesis turns the endosteal BM niche which promotes LSC function and impairs the normal HSCs maintenance (figure 1). [1-9] Told all, pre-leukemic HSCs progeny not only inherit the mutations baggage but also can be affected by genetic hit themselves. In fact, neoplastic mutation occurs at the progenitor cell level. The combination of all mutations leads to adaptive self-renewal activation or/and block differentiation. So the completely transformed progenitor with acquired self-renewal capacity is called LSC which gives rise to bulk leukemic blasts and sustains their production. Importantly, the pre-leukemic HSCs involvement in leukemogenesis is well evidenced in two types of leukemia including AML and CML that called as paradigmatic HSC-source diseases together. Hence in these diseases, diversity within malignant cells at the genetic and functional together with their coexistence with the microenvironment also increases tumor fitness allowing the tumor growth that demonstrate three facets includes genetic diversity, epigenetics and the tumor microenvironment contribute to tumor heterogeneity as well as the tumor shape progression (figure 1). So, as a result, the niche compartment can become a traitor that can do harm to normal hematopoietic cell. In other words, we know the complex process produces one million cells in human cells which may acquire genetic and epigenetic abnormalities that enhance proliferation or block differentiation (figure 1 &2). It leads to an excess of immature progenitors in the BM as well as peripheral blood resulting in different leukemic diseases. [7-14]

Figure 2 : Cancer stem cell hypothesis stated that the cells in a tumor are organized as a hierarchy similar to normal tissue which are responsible for tumor formation and growth. This is while, normal transient amplifying cells usually differentiate and die but in leukemia, cells fail to differentiate normally and instead accumulate, notably some of LSCs are born while others are made by the tumor milieu.(1-5, google.com, 25)

Therefore, if we accept the malignancy creates an aberrant niche (tumor microenvironment), so the niche can be as a driver for the malignancy. If the niche as an accomplice for malignancy, there is increasing that malignant cells are able to shape the niche, hence creating a malignancy-favorable environment to support their survival at the expense of normal hematopoiesis. That’s why, HSCs have proven their clinical relevance in stem cell transplantation to cure patients with hematological disorders, key to regenerative potential is their natural microenvironment in the BM (figures 1,2&3).

II- Chronic myeloid leukemia (CML): Some points are necessary as follows: 1) In most newly diagnosed CML patients the majority of the most primitive cells (LTC-ICs) or quiescent CD34+ cells are Ph negative partially or even predominantly, so however there’s lineage imbalance in favor of myeloid series and the granulopoiesis left shift go to delayed in maturation stage but cellular function is normal mostly. Although BCR-ABL1 is detected in cells belonging to all hematopoietic lineages includes B & T cells. 2) In CML models, leukemic cells induced BM stromal cells to overproduce placental growth factor, thus in turn promoting leukemia cell proliferation. 3) CML cells could progressively remodel the endosteal BM niche to a self reinforcing leukemia niche. Notably, CML cells stimulate MSCs to overproduce functionally altered osteoblasts with compromised HSC-supportive activity resulting in the development of myelofibrosis that was induced by TPO, CCL3 and cell-cell interactions among CML cells and MSCs directly. 4) Are CML stem cells dependent to BCR/ABL1 or not? In fact, residual BCR/ABL1+ cells remain detectable by PCR in most patients on TKIs has been interpreted as evidence for LSCs persistence despite continued BCR/ABL1kinase  activity suppression In this regard, long-lived B-cells and less T-cells account frequently for most the positive PCR results in the patients with treatment free resistance(TFR), while granulocytes are BCR/ABL1 negative consistently. That’s why, they explain the puzzling in positive results patients observation with stable TFR for years. So this is a question that is LSC with kinase-disabled BCR/ABL1 equal to the normal HSC or it’s still rewired? 5)The terms HSC and LSC denote cell population frequently which are enriched for functionally defined true stem cells but not pure for them, For example lineasge-CD34+CD38- but less than 10% of them are stem cells functionally. As of 2020 no universal phenotypic marker had been identified that separates HSCs from LSCs but strategies achieve enrichment considerably for BCR/ABL1+ cells. So the issue is complicated by the fact that some of them but not all markers are regulated by BCR/ABL1 kinase activity and that TKIs eliminate proliferating LSCs with the late myeloid signature predominantly, although sparing the most primitive LSCs relatively. [10-18,22-23]

III- Myeloproliferative neoplasm (MPN): Despite an improved understanding of the MPNs clonal structure, the mechanism leading to the clonal selection once the mutations are acquired remain poorly understood, actually MPNs were possibly oligo-clonal diseases with a coexistence of some molecularly distinct clones which rather than single founder clone. Anyhow, some disorders such as MPN remodels the endosteal BM niche progressively to a self-reinforcing leukemic niche that impairs normal hematopoiesis, favors leukemic stem cell(LSC) function and contributes to BM fibrosis development. These results expand our standing of the effects of leukemic hematopoiesis on the BM microenvironment and the contribution of the endosteal BM niche to MPN pathogenesis.

Figure 3 : This figure provide an ideal platform for understanding how leukemic hematopoiesis disrupts the normal mechanisms controlling HSC function and blood production, which these changes create a self-reinforcing cycle of damage to demonstrate the leukemic myeloid cells resulting to a self-reinforcing leukemic niche remodeling. [1-2,21,25- 27]

It’s mentionable that whereas the endosteal BM niche contributes to HSC maintenance and regulated production of myeloid cells, but during the MPN , transformed HSCs with LSC properties overproduce leukemic myeloid cells that secrete high levels of pro-inflammatory cytokines, thus creating a paracrine feedback loop that drives myeloid differentiation. Leukemic myeloid cells also stimulate MSCs to overproduce functionally altered OBCs that accumulate in the BM cavity as inflammatory myelofibrotic cells. These MPN expanded OBCs are compromised severely in their ability to maintain normal but not transformed, HSCs and promote myeloid differentiation. It's noticeable that in some MPN patients such as polycythemia vera (PV) cases, we have two distinct populations of erythroid progenitor cells that indicating the coexistence of a malignant and non-malignant population of hematopoietic progenitor cells, so in the progression of PV to be associated with a significant decreasing in normal colony and increasing preponderance of a malignant clone which should be an autonomous clone with genetic abnormality or even without it, in the result the clonal hematopoietic progenitor cells are dominant vs prevent the proliferation of normal progenitor cells by unknown mechanisms because this is as an independent mechanism that renders proliferation and expansion of stem cells independent from the hemostatic control of the niche which can be as an important point, namely , the tumor cell niche plays a critical role in the niche particularly in epithelial tumorigenesis. Also, HSC of PV has an intrinsic defect that can alter a number of cellular function and so it’s not restricted only to cytokine receptor signal transduction. On the other hand, some researchers stated that 20% of essential thrombocythemia (ET) cases  approximately, did not harbor any mutations with a capacity for cell-autonomous megakaryopoiesis, the hallmark of ET and so are called triple-negative ET ( namely JAK2V617F-, MPL exon 10 - and CALR exon -) that demonstrated with the female patients dominance in these population as well as other parameters like platelet count and age. Overall, the results stated which MPN development remodels the endosteal BM niche into a self-reinforcing malignant niche that impairs normal hematopoiesis into leukemic hematopoiesis which presented dysfunction BM microenvironment (tumor microenvironment). Notably, the overproduction of inflammatory myelofibrotic cells contributes to progressive BM fibrosis observed in the advanced stages of these diseases. Also, the loss of normal HSCs contributes to the clonal dominance of BCR/ABL expressing LSCs in transplanted mice probably and CML patients as well. Hereby, impairing in support for normal HSCs whereas osteoblastic defects demonstrated in accelerated situation which mostly go to bone major loss resulting in MPN development to a self-reinforcing leukemic niche remodeling. [15-21,23-28] At the same time excessive osteoblast production perpetuates clonal-MPN proliferation. In short, the MPN mutated-HSC disrupts the harmony at the BM niche, promoting a self-reinforcing environment that facilities their proliferation at the expense of normal hematopoiesis. Or in other words, reinforcing myeloid production in the erythroid and lymphoid commitment expense that may actively participate in the generation of self-reinforcing malignant niches at the expense of normal HSCs. Additionally, in CML models, leukemic cells induced BM stromal cells to overproduce placental growth factor, consequently promoting leukemic cell proliferation. So despite extensive efforts over past decades, it’s likely that we will soon see the added dimension of micro-environment directed agents in the combined agent approach, because we know the reciprocal interaction between malignant cells and their local BM microenvironment can contribute to the disease of myeloid initiation and progression. In this regard, current therapies focus on malignant cells selective elimination via cytotoxic chemotherapy. But targeting the complex interactions with the marrow microenvironment represents a complementary therapeutic opportunity and that’s why, leukemia cell infiltration exactly affects a number of BM niche populations and in so doing is thought to advantage abnormal over normal hematopoietic cells. In fact, leukemia per-sisters are characterized by their quiescence state, different energy consumption, BM microenvironment adaptation, changing identity and phenotypic plasticity. These mechanisms that cause their persistence include a variety of epigenetic, transcriptional and metabolic processes as well as environmental changes which often co-exist. [16-17, 20,23] 

IV- Acute myeloid leukemia (AML): it’s characterized by clonal expansion and accumulation of abnormally or incompletely differentiated immature progenitors or blasts of myeloid lineage with associated normal hematopoiesis impairment. In fact, in principle primitive HSCs harboring mutation(s) and epigenetic aberrations expand to pre-leukemia cells initially and then transform to LSCs ultimately that can regenerate and maintain the disease and also form the disease relapse cellular reservoir at post-therapy. Actually, AML LSCs transcriptional profiling revealed a molecular signature that is associated with leukemia stem-ness and leukemia initiating capacity and that is highly correlative with AML prognosis indicating that the presence of a transcriptional stem-ness profiles affects response to therapy. Also, reactivation of a self-renewal associated transcriptional signature was shown to be an important characteristic of the normal progenitors transformation into LSCs. The results implicated together that there’s a high degree of plasticity in imposing stem-ness on leukemic cells and showed the stem cell features importance for the response to AML cells chemotherapy. Also, serial transplantations and leukemia cells clonal tracing from AML specimens identified quiescent long-term leukemia initiating cells(LICs)that could self-renew and give rise to short-term LICs, ultimately generating bulk tumor cells with impaired self-renewal and differentiation capabilities. [10,13, 20,25-28]

1-There is a phenotype plasticity high degree to impose stem-ness on leukemia cells and shows the importance of studying leukemia re-initiating cells after the initial therapy. 2- The malignant stem cells can provide a critical reservoir for the malignant progression, in other words, there is a feedback mechanism between genetic alterations and the malignant hematopoietic microenvironment in transferring process of information about the results of abnormal actions that the observations endorse on the view of malignant stem cells and its microenvironment as the units of selection. 3- There’s a complex interplay between malignant cells like AML cells particularly stem cells (LSCs), their BM niche and the treatment outcome. This interplay is significantly influenced by signaling events from the BM niche affecting metabolism, epigenetic processes, stress responses and AML cells transcriptomes and all subsequently affecting the leukemia stem cell death level induced by therapy. Hereby, AML LSCs can remodel the BM niche into a leukemic-permissive microenvironment, thereby suppressing normal hematopoiesis, in fact this complex interplay between LSCs and their microenvironment including adhesion molecules, cytokines and chemokines that contribute to LSCs survival, therapy resistance and the disease relapse.