Hepatocyte Growth Factor Is Overexpressed in AML and Remodels the Mesenchymal Stromal Cell Niche

Acute Myeloid Leukemia (AML) is a highly heterogenous and aggressive cancer characterized by the rapid proliferation of abnormal myeloid cells in the bone marrow and blood. Despite advances in chemotherapy, targeted therapy, and stem cell transplantation, these treatments are often insufficient, resulting in disease relapse and a 5-year survival rate of under 32% (National Cancer Institute, 2021).

Ficlatuzumab is an anti-HGF (hepatocyte growth factor) antibody that has demonstrated effectiveness in relapsed/refractory AML with an overall response rate of 53% observed in a phase 1b clinical trial (Wang et al, 2021). Aberrant expression of HGF and activation of its receptor, c-MET, is frequently observed in cancers, including AML (Fu, et. al 2021; Kentsis et al., 2012). Complete response to ficlatuzumab was associated with suppression of MET signaling while non-response to ficlatuzumab was associated with augmented expression of HGF by a subpopulation of AML blasts (Wang et al., 2021). Effects of HGF signaling on the hematopoietic microenvironment in this context are unknown.

To address this gap in knowledge, we generated a transgenic zebrafish model in which expression of zebrafish HGF (hgfa) is enforced in hematopoietic stem cells (HSCs) using the Runx1 enhancer element. By fluorescent microscopy, there were no significant changes in HSC numbers in 72 hour-post-fertilization (hpf) embryos with hgfa overexpression compared to clutchmate controls. This suggests that overexpression of hgfa alone may not be sufficient to alter HSC numbers in short term assays.

However, single cell RNA-sequencing (scRNA-seq) performed at this timepoint did show a 1.7-fold expansion of lepr+ mesenchymal stromal cells (MSCs) in Runx1:hgfa transgenic animals compared to controls. Gene expression profiling showed nearly exclusive expression of met in the MSC cluster, consistent with this expansion being a direct consequence of hgf signaling. To identify potential downstream effects of MSC expansion, scRNA-seq data from the ficlatuzumab trial was reanalyzed to identify markers specific for treatment-resistant blasts. One such marker, CD44, was expressed specifically in a population of hematopoietic progenitor cells that was overrepresented by 2.1-fold in Runx1:hgfa transgenic animals. CD44 is a receptor for extracellular matrix proteins including collagen, which is abundantly produced by the MSC population in this study. CD44 transcription is controlled in part by WNT signaling (Wielenga, et. al, 1999; Schmitt, et. al 2014); MSCs were the dominant producer of wnt3, 4, 6 and 9.

Together, these in vivo data point to a novel set of complex interactions between hematopoietic cells and MSCs which may have relevance for therapeutic resistance in AML.