Regulation of Transcription Factors Foxp1, E2A and Egr-1 By microRNA-191 Constrains B Lymphocyte Development and Suppresses Development of Diffuse Large B Cell Lymphoma

Development of B-cells in the bone marrow is a tightly controlled, multi-step process. Whereas its major transcriptional regulators are well characterized, less is known about its post-transcriptional regulation. It has been demonstrated that B cell development critically depends on microRNAs. Yet, the role of individual members still remains to be uncovered.

Relatively subtle regulatory effects of miRNAs are well-visible in the pathways which require tight control. Disruption of such a pathway, where the protein function (i.e. transcription factor) highly depends on the gene dosage, is major cause of malignant transformation. 

We identified transcription factors Foxp1, E2A and Egr1 as direct targets of microRNA miR-191. Reduced expression of these transcription factors through ectopic expression of miR-191 in B-cell progenitors resulted in numerous secondary effects, such as downregulation of Rag1, Rag2, Dntt and Il7r alpha genes. In consequence, we observed impaired immunoglobulin heavy chain rearrangement and limited responsiveness to IL-7 of B cell progenitors resulting in partial developmental arrest and predisposition of immature B-cells to death during negative selection. Finally, we showed that co-overexpression of E2A or FoxP1 and miR-191 restored normal B cell development, supporting the hypothesis that both transcription factors are functionally relevant targets of miR-191.

Having established a regulatory cascade comprising miR-191 at the top of hierarchy and the transcription factors Foxp1, E2A and Egr1 as its direct targets we sought to identify a pathophysiological scenario in which this cascade might be explored therapeutically. Given that the most aggressive forms of human diffuse large B cell lymphoma (DLBCL) are FOXP1 dependent, we assessed whether miR-191 was able to limit growth of DLBCL. Of note, in vivo ectopic expression of miR-191 prevented the growth of DLBCL. Consistent with murine data, expression of the three target genes FOXP1, EGR1 and TCF3 (encoding E2A) was decreased upon miR-191 overexpression. Interestingly, treatment with the histone deacetylase inhibitor vorinostat revealed decreased resistance to cell death of DLBCL lines overexpressing miR-191 in vitro.

Thus, our data describe a new regulatory pathway, where miR-191 by fine-tuning key B-lineage transcription factors acts as modulator of B-cell development as well as tumor suppressor in B-lineage malignancy.