The miR-17-92 microRNA polycistron regulates MLL leukemia stem cell potential by modulating p21 expression.

Despite advances in defining the critical molecular determinants for leukemia stem cell (LSC) generation and maintenance, little is known about the roles of microRNAs in LSC biology. Here, we identify microRNAs that are differentially expressed in LSC-enriched cell fractions (c-kit(+)) in a mouse model of MLL leukemia. Members of the miR-17 family were notably more abundant in LSCs compared with their normal counterpart granulocyte-macrophage progenitors and myeloblast precursors. Expression of miR-17 family microRNAs was substantially reduced concomitant with leukemia cell differentiation and loss of self-renewal, whereas forced expression of a polycistron construct encoding miR-17-19b miRNAs significantly shortened the latency for MLL leukemia development. Leukemias expressing increased levels of the miR-17-19b construct displayed a higher frequency of LSCs, more stringent block of differentiation, and enhanced proliferation associated with reduced expression of p21, a cyclin-dependent kinase inhibitor previously implicated as a direct target of miR-17 microRNAs. Knockdown of p21 in MLL-transformed cells phenocopied the overexpression of the miR-17 polycistron, including a significant decrease in leukemia latency, validating p21 as a biologically relevant and direct in vivo target of the miR-17 polycistron in MLL leukemia. Expression of c-myc, a crucial upstream regulator of the miR-17 polycistron, correlated with miR-17-92 levels, enhanced self-renewal, and LSC potential. Thus, microRNAs quantitatively regulate LSC self-renewal in MLL-associated leukemia in part by modulating the expression of p21, a known regulator of normal stem cell function.