The infiltration of the bone marrow by leukemia cells has long been associated with a profound dysfunction in the stem and progenitor cells that form normal blood. Patients with acute myeloid leukemia (AML) thus experience infections, bleeding complications, and anemia as a result of stem/progenitor dysfunction. In this issue of Blood, Cheng et al1 demonstrate that this dysfunction of blood stem cells is mediated by a distinct and reversible pathway.
Emil “Jay” Freireich, the physician-scientist who pioneered the use of multidrug chemotherapy to cure childhood acute lymphoblastic leukemia (ALL) in the 1960s, was once asked about the most challenging aspect of ALL treatment. As Freireich recalled,2 it was not the spiraling growth of the tumor or the collateral toxicities of chemotherapy. “At the time, eighty percent of the children died because of bleeding complications. Leukemia didn’t have a chance to kill ’em. There was blood on the sheets. There’s blood on the uniforms. The nurses were covered head to toe in blood.”
Freireich’s clinical observations have puzzled hematologists for decades. Why do patients with various forms of leukemia have such profound bleeding complications? One simple answer is that their bone marrow, densely infiltrated by leukemic cells, fails to make functional platelets (and other mature cells, such as neutrophils and erythrocytes), and thus the patients have severely compromised clotting capacity. But that answer begs a deeper understanding: why are these mature cells not generated from the bone marrow in the first place? Perhaps more generally, what is the effect of leukemic infiltration of the bone marrow on marrow-resident stem and progenitor cells? Lung cancer (as far as we know) does not seem to arrest the maturation of normal lung cells in its neighborhood. How does leukemia arrest the development of normal blood-forming stem and progenitor cells in the marrow?
Cheng et al address some of these crucial questions using a model of acute myelogenous leukemia (AML). Earlier work by the same group demonstrated that the infiltration of marrow by AML cells causes profound bone marrow stem and progenitor dysfunction. The most severely affected progenitor population is the megakaryocyte-erythroid-progenitor (MEP)—the progenitors responsible for the genesis of erythroid cells and platelets. In this study, Cheng et al do not address why MEPs are so drastically and selectively affected, leading to the bleeding complications so frequently observed in leukemias, but their model will doubtlessly be used to investigate this clinically important feature in the future.
Instead, Cheng et al focus on blood-forming stem cells. As AML infiltrates the marrow, they observed, hematopoietic stem cells (HSCs) became progressively quiescent and ceased cycling. Notably (and fortunately for patients), the quiescence is reversible: transplanted into nonleukemic hosts, these stem cells reacquired their functional capacity and resumed the genesis of mature cells.
Two broad mechanisms have been proposed to explain these effects of leukemia on normal stem cells. The first suggests that the effect occurs because AML cells “hijack” the normal microenvironments, or “niches,” of normal stem and progenitor cells, thereby leading to their dysfunction. Work from several groups has demonstrated that both normal and leukemia cells are dependent on specific stromal cells in the bone marrow for their function.3-7 For normal HSCs, the “niche” is now thought to be anatomically located in the perivascular region and is created by stromal cell types, including mesenchymal stromal cells and endothelial cells. Neuronal cells and bone-forming osteoprogenitors also regulate the physiology of HSCs.8
These microenvironmental interactions are known to maintain stem cell quiescence and can affect progenitor maturation. By disrupting these microenvironments, leukemia cells might cause a global disruption of stem and progenitor function. We might imagine this as a “broken windows/bad neighborhoods” theory: as AML cells infiltrate a perfectly viable “neighborhood” of a normal marrow, they disrupt the homes for stem and progenitor cells, and thereby cause the observed dysfunction of stem/progenitor cells.
A second theory posits that the AML cells directly alter the function of stem and progenitor cells. In this model, which we might call the “blood feud” theory, AML cells secrete factors, or interact directly with normal cells through cell-cell contacts, to cause their dysfunction. Cheng et al focused on this second theory. First, they demonstrate that as AML infiltrates the bone marrow, normal HSCs acquire progressive quiescence. What might explain the quiescence of HSCs in AML-infiltrated marrow? Bone marrow serum isolated from leukemia-infiltrated marrow was sufficient to enforce HSC quiescence, suggesting the presence of a soluble factor, or factors, that impinge on a signaling pathway in HSCs, restricting their proliferation.
To further elucidate this mechanism, Cheng et al flow-sorted HSCs from leukemia-infiltrated bone marrow and analyzed gene expression. As expected, cell cycle–related genes were profoundly downregulated. Notably, several genes were also upregulated. These included Hes-1 and a zinc finger–related transcription factor named Egr3. When HSCs from AML-occupied marrows were transplanted into normal recipients, the quiescence of the stem cells was relieved, and Egr3 levels returned to a lower baseline.
These experiments suggested that Egr3 might be a “strong limiting factor” that restricts the proliferation of HSCs. To determine if this was the case, Cheng et al knocked down Egr3 in HSCs, transplanted the cells into bone marrow, and then transplanted AML cells. In control HSCs (with intact Egr3), the previously observed quiescence was recapitulated. In Egr3 knockdown HSCs, in contrast, the AML infiltration was unable to induce HSC quiescence.
One notable feature of the experiment is that the effect of AML on hematopoietic stem cells, although mediated by Egr3, was still dependent on the “niche.” Although marrow-resident HSCs were rendered quiescent by AML, spleen-resident stem cells seemed to be unaffected by leukemic infiltration. The precise contributions of “niche-dependent” and “niche-independent” effects remains to be fully elucidated, but bad neighborhoods and bad residents clearly collaborate in altering hematopoiesis in leukemic patients. These observations serve as potent reminders that successful AML therapy may depend on not just killing leukemia cells in their niches, but also restoring the normalcy of bone marrow function.
Conflict-of-interest disclosure: The author declares no competing financial interests.
