The blood system is maintained by a small pool of haematopoietic stem cells (HSCs), which are required and sufficient for replenishing all human blood cell lineages at millions of cells per second throughout life. Megakaryocytes in the bone marrow are responsible for the continuous production of platelets in the blood, crucial for preventing bleeding--a common and life-threatening side effect of many cancer therapies--and major efforts are focused at identifying the most suitable cellular and molecular targets to enhance platelet production after bone marrow transplantation or chemotherapy. Although it has become clear that distinct HSC subsets exist that are stably biased towards the generation of lymphoid or myeloid blood cells, we are yet to learn whether other types of lineage-biased HSC exist or understand their inter-relationships and how differently lineage-biased HSCs are generated and maintained. The functional relevance of notable phenotypic and molecular similarities between megakaryocytes and bone marrow cells with an HSC cell-surface phenotype remains unclear. Here we identify and prospectively isolate a molecularly and functionally distinct mouse HSC subset primed for platelet-specific gene expression, with enhanced propensity for short- and long-term reconstitution of platelets. Maintenance of platelet-biased HSCs crucially depends on thrombopoietin, the primary extrinsic regulator of platelet development. Platelet-primed HSCs also frequently have a long-term myeloid lineage bias, can self-renew and give rise to lymphoid-biased HSCs. These findings show that HSC subtypes can be organized into a cellular hierarchy, with platelet-primed HSCs at the apex. They also demonstrate that molecular and functional priming for platelet development initiates already in a distinct HSC population. The identification of a platelet-primed HSC population should enable the rational design of therapies enhancing platelet output.

Rare multipotent haematopoietic stem cells (HSCs) in adult bone marrow with extensive self-renewal potential can efficiently replenish all myeloid and lymphoid blood cells, securing long-term multilineage reconstitution after physiological and clinical challenges such as chemotherapy and haematopoietic transplantations. HSC transplantation remains the only curative treatment for many haematological malignancies, but inefficient blood-lineage replenishment remains a major cause of morbidity and mortality. Single-cell transplantation has uncovered considerable heterogeneity among reconstituting HSCs, a finding that is supported by studies of unperturbed haematopoiesis and may reflect different propensities for lineage-fate decisions by distinct myeloid-, lymphoid- and platelet-biased HSCs. Other studies suggested that such lineage bias might reflect generation of unipotent or oligopotent self-renewing progenitors within the phenotypic HSC compartment, and implicated uncoupling of the defining HSC properties of self-renewal and multipotency. Here we use highly sensitive tracking of progenitors and mature cells of the megakaryocyte/platelet, erythroid, myeloid and B and T cell lineages, produced from singly transplanted HSCs, to reveal a highly organized, predictable and stable framework for lineage-restricted fates of long-term self-renewing HSCs. Most notably, a distinct class of HSCs adopts a fate towards effective and stable replenishment of a megakaryocyte/platelet-lineage tree but not of other blood cell lineages, despite sustained multipotency. No HSCs contribute exclusively to any other single blood-cell lineage. Single multipotent HSCs can also fully restrict towards simultaneous replenishment of megakaryocyte, erythroid and myeloid lineages without executing their sustained lymphoid lineage potential. Genetic lineage-tracing analysis also provides evidence for an important role of platelet-biased HSCs in unperturbed adult haematopoiesis. These findings uncover a limited repertoire of distinct HSC subsets, defined by a predictable and hierarchical propensity to adopt a fate towards replenishment of a restricted set of blood lineages, before loss of self-renewal and multipotency.

The CEBPA gene is mutated in 9% of patients with acute myeloid leukemia (AML). Selective expression of a short 30 kDa C/EBPα translational isoform, termed p30, represents the most common type of CEBPA mutations in AML. The molecular mechanisms underlying p30-mediated transformation remain incompletely understood. We show that C/EBPα p30, but not the normal p42 isoform, preferentially interacts with Wdr5, a key component of SET/MLL histone-methyltransferase complexes. Accordingly, p30-bound genomic regions were enriched for MLL-dependent H3K4me3 marks. The p30-dependent increase in self-renewal and inhibition of myeloid differentiation required Wdr5, as its down-regulation inhibited proliferation and restored differentiation in p30-dependent AML models. OICR-9429 is a novel small-molecule antagonist of the Wdr5-MLL interaction. This compound selectively inhibited proliferation and induced differentiation in p30-expressing human AML cells. Our data reveal the mechanism of p30-dependent transformation and establish the essential p30-cofactor Wdr5 as a therapeutic target in CEBPA-mutant AML.