HDAC8 Promotes MLL-Rearranged Acute Myeloid Leukemia By Inhibiting p53 Acetylation

Chromosomal rearrangements involving the mixed lineage leukemia (MLL) gene at the 11q23 locus are found in a subset of acute myeloid leukemia (AML) generally associated with intermediate to poor prognosis. MLL-AF9 resulting from t(9;11)(p22;q23) is the most common in MLL-rearranged (MLL-r) AML. Chemotherapy is the current standard of care for AML and the overall 5-year survival rate is only 25%. We previously reported that HDAC8 functions to control p53 activity in hematopoietic stem cells via deacetylation of p53 protein (Hua et al, Blood 2017). Here, we investigated the contribution of HDAC8 in MLL-r AML pathogenesis and maintenance. We have previously generated a conditional Hdac8 floxed (f) allele and combined with Mx1-Cre (Mx1-Cre/Hdac8^f/f(y)) to generate HDAC8^△/△ mice upon induction with poly (I:C). To examine the role of HDAC8 in MLL-AF9-AML, we transduced Hdac8^△/△ or control LSK (Lin-Sca1+cKit+) with a MSCV-ires-GFP (MIG)-MLL-AF9 (MA9) vector and sorted GFP⁺ cells for CFC assay. Hdac8^△/△/MA9 cells generated significantly less CFC colonies compared to control, suggesting that HDAC8 promotes MA9 clonogenic activity. To assess their leukemogenic potential in vivo, we transplanted GFP⁺ control MA9 or Hdac8^△/△/MA9 cells (5x10³-10⁴) into sublethally irradiated (6 Gy) wild-type (WT) recipients. All MA9 recipients (n=4) developed lethal AML with a median survival of 69 days while none of the Hdac8^△/△/MA9 recipients (n=4) developed AML up to 4 months.

To examine the contribution of HDAC8 in established AML, we transduced LSK from uninduced Hdac8^f/f(y)/Mx1-Cre mice with MIG-MA9 and transplanted into WT recipients. Upon development of moribund leukemia, AML cells (5x 10³) were transplanted into a cohort of recipients (n=16), which were treated with poly (I:C) (n=8; 14 mg/kg/dose) or PBS every other day for 2 weeks starting at day 10 (5-10% GFP⁺ in blood). Mice induced with poly (I:C) to delete Hdac8 showed dramatically lower white blood count [9.0 (10³ /ul) vs. 109.7 (10³ /ul); n=8, P<0.05] and lower AML burden (GFP+) in peripheral blood (9.3% vs 42.3%; n=8, P<0.05) at 3 weeks. Prolonged survival was observed in poly (I:C) treated mice compared to control (median survival 36 days vs 27 days, n=8, P<0.01). These results indicate that HDAC8 deletion impairs AML progression. Thus, we examined the effects of HDAC8-selective inhibitor (HDAC8i) on MA9 AML cell proliferation and survival. Treatment of MA9 AML cells with HDAC8i (22d) led to dose-dependent inhibition of cell survival (IC50 = 4.724 - 8.453 mM) and increased apoptosis (10 mM, 48 h) compared to vehicle treated control (40% vs. 7%). HDAC8i treatment significantly enhanced p53 acetylation, and upregulated p53 target genes, Gadd45d (fold change = 3.2; p< 0.01), Puma (fold change = 5.7; p< 0.01), Mdm2 (fold change = 3.0; p< 0.01) and Cdkn1a (fold change = 3.6; p< 0.01). To assess the effects on AML growth and leukemia-initiating capacity in vivo, we transplanted MA9 AML cells treated with HDAC8i (1 x 10⁵, 2.5x10⁵) or vehicle (2.5x10⁵) into sublethally irradiated WT recipients (n=8-11). We observed significantly (p<0.0001) reduced GFP⁺ AML burden in recipients of HDAC8i-treated cells (both 1x10⁵ & 2 x10⁵) and significantly (p<0.0001) improved survival. Secondary transplantation of AML cells from HDAC8i- or vehicle-treated (2 x10⁵) recipients revealed significantly improved survival in HDAC8i-treated group (22d 105 days n=4 vs. vehicle 33 days n=5; p< 0.01). These results indicate that HDAC8 inhibition enhanced elimination of MA9 AML cells and leukemia-initiating cells.

We further evaluated the effect of HDAC8i in human AML cell lines and primary MLL-r AML patient cells. HDAC8i treatment increased p53 acetylation and enhanced p53-dependent apoptosis in MOLM-13 AML cells (22d: 50.8% vs. vehicle: 11.89%). Knock-down of p53 rescued the apoptosis induced by HDAC8i, indicating that HDAC8i induced apoptosis is mediated by p53. Similarly, HDAC8i (22d) treatment significantly increased apoptosis accompanied by elevated levels of p53 acetylation and p53 target gene expression in MLL-r AML CD34+ cells compared to normal (NL) CD34+ cells. Thus, HDAC8 inhibition targets human MLL-r AML cells and AML CD34+ cells by restoring p53 acetylation and inducing apoptosis.

In conclusion, this study reveals a critical role of HDAC8 in promoting initiation and maintenance of MLL-AF9 AML and highlights HDAC8 inhibition as a promising approach to target MLL-r AML.