Grimes Lab

Understanding granulocyte homeostasis is crucial because producing too few granulocytes results in increased risk for infection (neutropenia), while producing too many granulocytes can result in severe tissue damage and death (neutrophilia, myeloproliferative disorders).

Current knowledge illustrates many factors that can modify granulocyte versus monocyte production; however, we still do not understand a fundamental question: which specific factors control the granulocyte-monocyte lineage decision during homeostatic production in vivo?

To discern the transcriptional network underlying the binary fate decision between neutrophil granulocyte and monocyte lineage decisions as they occur at steady state (homeostatic control), we have performed single-cell RNA-seq on bone-marrow granulocyte-monocyte progenitors (GMP).

Our bioinformatic analyses reveal a varied but coherent spectrum of gene expression patterns in individual murine GMPs.

The majority of cells could be clustered into ones expressing either granulocytic or monocytic genes, suggesting that they were primed for lineage determination.

A minority of GMPs expressed a mixed-lineage pattern of genes.

Deeper analyses of the single-cell data implicate the repression of a key requisite factor for monopoiesis in mice and man (Irf8) by a key requisite factor for granulopoiesis in mice and man (Gfi1) as the central mechanism for homeostatic control of the granulocyte-monocyte lineage decision in vivo.

We propose to determine the transcriptional program underlying granulocyte-monocyte lineage fate decisions during homeostasis, and then define the impact of severe congenital neutropenia (SCN)-associated mutations on transcriptional control of granulopoiesis in mice and man.

We expect to delineate a cross-species transcriptional signature of granulopoiesis, and define the impact of neutropenic stress.

The function of the HoxA9 transcription factor is of critical interest in human acute myeloid leukemia (AML) since oncogenic activation of HoxA9 is induced by multiple chromosomal translocations; however, independent studies indicate that the expression level of HoxA9 is prognostic in human AML lacking these chromosomal abnormalities. Thus, an understanding of HoxA9 signaling would significantly impact patients with AML.

Importantly, the direct transcriptional targets of HoxA9 and thus mechanism of HoxA9-mediated transformation remain largely unknown. The growth factor independent-1 (Gfi1) transcriptional repressor is known to induce granulopoiesis, inhibit myeloid progenitor proliferation and is mutated in patients with severe congenital neutropenia (SCN). SCN patients are at increased risk for AML.

We have recently shown that:

• Gfi1 represses HoxA9, Meis1 and Pbx1 expression
• Gfi1 and HoxA9 demonstrate dramatic epistatic relationships
• Gfi1 loss of function is potently preleukemic

The antagonism between Gfi1 and HoxA9 is conserved to Drosophila, and our preliminary data indicate that in mammalian myeloid progenitors centers upon the expression of microRNA encoding genes.

We hypothesize that these microRNA mediate Hox-based leukemic signaling, and that specific microRNA inhibitors abort the maintenance of Hox-based leukemia initiating cells.

The proposed research will delineate the functional role of microRNA as molecular signaling effectors / clients of Hox-based leukemia oncoproteins.