Avian erythropoiesis and erythroleukemia: towards understanding the role of the biomolecules involved

Hi-C analysis of genomic contacts revealed karyotype abnormalities in chicken HD3 cell line

BACKGROUND

Karyotype abnormalities are frequent in immortalized continuous cell lines either transformed or derived from primary tumors. Chromosomal rearrangements can cause dramatic changes in gene expression and affect cellular phenotype and behavior during in vitro culture. Structural variations of chromosomes in many continuous mammalian cell lines are well documented, but chromosome aberrations in cell lines from other vertebrate models often remain understudied. The chicken LSCC-HD3 cell line (HD3), generated from erythroid precursors, was used as an avian model for erythroid differentiation and lineage-specific gene expression. However, karyotype abnormalities in the HD3 cell line were not assessed. In the present study, we applied high-throughput chromosome conformation capture to analyze 3D genome organization and to detect chromosome rearrangements in the HD3 cell line.

RESULTS

We obtained Hi-C maps of genomic interactions for the HD3 cell line and compared A/B compartments and topologically associating domains between HD3 and several other cell types. By analysis of contact patterns in the Hi-C maps of HD3 cells, we identified more than 25 interchromosomal translocations of regions ≥ 200 kb on both micro- and macrochromosomes. We classified most of the observed translocations as unbalanced, leading to the formation of heteromorphic chromosomes. In many cases of microchromosome rearrangements, an entire microchromosome together with other macro- and microchromosomes participated in the emergence of a derivative chromosome, resembling "chromosomal fusions'' between acrocentric microchromosomes. Intrachromosomal inversions, deletions and duplications were also detected in HD3 cells. Several of the identified simple and complex chromosomal rearrangements, such as between GGA2 and GGA1qter; GGA5, GGA4p and GGA7p; GGA4q, GGA6 and GGA19; and duplication of the sex chromosome GGAW, were confirmed by FISH.

CONCLUSIONS

In the erythroid progenitor HD3 cell line, in contrast to mature and immature erythrocytes, the genome is organized into distinct topologically associating domains. The HD3 cell line has a severely rearranged karyotype with most of the chromosomes engaged in translocations and can be used in studies of genome structure-function relationships. Hi-C proved to be a reliable tool for simultaneous assessment of the spatial genome organization and chromosomal aberrations in karyotypes of birds with a large number of microchromosomes.

Molecular Landscapes and Models of Acute Erythroleukemia

Malignancies of the erythroid lineage are rare but aggressive diseases. Notably, the first insights into their biology emerged over half a century ago from avian and murine tumor viruses-induced erythroleukemia models providing the rationale for several transgenic mouse models that unraveled the transforming potential of signaling effectors and transcription factors in the erythroid lineage. More recently, genetic roadmaps have fueled efforts to establish models that are based on the epigenomic lesions observed in patients with erythroid malignancies. These models, together with often unexpected erythroid phenotypes in genetically modified mice, provided further insights into the molecular mechanisms of disease initiation and maintenance. Here, we review how the increasing knowledge of human erythroleukemia genetics combined with those from various mouse models indicate that the pathogenesis of the disease is based on the interplay between signaling mutations, impaired TP53 function, and altered chromatin organization. These alterations lead to aberrant activity of erythroid transcriptional master regulators like GATA1, indicating that erythroleukemia will most likely require combinatorial targeting for efficient therapeutic interventions.

Thyroid hormone receptor mutations in cancer and resistance to thyroid hormone: perspective and prognosis

Thyroid hormone, operating through its receptors, plays crucial roles in the control of normal human physiology and development; deviations from the norm can give rise to disease. Clinical endocrinologists often must confront and correct the consequences of inappropriately high or low thyroid hormone synthesis. Although more rare, disruptions in thyroid hormone endocrinology due to aberrations in the receptor also have severe medical consequences. This review will focus on the afflictions that are caused by, or are closely associated with, mutated thyroid hormone receptors. These include Resistance to Thyroid Hormone Syndrome, erythroleukemia, hepatocellular carcinoma, renal clear cell carcinoma, and thyroid cancer. We will describe current views on the molecular bases of these diseases, and what distinguishes the neoplastic from the non-neoplastic. We will also touch on studies that implicate alterations in receptor expression, and thyroid hormone levels, in certain oncogenic processes.

Recruitment of the oncoprotein v-ErbA to aggresomes

Aggresome formation, a cellular response to misfolded protein aggregates, is linked to cancer and neurodegenerative disorders. Previously we showed that Gag-v-ErbA (v-ErbA), a retroviral variant of the thyroid hormone receptor (TRα1), accumulates in and sequesters TRα1 into cytoplasmic foci. Here, we show that foci represent v-ErbA targeting to aggresomes. v-ErbA colocalizes with aggresomal markers, proteasomes, hsp70, HDAC6, and mitochondria. Foci have hallmark characteristics of aggresomes: formation is microtubule-dependent, accelerated by proteasome inhibitors, and they disrupt intermediate filaments. Proteasome-mediated degradation is critical for clearance of v-ErbA and T(3)-dependent TRα1 clearance. Our studies highlight v-ErbA's complex mode of action: the oncoprotein is highly mobile and trafficks between the nucleus, cytoplasm, and aggresome, carrying out distinct activities within each compartment. Dynamic trafficking to aggresomes contributes to the dominant negative activity of v-ErbA and may be enhanced by the viral Gag sequence. These studies provide insight into novel modes of oncogenesis across multiple cellular compartments.

Hematopoietic stem cells and retroviral infection

Retroviral induced malignancies serve as ideal models to help us better understand the molecular mechanisms associated with the initiation and progression of leukemogenesis. Numerous retroviruses including AEV, FLV, M-MuLV and HTLV-1 have the ability to infect hematopoietic stem and progenitor cells, resulting in the deregulation of normal hematopoiesis and the development of leukemia/lymphoma. Research over the last few decades has elucidated similarities between retroviral-induced leukemogenesis, initiated by deregulation of innate hematopoietic stem cell traits, and the cancer stem cell hypothesis. Ongoing research in some of these models may provide a better understanding of the processes of normal hematopoiesis and cancer stem cells. Research on retroviral induced leukemias and lymphomas may identify the molecular events which trigger the initial cellular transformation and subsequent maintenance of hematologic malignancies, including the generation of cancer stem cells. This review focuses on the role of retroviral infection in hematopoietic stem cells and the initiation, maintenance and progression of hematological malignancies.

The carbonic anhydrase I locus contains a c-Myb target promoter and modulates differentiation of murine erythroleukemia cells

The Myb proto-oncogene encodes a transcription factor (c-Myb) that is essential for normal hematopoiesis and is thought to regulate hematopoietic cell proliferation and differentiation by regulating expression of specific target genes. We identify the mouse erythroid-specific carbonic anhydrase I promoter (CAIe) as a target of c-Myb activity and demonstrate that Myb activity is critical for carbonic anhydrase I (CAI) expression in C19 MEL cells. CAI expression is downregulated when MEL cells differentiate in response to MEnT or treatment with N, N-hexamethylene bisacetamide (HMBA). Coexpression of GATA-1 with c-Myb results in synergistic activation of transcription from the CAIe promoter and both transcription factors interact with the CAIe promoter in vivo. We identify a novel 20 bp sequence in the CAIe promoter that is sufficient to mediate synergistic activation of the CAIe promoter by c-Myb and GATA-1. c-Myb and GATA-1 interact with this DNA sequence suggesting that c-Myb and GATA-1 may be contained in a complex that interacts with this region of the CAIe promoter. Forced expression of CAI delayed HMBA-induced differentiation of MEL cells and maintained them in a proliferating state. These data strongly suggest that CAI is a c-Myb target and is involved in regulating MEL cell proliferation and differentiation.

Erythroid progenitor renewal versus differentiation: genetic evidence for cell autonomous, essential functions of EpoR, Stat5 and the GR

The balance between hematopoietic progenitor commitment and self-renewal versus differentiation is controlled by various transcriptional regulators cooperating with cytokine receptors. Disruption of this balance is increasingly recognized as important in the development of leukemia, by causing enhanced renewal and differentiation arrest. We studied regulation of renewal versus differentiation in primary murine erythroid progenitors that require cooperation of erythropoietin receptor (EpoR), the receptor tyrosine kinase c-Kit and a transcriptional regulator (glucocorticoid receptor; GR) for sustained renewal. However, mice defective for GR- (GR(dim/dim)), EpoR- (EpoR(H)) or STAT5ab function (Stat5ab(-/-)) show no severe erythropoiesis defects in vivo. Using primary erythroblast cultures from these mutants, we present genetic evidence that functional GR, EpoR, and Stat5 are essential for erythroblast renewal in vitro. Cells from GR(dim/dim), EpoR(H), and Stat5ab(-/-) mice showed enhanced differentiation instead of renewal, causing accumulation of mature cells and gradual proliferation arrest. Stat5ab was additionally required for Epo-induced terminal differentiation: differentiating Stat5ab(-/-) erythroblasts underwent apoptosis instead of erythrocyte maturation, due to absent induction of the antiapoptotic protein Bcl-X(L). This defect could be fully rescued by exogenous Bcl-X(L). These data suggest that signaling molecules driving leukemic proliferation may also be essential for prolonged self-renewal of normal erythroid progenitors.

Multiple mutations contribute to repression by the v-Erb A oncoprotein

The v-Erb A oncoprotein of avian erythroblastosis virus is derived from c-Erb A, a hormone-activated transcription factor. Notably, v-Erb A has sustained multiple mutations relative to c-Erb A and functions as a constitutive transcriptional repressor. We report here an analysis of the contributions of these different mutations to v-Erb A function. Our experiments demonstrate that two amino-acid differences between v-Erb A and c-Erb A, located in the 'I-box,' alter the dimerization properties of the viral protein, resulting in more stable homodimer formation, increased corepressor binding, and increased target gene repression. An additional amino-acid difference between v- and c-Erb A, located in helix 3 of the hormone binding domain, renders corepressor binding by the viral protein more resistant to release by thyroid hormone. Finally, we report that a C-terminal truncation in v-Erb A not only inhibits exchange of corepressor and coactivator, as previously noted, but also permits v-Erb A to recruit both SMRT and N-CoR corepressors, whereas c-Erb A is selective for N-CoR. The latter two mutations in v-Erb A also impair its ability to suppress c-Jun function in response to T3 hormone. We propose that the acquisition of oncogenic potential by the v-Erb A protein was a multistep process involving a series of mutations that alter the transcriptional repressive properties of the viral protein through multiple mechanisms.

Leukemic transformation of normal murine erythroid progenitors: v- and c-ErbB act through signaling pathways activated by the EpoR and c-Kit in stress erythropoiesis

Primary erythroid progenitors can be expanded by the synergistic action of erythropoietin (Epo), stem cell factor (SCF) and glucocorticoids. While Epo is required for erythropoiesis in general, glucocorticoids and SCF mainly contribute to stress erythropoiesis in hypoxic mice. This ability of normal erythroid progenitors to undergo expansion under stress conditions is targeted by the avian erythroblastosis virus (AEV), harboring the oncogenes v-ErbB and v-ErbA. We investigated the signaling pathways required for progenitor expansion under stress conditions and in leukemic transformation. Immortal strains of erythroid progenitors, able to undergo normal, terminal differentiation under appropriate conditions, were established from fetal livers of p53-/- mice. Expression and activation of the EGF-receptor (HER-1/c-ErbB) or its mutated oncogenic version (v-ErbB) in these cells abrogated the requirement for Epo and SCF in expansion of these progenitors and blocked terminal differentiation. Upon inhibition of ErbB function, differentiation into erythrocytes occurred. Signal transducing molecules important for renewal induction, i.e. Stat5- and phosphoinositide 3-kinase (PI3K), are utilized by both EpoR/c-Kit and v/c-ErbB. However, while v-ErbB transformed cells and normal progenitors depended on PI3K signaling for renewal, c-ErbB also induces progenitor expansion by PI3K-independent mechanisms.

Avian erythroleukemia: a model for corepressor function in cancer

Transcriptional regulation at the level of chromatin plays crucial roles during eukaryotic development and differentiation. A plethora of studies revealed that the acetylation status of histones is controlled by multi-protein complexes containing (de)acetylase activities. In the current model, histone deacetylases and acetyltransferases are recruited to chromatin by DNA-bound repressors and activators, respectively. Shifting the balance between deacetylation, i.e. repressive chromatin and acetylation, i.e. active chromatin can lead to aberrant gene transcription and cancer. In human acute promyelocytic leukemia (APL) and avian erythroleukemia (AEL), chromosomal translocations and/or mutations in nuclear hormone receptors, RARalpha [NR1B1] and TRalpha [NR1A1], yielded oncoproteins that deregulate transcription and alter chromatin structure. The oncogenic receptors are locked in their 'off' mode thereby constitutively repressing transcription of genes that are critical for differentiation of hematopoietic cells. AEL involves an oncogenic version of the chicken TRalpha, v-ErbA. Apart from repression by v-ErbA via recruitment of corepressor complexes, other repressors and corepressors appear to be involved in repression of v-ErbA target genes, such as carbonic anhydrase II (CAII). Reactivation of repressed genes in APL and AEL by chromatin modifying agents such as inhibitors of histone deacetylase or of methylation provides new therapeutic strategies in the treatment of acute myeloid leukemia.

The v-ErbA oncoprotein quenches the activity of an erythroid-specific enhancer

v-ErbA is a mutated variant of thyroid hormone receptor (TRalpha/NR1A1) borne by the Avian Erythroblastosis virus causing erythroleukemia. TRalpha is known to activate transcription of specific genes in the presence of its cognate ligand, T3 hormone, while in its absence it represses it. v-ErbA is unable to bind ligand, and hence is thought to contribute to leukemogenesis by actively repressing erythroid-specific genes such as the carbonic anhydrase II gene (CA II). In the prevailing model, v-ErbA occludes liganded TR from binding to its cognate elements and constitutively interacts with the corepressors NCoR/SMRT. We previously identified a v-ErbA responsive element (VRE) within a DNase I hypersensitive region (HS2) located in the second intron of the CA II gene. We now show that HS2 fulfils all the requirements for a genuine enhancer that functions independent of its orientation and position with a profound erythroid-specific activity in normal erythroid progenitors (T2ECs) and in leukemic erythroid cell lines. We find that the HS2 enhancer activity is governed by two adjacent GATA-factor binding sites. v-ErbA as well as unliganded TR prevent HS2 activity by nullifying the positive function of factors bound to GATA-sites. However, v-ErbA, in contrast to TR, does not convey active repression to silence the transcriptional activity intrinsic to a heterologous tk promoter. We propose that depending on the sequence and context of the binding site, v-ErbA contributes to leukemogenesis by occluding liganded TR as well as unliganded TR thereby preventing activation or repression, respectively.

Targeting of N-CoR and histone deacetylase 3 by the oncoprotein v-erbA yields a chromatin infrastructure-dependent transcriptional repression pathway

Transcriptional repression by nuclear hormone receptors is thought to result from a unison of targeting chromatin modification and disabling the basal transcriptional machinery. We used Xenopus oocytes to compare silencing effected by the thyroid hormone receptor (TR) and its mutated version, the oncoprotein v-ErbA, on partly and fully chromatinized TR-responsive templates in vivo. Repression by v-ErbA was not as efficient as that mediated by TR, was significantly more sensitive to histone deacetylase (HDAC) inhibitor treatment and, unlike TR, v-ErbA required mature chromatin to effect repression. We find that both v-ErbA and TR can recruit the corepressor N-CoR, but, in contrast to existing models, show a concomitant enrichment for HDAC3 that occurs without an association with Sin3, HDAC1/RPD3, Mi-2 or HDAC5. We propose a requirement for chromatin infrastructure in N-CoR/HDAC3-effected repression and suggest that the inability of v-ErbA to silence on partly chromatinized templates may stem from its impaired capacity to interfere with basal transcriptional machinery function. In support of this notion, we find v-ErbA to be less competent than TR for binding to TFIIB in vitro and in vivo.

Physical and functional interactions between the transcription factor PU.1 and the coactivator CBP

Yeast two-hybrid system was employed to isolate novel proteins that physically interact with PU.1, a member of Ets family transcription factors. Sequence analyses of several isolated clones positive for beta-galactosidase activity revealed that one of these clones was confirmed to encode a transcriptional coactivator, CREB binding protein (CBP). GST binding assay showed that the interacting sites were located at the transcriptional activation domain of PU.1 through 74-122 and the region spanning residues 1283-1915 of CBP. CBP potentiated PU.1-mediated transcription of the reporter gene driven by the multimerized PU.1-binding sites, suggesting that CBP functions as a coactivator for PU.1. Considering that CBP is a limited cellular component to function as a coactivator for several transcription factors, CBP may mediate synergistic and antagonistic interactions between PU.1 and other transcription factors during the process of hematopoietic cell differentiation.

Regulation of embryonic/fetal globin genes by nuclear hormone receptors: a novel perspective on hemoglobin switching

The CCAAT box is one of the conserved motifs found in globin promoters. It binds the CP1 protein. We noticed that the CCAAT-box region of embryonic/fetal, but not adult, globin promoters also contains one or two direct repeats of a short motif analogous to DR-1 binding sites for non-steroid nuclear hormone receptors. We show that a complex previously named NF-E3 binds to these repeats. In transgenic mice, destruction of the CCAAT motif within the human epsilon-globin promoter leads to substantial reduction in epsilon expression in embryonic erythroid cells, indicating that CP1 activates epsilon expression; in contrast, destruction of the DR-1 elements yields striking epsilon expression in definitive erythropoiesis, indicating that the NF-E3 complex acts as a developmental repressor of the epsilon gene. We also show that NF-E3 is immunologically related to COUP-TF orphan nuclear receptors. One of these, COUP-TF II, is expressed in embryonic/fetal erythroid cell lines, murine yolk sac, intra-embryonic splanchnopleura and fetal liver. In addition, the structure and abundance of NF-E3/COUP-TF complexes vary during fetal liver development. These results elucidate the structure as well as the role of NF-E3 in globin gene expression and provide evidence that nuclear hormone receptors are involved in the control of globin gene switching.

Leukemic transformation by the v-ErbA oncoprotein entails constitutive binding to and repression of an erythroid enhancer in vivo

v-ErbA, a mutated thyroid hormone receptor alpha (TRalpha), is thought to contribute to avian erythroblastosis virus (AEV)-induced leukemic transformation by constitutively repressing transcription of target genes. However, the binding of v-ErbA or any unliganded nuclear receptor to a chromatin-embedded response element as well as the role of the N-CoR-SMRT-HDAC co-repressor complex in mediating repression remain hypothetical. Here we identify a v-ErbA-response element, VRE, in an intronic DNase I hypersensitive site (HS2) of the chicken erythroid carbonic anhydrase II (CAII) gene. In vivo footprinting shows that v-ErbA is constitutively bound to this HS2-VRE in transformed, undifferentiated erythroblasts along with other transcription factors like GATA-1. Transfection assays show that the repressed HS2 region can be turned into a potent enhancer in v-ErbA-expressing cells by mutation of the VRE. Differentiation of transformed cells alleviates v-ErbA binding concomitant with activation of CAII transcription. Co-expression of a gag-TRalpha fusion protein in AEV-transformed cells and addition of ligand derepresses CAII transcription. Treatment of transformed cells with the histone deacetylase inhibitor, trichostatin A, derepresses the endogenous, chromatin-embedded CAII gene, while a transfected HS2-enhancer construct remains repressed. Taken together, our data suggest that v-ErbA prevents CAII activation by 'neutralizing' in cis the activity of erythroid transcription factors.

The thyroid hormone receptor functions as a ligand-operated developmental switch between proliferation and differentiation of erythroid progenitors

The avian erythroblastosis virus (AEV) oncoprotein v-ErbA represents a mutated, oncogenic thyroid hormone receptor alpha (c-ErbA/ TRalpha). v-ErbA cooperates with the stem cell factor-activated, endogenous receptor tyrosine kinase c-Kit to induce self-renewal and to arrest differentiation of primary avian erythroblasts, the AEV transformation target cells. In this cooperation, v-ErbA substitutes for endogenous steroid hormone receptor function required for sustained proliferation of non-transformed erythroid progenitors. In this paper, we propose a novel concept of how v-ErbA transforms erythroblasts. Using culture media strictly depleted from thyroid hormone (T3) and retinoids, the ligands for c-ErbA/TRalpha and its co-receptor RXR, we show that overexpressed, unliganded c-ErbA/ TRalpha closely resembles v-ErbA in its activity on primary erythroblasts. In cooperation with ligand-activated c-Kit, c-ErbA/ TRalpha causes steroid-independent, long-term proliferation and tightly blocks differentiation. Activation of c-ErbA/ TRalpha by physiological T3 levels causes the loss of self-renewal capacity and induces synchronous, terminal differentiation under otherwise identical conditions. This T3-induced switch in erythroid progenitor development is correlated with a decrease of c-ErbA-associated histone deacetylase activity. Our results suggest that the crucial role of the mutations activating v-erbA as an oncogene is to 'freeze' c-ErbA/ TRalpha in its non-liganded, repressive conformation and to facilitate its overexpression.