**4. Leukemia and leukemic development**

Leukemia is the consequence of stepwise genetic alterations that confer both proliferative and survival advantage, as well as self-renewal capacity to the malignant cells (Lane et al., 2011). When the HSCs processes of self-renewal and differentiation become deregulated or uncoupled, leukemias can result, characterized by an accumulation of immature blast cells that fail to differentiate into functional cells. Two types of abnormal events can lead to leukemia. First, a normal stem cell acquires several mutations (from different types of genetic events) and, due to epigenetic changes that alter its growth control, the resistance to apoptosis is increased, interfering with the ability of its progeny to differentiate. Second, partially differentiated cells restore gene expression patterns that allow them to reacquire the unique self-renewal properties of stem cells while also interferes with their subsequent ability to differentiate (Testa, 2011).

Hanahan & Weinberg (2000) described the rules that govern the transformation of normal cells into a malignant cell. The six main properties that define malignant cells are: selfsufficiency in growth signals; insensitivity to growth inhibitory signals; evasion of

Hematopoietic Stem Cell in Acute Myeloid Leukemia Development 269

LSCs infiltrate the bone marrow and interfere with the normal HSC-microenvironment homeostasis. Available data indicate that LSCs also interact with the hematopoietic microenvironment to maintain self-renewal and to mitigate the effects of cytotoxic

The immunophenotype and isolation of LSCs were first described by Lapidot et al.(1994) from primary human AML samples and, later, studies have shown that LSCs can be defined as expressing CD34, CD382, HLA-DR2, CD902, CD117 and CD123. Some of these markers are also detected in HSCs, but the expression of CD123 seems to be leukemic-specific (Blair

Another LSC specific antigen is C-type lectin-like molecule-1, CLL-1. This antigen was demonstrated to be capable of identifying residual leukemic CD34+CD38− cells in clinical remission bone marrow samples. However, more recent data indicate that the phenotype of LSCs may be somewhat variable from patient to patient and that, in some cases, more than one phenotypically distinct subpopulation may possess LSC activity (Konopleva & Jordan, 2011). Expression of Oct-4 is another similarity between normal and cancer stem cells. Oct-4, a member of the family of POU-domain transcription factors, is expressed in pluripotent embryonic stem and germ cells. Oct-4 mRNA is normally found in totipotent and pluripotent stem cells of pregastrulation embryos (Soltanian & Matin, 2011). Expression of this factor plays a crucial role in maintaining the self-renewing, cancer stem-cell-like, and chemoradioresistant properties in lung cancer-derived CD133+ cells (Chen et al., 2008).

Oct-4 gene product is expressed in several types of adult pluripotent stem cells including kidney, breast, epithelial, pancreatic, mesenchymal, gastric and liver, as well as in tumor cell lines derived from pancreas and liver (Tai et al., 2005). According to Marques et al. (2010) it is also possible that the resistance phenotype developed by leukemic cells is determined by ABC transporter expression which is probably activated by the induction of the Oct-4 transcription factor. The ABCB1, ABCG2 and ABCC1 transporters exhibit binding sites (octamer-ATGCAAAT) for the Oct-4 transcription factor. The presence of these binding sites in the gene promoter of these transport proteins suggests that the transporter regulation pathways may be initiated at the Oct-4 recognized binding sites. However, the presence of Oct-4 alone is not always sufficient for induction of transporter genes. Transporter expression levels are often

The Wnt/Catenin signaling has been implicated in the self-renewal of LSCs. Wnt proteins are a large family of glycoproteins that bind to Frizzled receptors and LRP5/6 coreceptors. By stabilizing the mediator β-catenin, they start a complex signaling cascade that plays a significant role in regulating cell proliferation and differentiation. Wnt cascade has appeared as a critical regulator of stem cells self-renewal. Comparing the expression of normal hematopoietic stem cells to that of AML leukemic stem cells, evidences show that the Wnt

dependent upon Oct-4 interactions with other transcription factors.

signaling pathway is aberrantly regulated in leukemic stem cells.

**4.2 Genetic pathways of LSCs** 

**4.2.1 Wnt/Catenin** 

chemotherapy (Lane et al., 2011)

et al., 1998).

**4.1 Leukemia stem cells characterization** 

programmed cell death (apoptosis); limitless replicative potential; sustained angiogenesis; and tissue invasion and metastasis.

Fig. 5. Cancer stem cell hypothesis. A normal stem cell acquires several mutations and in consequence, by epigenetic changes that alter its growth control, its resistance to apoptosis increased and the ability of its progeny to differentiate is changed. Partially differentiated cells restore gene expression patterns that allow these cells to reacquire the unique self-renewal properties of stem cells while also interfere with their subsequent ability to differentiate.

The "cancer stem cell hypothesis" has gained considerable interest in recent years. This theory states that cells in a tumor are organized as a hierarchy similar to that of normal tissues, and are maintained by a small subset of tumor cells that are ultimately responsible for tumor formation and growth. These cells, defined as "cancer stem cells" (CSCs) or "tumor initiating cells" (TICs), possess several key properties of normal tissue stem cells including self-renewal, unlimited proliferative potential (i.e., the ability of a cell to renew itself indefinitely in an undifferentiated state), infrequent or slow replication, resistance to toxic xenobiotics, high DNA repair capacity, and the ability to give rise to daughter cells that differentiate. However, the major difference between cancer growth and normal tissue renewal is that whereas normal transit amplifying cells usually differentiate and die, at various levels of differentiation, the cancer transit-amplifying cells fail to differentiate normally and instead, accumulate (i.e. they undergo maturation arrest), resulting in cancer growth (Soltanian & Matin, 2011).

In the last years, studies have also clearly demonstrated that leukemia populations are highly heterogeneous and that the disease is propagated by a subpopulation of leukemia stem cells (LSC). LSCs, like normal hematopoietic stem cells, possess a range of biological characteristics that enable their long-term survival. Therefore, LSCs reside in a mostly quiescent state, and as a consequence, the overall activity of many chemotherapeutic agents that function by targeting cycling cells is likely diminished (Konopleva & Jordan, 2011).

LSCs infiltrate the bone marrow and interfere with the normal HSC-microenvironment homeostasis. Available data indicate that LSCs also interact with the hematopoietic microenvironment to maintain self-renewal and to mitigate the effects of cytotoxic chemotherapy (Lane et al., 2011)
