IDH2 mutant hematopoietic stem and progenitor cells promote endothelial niche remodeling Free

Myelodysplastic syndromes (MDS) are a group of hematologic disorders, characterized by a block in differentiation of myeloid-lineage hematopoietic stem and progenitor cells (HSPCs), leading to abnormal hematopoiesis and progressive bone marrow failure. In addition to intrinsic factors, self-renewal and differentiation of normal and malignant HSPCs can be regulated by cues from the local bone marrow niche. This niche includes cells such as mesenchymal stromal cells, osteoblasts, and endothelial cells. Endothelial cells (ECs) can secrete growth factors to regulate HSPC self-renewal, differentiation, and aid in bone marrow homing. Prior work suggests that hematological malignances such as acute myeloid leukemia (AML) and MDS are associated with morphological and functional changes in the bone marrow microenvironment. For instance, immunohistochemistry-based analysis of bone marrow biopsies of MDS patients has shown an increase in micro vessel density, indicating remodeling of the endothelial niche. However, it is not known if endothelial niche remodeling can precede the onset of MDS phenotype and thus promote the expansion of malignant cells. To determine this, we used murine models of MDS driven by Isocitrate Dehydrogenase 2 (IDH2) mutations. Presence of IDH2 mutations in MDS is associated with a high risk of developing secondary AML, highlighting a key role of this mutation in disease progression.

To define the impact of IDH2 mutant cells on the endothelial niche, we established an in vivo murine model by transplanting HSPCs from 8–12-week-old conditional knock-in Idh2^R140Q/+; Vav-Cre⁺ mice in wild-type (WT) mice conditioned with ultra-low dose irradiation that minimally impacts the ECs. Chimerism of Idh2^R140Q/+ HSPCs in recipient mice increased over time relative to controls engrafted with WT HSPCs. Our data shows that Idh2^R140Q/+ recipients have 15% lower platelet counts at 5 months post-transplant compared to WT controls. To determine if the endothelial niche is altered before development of MDS associated thrombocytopenia and anemia, we analyzed ECs 2-months post-transplant. Our analysis identified a nearly 2-fold increase in the CD45^-Ter119^-CD31⁺ ECs in Idh2^R140Q/+ recipients compared to the WT controls, even in the absence of a frank MDS associated phenotype in the blood. These data indicate that remodeling of the endothelial niche precedes onset of pathological parameters associated with MDS. Prior work has shown that Idh2^R140Q/+; Vav-Cre⁺ mice start to display features of MDS starting at 20 weeks. We thus tested if HSPCs from 20-week-old Idh2^R140Q/+; Vav-Cre⁺ mice can accelerate development of endothelial defects in WT recipients. We find that transplanting 20-week Idh2^R140Q/+ HSPCs can lead to 3 to 4-fold higher chimerism at 2 months compared to that seen with 8-12-week-old Idh2^R140Q/+ donors. Importantly, these mice also showed about 40% lower platelet counts indicative of MDS onset at 2 months, suggesting accelerated disease onset.

To test if the expanded endothelial niche is associated with defects in endothelial function, we used in vitro co-culture assays. ECs co-cultured with Idh2^R140Q/+ HSPCs from 8–12-week-old mice formed 18% longer and 16% more tubes than those co-cultured with age and sex-matched WT cells, suggesting that the Idh2 mutation can promote features of neo-angiogenesis. Finally, our experiments using a novel microdevice featuring a silicon membrane (µSiM) platform indicate that ECs co-cultured with Idh2^R140Q/+ HSPCs show 1.4-fold increased FITC dextran permeability. Our findings reveal that the endothelial niche permeability and neo-angiogenesis precede the onset of MDS associated cytopenia and may promote disease progression.