Targeted down-regulation of MLL-AF9 with antisense oligodeoxyribonucleotide reduces the expression of the HOXA7and -A10 genes and induces apoptosis in a human leukemia cell line, THP-1

The MLL gene is frequently rearranged in leukemias, and MLL chimeric proteins generated by chromosomal translocations play crucial roles in leukemogenesis. Targets of murine Mll include HOX proteins that regulate body pattern formation and hematopoiesis. However, it is not known whether or not the MLL chimeric proteins regulate the HOX gene expression in human leukemia. To address this issue, THP-1 cells, a human leukemia cell line expressing MLL-AF9, were treated with antisense oligodeoxyribonucleotide (ODN) complementary to the coding sequence of the MLL-AF9 junction. Down-regulation of the MLL-AF9 transcript was accompanied by the reduced expression of the HOXA7 and -A10 genes, but not of the HOXA2, -A4, -A5, and -A9 genes. The number of viable cells cultured with 20 μM antisense ODN for 5 days was 10-fold lower than that of the sense ODN-treated control. And the number of the annexin V−/propidium iodide− apoptotic cells in the antisense ODN-treated cells after 3 days of culture was two-fold higher than that in the control. Staining of the antisense ODN-treated cells with Hoechst 33258 showed the morphology characteristic to apoptosis. These results indicate that MLL-AF9 regulates the expression of the selected HOX genes as well as prevents the leukemic cells from apoptosis.