Unlike CML, AML is a very heterogeneous disease and represents much more context-dependent complexity regarding the role of autophagy in disease initiation and progression. Acute leukemias are characterized by the rapid accumulation of clonal, immature myeloid blasts, with incomplete differentiation, in the patients’ bone marrow and peripheral blood, and are often accompanied by multi-lineage cytopenias
[34][35]. Although major progress has been made in identifying different molecular and genetic subgroups, AML therapies and long-term patient outcomes have not improved significantly over the past 40 years
[36][37]. One of the foremost hurdles to a cure is thought to be the presence of LSCs or leukemia-initiating cells that, in many patients, constitute a small subpopulation of self-renewing cells at the top of a hierarchical organization with resistance to chemotherapy, causing treatment failure and relapse
[38][39][40][41]. Immunodeficient mouse models show that these leukemic cells can produce AML upon xenotransplantation, including in secondary and tertiary recipients, and generate non-LSC committed progenitors unable to be serially transplanted, all features of functionally true LSCs
[39][40][42][43][44]. Initial immunophenotypic analyses demonstrated that the majority of AML patient cells express the surface antigen CD34
+, with LSCs mostly residing in the CD34
+CD38
− subfraction
[40][45][46]. However, more recent studies revealed that many patients harbor, at minimum, two distinct LSC populations that coexist in the CD34
+CD38
− (Lymphoid-primed multipotent progenitor (LMPP)- or multipotent progenitor (MPP)-like LSCs) and in the CD34
+CD38
+ (granulocyte-macrophage progenitor (GMP)-like LSCs) subfractions or less often in the CD34
− subfraction, with the former, more immature CD34
+CD38
− subpopulation containing a higher LSC frequency in comparative xenotransplantation experiments
[47][48][49][50]. Moreover, some AML patients lack expression of CD34, the CD34
− AML cells, and here, LSCs can mostly be detected in the CD34
− compartment, with very few LSCs being present in the much smaller CD34
+ fraction
[48][49][51][52], highlighting the heterogeneity of AML and the complexity of studying and understanding the disease.
AML patients usually harbor genetic abnormalities that often originate from chromosomal translocations and rearrangements, such as t(8;21), t(15;17), inv(16), t(6;9), t(9;11), or t(11;19), and characterize, in some cases, a particular prognostic leukemic subtype
[53][54][55][56]. Recent molecular studies demonstrated that additional mutations in receptor kinases, key signaling mediators, proto-oncogenes, or epigenetic enzymes, for example FLT3-ITD, TP53, c-KIT, or IDH1/2 mutations, often determine the course and severity of the disease
[57][58][59][60][61][62]. Interestingly, sequencing and in silico studies have shown that a high frequency of AML patients carry often heterozygous deletions, missense mutations, or copy number changes of key autophagy genes, particularly AML patients with complex karyotypes
[63][64][65]. For example, heterozygous chromosomal loss of 5q, 16q, or 17p correlate with the encoded regions for the autophagy genes
ATG10 and
ATG12,
GABARAPL2 and
MAP1LC3B, or
GABARAP, respectively
[63]. In line with these observations, Watson et al. also demonstrated that human AML blasts exhibit low expression of several autophagy genes, including
ATG10,
ATG5,
ATG7,
BECN1,
GABARAP,
GABARAPL1/2, and
MAP1LC3B, decreased autophagic flux, and high ROS levels
[63]. Furthermore, this study showed that heterozygous loss of
Atg5 or
Atg7 in a MLL-ENL AML mouse model led to more aggressive leukemia progression, suggesting a tumor-suppressive role for autophagy
[63]. Similarly, Jin et al. confirmed that Ficoll-enriched leukemic blasts from AML patients express significantly lower transcript levels of
ULK1,
FIP200,
ATG14,
ATG5,
ATG7,
ATG3,
ATG4B, and
ATG4D compared to granulocytes from healthy donors
[66]. In addition, Rudat et al. determined, in a large RNAi screen for “rearranged during transfection” receptor tyrosine kinase (RET) effectors, that mTORC1-mediated suppression of autophagy can stabilize mutant FLT3 in AML, while an increase in autophagy was achieved through RET inhibition and led to FLT3 depletion
[67]. In contrast, Heydt et al. showed that FLT3-ITD increases autophagy in AML cell lines and patient cells via ATF4 and that inhibition of autophagy or ATF4 abolishes FLT3 inhibitor resistance
[68]. Moreover, a recent investigation by Folkerts et al. revealed that various leukemic cell lines, and purified CD34
+ cells from AML patients, exhibit inconsistent levels of basal autophagic flux, with particularly high levels in immature ROS
low LSC blasts and adverse AML risk groups, such as those with TP53 mutations
[69]. In this study, knockdown of ATG5 in primary AML cells resulted in impaired engraftment of human cells in immunodeficient NSG mice, an observation that is in contrast to previous work and would rather suggest a tumor-promoting role for autophagy in this context
[69]. Interestingly, other reports showed that ATG5 or ATG7 are required for the efficient initiation of AML in the context of MLL-AF9, the most common alteration found in infant AML, with poor prognosis, but that autophagy is no longer needed for the maintenance of established AML or LSC functions in secondary xenotransplantation experiments
[70][71][72][73]. However, in a different murine MLL-ENL AML model, knockout of
Atg5 or
Atg7 decreased the number of functional LSCs, increased activation of mitochondria and ROS levels in these cells, and extended survival of leukemic mice
[74]. Together, these representative, variable data suggest a highly complex, context-dependent role for autophagy in leukemic transformation vs. maintenance and LSC properties in AML.
Notably, and similar to CML, many studies in AML have shown another consistently critical role for autophagy with treatment implications: pro-survival protection of leukemic cells upon chemotherapy. For instance, it has been demonstrated that treatment of AML cells with cytarabine (AraC), anthracyclines, or sorafenib activates and increases autophagic flux in these cells, including LSCs, allowing them to resist chemotherapy
[70][74][75]. However, in these cases, targeting of autophagy by genetic or pharmacological means, combined with chemotherapy, seems to hold great promise in developing more effective treatment strategies, as discussed later.