Yang Jo Chung, Ryan Bertoli, and Peter D. Aplan

• Submitter: Yang Jo Chung (Staff scientist, staff clinician)
• email: chungya@mail.nih.gov

The ineffective hematopoiesis that is characteristic of myelodysplastic syndrome (MDS) suggests the presence of defective hematopoietic stem and progenitor cells (HSPC). NUP98-HOXD13 (NHD13) transgenic mice recapitulate many features of human MDS such as ineffective hematopoiesis, peripheral blood cytopenias, dysplasia, and transformation to acute myeloid leukemia (AML), and have been used as a pre-clinical model for human MDS. NHD13 mice universally develop signs of MDS (e.g., peripheral blood cytopenia, macrocytosis, dysplasia) at approximately 5 months of age, with median survival of 10 months.
Immunophenotypic analysis of bone marrow nucleated cells (BMNC) from NHD13 mice demonstrates progressive abnormalities of HSPC with age. There are decreased numbers of multipotential progenitor (MPP) cells, as well as Short-Term and Long-Term Hematopoietic Stem Cells (HSC) in older (about 6 to 7 months) mice compared with younger (2 months) mice. Furthermore, in vitro colony forming and in vivo transplantation assays, which assess the function of NHD13 MPPs and HSCs, showed diminished self-renewal activity in both assays. These findings suggest that the MDS mice have decreased numbers of functional HSCs and that the loss of HSCs with age correlates with progression of bone marrow failure associated with MDS. The observation that both humans and mice with MDS have normal or increased BM cellularity despite ineffective hematopoietic output (i.e., production of mature blood cells) is counterintuitive, but consistent with the maturation arrest classically recognized in MDS patients. However, in the NHD13 MDS model, the number and function of HSPC can be carefully characterized using in vivo reconstitution assays. Wild-type hematopoiesis is characterized by terminal differentiation and limited self-renewal of differentiated cells, which are marked by acquisition of cell surface markers of differentiation (designated lineage positive or LP cells). In contrast, undifferentiated lineage negative (LN) cells contain hematopoietic stem cells that can self-renew indefinitely. Of note, whereas LP cells from WT donors were unable to self-renew and engraft lethally irradiated recipients. LP cells from NHD13 donors showed robust, durable engraftment for at least 12 months. The LP cells from NHD13 donors displayed long term repopulation activity with differentiation to myeloid and erythroid cells, further proven by serial HSCT. These results demonstrate that NHD13 LP cells have acquired the ability to self-renew and contribute to long term hematopoiesis in the NHD13 mice with MDS. Taken together, these findings suggest that the HSPCs from NHD13 BM become exhausted with age, resulting in findings consistent with MDS (peripheral blood cytopenia, macrocytosis). This is followed by acquisition of self-renewal activity in NHD13 LP BMCs, suggesting the existence of a compensatory mechanism for homeostasis of hematopoiesis in MDS.