Dysregulated haematopoietic stem cell behaviour in myeloid leukaemogenesis

Abstract

Haematopoiesis is governed by haematopoietic stem cells (HSCs) that produce all lineages of blood and immune cells. The maintenance of blood homeostasis requires a dynamic response of HSCs to stress, and dysregulation of these adaptive-response mechanisms underlies the development of myeloid leukaemia. Leukaemogenesis often occurs in a stepwise manner, with genetic and epigenetic changes accumulating in pre-leukaemic HSCs prior to the emergence of leukaemic stem cells (LSCs) and the development of acute myeloid leukaemia. Clinical data have revealed the existence of age-related clonal haematopoiesis, or the asymptomatic clonal expansion of mutated blood cells in the elderly, and this phenomenon is connected to susceptibility to leukaemic transformation. Here we describe how selection for specific mutations that increase HSC competitive fitness, in conjunction with additional endogenous and environmental changes, drives leukaemic transformation. We review the ways in which LSCs take advantage of normal HSC properties to promote survival and expansion, thus underlying disease recurrence and resistance to conventional therapies, and we detail our current understanding of leukaemic 'stemness' regulation. Overall, we link the cellular and molecular mechanisms regulating HSC behaviour with the functional dysregulation of these mechanisms in myeloid leukaemia and discuss opportunities for targeting LSC-specific mechanisms for the prevention or cure of malignant diseases.

Fig. 1 |. Dynamic and coordinated regulation of HSC activity.

In the steady state, the majority of haematopoietic stem cells (HSCs) are maintained in a quiescent, G₀ state. The cell cycle of quiescent HSCs is reversibly arrested owing to limited activity of the cyclin D-CDK6 complex, which is mediated by low CDK6 expression or high levels of endogenous CDK6 inhibitors such as p57 (encoded by CDKN1C) and p18 (encoded by CDKN2C). Once stimulated by regenerative signals, CDK6 activity is upregulated at least in part through mTORC1 activity, and HSCs enter the cell cycle. Quiescent HSCs rely on glycolysis for their metabolic needs, whereas cycling HSCs activate mitochondrial oxidative phosphorylation (OXPHOS). This metabolic switch results in increasing the amounts of tricarboxylic acid (TCA) cycle products, including acetyl-CoA, nicotinamide adenine dinucleotide (NAD⁺), S-adenosylmethionine (SAM) and α-ketoglutarate (αKG), which are essential to key epigenetic modifiers, such as histone acetyl transferase (HAT), sirtuins (SIRT), DNA methyltransferases (DNMT), histone methyltransferases (HMT), ten–eleven translocation 2 (TET2) and jumonji C domain-containing histone lysine demethylases (JmjC). Epigenetic alteration controls HSC fate decisions, such as self-renewal versus differentiation, through modulation of key transcription factor activity. HSCs are robustly protected from programmed cell death mechanisms, such as apoptosis and necroptosis, via the upregulation of pro-survival BCL-2 genes and the TNF-NF-κB-p65-cIAP2 axis. HSCs return to the quiescent state by deactivating their cell cycle machinery and switching their metabolism back to glycolysis, in part through autophagy-dependent mitochondrial clearance. LP, lymphoid progenitor; MP, myeloid progenitor; MPP, multipotent progenitor; NK cell, natural killer cell; ROS, reactive oxygen species.

Fig. 2 |. Dysregulation of HSC properties during LSC emergence and leukaemia development.

a | Leukaemia develops through the accumulation of mutations that dysregulate haematopoietic stem cell (HSC) self-renewal, activate HSC proliferation and inhibit differentiation into progenitor cells. In age-related clonal haematopoiesis (ARCH), somatic mutations in HSCs lead to competitive fitness and cause the relative expansion of single clones, without obvious changes to the lineage output of the haematopoietic system. In myeloproliferative neoplasms (MPNs), HSCs accrue mutations that enhance self-renewal and activate emergency haematopoiesis pathways, leading to excessive production of mature cells. Separately, myelodysplastic syndromes (MDSs) arise when increased HSC self-renewal and the inhibition of progenitor differentiation cause dysplasia of the haematopoietic stem and multipotent progenitor cell (HSPC) compartment and mature cell cytopenia. In acute myeloid leukaemia (AML), the activation of emergency haematopoiesis pathways and the inhibition of differentiation combine to drive aggressive proliferation and expansion of leukaemic blasts. Although the interrelated malignancies can progress in a stepwise manner, MPNs, MDSs and AML can also arise de novo from normal haematopoiesis, and ARCH can probably progress directly to AML. Commonly mutated genes that drive each dysregulated process in HSPCs are listed. b–e | Leukaemia also hijacks the bone marrow niche microenvironment to drive leukaemic stem cell (LSC) emergence and disease evolution. b | Initiation phase: HSCs reside in a specialized niche that provides supportive factors for maintaining quiescence and stemness. However, quiescent HSCs are intrinsically vulnerable to mutagenesis driven by erroneous DNA repair and age-associated single-base substitutions. In this context, pre-leukaemic HSCs gain founder mutations that confer competitive fitness over normal HSCs in the changing selective pressure provided by the bone marrow niche microenvironment. c | Pre-leukaemic phase: mutated pre-leukaemic HSCs gradually expand in an increasingly inflammatory bone marrow milieu, which also allows for the accumulation of additional mutations, leading to the eventual transformation of pre-leukaemic HSCs into LSCs. d | Leukaemic phase: LSCs emerge from either pre-leukaemic HSCs or their progeny, produce leukaemic blasts and further remodel the bone marrow niche, thereby suppressing normal HSC function and haematopoiesis. e | Relapse phase: relapse can occur through different mechanisms. Upon treatment with conventional chemotherapeutics, leukaemic blasts but not LSCs are killed, allowing pre-existing LSCs to re-establish the leukaemic hierarchy. Alternatively, after successful eradication of LSCs, the therapy-resistant pre-leukaemic HSC clones can acquire other driver mutations and transform into new LSC clones. Leukaemic blasts, if not killed by the therapy, can also become LSCs by acquiring additional mutations conferring self-renewal.

Fig. 3 |. Mechanisms of LSC resistance.

Leukaemic stem cells (LSCs) are both intrinsically genetically heterogeneous and functionally plastic, features that underlie therapeutic resistance in acute myeloid leukaemia (AML). LSCs expand 10-fold to 100-fold following initial therapy and become immunophenotypically and molecularly more heterogeneous, which makes subsequent targeting more challenging. Many of the haematopoietic stem cell (HSC)-intrinsic pro-survival and immune-evasion pathways are co-opted by LSCs. Signalling through several cell surface receptors is upregulated in LSCs compared with HSCs, including the potentiation of chronic inflammatory signalling pathways via TNF receptors (TNFRs) or IL-1 receptor accessory protein (IL1RAP), along with proliferation and survival signalling via c-KIT and FLT3. The upregulation of CD47, PDL1 and PDL2 might also have an immune-inhibitory role in LSCs. The metabolism of LSCs is also rewired, most notably by increased utilization of mitochondrial metabolism and non-glucose energy substrates such as amino acids and fatty acids. BCL-2 is upregulated in LSCs and is involved in maintaining mitochondrial oxidative phosphorylation (OXPHOS). The mitochondrial enzyme dihydroorotate dehydrogenase (DHODH), which catalyses pyrimidine synthesis and downstream anabolic synthesis, is dependent on amino acid utilization by the mitochondria and has a key role in blocking the differentiation of LSCs^,. Metabolic rewiring and specific mutations in proteins such as IDH1 and IDH2 can lead to the depletion of α-ketoglutarate (αKG), restricting TET2 activity. BCAT1 upregulation and ascorbate depletion also restrict TET2 function. Direct mutations in TET2, DNMT3A and ASXL1, alongside upregulation of LSD1 and DOT1L activity, profoundly alter the epigenetic landscape of AML stem cells and promote LSC survival and differentiation blockade. Enhanced mitophagy also has an important role in maintaining AML stem cell self-renewal.