Characterization of the Hematopoietic Stem and Progenitor Cell Hierarchy in Myelodysplastic Syndromes Patients with Monosomy 7 As the Sole Cytogenetic Abnormality

Deletions involving chromosome 7 constitute the second most common cytogenetic abnormality in myelodysplastic syndromes (MDS), associated with an overall poor outcome and high risk of transformation to acute myeloid leukemia (AML). Previous studies focusing on low to intermediate risk MDS, including cases with del(5q) as the only cytogenetic abnormality, have revealed a remarkably preserved stem and myeloid progenitor cell hierarchy, more characteristic of normal hematopoiesis than that of AML. Herein we explored this in a group of seven higher-risk MDS cases, all with an isolated deletion of the entire chromosome 7 (-7; monosomy 7), as the only cytogenetic abnormality.

In all investigated isolated monosomy 7 cases, the expression of cell surface markers used to define the normal hematopoietic stem and myeloid progenitor cell hierarchy in bone marrow resembled more the expression pattern observed in AML than in low-risk MDS, irrespectively of the -7 MDS patients blast counts. Specifically, when compared to healthy age-matched controls, we observed a 15-fold reduction of LIN^-CD34⁺CD38^-CD45RA^-CD90⁺ cells (hematopoietic stem cells; HSCs, p=0.010), a 15-fold increase in LIN^-CD34⁺CD38⁺CD123⁺CD45RA⁺ cells (Granulocyte-macrophage progenitors; GMPs, p=0.038) and a 300-fold increase of LIN^-CD34⁺CD38^-CD90^-CD45RA⁺ cells (lymphoid-primed multipotent progenitors; LMPPs, p=0.050) within total bone marrow mononuclear cells of MDS patients with isolated monosomy 7. Fluorescence in situ hybridization (FISH) analysis revealed high -7 clonal involvement (>80%) for all investigated progenitor populations, whereas it was much lower (<20%) in the HSC compartment. Functional assays (colony forming unit assays, long term culture initiating cell assays and mouse xenotransplantation in-vivo) revealed dramatic reductions in stem and progenitor activities in the investigated -7 patients, more resembling that previously reported in AML than in low risk MDS cases. Various differentiation assays supported that the -7 clone was much less affected in its myeloid than erythroid differentiation pathway. Similar to -7, also identifiedconcomitant recurrent driver mutations showed high clonal involvement. Notably, while a 3- to 4-fold reduction of -7 clonal involvement was observed in total bone marrow cells in patients achieving good partial remissions in response to azacytidine, the treatment failed to significantly impact on the high -7 clonal involvement in expanded GMPs and LMPPs.

In conclusion, investigations of the stem and progenitor cell compartments of MDS patients with loss of chromosome 7 as the sole cytogenetic abnormality, is associated with a loss of HSC phenotype and function, and an expansion of cells with a GMP and LMPP phenotype, remarkably similar to what is frequently observed in AML, and in contrast to reported low-risk MDS cases. While azacytidine treatment significantly reduces the overall clonal involvement in bone marrow, it fails to significantly affect the -7 clonal involvement in notably expanded progenitor cell populations, providing an explanation for the typically short duration of remissions in these patients, and identifying distinct and potentially important target cells for future treatments.