Where does transformation occur in acute leukemia?

Acute Myeloid Leukemia Stem Cells in Minimal/Measurable Residual Disease Detection

Acute myeloid leukemia (AML) is a hematological malignancy characterized by an abundance of incompletely matured or immature clonally derived hematopoietic precursors called leukemic blasts. Rare leukemia stem cells (LSCs) that can self-renew as well as give rise to leukemic progenitors comprising the bulk of leukemic blasts are considered the cellular reservoir of disease initiation and maintenance. LSCs are widely thought to be relatively resistant as well as adaptive to chemotherapy and can cause disease relapse. Therefore, it is imperative to understand the molecular bases of LSC forms and functions during different stages of disease progression, so we can more accurately identify these cells and design therapies to target them. Irrespective of the morphological, cytogenetic, and cellular heterogeneity of AML, the uniform, singularly important and independently significant prognosticator of disease response to therapy and patient outcome is measurable or minimal residual disease (MRD) detection, defined by residual disease detection below the morphology-based 5% blast threshold. The importance of LSC identification and frequency estimation during MRD detection, in order to make MRD more effective in predicting disease relapse and modifying therapeutic regimen is becoming increasingly apparent. This review focuses on summarizing functional and cellular composition-based LSC identification and linking those studies to current techniques of MRD detection to suggest LSC-inclusive MRD detection as well as outline outstanding questions that need to be addressed to improve the future of AML clinical management and treatment outcomes.

Targeting Immunophenotypic Markers on Leukemic Stem Cells: How Lessons from Current Approaches and Advances in the Leukemia Stem Cell (LSC) Model Can Inform Better Strategies for Treating Acute Myeloid Leukemia (AML)

Therapies targeting cell-surface antigens in acute myeloid leukemia (AML) have been tested over the past 20 years with limited improvement in overall survival. Recent advances in the understanding of AML pathogenesis support therapeutic targeting of leukemia stem cells as the most promising avenue toward a cure. In this review, we provide an overview of the evolving leukemia stem cell (LSC) model, including evidence of the cell of origin, cellular and molecular disease architecture, and source of relapse in AML. In addition, we explore limitations of current targeted strategies utilized in AML and describe the various immunophenotypic antigens that have been proposed as LSC-directed therapeutic targets. We draw lessons from current approaches as well as from the (pre)-LSC model to suggest criteria that immunophenotypic targets should meet for more specific and effective elimination of disease-initiating clones, highlighting in detail a few targets that we suggest fit these criteria most completely.

Transdifferentiation and nuclear reprogramming in hematopoietic development and neoplasia

Cell transplantation and tissue regeneration studies indicate a surprisingly broad developmental potential for lineage-committed hematopoietic stem cells (HSCs). Under these conditions HSCs transition into myocytes, neurons, hepatocytes or other types of nonhematopoietic effector cells. Equally impressive is the progression of committed neuronal stem cells (NSCs) to functional blood elements. Although critical cell-of-origin issues remain unresolved, the possibility of lineage switching is strengthened by a few well-controlled examples of cell-type conversion. At the molecular level, switching probably initiates from environmental signals that induce epigenetic modifications, resulting in changes in chromatin configuration. In turn, these changes affect patterns of gene expression that mediate divergent developmental programs. This review examines recent findings in nuclear reprogramming and cell fusion as potential causative mechanisms for transdifferentiation during normal and malignant hematopoiesis.

Overgrowth of a leukemic culture by a minor CD34+ population

We have investigated the differentiation potential of blast cells in a case of acute myeloid leukemia which comprised a majority CD34- population and a minor (2%) CD34+ fraction. Blasts were cultured for 2 weeks in a combination of cytokines--c-Kit ligand, interleukin 3 and granulocyte macrophage colony-stimulating factor (SIGm mix)--together with all-trans retinoic acid or 1alpha ,25-dihydroxy vitamin D3. Maturation of blasts was assessed by morphology on Romanowsky-stained slides, changes in surface CD markers and clonogenic culture. After 7 days of culture of unseparated blasts in SIGm, most maturation was monocytic, but with retinoic acid 63% of blasts had matured into granulocytes. Vitamin D3 enhanced monocytic differentiation, with 60% of cells becoming monocytic. The percentage of CD14 and CD15 positive cells decreased over 7 days in SIGm (from 62% to 17% and from 76% to 39% for CD14 and CD15, respectively). CD14+ cell numbers were maintained, or recovered, in cultures supplemented with vitamin D3 (59% at day 7), and CD15+ cell numbers, too, remained unchanged in the presence of retinoic acid (67%) or vitamin D3 (66%). Aberrant markers CD7 and CD56 declined under any conditions. When separated, both the CD34- and CD34+ fractions showed similar changes in morphology and surface maturation markers, suggesting that these two populations may be closely related. However, only a few CD34+ cells expressed the aberrant markers present on the majority blast population. The CD34- population declined in culture while the CD34+ fraction rapidly expanded. This probably reflects the difference in progenitor content; high numbers of colony-forming cells were concentrated in the CD34+ subpopulation. We conclude that both CD34- and CD34+ populations can differentiate but only the CD34+ fraction proliferates. Primitive clonogenic CD34+ cells from this patient may generate occasional aberrant CD34+ blasts which could then differentiate into the accumulating aberrant CD34- blast population.