Deciphering the molecular mechanism of ineffective erythropoiesis in MDS-5q

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(PLEASE KEEP IN WORD, DO NOT PDF) Approximately 80% of MDS patients present with anemia, which is the major cause of morbidity. The anemia is generally macrocytic and always reflects ineffective erythropoiesis. However, why erythroid cells die while maturing in the bone marrow is uncertain, prompting this investigation. Especially unclear is why anemia occurs early in MDS (i.e. at presentation) when often 50-75% of the marrow is replaced by neoplastic cells and many normal (non-neoplastic) cells remain. We previously have shown that CFU-E/proerythroblasts die when their intracellular heme (a toxic chelate synthesized enzymatically) exceeds globin (a protein). This can result from failed heme export (Flvcr1-deleted mice) or from ribosomal protein haploinsufficiency, impaired translation, and slowed globin synthesis (MDS-5q and Diamond Blackfan anemia (DBA) patients). Since heme synthesis initiates normally, but globin translation is slowed, heme exceeds the export capacity of FLVCR and induces high levels of ROS and cell death. To delineate the shared mechanisms that lead to the death of maturing MDS-5q and DBA cells erythroid cells, we studied single marrow cells with CITE-seq (antibody barcoding of cell surface proteins) and RNA sequencing. We aligned total transcriptomes with SCVelo pseudotime analysis, which includes incompletely processed mRNAs, and then linked a cell’s surface protein expression to its unique transcriptome. Using this approach in preliminary studies, we showed that all erythroid cells in MDS-5q patient marrow have transcriptional changes implicating heme toxicity. Nearly all marrow erythropoiesis takes place within erythroblastic islands (EBIs), a structure comprised of a central macrophage (“nurse cell”) and ~10-50 maturing red cells. We suspect that the role of the central macrophage is to safely and efficiently recycle heme from CFU-E/proerythroblasts to neighboring iron-avid later erythroid cells. Interestingly, the 5q+ (non-neoplastic) cells are also compromised and fail to expand and reconstitute erythropoiesis. We hypothesize, that the large quantities of heme influx from neoplastic (5q-) erythroid precursors into the central macrophage exceeds that which can be metabolized to iron then stored as ferritin or exported via ferroportin. This compromises the macrophage’s ability to support the maturation of co-adherent normal (5q+) cells, and thus the differentiation of both normal and neoplastic erythroid precursors fail. In order to test this hypothesis we will first show the feasibility and the relevance of using an EBI culture system to model human EBI in this R56-funded study. This would provide the data needed to justify studies of heme-iron trafficking and its impact on red cell differentiation in MDS-5q patients, and perhaps other low and low-intermediate risk MDS patients with disabling anemia.