Transcriptional repression of the p15 gene predicts the clinical outcome of acute myeloblastic leukemia with intermediate and adverse cytogenetics

A vicious loop of fatty acid-binding protein 4 and DNA methyltransferase 1 promotes acute myeloid leukemia and acts as a therapeutic target

Aberrant DNA methylation mediated by deregulation of DNA methyltransferases (DNMT) is a key hallmark of acute myeloid leukemia (AML), yet efforts to target DNMT deregulation for drug development have lagged. We previously demonstrated that upregulation of fatty acid-binding protein 4 (FABP4) promotes AML aggressiveness through enhanced DNMT1-dependent DNA methylation. Here, we demonstrate that FABP4 upregulation in AML cells occurs through vascular endothelial growth factor (VEGF) signaling, thus elucidating a crucial FABP4-DNMT1 regulatory feedback loop in AML biology. We show that FABP4 dysfunction by its selective inhibitor BMS309403 leads to downregulation of DNMT1, decrease of global DNA methylation and re-expression of p15INK4B tumor suppressor gene by promoter DNA hypomethylation in vitro, ex vivo and in vivo. Functionally, BMS309403 suppresses cell colony formation, induces cell differentiation, and, importantly, impairs leukemic disease progression in mouse models of leukemia. Our findings highlight AML-promoting properties of the FABP4-DNMT1 vicious loop, and identify an attractive class of therapeutic agents with a high potential for clinical use in AML patients. The results will also assist in establishing the FABP4-DNMT1 loop as a target for therapeutic discovery to enhance the index of current epigenetic therapies.

Reversal of p15/INK4b hypermethylation in AML1/ETO-positive and -negative myeloid leukemia cell lines

In vitro and in vivo, myeloid leukemic and preleukemic cells exhibit variable sensitivity to the antiproliferative and proapoptotic effects induced already at low concentrations of DNA methyltransferase (DNMT) inhibitors. The molecular mechanisms underlying this variable sensitivity of leukemic blasts to azanucleosides such as 5-azacytidine and 5-aza-2'-deoxycytidine (DAC) may involve modifier effects of specific fusion proteins such as AML1/ETO. The cyclin-dependent kinase inhibitor p15/INK4b is one potential target of DNA demethylating activity in AML and MDS where it is frequently silenced by hypermethylation. To study sensitivity to DAC in myeloid leukemia cells, we chose the myeloid cell lines Kasumi-1 (expressing AML1/ETO), KG-1 and KG-1a (both AML1/ETO-negative) all of which a highly methylated p15/INK4b gene. Treatment with DAC resulted in dose-dependent regional demethylation of p15/INK4b in Kasumi-1 and KG-1, but only to a modest degree in KG-1a cells. Demethylation was associated with induction of p15/INK4b protein expression. Growth-inhibitory and proapoptotic activity of DAC was significantly higher in Kasumi-1 than in KG-1a cells, and sensitization of cells to a cooperating effect of All-trans retinoic acid and of the histone deacetylase (HDAC) inhibitor Trichostatin A was observed. DAC-induced growth inhibition and apoptosis were enhanced when AML1/ETO was conditionally expressed in AML1/ETO-negative U-937 cells. In conclusion, hypomethylation and reactivation of p15/INK4b in myeloid cell lines are among the molecular events associated with DAC-induced growth arrest and apoptosis. Further studies of AML1/ETO as a modifier of the epigenotype and sensitivity of myeloid cells to inhibitors of DNMTs and HDACs appear warranted.

In vivo effects of decitabine in myelodysplasia and acute myeloid leukemia: review of cytogenetic and molecular studies

Low-dose demethylating agents such as 5-aza-2'-deoxycytidine (decitabine, DAC) and 5-azacytidine (azacitidine, Vidaza) have been explored for the treatment of myelodysplasia, acute myeloid leukemia, and hemoglobinopathies since the early 1980s, aiming to revert a methylator phenotype. Originally, the treatment rationale in hemoglobinopathies was to achieve demethylation of the hypermethylated and hence silent gamma-globin gene locus, thus reactivating synthesis of hemoglobin F (HbF). In myelodysplastic syndrome (MDS), cytogenetic analyses are mandatory for risk stratification and for monitoring response to drug treatment. The current knowledge regarding cytogenetic subgroups as predictors of response to low-dose decitabine in MDS as well as cytogenetic responses caused by demethylating agents is summarized in this review. Decitabine treatment is associated with a response rate that is higher in patients with high-risk cytogenetics (i.e., complex karyotype and/or abnormalities of chromosome 7) than in patients with intermediate-risk cytogenetics (two abnormalities or single abnormalities excluding 5q-, 20q-, and -Y). Following decitabine treatment of patients with abnormal karyotype, approximately one-third achieve a major cytogenetic response that can be confirmed by FISH analyses, while in two-thirds of patients, the abnormal karyotype persists but hematologic improvement may be observed during continued treatment. The most frequently studied gene in myelodysplasia is the cell cycle regulator p15(INK4b). Hypermethylation of p15(INK4b) in MDS is reversed during treatment with decitabine, resulting in reactivation of this gene. In hemoglobinopathies, treatment with demethylating agents leads to reactivation of fetal HbF (the gamma-globin gene locus also possibly being another target for reactivation in MDS), and thus, HbF may potentially act as surrogate marker for activity of decitabine. Other, thus far unidentified hypermethylated genes may also be targets for demethylating agents.

Mutations in the receptor tyrosine kinase pathway are associated with clinical outcome in patients with acute myeloblastic leukemia harboring t(8;21)(q22;q22)

AML1-MTG8 generated by t(8;21) contributes to leukemic transformation, but additional events are required for full leukemogenesis. We examined whether mutations in the receptor tyrosine kinase (RTK) pathway could be the genetic events that cause acute myeloblastic leukemia (AML) harboring t(8;21). Mutations in the second tyrosine kinase domain, juxtamembrane (JM) domain and exon 8 of the C-KIT gene were observed in 10, one and three of 37 AML patients with t(8;21), respectively. Three patients showed an internal tandem duplication in the JM domain of the FLT3 gene. One patient had a mutation in the K-Ras gene at codon 12. As the occurrence of these mutations was mutually exclusive, a total of 18 (49%) patients showed mutations in the RTK pathway. These results suggest that activating mutations in the RTK pathway play a role in part as an additional event leading to the development of t(8;21) AML. The 6-year cumulative incidence of relapse in patients with RTK pathway mutations was 79.8%, compared with 13.5% in patients lacking such mutations (P=0.0029). Furthermore, the 6-year relapse-free survival in patients with mutations was 18% compared to 60% in those without mutations (P=0.0340), indicating that RTK mutations are associated with the clinical outcome in t(8;21) AML.

p15 mRNA expression detected by real-time quantitative reverse transcriptase-polymerase chain reaction correlates with the methylation density of the gene in adult acute leukemia

Cyclin-dependent kinase inhibitor p15 is frequently inactivated by either methylation or deletion in patients with acute leukemia. To examine pathologic and clinical significance of the p15 gene inactivation, we established a quantitative assay of p15 mRNA expression in the bone marrow cells by real-time quantitative reverse transcriptase-polymerase chain reaction. p15 mRNA expression in 14 patients with precursor B-cell acute lymphoblastic leukemia (PBC-ALL) well correlated with status of deletion and methylation in the p15 gene analyzed by Southern blotting. Furthermore, two patients with PBC-ALL and 11 acute myeloblastic leukemia (AML) were quantitatively examined for p15 gene methylation using bisulfite genomic sequencing. The data showed that p15 mRNA expression significantly correlated with the CpG island methylation density. Among 108 AML patients, p15 mRNA expression was significantly lower in the myeloid lineage (M1, M2, M3) than the monocytic lineage (M4, M5) (P = 0.0019). Above all, the majority of M3 patients showed low p15 expression compared with M1 and M2 patients (P = 0.029). These observations suggest that quantitative analysis of p15 mRNA will be useful to evaluate transcriptional repression of the p15 gene caused by various degrees of methylation.