Leukemia with distinct phenotypes in transgenic mice expressing PML/RAR alpha, PLZF/RAR alpha or NPM/RAR alpha

Recurrent chromosomal translocations involving the RAR alpha locus on chromosome 17 are the hallmark of acute promyelocytic leukemia (APL). The RAR alpha gene fuses to variable partners (PML, PLZF, NPM, NuMA and STAT5B: X genes) leading to the expression of APL-specific fusion proteins with identical RAR alpha moieties. To analyse whether the variable X moiety could affect the activity of the fusion protein in vivo, we generated and characterized, on a comparative basis, NPM/RAR alpha transgenic mice (TM) in which the fusion gene is expressed under the control of a human Cathepsin G (hCG) minigene. We compared the features of the leukemia observed in these TM with those in hCG-PML/RAR alpha and hCG-PLZF/RAR alpha TM. In all three transgenic models, leukemia developed after a variably long latency, with variable penetrance. However, the three leukemias displayed distinct cytomorphological features. hCG-NPM/RAR alpha leukemic cells resembled monoblasts. This phenotype contrasts with what was observed in the hCG-PML/RAR alpha TM model in which the leukemic phase was characterized by the proliferation of promyelocytic blasts. Similarly, hCG-PLZF/RAR alpha TM displayed a different phenotype where terminally differentiated myeloid cells predominated. Importantly, the NPM/RAR alpha oncoprotein was found to localize in the nucleolus, unlike PML/RAR alpha and PLZF/RAR alpha, thus possibly interfering with the normal function of NPM. Similarly to what was observed in human APL patients, we found that NPM/RAR alpha and PML/RAR alpha, but not PLZF/RAR alpha leukemia, was responsive to all-trans retinoic acid (ATRA) or As2O3 treatments. Taken together, our results underscore the critical relevance of the X moiety in dictating the biology of the disease and the activity of the APL fusion oncoprotein.

Variations on a theme: the alternate translocations in APL

The t(15;17)(q22;q21) translocation is tightly linked to the APL phenotype, and the resultant PML-RAR fusion can be demonstrated in 98% of APL cases. Rare variant translocations have been reported, the majority of which on detailed analysis represent cryptic PML-RAR fusions. However, a handful of APL cases have been described with different genotypes. These include the t(11;17)(q23;q21) that produces the PLZF-RAR fusion, t(5;17)(q35;q21) that forms NPM-RAR, t(11;17)(q13;q21) that generates NUMA-RAR, and der(17) that creates STAT5b-RAR. In this review we will discuss these variant translocations, and discuss the insights that we have gained from their study.

The t(5;17) acute promyelocytic leukemia fusion protein NPM-RAR interacts with co-repressor and co-activator proteins and exhibits both positive and negative transcriptional properties

The t(5;17) variant of acute promyelocytic leukemia (APL) fuses the genes for nucleophosmin (NPM) and the retinoic acid receptor alpha (RARalpha). Two NPM-RAR molecules are expressed as a result of alternative RNA splicing. Both contain RARalpha sequences that encode the DNA binding, heterodimerization, and ligand activation domains of RARalpha. This study was designed to test the ability of these fusion proteins to act as transcriptional activators of retinoic acid responsive promoters. The NPM-RAR fusion proteins bind to retinoic acid response element sequences as either homodimers or as heterodimers with RXR. Transcription of retinoic acid-inducible promoters is activated by the fusion proteins in the presence of retinoic acid. The level of transactivation induced by the NPM-RAR fusions differs from the level of transactivation induced by wild-type RARalpha in both a promoter and cell specific fashion, and more closely parallels the pattern of activation of the PML-RAR fusion than wild-type RARalpha. In addition, NPM-RAR decreases basal transcription from some promoters and acts in a dominant-negative fashion when co-transfected with wild-type RARalpha. Both NPM-RAR and PML-RAR interact with the co-repressor protein SMRTe in a manner that is less sensitive than RARalpha to dissociation by retinoic acid. Retinoic acid induces binding of the co-activator protein RAC3. These data indicate that the NPM-RAR fusion proteins can modulate expression of retinoid-responsive genes in a positive or negative manner, depending on context of the promoter, and lend support to the hypothesis that aberrant transcriptional activation underlies the APL phenotype. (Blood. 2000;95:2683-2690)

Overexpression of wild-type retinoic acid receptor alpha (RARalpha) recapitulates retinoic acid-sensitive transformation of primary myeloid progenitors by acute promyelocytic leukemia RARalpha-fusion genes

Retinoic acid receptor alpha (RARalpha) is the target of several chromosomal translocations associated with acute promyelocytic leukemias (APLs). These rearrangements fuse RARalpha to different partner genes creating the chimeric proteins: PML-RARalpha, PLZF-RARalpha, and NPM-RARalpha. Although the vast majority of APLs respond to retinoic acid therapy, those associated with PLZF-RARalpha are resistant. We have used retroviruses to express PML-RARalpha, PLZF-RARalpha, NPM-RARalpha, RARalpha403 (a dominant negative mutant of RARalpha), and wild-type RARalpha in murine bone marrow progenitors and found that all of these constructs blocked differentiation and led to the immortalization of myeloid progenitors. This cellular transformation is specific to an alteration of the RARalpha pathway because overexpression of RARbeta, RARgamma, or RXRalpha did not result in similar growth perturbations. Pharmacological doses of RA induced differentiation and inhibited proliferation of cells transformed with either of the APL fusion genes, including PLZF-RARalpha, whereas physiological retinoic acid concentrations were sufficient to reverse the phenotype of cells transformed with wild-type RARalpha. The cellular responses to retinoic acid were accompanied by a sharp decrease in the amount of the RARalpha-fusion proteins expressed in the cells. Our findings suggest that the oncogenicity of RARalpha-fusion proteins results from their nature to behave as unliganded RARalpha in the presence of physiological concentrations of retinoic acid.

Inhibitors of histone deacetylase relieve ETO-mediated repression and induce differentiation of AML1-ETO leukemia cells

The (8;21) translocation, found in 12% of acute myeloid leukemia (AML), creates the chimeric fusion product, AML1-ETO. Previously, we demonstrated that the ETO moiety recruits a transcription repression complex that includes the histone deacetylase (HDAC1) enzyme. Here, we used inhibitors of HDAC1 to study the pathophysiology of AML1-ETO. Both the potent inhibitor, trichostatin (TSA), and the well-known but less specific inhibitor, phenylbutyrate (PB), could partially reverse ETO-mediated transcriptional repression. PB was also able to induce partial differentiation of the AML1-ETO cell line, Kasumi-1. With the intention of developing a clinically useful protocol, we combined PB with a number of other agents that induced differentiation and apoptosis of Kasumi-1 cells. In summary, transcriptional repression mediated by AML1-ETO appears to play a mechanistic role in the t(8;21) AML, and relief of repression using agents such as PB (alone or in combination) may prove to be therapeutically useful.

ETO, fusion partner in t(8;21) acute myeloid leukemia, represses transcription by interaction with the human N-CoR/mSin3/HDAC1 complex

The t(8;21) translocation between two genes known as AML1 and ETO is seen in approximately 12-15% of all acute myeloid leukemia (AML) and is the second-most-frequently observed nonrandom genetic alteration associated with AML. AML1 up-regulates a number of target genes critical to normal hematopoiesis, whereas the AML1/ETO fusion interferes with this trans-activation. We discovered that the fusion partner ETO binds to the human homolog of the murine nuclear receptor corepressor (N-CoR). The interaction is mediated by two unusual zinc finger motifs present at the carboxyl terminus of ETO. Human N-CoR (HuN-CoR), which we cloned and sequenced in its entirety, encodes a 2,440-amino acid polypeptide and has a central domain that binds ETO. N-CoR, mammalian Sin3 (mSin3A and B), and histone deacetylase 1 (HDAC1) form a complex that alters chromatin structure and mediates transcriptional repression by nuclear receptors and by a number of oncoregulatory proteins. We found that ETO, through its interaction with the N-CoR/mSin3/HDAC1 complex, is also a potent repressor of transcription. This observation provides a mechanism for how the AML1/ETO fusion may inhibit expression of AML1-responsive target genes and disturb normal hematopoiesis.

Isolated deep cerebral venous thrombosis treated by direct endovascular thrombolysis

BACKGROUND

Isolated thrombosis of the deep cerebral venous system is very rare and is associated with a poor prognosis. Antithrombin III (AT III) deficiency is a disorder of hypercoagulability associated with deep venous thrombosis and recurrent pulmonary emboli. We report a case of an 18-year-old man who presented with spontaneous thrombosis of the deep cerebral veins and straight dural sinus as the initial presentation of a previously undiagnosed AT III deficiency.

METHODS

The patient was managed using direct endovascular infusion of the fibrinolytic agent urokinase followed by intravenous heparin.

RESULTS

The technique was successful in establishing patency of the deep cerebral venous system. The patient experienced a good clinical outcome.

CONCLUSIONS

Direct endovascular thrombolysis is a potentially effective management strategy for isolated thrombosis of the deep cerebral venous system.

Differentiation of t(5;17) variant acute promyelocytic leukemic blasts by all-trans retinoic acid

All-trans retinoic acid (ATRA) induces differentiation of acute promyelocytic leukemic (APL) blasts from patients with t(15;17) APL. However, blasts from patients with the t(11;17) variant do not differentiate in response to ATRA. Our group has identified a variant of APL characterized by t(5;17) and expression of the NPM-RAR fusion gene product. From case reports it has been difficult to establish whether ATRA induces clinical responses in patients with this variant. In order to determine whether t(5;17) blasts differentiate with ATRA, we harvested mononuclear bone marrow cells from a patient with t(5;17) APL at time of relapse and cultured them in medium containing ATRA. Morphologic analysis of cytospins after 7 days of culture revealed that 60% of cells in the ATRA-treated culture had differentiated into mature neutrophilic forms, as opposed to less than 1% in the control culture. Seventy-three percent of cells acquired NBT positivity after exposure to ATRA, compared with 1% in the control culture. These results indicate that t(5;17) blasts retain the ability to terminally differentiate in response to retinoic acid.

The t(5;17) variant of acute promyelocytic leukemia expresses a nucleophosmin-retinoic acid receptor fusion

We have studied an acute promyelocytic leukemia (APL) patient with a variant t(5;17)(q32;q12). This translocation fuses the gene for the nucleolar phosphoprotein nucleophosmin (NPM) to the retinoic acid receptor alpha (RARA). Two alternatively spliced transcripts are expressed, which differ in 129 bases immediately upstream of the RARA sequence. The NPM sequences contained in the shorter NPM-RAR cDNA are identical to the NPM sequences contained in the NPM-ALK fusion gene expressed in t(2;5) lymphomas. The RARA sequences are the same as the RARA sequences found in the PML-RAR and PLZF-RAR fusion seen in t(15;17) and t(11;17) APL, respectively. Both NPM-RAR transcripts fuse NPM and RARA sequence in the same reading frame, to generate translation products of 57 kD and 62 kD. Both NPM-RAR proteins are expressed in the patient's leukemic cells, along with wild-type RARA derived from the uninvolved allele. In transcriptional assays using a retinoic acid response element reporter construct, both NPM-RAR fusion proteins act as retinoic acid-dependent transcriptional activators. This case defines a third class of APL rearrangements, all of which generate fusion proteins of RARA.

A non-classical translocation involving 17q12 (retinoic acid receptor alpha) in acute promyelocytic leukemia (APML) with atypical features

Acute promyelocytic leukemia (APML) almost always involves a chromosomal translocation t(15:17) that results in the fusion of the retinoic acid receptor alpha (RAR alpha) gene with a transcription factor gene called PML. Several cases of APML with t(11;17) have recently been described, involving fusion of the RAR alpha gene with a new zinc finger gene named PLZF. We report here a second non-classical translocation, t(5;17), with a rearranged RAR alpha gene in a child with APML. Based on restriction endonuclease analysis, the rearrangement of RAR alpha occurred within the second intron, the common breakpoint site for t(15;17). The leukemic cells in the bone marrow aspirate were a mixture of hypergranular and hypogranular bilobed promyelocytes. Although less than 1% abnormal promyelocytes were identified after induction therapy, cytogenetics revealed persistent t(5;17). Therefore, the child was treated with all-trans-retinoic acid (ATRA). There was no disease progression, and one marrow was interpreted as remission, with confirmation by cytogenetics which failed to reveal the translocation. However, disease reoccurred shortly after completion of ATRA. This poor response to ATRA may be an additional characteristic associated with non-classical translocations in APML. The identification of a second variant translocation involving the RAR alpha gene in APML suggests yet another RAR alpha rearrangement related to neoplastic myelopoiesis.

Patterns of infection in patients with aplastic anemia and the emergence of Aspergillus as a major cause of death

Patterns of infection were studied in 150 patients with aplastic anemia who were admitted to the Clinical Hematology Branch, National Institutes of Health, between January 1978 and December 1989 for immunosuppressive therapy. Sixty percent of the patients were males, 71% were white, their mean age was 33.6 years (median, 27.5; range, 1-75), and 83% had severe aplastic anemia. One hundred three patients developed 1 or more febrile episodes during the study period. The risk factors for developing a febrile episode included a low Absolute Neutrophil Count (ANC) and Absolute Monocyte Count (AMC) at admission and the presence of an indwelling central venous catheter (Hickman-Broviack or Port-A-Cath). A total of 289 febrile events were studied, including unexplained fever (FUO) in 89 (31%), microbiologically documented infection (MBDI) in 137 (47%), and clinically documented infection (CDI) in 63 patients (22%). Compared to documented infections (MBDI) or CDI), FUO events were associated with a higher frequency of rigors, signs and symptoms of serum sickness, and treatment regimens known to cause fevers. None of the FUO events had a fatal outcome, even if the antibiotic therapy was discontinued before day 7. Among CDI events, bacteria were the most commonly defined etiologic agent (67%), followed by fungi (23%), viruses (7%), and parasites (3%). The patterns of bacterial infections in patients with aplastic anemia were similar to those observed in patients with cancer-related neutropenia. Twenty-one patients (15%) developed invasive fungal infections (aspergillus, 11; candida, 7; and both, 3), which were fatal in 19 (90%). Fungal infections accounted for 30% of the secondary infectious events and for 55% of fatal infectious events. The only identifiable risk factors for developing a fungal infection were the degree of neutropenia and monocytopenia at initial admission or final evaluation. Invasive pulmonary aspergillosis developed despite empirical amphotericin B therapy and was associated with a high incidence of fatal pulmonary hemorrhage (10 of 13 patients [77%]). Infection was responsible for 36 (62%) of the deaths observed during the study period and hemorrhage alone for 4 (7%). However, 20 of the patients who died of infection had concomitant hemorrhage. No significant drop in ANC, AMC, or platelet count could be demonstrated during a fatal infectious event as compared to a nonfatal infectious event. Invasive fungal infections, predominantly with aspergillus and candida, emerged in our study as the major causes of mortality in patients with aplastic anemia. Without bone marrow recovery the prognosis associated with invasive mycoses was grave.

Variable pattern of jun and fos gene expression in different hematopoietic cell lines during interleukin 3-induced entry into the cell cycle

Jun (c-jun, jun-B and jun-D) and fos (c-fos, fos-B and fra) proteins dimerize to form the family of AP-1 transcriptional activators. If each dimer exhibits unique transactivating properties, then any phenotypic change should show a characteristic pattern of jun and fos expression. To test this hypothesis we have assessed jun and fos RNA expression after stimulation of the factor-dependent cell lines 32D and FDCP1. These hematopoietic progenitor lines become quiescent in G0/G1 after interleukin 3 (IL-3) deprivation, and upon stimulation synchronously enter the cell cycle. 32D cells respond to IL-3 with rapid induction of jun-B and c-fos, followed by induction of jun-D and fra-1, but no rise in c-jun expression. FDCP1 cells show a very different pattern, with induction of c-jun, jun-D and fra-1. To investigate the response of a single cell line to different physiological stimuli we used a 32D subclone engineered to respond to colony stimulating factor 1 (CSF-1). This subclone showed identical induction of jun and fos after stimulation with either CSF-1 or IL-3. The conservation of response of a single cell line, but the disparate patterns demonstrated by different cells, suggest a fundamental difference in both the regulation and function of the fos/jun complexes in these cells.

Tumour-specific inhibition of lymphoma growth by an antisense oligodeoxynucleotide

In a high proportion of Burkitt lymphomas, transcription of the c-myc gene is initiated from a cryptic promoter in the first intron, creating abnormal messenger RNA molecules in which intron sequences, normally spliced out of the nascent transcripts, persist. An antisense oligodeoxynucleotide directed against these intron sequences greatly inhibited the proliferation of Burkitt lymphoma cell lines containing the abnormal transcripts (ST486 and JD38), but not that of cell lines containing normal c-myc transcripts (KK124). Flow cytometry showed a pronounced reduction in intracellular c-myc protein levels in cell lines containing aberrant myc transcripts, but no change in other cellular proteins. Control oligonucleotide did not inhibit c-myc protein expression or growth. These experiments provide evidence that antisense oligonucleotides targeted against tumour-specific, aberrant RNA species could be effective in controlling the proliferation of tumour cells without affecting normal cells.

An oligomer complementary to c-myc mRNA inhibits proliferation of HL-60 promyelocytic cells and induces differentiation

To study the role of a nuclear proto-oncogene in the regulation of cell growth and differentiation, we inhibited HL-60 c-myc expression with a complementary antisense oligomer. This oligomer was stable in culture and entered cells, forming an intracellular duplex. Incubation of cells with the anti-myc oligomer decreased the steady-state levels of c-myc protein by 50 to 80%, whereas a control oligomer did not significantly affect the c-myc protein concentration. Direct inhibition of c-myc expression with the anti-myc oligomer was associated with a decreased cell growth rate and an induction of myeloid differentiation. Related antisense oligomers with 2- to 12-base-pair mismatches with c-myc mRNA did not influence HL-60 cells. Thus, the effects of the antisense oligomer exhibited sequence specificity, and furthermore, these effects could be reversed by hybridization competition with another complementary oligomer. Antisense inhibition of a nuclear proto-oncogene apparently bypasses cell surface events in affecting cell proliferation and differentiation.