Key Points Atovaquone induces AML blast apoptosis and prolongs survival in AML xenografts. Atovaquone induces proapoptotic signaling and inhibits the mTOR pathway through upregulation of ATF4 and also suppresses OXPHOS.

HSCT significantly improves splenic function for most pediatric patients with SCD, but older patient age at time of HSCT and extensive chronic GVHD appear to be risk factors for poor post-HSCT splenic function.

Despite aggressive chemotherapy, relapse occurs in almost half of children with acute myeloid leukemia (AML), with very dismal survival. Novel and mechanism-driven therapies are desperately needed to conquer chemotherapy resistance and leukemic stemness in pediatric AML. Within the bone marrow niche, stromal cells protect leukemia cells from chemotherapy, maintain leukemic stemness, and eventually lead to disease recurrence. We developed an in vitro AML cell-stromal cell co-culture model to mimic bone marrow microenvironment. Stroma-leukemia cell interaction leads to activation of various signaling molecules in AML cells that allow them to evade apoptosis. One such example is extracellular signal-regulated kinases 1/2 (ERK1/2), important pro-survival proteins. ERK1/2 are activated by the Ras/Raf/ mitogen-activated protein kinase kinase (MAPK/ERK kinase or MEK) pathway downstream of signals from the stroma. We recently showed that stromal co-culture activates ERK1/2 in pediatric AML samples, contributing to chemotherapy resistance (Long, et al, 2017, Oncotarget, 8:90037). To identify genes that are regulated in AML cells by ERK1/2 activation, 4 pediatric AML samples were cultured alone, or co-cultured with mOrange-expressing stroma for 24 hours, in the presence or absence of a selective MEK inhibitor, selumetinib (1 μM). Thereafter, cells were flow sorted to exclude mOrange+ stroma and CD45high/SSClow lymphocytes. Sorted AML cells underwent total RNA extraction for nCounter® PanCancer Pathways Panel (Nanostring Technologies) gene expression profiling study. We focused on the genes that were either up- or down-regulated by co-culture with stroma, and changed in the reverse direction by the addition of selumetinib. We chose a few genes among the list (BMP2, BNIP3, H2AFX, DUSP2, FZD3, BCL2L1, CHEK2) that are reported to be involved in oncogenesis, chemotherapy resistance, cell growth and survival. Using qRT-PCR, we confirmed bone morphogenic protein 2 (BMP2) to be upregulated in AML cells by stroma, and the effect of stroma was reversed by selumetinib. Further, we confirmed the same change of BMP2 at the protein level by FACS. Smad1, 5 and 8 are transcriptional factors immediately downstream from BMP receptors and play a central role in BMP signal transduction. Using FACS we discovered stroma-induced activation of Smad 1/8 in pediatric AML patient samples, which was partially alleviated by selumetinib and a selective BMP inhibitor, K02288 (10 μM). BMPs are growth factors that belong to the transforming growth factor beta (TGF-beta) superfamily and are thought to be involved in stem cell properties such as self-renewal. To determine if the BMP-Smad pathway plays a role in chemotherapy resistance, 4 pediatric AML patient samples were cultured on or off stromal cells, and treated with cytarabine (10 μM) with or without K02288 (10 μM) for 24h. Cells were analyzed for cytarabine-induced apoptosis with Annexin V staining by FACS, excluding stromal cells and lymphocytes. K02288 treatment did not alter cytarabine-induced apoptosis. We next tested the potential role of BMP-Smad pathway in leukemic self-renewal in pediatric AML samples. Pediatric AML patient samples were plated at 50,000/ml or 100,000/ml in Methocult medium (H-4535) to quantify stem and progenitor cells. Samples were treated with vehicle or K02288 (10 μM). Colonies and viable cells were counted and normalized to control 7-14 days after plating. Harvested cells were stained for CD34, CD14 and CD11b to evaluate differentiation by FACS. Remaining cells were replated at the same density for 2 more rounds. We found that K02288 reduced colony counts (e.g., 100±0% in vehicle control, v. 25±10% in K02288, n=5, p<0.002, post 2nd plating) and decreased the number of viable cells at 2nd and 3rd plating (Figure 1). The BMP inhibitor also promoted differentiation of leukemic stem cell in pediatric AML samples, as evidenced by increased CD14 and CD11b expression. In contrast, for normal pediatric bone marrow samples, K02288 did not change colony counts or the number of viable cells, and it did not promote differentiation. Those data suggested BMP-Smad is likely to contribute to leukemic stemness in pediatric AML without disrupting normal hematopoietic stem cells. Therefore, BMP-Smad pathway may be a promising therapeutic target to reduce leukemia burden and improve survival for children with AML. Disclosures No relevant conflicts of interest to declare.

Introduction: Splenic dysfunction is a critical complication of SCD that begins in early childhood and can cause fatal sepsis. Hematopoietic stem cell transplant (HSCT) is a proven cure for SCD, however, its long-term effect on the spleen in patients who had SCD is not well characterized. This information could be helpful to further inform medical professionals and families considering HSCT for SCD. Better understanding of the splenic function of patients after transplant for SCD could also provide important information regarding these patients' future risk of infection and other possible problems related to asplenia. Methods: We conducted a retrospective cohort study of pediatric patients who had HSCT for SCD at two transplant centers. Patients were excluded for analysis if they had pre-HSCT splenectomy, died less than one year post-transplant, failed to engraft, or had no post-HSCT spleen function testing. Tc99m liver-spleen (LS) scans and red blood cell (RBC) pit counts, validated tests to evaluate the spleen in SCD, were used to assess splenic function. Splenic uptake on scans was classified as either present or absent. RBC pit count was classified as either normal or abnormal. If a patient had multiple LS scans or RBC pit counts post-HSCT, then the most recent test was used for this analysis. Results: Fifty-five evaluable patients were identified (53 patients with a post-HSCT LS scan, 29 patients with a post-HSCT RBC pit count, and 27 patients with both tests). Almost all (52/55, 95%) patients had hemoglobin SS or Sβ0 disease. Median age at transplant was 8.8 years (range 1.8-22.0 years). Conditioning regimen and stem cell source varied, however, 48/55 (87%) received myeloablative conditioning and an HLA-identical sibling donor HSCT. At a median 2.0 years post-HSCT (range 0.9-8.9 years), 48/53 (91%) had spleen uptake present on LS scan. At a median 3.2 years post-HSCT (range 1.0-11.4 years), 24/29 (83%) had a normal RBC pit count. Analysis of patients who had both pre- and post-HSCT tests showed that patients had significantly improved outcomes post-HSCT: Table Pre-HSCT Post-HSCT p-value Spleen Uptake Present on LS scan 17/36 (47%) 32/36 (89%) 0.0002 Normal RBC pit count 6/20 (30%) 17/20 (85%) 0.0004 All patients (17/17, 100%) with LS scan spleen uptake pre-HSCT had uptake present post-HSCT compared to 15/19 (79%) patients with absent spleen uptake pre-HSCT had uptake present post-HSCT (p=0.11). Four patients had absent spleen uptake at 1 year post-HSCT but had uptake present at 2 years post-HSCT. Conclusion: Our study represents the largest comprehensive evaluation of long-term splenic function in patients after HSCT for SCD. Our results suggest that successful HSCT mitigates damage to the spleen induced by SCD. In patients with splenic function pre-transplant, HSCT appears to preserve this function. In patients lacking splenic function, HSCT appears to engender the return of function in most, but not all patients. Disclosures No relevant conflicts of interest to declare.