A Myb enhancer-guided analysis of basophil and mast cell differentiation.

The transcription factor MYB is a crucial regulator of hematopoietic stem and progenitor cells. However, the nature of lineage-specific enhancer usage of the Myb gene is largely unknown. We identify the Myb -68 enhancer, a regulatory element which marks basophils and mast cells. Using the Myb -68 enhancer activity, we show a population of granulocyte-macrophage progenitors with higher potential to differentiate into basophils and mast cells. Single cell RNA-seq demonstrates the differentiation trajectory is continuous from progenitors to mature basophils in vivo, characterizes bone marrow cells with a gene signature of mast cells, and identifies LILRB4 as a surface marker of basophil maturation. Together, our study leads to a better understanding of how MYB expression is regulated in a lineage-associated manner, and also shows how a combination of lineage-related reporter mice and single-cell transcriptomics can overcome the rarity of target cells and enhance our understanding of gene expression programs that control cell differentiation in vivo.

Loss of METTL3 attenuates blastic plasmacytoid dendritic cell neoplasm response to PRMT5 inhibition via IFN signaling.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and aggressive hematologic malignancy with poor clinical outcomes. Dysregulated MYC expression, which is associated with protein arginine methyltransferase 5 (PRMT5) dependency, is a recurrent feature of BPDCN. Although recent studies have reported a PRMT5 gene signature in BPDCN patient samples, the role of PRMT5 in BPDCN remains unexplored. Here, we demonstrate that BPDCN is highly sensitive to PRMT5 inhibition. Consistent with the upregulation of PRMT5 in BPDCN, we show that pharmacological inhibition (GSK3326595) of PRMT5 inhibits the growth of the patient-derived BPDCN cell line CAL-1 in vitro and mitigated tumor progression in our mouse xenograft model. Interestingly, RNA-sequencing (RNA-seq) analysis revealed that PRMT5 inhibition increases intron retention in several key RNA methylation genes, including METTL3, which was accompanied by a dose-dependent decrease in METTL3 expression. Notably, the function of cellular m6A RNA modification of METTL3 was also affected by PRMT5 inhibition in CAL-1 cells. Intriguingly, METTL3 depletion in CAL-1 caused a significant increase in interferon (IFN) signaling, which was further elevated upon PRMT5 inhibition. Importantly, we discovered that this increase in IFN signaling attenuated the sensitivity of METTL3-depleted CAL-1 cells to PRMT5 inhibition. Correspondingly, stimulation of IFN signaling via TLR7 agonists weakened CAL-1 cell sensitivity to PRMT5 inhibition. Overall, our findings implicate PRMT5 as a therapeutic target in BPDCN and provide insight into the involvement of METTL3 and the IFN pathway in regulating the response to PRMT5 inhibition.

Myeloma cells self-promote migration by regulating TAB1-driven TIMP-1 expression in mesenchymal stem cells.

Multiple myeloma (MM) is an intractable hematological malignancy characterized by abnormal plasma cells in the bone marrow (BM) and increased osteolytic lesions. Within the BM niche, mesenchymal stem cells (MSCs) have been proposed to contribute to functionally important MM-MSC interactions. However, despite various studies on MM pathology, the impact of MM on MSCs during the early stages of malignancy has not been adequately addressed. We previously identified tissue inhibitor of matrix metalloproteinase 1 (TIMP-1) as a cytokine that is modulated in vivo within the MM BM niche, and highlighted its potential relevance in MM. Given the role of TIMP-1 in preventing migration of breast cancer cells, this study aimed to investigate the relationship between MSC-secreted TIMP-1 and MM progression. Here, we examined the effect of MSC-derived TIMP-1 on MM cell migration, and found that TIMP-1 secreted by human MSCs play a role in preventing migration of MM cells by reducing the levels of MM cell-derived MMP-9. We also investigated how MM cells regulate expression of TIMP-1 in MSCs. Using a knockdown approach in MSCs, we implicated TGF-B activated kinase 1 binding protein 1 (TAB1) as an upstream effector of TIMP-1 that was downregulated in the presence of MM cells, which resulted in reduced TIMP-1 secretion. Overall, our findings uncover how MSCs in the MM BM niche are modulated to promote MM progression, and unravel a previously unreported role of the TAB1-TIMP-1 axis in the context of the MM BM niche.

Modification of the bone marrow MSC population in a xenograft model of early multiple myeloma.

Multiple myeloma (MM) is a hematological malignancy characterized by clonal proliferation of abnormal plasma cells. MM dysregulates the homeostasis of the bone niche cells like osteoclasts and osteoblasts, responsible for the bone maintenance leading to bone loss and hypercalcemia, as well as the normal immune cells leading to immunodeficiency and anemia. Osteoblasts are part of the cell population differentiating from mesenchymal stem cells (MSC). MSC also gives rise to other cell types such as adipocytes and chondrocytes. It has been observed that adipocytes support MM growth by increasing its survival and chemo-resistance. As adipocytes originate from MSC, the understanding of early modifications in the MSC population during the disease progression is of paramount importance and may help for early diagnosis of MM. Herein, we have evaluated the modification of the MSC population in the bone niche in an in vivo model of MM. Our results showed that before an observable engraftment of MM in the bone niche, the proportion of MSC population is significantly decreased, while a significant increase in adipocyte related genes such as PPARγ and CEBPα expression appears, with no difference in osteogenic differentiation. These results suggest that the bone niche is switching to a "fatty" marrow which would create an adequate microenvironment for MM. This led us to screen for and identify modulated adipokines in the sera of this in vivo MM-mice model. Such changes could reflect early signs of MM and potentially be exploited as detection biomarkers of the disease.