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Summary: Severe congenital neutropenia (SCN) patients are prone to develop myelodysplastic syndrome (MDS) or acute myeloid leukaemia (AML). Leukaemic progression of SCN is associated with the early acquisition of CSF3R mutations in haematopoietic progenitor cells (HPCs), which truncate the colony‐stimulating factor 3 receptor (CSF3R). These mutant clones may arise years before MDS/AML becomes overt. Introduction and activation of CSF3R truncation mutants in normal HPCs causes a clonally dominant myeloproliferative state in mice treated with CSF3. Paradoxically, in SCN patients receiving CSF3 therapy, clonal dominance of CSF3R mutant clones usually occurs only after the acquisition of additional mutations shortly before frank MDS or AML is diagnosed. To seek an explanation for this discrepancy, we introduced a patient‐derived CSF3R‐truncating mutation in ELANE‐SCN and HAX1‐SCN derived and control induced pluripotent stem cells and compared the CSF3 responses of HPCs generated from these lines. In contrast to CSF3R‐mutant control HPCs, CSF3R‐mutant HPCs from SCN patients do not show increased proliferation but display elevated levels of inflammatory signalling. Thus, activation of the truncated CSF3R in SCN‐HPCs does not evoke clonal outgrowth but causes a sustained pro‐inflammatory state, which has ramifications for how these CSF3R mutants contribute to the leukaemic transformation of SCN. [ABSTRACT FROM AUTHOR]

Summary The transcription factor GATA2 has a pivotal role in haematopoiesis. Heterozygous germline GATA2 mutations result in a syndrome characterized by immunodeficiency, bone marrow failure and predispositions to myelodysplastic syndrome (MDS) and acute myeloid leukaemia. Clinical symptoms in these patients are diverse and mechanisms driving GATA2‐related phenotypes are largely unknown. To explore the impact of GATA2 haploinsufficiency on haematopoiesis, we generated a zebrafish model carrying a heterozygous mutation of gata2b (gata2b+/−), an orthologue of GATA2. Morphological analysis revealed myeloid and erythroid dysplasia in gata2b+/− kidney marrow. Because Gata2b could affect both transcription and chromatin accessibility during lineage differentiation, this was assessed by single‐cell (sc) RNA‐seq and single‐nucleus (sn) ATAC‐seq. Sn‐ATAC‐seq showed that the co‐accessibility between the transcription start site (TSS) and a −3.5–4.1 kb putative enhancer was more robust in gata2b+/− zebrafish HSPCs compared to wild type, increasing gata2b expression and resulting in higher genome‐wide Gata2b motif use in HSPCs. As a result of increased accessibility of the gata2b locus, gata2b+/− chromatin was also more accessible during lineage differentiation. scRNA‐seq data revealed myeloid differentiation defects, that is, impaired cell cycle progression, reduced expression of cebpa and cebpb and increased signatures of ribosome biogenesis. These data also revealed a differentiation delay in erythroid progenitors, aberrant proliferative signatures and down‐regulation of Gata1a, a master regulator of erythropoiesis, which worsened with age. These findings suggest that cell‐intrinsic compensatory mechanisms, needed to obtain normal levels of Gata2b in heterozygous HSPCs to maintain their integrity, result in aberrant lineage differentiation, thereby representing a critical step in the predisposition to MDS. [ABSTRACT FROM AUTHOR]