Addiction of B Cell Lymphomas to microRNA-17-92 Is Mediated By a Single Target Gene

Mature microRNAs (miRNA) are short non-coding RNAs that regulate gene expression by binding to messenger RNAs (mRNA) in a sequence-specific manner, causing translational repression and/or mRNA decay. The mammalian genome harbors thousands of miRNA genes, many of which are organized into transcriptionally co-regulated clusters such as miR-17-92.

Knockout of the miR-17-92 cluster gene in mice blocked B lymphopoiesis, and ectopic miR-17-92 expression sufficed to initiate B cell lymphomas and autoimmunity. In humans, the miR-17-92 gene is commonly amplified or overexpressed via MYC-driven transcription in diffuse large B cell and Burkitt's lymphomas.

A computationally predicted shared target of the miR-17-92 miRNAs is the pro-apoptotic BCL-2 family protein BIM, central to life-death decisions in mammalian cells. To clarify the contribution of miR-17-92:Bim interactions to the miR-17-92 knockout and overexpression phenotypes, we engineered a unique in vivo system of conditional mutagenesis of the nine Bim 3'UTR miR-17-92 binding sites. Instead of causing the predicted B cell developmental block, interruption of miR-17-92:Bim interactions produced a selective inability of B cells to resist cellular stress; and prevented lymphocyte hyperplasia caused by Bim haploinsufficiency.

Surprisingly, partial genetic disruption of miR-17-92:Bim interactions was sufficient to fully prevent B cell lymphoma formation in two out of three mice using two independent pre-clinical MYC-driven cancer models. This protective effect could be attributed to an increased activity of the mitochondrial apoptosis pathway in pre-malignant B cells, as apoptosis was abolished by concomitant overexpression of an anti-apoptotic BCL-2 protein. MYC-driven B lymphoma cells are addicted to miR-17-92 function. Our data build on these results and strongly suggest that miR-17-92:Bim interactions are vital in this context as acute ablation of miR-17-92:Bim interactions effectively promoted lymphoma cell apoptosis, both in vitro and in vivo.

In conclusion, among hundreds of putative miR-17-92 target mRNAs a single direct binding partner is vital for lymphoma development and maintenance, a discovery whose therapeutic exploitation is of major relevance.