MEIS1 Is a Promising New Target for Leukemia Therapy.

The homeobox transcription factor MEIS1 is expressed in most human acute myeloid leukemias and MLL-rearranged leukemias of both myeloid and lymphoid phenotypes. MEIS1 over-expression in murine hematopoietic progenitor cells accelerates transformation initiated by other oncogenes. All leukemias developing in knock-in MLL-AF9 mice display abundant expression of MEIS1. However, it remains unknown if MEIS1 is essential for the survival of established leukemias. We studied the effect of MEIS1 silencing in both human and murine leukemia cell lines. To knock down the expression of MEIS1, we screened multiple shRNA constructs cloned into a lentivirus expression vector (The RNAi Consortium). Two MEIS1 shRNA lentivirus clones reduced the growth of human leukemia cells RS4-11 and SEMK2 by 50-70% at 5 days post-infection compared to control virus at a multiplicity of infection (MOI) of 10. The murine cell line (4166) was derived from a transgenic MLL-AF9 knock-in leukemic mouse and displays a myelo-monocytic phenotype, similar to human MLL-AF9 leukemias. When transplanted into wild type mice, this cell line induces leukemia with high efficiency. Two murine MEIS1 shRNA clones (M26 and M27) consistently inhibited the growth of 4166 cells at MOI of 10 or greater, detected as early as 24-48h. At 5 days post-transduction with an MOI of 100, growth of M26 and M27 transduced 4166 cells was reduced by 88% and 58% respectively in a representative experiment compared to those transduced with a control virus. In M26 transduced cells, MEIS1 mRNA levels were reduced by > 70% when compared to cells infected with a control virus. Analysis of DNA content by flow cytometry showed that MEIS1 silencing induced apoptosis, as indicated by an increase in the amount sub-diploid DNA, with the sub-diploid fractions in the non-transduced, control virus and M26 transduced cells being 17.4%, 21.7% and 63.9% respectively. When cultured in semi-solid methylcellulose media for 5 days, M26 transduced cells formed significantly fewer colonies than those transduced with a control virus (439±24 and 974±31 colonies per 1000 cells respectively, p<0.0006). The proportion of dense compact colonies, which are composed predominantly of undifferentiated myeloid progenitors was also reduced (37.9±4.8% Vs 90.5±0.8% respectively). To investigate MEIS1 as a molecular target for leukemia therapy, 10,000 cells untreated or infected with either M26 or control virus were injected into lethally irradiated wild type mice (7 mice/group). Animals were sacrificed when moribund. Necropsies with assessment of white blood cell count, spleen weight and FACS analysis were performed to confirm the cause of death. By day 115 post-transplant, 100% of the mice given untreated or control virus-transduced 4166 cells had succumbed to myeloid leukemia, while 43% of the MEIS1 shRNA treated group were still alive. Notably, 28% of mice in this group have survived past 170 days. Log-rank analysis revealed that survival of the three groups was significantly different (p<0.04). Overall, these results show that MEIS1 silencing in leukemia decreases cell growth, induces apoptosis and delays the development of overt leukemia in vivo. These results demonstrate that MEIS1 is an attractive molecular therapeutic target for leukemias that express this transcription factor critical for leukemia cell survival.