Malignant transformation of normal hematopoietic transplants induced by residual leukemia cells is considered as a pivotal mechanism of donor cell leukemia (DCL). The effects of leukemia cell-derived microvesicles (MVs) in this transformation were examined. We found that MVs derived from K562 leukemia cells contained the breakpoint cluster region-Abelson leukemia gene human homolog 1 (BCR-ABL1) mRNA. Following incubation with BCR-ABL1-positive MVs, mononuclear cells derived from normal transplants exhibited a leukemia-like malignant phenotype both in vitro and in vivo. Horizontal transfer of BCR-ABL1 mRNA from MVs into the recipient cells was critical to the transformation. Relative genomic instability was observed and considered the main mechanism in the recipient cells. MVs contributed to genomic instability by two distinct pathways: via consequent overexpression of activation-induced cytidine deaminase and reactive oxygen species, which mediated DNA breakage and recombination; and via upregulation of methyltransferases and global DNA hypermethylation. We demonstrated that BCR-ABL1-positive MVs could initiate malignant transformation of normal hematopoietic transplants through genomic instability, which might serve as a convenient and operable model for investigating leukemogenesis, especially for DCL. Furthermore, MVs themselves could act as an early warning indicator and a novel tool to detect and prevent the occurrence of DCL.

Background: As a hallmark driver of multiple myeloma (MM), MM bone disease (MBD) is unique in that it is characterized by severely impaired osteoblast activity resulting from blocked osteogenesis in bone marrow-derived mesenchymal stem cells (BM-MSCs). The mechanisms underlying this preferential blockade are incompletely understood.Methods: miRNA expression of MM cell-derived extracellular vesicles (MM-EVs) was detected by RNA sequencing. MM-EVs impaired osteogenesis and exacerbated MBD were in vitro and in vivo validated by histochemical staining, qPCR and micro-CT. We additionally examined the correlation between CD138+ circulating EVs (cirEVs) count and bone lesion in de novo MM patients.Results: Here, by sequencing and bioinformatics analysis, we found that MM-EVs were enriched in various molecules negatively regulating osteogenesis. We experimentally verified that MM-EVs inhibited BM-MSC osteogenesis, induced elevated expression of miR-103a-3p inhibiting osteogenesis in BM-MSCs, and increased cell viability and interleukin-6 secretion in MM cells. In a mouse model, MM-EVs that were injected into the marrow space of the left tibia led to impaired osteogenesis and exacerbated MBD and MM progression. Furthermore, the levels of CD138+ cirEVs in the peripheral blood were positively correlated with the number of MM bone lesions in MM patients.Conclusions: These findings suggest that MM-EVs play a pivotal role in the development of severely impaired osteoblast activity, which represents a novel biomarker for the precise diagnosis of MBD and a compelling rationale for exploring MM-EVs as a therapeutic target.

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