Effects of a Histone Deacetylase Inhibitor On Myeloid Leukemia Cell Line with Mixed-Lineage Leukemia Partial Tandem Duplication

Mixed lineage leukemia-Partial tandem duplication (MLL-PTD) is one of the MLL gene rearrangements accompanying leukemia, and is associated with a worse prognosis. Since MLL regulates a gene transcription, histone deacetylase inhibitors (HDACi) have been considered to exert antitumor effects through epigenetic modification. We established a leukemia cell line KOPM-88 containing MLL-PTD from a child with acute myeloid leukemia (AML). Non-obese diabetic severe combined immunodeficient (NOD/SCID) mice inoculated with KOPM-88 cells exhibited leukemic infiltrations in the bone marrow and hemiparalysis because of compression myelopathy (Hayashi et al., Br J Haematol, 2007). We therefore investigated the effects of HDACi on KOPM-88 cells and NOD/SCID mice inoculated with KOPM-88 cells.

The cell lines employed were KOPM-88 and K-562. We chose K-562 as a control because it contains a wild type MLL. Firstly, a HDACi, trichostatin A (TSA), was added with 100 nM/L concentration in KOPM-88 cells for 24 hours and in K-562 cells for 12 hours. Apoptosis was evaluated by using a flow cytometer. MLL-PTD mRNA expression was measured by using a real time PCR before and after incubation with TSA. Secondary, 5×10⁶ of KOPM-88 cells were transplanted in NOD/SCID mice (n=32). Then, TSA was injected intraperitoneally into a half of the transplanted mice three times a week for 5 weeks (TSA group, n=16). DMSO, which was used as a solution of TSA, was injected intraperitoneally into the half rest of mice (control group, n=16). Hemiparalysis was counted as an event. The probability of event-free survival (EFS) and the probability of overall survival (OS) were analyzed using the Kaplan-Meier method.

TSA significantly increased the apoptosis in KOPM-88 cells (30.03±2.83% vs 53.17±2.11%,p <0.01), but it did not significantly increased the apoptosis in K-562 cells (8.15±0.20% vs 14.04±0.23%,p=0.08). TSA also significantly decreased the level of MLL-PTD mRNA in KOPM-88 cells (1.78±0.35 copies/μgRNA vs 0.38±0.33 copies/μgRNA, p <0.05). The median follow-up of the TSA group was 178 days (range; 30–210), and that of the control group was 149 days (range; 98–210). Although seven months-EFS was significantly better in the TSA group than in the control group (78±0.8% vs 43.8±0.8%,p <0.05), seven months-OS was similar between the TSA group and the control group (43.8±0.8% vs 18.8±0.6%,p=0.25).

Our study suggests that HDACi exerts an anti-leukemic effect partly from suppression of MLL-PTD transcription on leukemia cells with MLL-PTD. This anti-leukemic effect of HDACi was also confirmed in vivo mice model. The patients with MLL-PTD leukemia may benefit from HDACi treatment.

No relevant conflicts of interest to declare.