Effects of miR-125b expression on the transcriptome of B cell precursors

Here, we have screened a miRNA expression library in a pre-B cell expression system to identify miRNAs with a potential oncogenic activity in early lymphocytes. We show that miR-125b is sufficient to transform B cell precursors in vitro as defined by growth factor independence, and demonstrate that continuous expression of miR-125b is necessary to keep cells in the transformed cells. Mechanistically, we find that expression of miR-125b protects against apoptosis induced by growth factor withdrawal, and that miR-125 interferes with the differentiation of pre-B to immature B cells. In consequence, transformed cells express the pre-BCR on the cell surface, which appears to provide signals for ongoing proliferation and survival. Using microarray analysis, we identified several previously known and novel putative target genes of miR-125b in B cell precursors. To evaluate their contribution to the miR-125b-mediated phenotype, transformed cells were reconstituted with expression constructs of validated target genes. Surprisingly, we only find that reconstitution of MAP3K11 interferes with the cellular fitness of transformed cells in vitro as well as in vivo. This indicates that MAP3K11 functions as an important tumor suppressor in the context of miR-125b. Gene expression in cells expressing a control vector or a vector encoding hsa-miR-125b-1 was measured either in the presence or 24 hours after withdrawal of the growth factor IL-7. Two independent experiments were performed, using independently generated cell clones for each repetition.