PML-RARα co-operates with Sox4 in acute myeloid leukemia development in mice.

Acute promyelocytic leukemia is characterized by a chromosomal translocation involving the retinoic acid receptor alpha gene. To identify co-operating pathways to leukemogenesis, we crossed MRP8-PML/RARA transgenic mice with BXH-2 mice which harbor an endogenous murine leukemia virus that causes acute myeloid leukemia. Approximately half of the leukemias that arose in this cross showed features of acute promyelocytic leukemia. We identified 22 proviral insertion sites in acute promyelocytic-like leukemias and focused our analysis on insertion at Sox4, a HMG box transcription factor. Using a transplant model, co-operation between PML-RARα and Sox4 was confirmed with increased penetrance and reduced latency of disease. Interestingly, karyotypic analysis revealed cytogenetic changes suggesting that the factors combined to initiate but not complete leukemic transformation. The cooperation between these transcription factors is consistent with the paradigm of multiple routes to the disease and reinforces the concept that transcription factor networks are important therapeutic targets in myeloid leukemias.

Gain of MYC underlies recurrent trisomy of the MYC chromosome in acute promyelocytic leukemia.

Gain of chromosome 8 is the most common chromosomal gain in human acute myeloid leukemia (AML). It has been hypothesized that gain of the MYC protooncogene is of central importance in trisomy 8, but the experimental data to support this are limited and controversial. In a mouse model of promyelocytic leukemia in which the MRP8 promoter drives expression of the PML-RARA fusion gene in myeloid cells, a Myc allele is gained in approximately two-thirds of cases as a result of trisomy for mouse chromosome 15. We used this model to test the idea that MYC underlies acquisition of trisomy in AML. We used a retroviral vector to drive expression of wild-type, hypermorphic, or hypomorphic MYC in bone marrow that expressed the PML-RARA transgene. MYC retroviruses cooperated in myeloid leukemogenesis and suppressed gain of chromosome 15. When the PML-RARA transgene was expressed in a Myc haploinsufficient background, we observed selection for increased copies of the wild-type Myc allele concomitant with leukemic transformation. In addition, we found that human myeloid leukemias with trisomy 8 have increased MYC. These data show that gain of MYC can contribute to the pathogenic effect of the most common trisomy of human AML.

CCAAT/enhancer binding proteins alpha and epsilon cooperate with all-trans retinoic acid in therapy but differ in their antileukemic activities.

CCAAT/enhancer binding proteins (C/EBPs) play critical roles in myelopoiesis. Dysregulation of these proteins likely contributes to the pathogenesis of myeloid disorders characterized by a block in granulopoiesis. In one such disease, acute promyelocytic leukemia (APL), a promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) fusion protein is expressed as a result of a t(15;17) chromosomal translocation. Treatment of PML-RARalpha leukemic cells with all-trans retinoic acid (ATRA) causes them to differentiate into mature neutrophils, an effect thought to be mediated by C/EBPs. In this study, we assess the potential for cooperativity between increased C/EBP activity and ATRA therapy. We demonstrate that although both C/EBPalpha and C/EBPepsilon can significantly prolong survival in a mouse model of APL, they are not functionally equivalent in this capacity. We also show that forced expression of C/EBPalpha or C/EBPepsilon in combination with ATRA treatment has a synergistic effect on survival of leukemic mice compared with either therapy alone.

Detection of aberrant gene expression in CD34+ hematopoietic stem cells from patients with agnogenic myeloid metaplasia using oligonucleotide microarrays.

Agnogenic myeloid metaplasia (AMM) is a clonal stem cell disorder that leads to ineffective hematopoiesis, bone marrow fibrosis, and extramedullary hematopoiesis. The molecular mechanisms underlying the development of this syndrome are currently unknown. Therefore, the aim of this study was to characterize aberrant gene expression in CD34+ hematopoietic stem cells from patients with AMM. We used oligonucleotide microarrays to analyze gene expression profiles in CD34+ hematopoietic stem cells from patients with AMM compared with expression in CD34+ cells from healthy individuals. We identified 95 highly differentially expressed genes (48 upregulated and 47 down-regulated) that are potentially involved in regulating abnormal hematopoietic proliferation and differentiation and confirmed many of them by quantitative polymerase chain reaction. Using class membership prediction analysis, we identified 75 genes whose expression profiles can accurately differentiate AMM samples from the controls. Using these 75 genes, we were able to discriminate patients with AMM from the controls by hierarchical clustering (Spearman's confidence correlation). The predictive power of these genes was verified by applying the algorithm to an unknown test set containing expression data from eight additional CD34+ samples (four AMM, four control). Our results indicate that a subset of genes may be used to differentiate patients with AMM from healthy individuals. Furthermore, we identify 95 genes whose aberrant expression may be involved in AMM.

Methylation analysis of the cell cycle control genes in myelofibrosis with myeloid metaplasia.

Promoter hypermethylation represents a primary mechanism in the inactivation of tumor suppressor genes during tumorigenesis. We analyzed the promoter methylation status of eight tumor-associated genes (p14 ARF, p15 INK4B, p16 INK4A, Rb, hMLH1, hMSH2, APC, and DAPK) in 30 patients with myelofibrosis with myeloid metaplasia (MMM) by methylation specific PCR. The study showed no hypermethylation of the promoters of p16(INK4A), Rb, hMLH1, hMSH2, APC, and DAPK genes. The p14 ARF, p15 INK4B promoters were hypermethylated in only one patient each. This study indicates that, although methylation of these genes is important in other cancers, it is rare in MMM and causation of this disease should be focused elsewhere.

RARbeta2 is a candidate tumor suppressor gene in myelofibrosis with myeloid metaplasia.

Myelofibrosis with myeloid metaplasia (MMM) is a clonal stem-cell disorder that leads to ineffective hematopoiesis, bone marrow fibrosis, and extramedullary hematopoiesis. The molecular mechanisms underlying the development of this myeloproliferative syndrome are currently unknown. In order to identify tumor suppressor genes that may be involved in the disease process, we performed an analysis for loss of heterozygosity (LOH) in CD34+ cells from 29 patients with MMM. We observed a frequency of allelic loss on chromosomal arm 3p in 24% of cases. Detailed mapping of 3p revealed a distinct region of deletion at 3p24. Among the genes known to map within this region is the retinoic acid receptor-beta (RARbeta2) gene. To determine whether RARbeta2 gene activity is diminished in this disease, we analysed its expression in CD34+ cells from 17 patients with MMM using quantitative PCR. Our results indicate that expression of RARbeta2 is significantly decreased in 100% of patient samples compared to that in CD34+ cells from 10 normal individuals. Since allelic loss at 3p24 occurs in <25% of patients, we investigated the contribution of epigenetic modifications to RARbeta2 inactivity. Using methylation-specific PCR, we found hypermethylation of RARbeta2 in 16 of 18 patients (89%), while the methylated form of the gene was absent in CD34+ cells from nine normal individuals. Our results suggest that RARbeta2 acts as a tumor suppressor gene in MMM and that epigenetic changes are the most significant determinants of RARbeta2 gene activity in these patients.

Presence of the BCR-ABL mutation Glu255Lys prior to STI571 (imatinib) treatment in patients with Ph+ acute lymphoblastic leukemia.

The tyrosine kinase inhibitor STI571 (imatinib) binds competitively to the adenosine triphosphate (ATP) binding site of the ABL kinase, thereby inhibiting auto- and substrate phosphorylation of the oncogenic protein BCR-ABL and preventing the activation of downstream signaling pathways. Comparative studies on leukemic cell samples obtained from chronic myelogenous leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL) patients before and after treatment with STI571 reported point mutations in resistant samples after a short time of therapy. The aim of this study was to determine whether patients with Ph+ ALL in whom resistance developed as a consequence of the Glu255Lys mutation already harbored this subclone prior to STI571 treatment. First, the migration pattern of cDNAs from 30 bone marrow samples from patients with Ph+ ALL was analyzed by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP). Thereafter, detailed mutational analysis using genomic DNA was performed on initial STI571-naive bone marrow samples of 4 individuals with Ph+ ALL, for whom the mutation Glu255Lys in association with STI571 treatment had been shown. A 166-bp PCR fragment spanning from nucleotide (nt) 862 to nt 1027 was cloned, and 108 clones per sample were analyzed by direct sequencing. This more sensitive technique revealed the presence of the Glu255Lys mutation in 2 initial samples, one clone each. We identified for the first time the mutation Glu255Lys in STI571-naive leukemic samples of Ph+ ALL patients. The findings suggest that the mutation exists in a very small subpopulation of leukemic cells at the beginning of STI571 therapy.

Myb-binding protein 1a augments AhR-dependent gene expression.

We have studied the mechanism by which an acidic domain (amino acids 515-583) of the aromatic hydrocarbon receptor (AhR) transactivates a target gene. Studies with glutathione S-transferase fusion proteins demonstrate that the wild-type acidic domain associates in vitro with Myb-binding protein 1a, whereas a mutant domain (F542A, I569A) does not. AhR-defective cells reconstituted with an AhR containing the wild-type acidic domain exhibit normal AhR function; however, cells reconstituted with an AhR containing the mutant acidic domain do not function normally. Transient transfection of Myb-binding protein 1a into mouse hepatoma cells is associated with augmentation of AhR-dependent gene expression. Such augmentation does not occur when Myb-binding protein 1a is transfected into AhR-defective cells that have been reconstituted with an AhR that lacks the acidic domain. We infer that 1) Myb-binding protein 1a associates with AhR, thereby enhancing transactivation, and 2) the presence of AhR's acidic domain is both necessary and sufficient for Myb-binding protein 1a to increase AhR-dependent gene expression.

Expression of C/EBPbeta from the C/ebpalpha gene locus is sufficient for normal hematopoiesis in vivo.

CCAAT/enhancer-binding proteins (C/EBPs) are critical transcriptional regulators of differentiation of hematopoietic cells. Previous studies have shown that targeted disruption of the C/ebpalpha gene results in a lack of granulocytic differentiation with an arrest at the stage of immature myeloblasts. By using a gene replacement strategy in which C/EBPbeta was expressed from the C/ebpalpha gene locus of C/EBPalpha-null mice, we have evaluated the ability of C/EBPbeta to function for C/EBPalpha in directing differentiation along the granulocytic pathway. We show that the morphology and the differential cell counts of the bone marrow and peripheral blood cells from C/EBPbeta knockin mice are indistinguishable from those of their wild-type littermates, indicating that hematopoiesis occurs normally in these animals. Additionally, we analyzed expression of 21 myeloid-specific genes, including markers for distinct stages of granulocytic differentiation, and found no significant differences in their levels of expression in the bone marrow of C/EBPbeta knockin and wild-type mice. These results imply that C/EBPbeta can substitute for C/EBPalpha during hematopoiesis when expressed from the C/ebpalpha gene locus.

Ph(+) acute lymphoblastic leukemia resistant to the tyrosine kinase inhibitor STI571 has a unique BCR-ABL gene mutation.

The tyrosine kinase inhibitor STI571 is a promising agent for the treatment of advanced Philadelphia chromosome positive (Ph(+)) acute lymphoblastic leukemia (ALL), but resistance develops rapidly in most patients after an initial response. To identify mechanisms of resistance to STI571, 30 complementary DNAs (including 9 matched samples) obtained from the bone marrow of individuals with Ph(+) ALL were analyzed by direct sequencing of a 714-base pair region of ABL encoding for the adenosine triphosphate (ATP)-binding site and the kinase activation loop. A single point mutation was found at nucleotide 1127 (GI6382056) resulting in Glu255Lys. This mutation occurred in 6 of 9 patients (67%) following their treatment with STI571 but not in the samples from patients before beginning treatment with STI571. Glu255Lys is within the motif important for forming the pocket of the ATP-binding site in ABL and it is highly conserved across species. In conclusion, Ph(+) ALL samples resistant to STI571 have a unique mutation Glu255Lys of BCR-ABL.