Therapeutic implications of activation of the host gene (Dleu2) promoter for miR-15a/16-1 in chronic lymphocytic leukemia

Genetic lesions and other regulatory events lead to silencing of the 13q14 locus in a majority of chronic lymphocytic leukemia (CLL) patients. This locus encodes a pair of critical proapoptotic microRNAs, miR-15a/16-1. Decreased levels of miR-15a/16-1 are critical for the increased survival exhibited by CLL cells. Similarly, in a de novo murine model of CLL, the NZB strain, germline-encoded regulation of the syntenic region resulted in decreased miR-15a/16-1. In this paper, we have identified additional molecular mechanisms regulating miR-15a/16-1 levels and have shown that the transcription factor BSAP (B-cell-specific activator protein) directly interacts with Dleu2, the host gene containing the miR-15a/16-1 loci, and by negative regulation of the Dleu2 promoter, results in repression of miR-15a/16-1 expression. CLL patient B-cell expression levels of BSAP were increased compared with control sources of B cells. With the use of small interfering RNA-mediated repression, the levels of BSAP were decreased in vitro in the NZB-derived malignant B-1 cell line, LNC, and in ex vivo CLL patient peripheral blood mononuclear cells (PBMCs). BSAP knockdown led to an increase in the expression of miR-15a/16-1 and an increase in apoptosis, and a cell cycle arrest in both the cell line and patient PBMCs. Moreover, using Dleu2 promoter analysis by chromatin immunoprecipitation assay, we have shown that BSAP directly interacts with the Dleu2 promoter. Derepression of the Dleu2 promoter via inhibition of histone deacetylation combined with BSAP knockdown increased miR-15a/16-1 expression, and also increased malignant B-cell death. In summary, therapy targeting enhanced host gene Dleu2 transcription may augment CLL therapy.