MLL-Fusion Proteins Induce a Stem Cell Program in Committed Progenitors To Generate Leukemia Stem Cells.

Leukemias are composed of a hierarchy of cells only a fraction of which have stem cell like properties, and are capable of self-renewal. MLL fusion proteins produced by translocations involving the Mixed Lineage Leukemia (MLL) gene on chromosome 11q23 confer stem cell-like properties on committed hematopoietic progenitors. This provides an opportunity to determine if global cellular reprogramming is necessary for leukemia stem cell (LSC) generation from committed progenitors or if induction of a more limited self-renewal signature in committed progenitors is sufficient. We transduced murine IL-7R⁻ Lin⁻ Sca-1⁻ c-Kit⁺ CD34⁺ FcγRII/III^hi granulocyte macrophage progenitors (GMPs) with retroviruses encoding the MLL-AF9 fusion protein, which led to the development of acute myelogenous leukemia. From the leukemias we isolated a population of IL-7R⁻ Lin⁻ Sca-1⁻ c-Kit⁺ CD34^int. FcγRII/III^int. LSCs which can transplant the disease when fewer than 20 cells are injected into secondary recipients. We used hierarchical clustering, K-means clustering and principal component analysis to compare gene expression profiles of the LSC population to the normal lin⁻ sca-1⁺ c-kit⁺ HSC-enriched population, IL-7R⁻ Lin⁻ Sca-1⁻ c-Kit⁺ CD34⁺ FcγRII/III^lo common myeloid progenitors (CMPs), IL-7R⁻ Lin⁻ Sca-1⁻ c-Kit⁺ CD34⁻ FcγRII/III⁻ megakaryocyte erythroid progenitors (MEPs) and GMPs and found that the global gene expression profile most resembles the normal GMP from which they arose. However, a leukemia self-renewal signature was identified that shows significant overlap with a group of genes normally highly expressed in HSCs whose expression decreases during the transition to normal committed progenitors. Supervised analysis and gene set enrichment analysis (GSEA) demonstrated approximately 300 genes in the leukemia self-renewal signature. This is only a subset of the approximately 1500 genes that are highly expressed in the normal HSC-enriched population that show decreased expression in CMPs, MEPs, and GMPs. Next, we determined if this 300-gene leukemia stem cell signature is directly regulated by MLL-AF9 or if there is a hierarchy of gene expression. Assessment of gene expression changes 48 hours after MLL-AF9 expression in isolated GMPs demonstrated increased expression of 23/300 genes in the leukemia self-renewal signature. Of interest, there is a high degree of similarity between the 23 MLL immediate response genes and human MLL-rearranged AMLs including HOXA5, HOXA7, HOXA9, HOXA10, MEIS1 and genes not previously known to have a role in MLL-mediated leukemogenesis such as myocyte enhancer factor 2C (MEF2C). Detailed loss-of-function studies using shRNA and dominant negative mutants show inhibition of MEF2C reduces LSC colony formation and serial replating in semi-solid culture to less than 20% of control. Furthermore shRNA mediated inhibition of MEF2C has a significant impact on proliferation of human MLL-AF9 dependent leukemia cell lines, but not cell lines from other subtypes of AML. These data demonstrate LSCs can be generated from committed progenitors without widespread reprogramming of gene expression, and a leukemia self-renewal signature is activated in the process. We have used this program to identify MEF2C as playing a role in MLL-AF9 induced AML. Identification of this program provides an opportunity to further assess its importance in normal tissue homeostasis and neoplastic self-renewal/proliferation, and defines the progression from normal hematopoietic progenitor to leukemia stem cell.