MLL-Rearrangements Result in Upregulation of Mir-9 and Subsequent Inhibition of the Tumor Suppressor TGFBI

Abstract 2453

Acute Leukemia represents one of the most deadly cancers in the United States. Clinical treatments in leukemia have progressed significantly through the use of therapies targeted specifically to chromosomal translocations. The success of these therapies has provided a model for future treatment in various cancers. However, there are various subtypes of leukemia where five-year survival and relapse rates have poor clinical outcome, indicating that new therapies are needed. A particular leukemia subtype, namely mixed lineage leukemia (MLL)-rearranged leukemia that is a result of chromosomal rearrangements leading to fusions between MLL and partner genes, is associated with a dismal outcome. Therapeutic targeting of MLL rearrangements has proven challenging as there have been dozens of described rearrangements. We have performed both messenger RNA (mRNA) and microRNA (a class of small non-coding RNA) microarray analyses on over 100 leukemia patient samples and have identified that microRNA-9 (miR-9) is highly upregulated in MLL-associated leukemias. Through correlating expression of miR-9 and those of its predicted target genes, we identified TGFbeta-induced protein (TGFBI) as a potential target of miR-9 that exhibited a significantly (P<0.05) inverse correlation of expression with miR-9 in acute leukemia. Our further luciferase reporter/mutagenesis assays show that TGFBI is a direct target of miR-9. TGFBI has been described as a tumor suppressor in breast, lung and ovarian tumors through two mechanisms, AKT inhibition and microtubule stabilization, but its involvement in leukemia has never been reported. We found that TGFBI expression was decreased while miR-9 was upregulated in MLL-rearrangement acute myeloid leukemias. Upon further testing we discovered that exogenous expression of miR-9 increased proliferation and enhanced the colony-forming ability of progenitor cells infected with MLL-AF9 fusions. Expression of TGFBI was sufficient to inhibit proliferation and significantly reduced the colony-forming ability of MLL-AF9. This reduction in colony-forming ability was also observed when MLL-AF9 progenitor cells were infected with TGFBI and miR-9 together, showing that TGFBI expression is sufficient to block the effects of miR-9 overexpression. We suspect that TGBFI expression modulates AKT in MLL-rearranged leukemias, thus making it susceptible to combination of standard leukemia chemotherapies with AKT-inhibitors. We will test the effects on NVP-BEZ235, a PI3K/mTOR inhibitor in phase II clinical trials, alone and in combination with other chemotherapies against MLL-rearrangement leukemias. We expect that the specific targeting of this pathway in these leukemias will be highly efficacious. Thus these studies could identify a novel therapeutic strategy for the treatment of MLL-rearrangement leukemias that is more effective and specific than the current therapies.