CBFβ-SMMHC creates aberrant megakaryocyte-erythroid progenitors prone to leukemia initiation in mice

Abstract

Acute myeloid leukemia (AML) arises through multistep clonal evolution characterized by stepwise accumulation of successive alterations affecting the homeostasis of differentiation, proliferation, self-renewal, and survival programs. The persistence and dynamic clonal evolution of leukemia-initiating cells and preleukemic stem cells during disease progression and treatment are thought to contribute to disease relapse and poor outcome. Inv(16)(p13q22) or t(16;16)(p13.1;q22), one of the most common cytogenetic abnormalities in AML, leads to expression of a fusion protein CBFβ-SMMHC (CM) known to disrupt myeloid and lymphoid differentiation. Anemia is often observed in AML but is presumed to be a secondary consequence of leukemic clonal expansion. Here, we show that CM expression induces marked deficiencies in erythroid lineage differentiation and early preleukemic expansion of a phenotypic pre-megakaryocyte/erythrocyte (Pre-Meg/E) progenitor population. Using dual-fluorescence reporter mice in lineage tracking and repopulation assays, we show that CM expression cell autonomously causes expansion of abnormal Pre-Meg/E progenitors with compromised erythroid specification and differentiation capacity. The preleukemic Pre-Meg/Es display dysregulated erythroid and megakaryocytic fate-determining factors including increased Spi-1, Gata2, and Gfi1b and reduced Zfpm1, Pf4, Vwf, and Mpl expression. Furthermore, these abnormal preleukemic Pre-Meg/Es have enhanced stress resistance and are prone to leukemia initiation upon acquiring cooperative signals. This study reveals that the leukemogenic CM fusion protein disrupts adult erythropoiesis and creates stress-resistant preleukemic Pre-Meg/E progenitors predisposed to malignant transformation. Abnormality in Meg/E or erythroid progenitors could potentially be considered an early predictive risk factor for leukemia evolution.

Modelling acute myeloid leukaemia in a continuum of differentiation states

“…Of note, this prediction of a mixed CMP-MEP cell type echoes the biological observation that abnormal myeloid progenitors seen during AML progression exhibit an MEP-like immunophenotype with a CMP-like functional readout (Kuo et al 2006). This mixed identity/functionality coincides with a strong differentiation block towards erythrocyte and megakaryocytes (Cai et al 2016). …”

“…Most notable was the increased in abnormal myeloid progenitors, which had an MEP-like immunopheno-type and a CMP-like differentiation potential (Kuo et al 2006). Further separation of myeloid-erythroid progenitors with additional phenotypic markers (Pronk et al 2007) show a predominant increase in pre-megakaryocyte/erythroid (Pre-Meg/E) population (ranging from 5 to 12 fold) accompanied by impaired erythroid lineage differentiation (Figure 6A)(Cai et al 2016). This refined phenotypic Pre-Meg/E population consists partly of the CMP and MEP populations using conventional markers (Akashi et al 2000)(Nestorowa et al 2016a).…”

“…These abnormal progenitors are predisposed to subsequent cooperating events necessary to transform to overt AML (Kuo et al 2006; Cai et al 2016; Castilla et al 1999). To simulate AML pathogenesis, we increase the proliferation rate of MEP (node 11) by 10 times, that is, r I [ i ,11] = 10 r normal , to reflect the abnormal expansion of MEP-like cells (ranging from 5 to 12 fold based on previous data) (Kuo et al 2006; Cai et al 2016). Here, r normal is the value that is used in the normal condition in section 3.1.…”

“…We model the induction of the leukemogenic fusion protein CBF β -SMMHC resulting from the chromosome inversion inv(16) (p13q22) as the “start of AML.” In this murine model of AML, inv(16) is the initial founder event that results in differentiation block and expansion of abnormal progenitors, which are predisposed to subsequent cooperating events necessary to transform to overt AML (Kuo et al 2006; Cai et al 2016; Castilla et al 1999). The approach used here directly models the sequence of events observed during leukemia initiation.…”

“…), which yields an incomplete differentiation trajectory. Although we note that the stem and progenitor cell populations are the leukemia-initiating cell populations most immediately relevant to the pathogenesis of inv(16) driven AML Cai et al (2016). Data sets covering the full spectrum of differentiation trajectory during normal and abnormal (AML) hematopoeisis will enable modeling of differentiation blocks occurring at later stages of differentiation.…”