Advances in Acute Myeloid Leukemia Stem Cells

*Xiaoxiao Yang, Xuewen Xu, Yanfang Liu, Aihua Gong, Dongqing Wang, Xiang Liao and Haitao Zhu*

### **Abstract**

As a common hematological malignant tumor, acute leukemia is believed to originate from a subpopulation of special cancer cells, named cancer stem cells. Cancer stem cells are recognized to be the main source of tumor origin, multidrug resistance, metastasis, and recurrence. Leukemic stem cells (LSCs) were first identified and confirmed to play an important role in the occurrence and development of leukemia. In this article, we summarize the following content: special markers and sorting methods for acute myeloid leukemia stem cells and the role of cancer stem cells in treatment resistance, metastasis and invasion, recurrence, and target treatment of acute leukemia.

**Keywords:** acute myeloid leukemia, cancer stem cells, leukemic stem cells, treatment resistance, metastasis

### **1. Introduction**

Acute myeloid leukemia (AML) is a group of heterogeneous diseases characterized by the uncontrolled proliferation of myeloid precursor cells and the replacement of normal hematopoiesis in the bone marrow. According to the latest survey, AML is a common cancer in adults and the second most common leukemia in children, with relatively higher rates observed in countries with high Human Development Index in North America, Oceania, and Europe [1]. The annual incidence rate of AML in the world is 2.25/100,000, and the incidence increases with age. The number is below 1/100,000 for people under 30 years of age and 17/100,000 for those above 75 years of age. Therefore, AML is actually a middle-aged and elderly disease, accounting for 80–90% of adult acute leukemia, but only accounts for 15–20% of children leukemia. Men have a higher incidence of AML than women, especially in North America, Oceania, Europe, and Asia. Epidemiology shows that environmental, occupational, and genetic factors are closely related to the pathogenesis of AML. Genetic changes in tumor cloning lead to a cascade of reactivity at the molecular level that cause abnormal proliferation and differentiation of malignant cells and inhibit normal hematopoiesis.

Tumorigenesis has been long known to resemble organogenesis and is a heterogeneous process involving many phenotypically and functionally different cells. The cancer stem cell (CSC) concept was first reported more than a century ago and refers to a very small subset of relatively quiescent cells in the tumor that are endowed with the ability to self-renew and differentiate into non-stem daughter cells that make the bulk of tumor [2]. Leukemic stem cells (LSCs) were first

identified and confirmed to play an important role in the occurrence and development of leukemia. In 1994, Lapidot et al. reported that AML contains LSC. It is believed that only 0.1–1% cells have the ability to produce AML [3]. The researchers transplanted sublethal doses of CD34<sup>+</sup> CD38+ and CD34+ CD38<sup>−</sup> subpopulations isolated from the bone marrow of a patient with AML into non-obese diabetic mice with severe combined immunodeficiency disease (NOD/SCID mice). After 4–8 weeks, human AML cells isolated from the engrafted murine bone marrow expression both of CD34+ CD38<sup>−</sup>. The recipient mouse, re-implanted with CD34+ CD38<sup>−</sup> cells, could survive and pass to the next generation. The researchers also found CD34+ CD38<sup>−</sup> cells can induce various subtypes of leukemia other than M3, thus indicating that this subpopulation of cells has stem cell-like strong selfrenewal and reproductive ability. In 1997, Bonnet et al. confirmed the presence of LSC in NOD/SCID mice [4]. Inoculation of 106 LSCs resulted in the formation of human AML in animals; this finding suggested that the "source of all evils" is LSC (**Figure 1**) [5]. Since then, the existence of LSC has been recognized, which is a significant milestone. The presence of LSC has been confirmed not only in hematological malignancies but also in some solid tumors.

Although LCSs were identified and thought to be the main cause of leukemia origin, recurrence, and drug resistance, there is still controversy regarding the origin of this distinct population [6]. Several hypotheses have been proposed with regard to the origin of LCS: (1) from hematopoietic stem cells (HSCs) [7]; (2) from partially differentiated hematopoietic progenitor cells [8]; (3) from blood vascular stem cells and granulocyte macrophage precursors (GMPs) [9–11]; and (4) from relatively mature leukemia cells [12]. Although the number of LSCs is small, LSCs have the same potential for self-renewal, multidirectional differentiation, and unlimited proliferation, resistance to cell death, multidrug resistance, metastasis, and recurrence. Because they can escape inhibition by most chemotherapeutic

**13**

**Table 1.**

*Summary of cell surface marker expression on AML LSCs.*

**2.1 CLL-1**

*Advances in Acute Myeloid Leukemia Stem Cells DOI: http://dx.doi.org/10.5772/intechopen.84263*

and niche may become a new research hotspot.

Bonnet et al. revealed that the CD34+

**2. Expression of special surface markers in LSCs**

drugs, LSCs in a relatively quiescent state can be latent for a long time. Once the conditions are appropriate, such as a certain stimulus into the cell cycle, they can escape the immune surveillance of the body, thus showing unlimited proliferation. Therefore, relevant research and analysis on the biological characteristics of LSCs may provide new ideas for therapeutic regimens. The discovery of LSCs has broadened the treatment of leukemia, and targeted therapy of related signaling pathways

HSCs with surface markers and can be used to identify cells with unlimited proliferation and differentiation in AML [4]. Subsequent studies have shown that the surface markers of LSCs are extremely complex and vary from person to person. Previous experiments have demonstrated that in some cases, subpopulations of cells with different phenotypes have LSC activity [13–15]. CD34 and CD38 are no longer specific markers that define LSCs. Recent studies have identified various new markers such as CLL-1, CD96, TIM3, CD47, CD32, and CD25. The current study summarizes some of the specific markers expressed by LSCs (**Table 1**) [16] and has

C-type lectin domain family 12 member A (CLL1, also known as CLEC12A) positive cells show high tumorigenicity in immunodeficient mice, indicating that this cell subpopulation has the characteristics of LSCs. Moreover, the side population cells enriched in LSCs isolated by flow cytometry from patients with AML also express CLL-1 [18]. Jiang et al. have reported that CLL1-antibody-drug conjugate (CLL1-ADC) could become an attractive target therapy for AML [19]. The use of a

**Marker Expression on LSC Reference** CD34 +/− [4, 13] CD38 +/− [4, 12, 13] CD90 −/+ [16] CD123 ++ [16, 22] CD45RA + [15] CD33 ++ [90, 91] CD13 ++ [15] CD44 ++ [15] CLL-1 + [17, 18] CD96 ++ [16, 19] TIM3 ++ [15] CD47 ++ [23–26] CD32 + [27–30] CD25 + [27–31]

been utilized to successfully validate LSCs in recent clinical trials [17].

CD38<sup>−</sup> subpopulation is similar to normal

**Figure 1.**

*Comparison of the normal and AML human hematopoietic systems.*

*Advances in Acute Myeloid Leukemia Stem Cells DOI: http://dx.doi.org/10.5772/intechopen.84263*

drugs, LSCs in a relatively quiescent state can be latent for a long time. Once the conditions are appropriate, such as a certain stimulus into the cell cycle, they can escape the immune surveillance of the body, thus showing unlimited proliferation. Therefore, relevant research and analysis on the biological characteristics of LSCs may provide new ideas for therapeutic regimens. The discovery of LSCs has broadened the treatment of leukemia, and targeted therapy of related signaling pathways and niche may become a new research hotspot.
