B-cell chronic lymphocytic leukemia (B-CLL), the most common human leukemia in the world, is a malignancy of mature B-lymphocytes. Deregulation of the TCL1 oncogene is a causal event in the pathogenesis of the aggressive form of this disease as was verified by using animal models. To study the mechanism of TCL1 regulation in CLL, we carried out microRNA expression profiling of three types of CLL: indolent CLL, aggressive CLL, and aggressive CLL showing 11q deletion. We identified distinct microRNA signatures corresponding to each group of CLL. We further determined that TCL1 expression is regulated by miR-29 and miR-181, two microRNAs differentially expressed in CLL. Expression levels of miR-29 and miR-181 generally inversely correlated with TCL1 expression in the CLL samples we examined. Our results suggest that TCL1 expression in CLL is, at least in part, regulated by miR-29 and miR-181 and that these microRNAs may be candidates for therapeutic agents in CLLs overexpressing TCL1.
Human B-CLL occurs in two forms: aggressive (showing high ZAP-70 expression and unmutated IgH VH) and indolent (showing low ZAP-70 expression and mutated IgH VH). We found that miR-29a is upregulated in indolent human B-CLL compared to aggressive B-CLL and normal CD19+ B-cells. To study the role of miR-29 in B-CLL, we generated Eµ-miR-29 transgenic mice overexpressing miR-29 in mouse B-cells. Flow cytometric analysis revealed a markedly expanded CD5+ population in the spleen of these mice starting at 2 months of age. 85% (34/40) of miR-29 transgenic mice exhibited an expanded population of CD5+ B-cells, a characteristic of the B-CLL phenotype. An average of 50% of the B-cell population in these transgenics was CD5 positive. At the age of 2 years these mice showed significantly enlarged spleens and an increase in CD5+ B-cell population of up to 100% of B-cells. Of 20 Eµ-miR-29 transgenic mice followed up to the age of 24-26 months, 4 (20%) developed frank leukemia and prematurely died from the disease. The expanded CD5+ B-cell population was found to be proliferative, with an increased number of cells in the S-phase of the cell cycle, compared to wild type CD19+ B-cells. These results suggest that deregulation of miR-29 can cause, or at least significantly contribute to the pathogenesis of indolent B-CLL.
Although many microRNAs are up- or down-regulated in a number of solid tumors and hematological malignancies and several are postulated to function as tumor suppressors or oncogenes, there have been only two reports demonstrating that up-regulation of a single microRNA can cause malignancy. Here we demonstrate that up-regulation of miR-29 in human indolent CLL is an important initiating event in the pathogenesis of this disease. These results show that overexpression of miR-29 is sufficient to cause the development of indolent CLL with high penetrance and provide a new mouse model for indolent CLL.