In the last 5 years, we have learned more about the intratumor heterogeneity of myeloid neoplasms than in the previous 5 decades. Knowledge about intratumor heterogeneity is of significant importance both for our understanding of the development and biology of leukemia and for our understanding of the basis of treatment success and failure. The internal architecture of leukemia generally reflects a hierarchical pyramid with the most immature stem cells at the apex of the tumor mass and maturing progeny with reduced renewal ability and possibly end cells with no proliferative abilities at its base. The most immature stem cells, which may belong to divergent subclones, are thought to sustain the malignancy and allow leukemic cell expansion. For a particular treatment to be effective, one must target these critical cells or a critical subpopulation of these cells. There may be substantial variations among the malignant stem and progenitor cells with regard to their functional properties as well as their genomic, epigenetic, and immunologic phenotypes. This diversity not only is apparent among different patients (interindividual variation) but may also be evident within the cellular composition of the malignancy in a single individual (intraindividual variation). These highly variable differences pose a therapeutic challenge as it appears that the more immature cell compartments must be effectively targeted to achieve complete and sustained disease control.

Genome-wide genetic studies have revealed that myeloid malignancies develop over many years. During these years, initial preleukemic genomic events may set the stage for subsequent malignant transformation. Cells harboring the earliest founding events may persist without necessarily progressing to leukemia. However, with the acquisition of particular additional genetic alterations, these founder cells may ultimately transform to overt leukemia through processes that remain poorly understood.

Thus, the emerging knowledge about the dynamics and intrinsic properties of leukemic stem cells during disease establishment and evolution has dramatically changed our understanding of the pathobiology and therapeutic responsiveness of these diseases. These novel insights have inspired the development of the review series titled “Leukemic Stem Cells” that is published in the current issue of Blood.

The reviews summarize the current state-of-the-art of this fascinating area of experimental and clinical hematology, as follows:

Daniel Thomas and Ravindra Majeti offer a discussion and update of the immunophenotypic, genotypic, and epigenetic heterogeneity and the biological properties and clinical relevance of human acute myeloid leukemia (AML) stem cells.

In their comprehensive review of myelodysplastic syndrome (MDS) as a stem cell disease, Aditi Shastri and colleagues discuss disease initiation, disease progression, stem cell alterations, and therapeutic approaches as well as the microenvironment sustaining MDS stem cells.

Tessa L. Holyoake and David Vetrie review our current understanding of the molecular mechanisms of persistence of leukemic stem cells involved in resistance to tyrosine kinase inhibitor therapies, and the challenges in targeting these critical cells and future directions of research.

Adam J. Mead and Ann Mullally provide a thoughtful comprehensive review on the features and role of the disease-initiating cells in myeloproliferative neoplasms (MPNs).

Hind Medyouf focuses on the microenvironment. She discusses the complex organization of the human bone marrow niche and the key signals of the hematopoietic niche: leukemic stem cell interactions and their impact for niche-mediated therapeutic targeting.

In the comprehensive final review in the series, Daniel A. Pollyea and Craig T. Jordan describe the biological studies and ongoing clinical trials with agents targeting leukemic stem cells.

It is my pleasure to introduce this review series on leukemic stem cells. These reviews present themselves as “must-reads” for those interested in leukemia research and the clinical management of patients with myeloid malignancies, especially AML, MDS, chronic myeloid leukemia, and MPNs.

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