Avian erythroleukemia: a model for corepressor function in cancer

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.

A conserved lysine in the thyroid hormone receptor-alpha1 DNA-binding domain, mutated in hepatocellular carcinoma, serves as a sensor for transcriptional regulation

Nuclear receptors are hormone-regulated transcription factors that play key roles in normal physiology and development; conversely, mutant nuclear receptors are associated with a wide variety of neoplastic and endocrine disorders. Typically, these receptor mutants function as dominant negatives and can interfere with wild-type receptor activity. Dominant-negative thyroid hormone receptor (TR) mutations have been identified in over 60% of the human hepatocellular carcinomas analyzed. Most of these mutant TRs are defective for corepressor release or coactivator binding in vitro, accounting for their transcriptional defects in vivo. However, two HCC-TR mutants that function as dominant-negative receptors in cells display near-normal properties in vitro, raising questions about the molecular basis behind their transcriptional defects. We report here that a single amino acid substitution, located at the same position in the DNA-binding domain of both mutants, is responsible for their impaired transcriptional activation and dominant-negative properties. Significantly, this amino acid, K74 in TRalpha, is highly conserved in all known nuclear receptors and seems to function as an allosteric sensor that regulates the transcriptional activity of these receptors in response to binding to their DNA recognition sequences. We provide evidence that these two human hepatocellular carcinoma mutants have acquired dominant-negative function as a result of disruption of this allosteric sensing. Our results suggest a novel mechanism by which nuclear receptors can acquire transcriptional defects and contribute to neoplastic disease.

Thyroid hormone receptor mutants implicated in human hepatocellular carcinoma display an altered target gene repertoire

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that control multiple aspects of normal physiology and development. Mutations in TRs have been identified at high frequency in certain cancers, including human hepatocellular carcinomas (HCCs). The majority of HCC-TR mutants bear lesions within their DNA recognition domains, and we have hypothesized that these lesions change the mutant receptors' target gene repertoire in a way crucial to their function as oncoproteins. Using stable cell transformants and expression array analysis, we determined that mutant TRs isolated from two different HCCs do, as hypothesized, display a target gene repertoire distinct from that of their normal TR progenitors. Only a subset of genes regulated by wild-type TRs was regulated by the corresponding HCC-TR mutants. More surprisingly, the HCC-TR mutants also gained the ability to regulate additional target genes not recognized by the wild-type receptors, and were not simply restricted to repression, but could also activate a subset of their target genes. We conclude that the TR mutants isolated from HCC have sustained multiple alterations from their normal progenitors that include not only changes in their transcriptional outputs, but also changes in the genes they target; both are likely to contribute to neoplasia.

Thyroid receptor: roles in cancer

The thyroid hormone receptors, encoded by the TRalpha and TRbeta genes, are ligand-dependent transcription factors that belong to the nuclear receptor superfamily. In addition to the role of these receptors in growth, development and metabolism, there is increasing evidence that they also inhibit transformation and act as tumor suppressors. Aberrant TR action, as well as receptor silencing, are common events in human cancer, and TRs also have an important role in tumor progression in experimental animal models, suggesting that these receptors constitute a novel therapeutic target in cancer. This review highlights recent studies on mechanisms by which loss of expression and/or function of these receptors results in a selective advantage for cellular transformation, tumor development and metastatic growth.

Thyroid hormone receptor mutations found in renal clear cell carcinomas alter corepressor release and reveal helix 12 as key determinant of corepressor specificity

Thyroid hormone receptors (TRs) regulate multiple normal physiological and developmental pathways, whereas mutations in TRs can result in endocrine and neoplastic disease. A particularly high rate of TR mutations has been found in human renal clear cell carcinomas (RCCCs). We report here that the majority of these RCCC TR mutants tested are defective for transcriptional activation and behave as dominant-negative inhibitors of wild-type receptor function. Although several of the dominant-negative RCCC TR mutants are impaired for hormone binding, all fail to release from corepressors appropriately in response to T(3), a trait that closely correlates with their defective transcriptional properties. Notably, many of these mutants exhibit additional changes in their specificity for different corepressor splice forms that may further contribute to the disease phenotype. Mapping of the relevant mutations reveals that the C-terminal receptor helix 12 is not simply a hormone-operated switch that either permits or prevents all corepressor binding, but is instead a selective gatekeeper that actively discriminates between different forms of corepressor even in the absence of T(3).

Association of v-ErbA with Smad4 disrupts TGF-beta signaling

Disruption of the transforming growth factor-beta (TGF-beta) pathway is observed in the majority of cancers. To further understand TGF-beta pathway inactivation in cancer, we stably expressed the v-ErbA oncoprotein in TGF-beta responsive cells. v-ErbA participates in erythroleukemic transformation of cells induced by the avian erythroblastosis virus (AEV). Here we demonstrate that expression of v-ErbA was sufficient to antagonize TGF-beta-induced cell growth inhibition and that dysregulation of TGF-beta signaling required that v-ErbA associate with the Smad4 which sequesters Smad4 in the cytoplasm. We also show that AEV-transformed erythroleukemia cells were resistant to TGF-beta-induced growth inhibition and that TGF-beta sensitivity could be recovered by reducing v-ErbA expression. Our results reveal a novel mechanism for oncogenic disruption of TGF-beta signaling and provide a mechanistic explanation of v-ErbA activity in AEV-induced erythroleukemia.

Modulation of expression of RA-regulated genes by the oncoprotein v-erbA

Retinoic acid (RA) modulates the expression of genes involved in embryogenesis, development and differentiation processes in vertebrates. The v-erbA oncogene is known to exert a dominant-negative effect on the expression of RA-responsive genes. v-erbA belongs to a superfamily of transcription factors called nuclear receptors, which includes the retinoic acid receptors (RARs) responsible for mediating the effects of retinoic acid. While RA inhibits cell proliferation and promotes cell differentiation and apoptosis in a variety of tissues, v-erbA seems to play a role in oncogenesis, namely in the development of hepatocellular carcinoma (HCC) in a transgenic mouse model. In order to study the effect of v-erbA on RA-responsive genes, we used microarray analysis to identify genes differentially expressed in murine hepatocytes in culture (AML12 cells) stably transfected with v-erbA and exposed to RA for 3 h or 24 h. We have identified RA-responsive genes that are affected by v-erbA, as well as genes that are regulated by v-erbA alone. We have found that v-erbA can affect gene expression in the presence of RA and at the level of basal transcription. We have also identified a number of v-erbA-responsive genes that are known to be involved in carcinogenesis and which may play a role in the development of HCC.

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.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

The telomeric protein Rap1 is conserved in vertebrates and is expressed from a bidirectional promoter positioned between the Rap1 and KARS genes

We have identified the chicken homolog of the mammalian telomere protein repression and activation protein 1 (Rap1). Although cRap1 has only 36% sequence identity to hRap1, it contains the same conserved BRCA1 C-terminal (BRCT), Myb and Rap C-terminus (RCT) domains. Two-hybrid analysis and immunolocalization experiments revealed that cRap1 interacts with the telomere-binding protein telomeric repeat binding factor (TRF)2 and localizes to telomeres. Thus, despite considerable sequence divergence, the identity and overall domain structure of telomere-associated proteins is conserved in vertebrates. Analysis of the cRap1 genomic locus revealed that the cRap1 gene lies immediately adjacent to the cKARS (lysyl-tRNA synthetase) gene with the two genes in a head-to-head orientation separated by only 57 nt. This same organization is conserved at the human Rap1-KARS locus. When 5' regions of the cRap1 and cKARS genes were tested for promoter activity, the promoters of both genes were found to lie in or near the intergenic spacer. The two promoters lack TATA boxes but appear to have downstream promoter elements (DPEs). Analysis of human Rap1 and KARS expressed sequence tags (ESTs) indicated that this localization of TATA-less promoters to the intergenic spacer is a conserved feature of the Rap1-KARS locus.

Sequences required for the transition from monomeric to homodimeric forms of thyroid hormone receptor alpha and v-erbA

Thyroid hormone receptor alpha (TRalpha) and the oncoprotein v-erbA (a mutated form of TRalpha incapable of binding T3) bind as heterodimers with retinoid X receptor (RXR) to DNA sequences with different orientations of AGGTCA half sites. v-erbA can also form homodimers, whereas, TRalpha1 homodimerizes poorly. Therefore, in order to obtain a better understanding for the distinct homodimerization properties between TRalpha1 and v-erbA, we created chimeras between these two receptors and tested their abilities to homodimerize on direct and everted repeats (DRs, ERs). We found that the enhanced homodimerization properties of v-erbA compared to TRalpha1 map to isoleucine at position 339 in conjunction with serine at position 351 and alanine at position 358. Our data indicate that the methyl group in isoleucine at position 339 plays an important role in v-erbA homodimerization, particularly on ER 6. Functional studies with I339V+S351P+A358T, a v-erbA mutant unable to homodimerize but still able to heterodimerize with RXR on ERs and DRs, indicate that v-erbA-RXR heterodimers mediate the dominant negative activity of v-erbA on DRs. However, the repressor activity of this mutant is weaker than that of the wild type v-erbA on ERs, suggesting that v-erbA homodimers rather than v-erbA-RXR heterodimers mediate the potent dominant negative activity of v-erbA on ERs.

In vivo repression of an erythroid-specific gene by distinct corepressor complexes

To assess the mechanisms of repression of the erythroid-specific carbonic anhydrase II (CAII) locus we used chromatin immunoprecipitation and show that an NCoR-histone deacetylase (HDAC)3 complex is recruited by the nuclear receptor v-ErbA to the intronic HS2 enhancer turning it into a potent silencer. Furthermore we demonstrate that efficient CAII silencing requires binding of a MeCP2-targeted HDAC-containing corepressor complex to the hypermethylated CpG-island at the promoter. Activation of transcription by either AZAdC or thyroid hormone results in loss of one of the two corepressor complexes. Thyroid hormone further replaces the enhancer-bound NCoR-corepressor complex by the TRAP220 coactivator. Treatment with the HDAC inhibitor trichostatin A (TSA) causes activation of CAII transcription and histone H3 and H4 hyperacetylation at the enhancer, apparently without affecting binding of the two corepressor complexes. Unexpectedly, histone H3 and H4 at the fully repressed promoter are already hyperacetylated despite the close apposition of the MeCP2-targeted HDAC complex. Acetylation of histone H4, but not H3, at the promoter is moderately increased following TSA treatment. Our data suggest that the hyperacetylated but repressed CAII promoter is (partially) remodeled and primed for activation in v-ErbA-transformed cells.

Critical residues within the BTB domain of PLZF and Bcl-6 modulate interaction with corepressors

The PLZF (promyelocytic leukemia zinc finger) transcriptional repressor, when fused to retinoic acid receptor alpha (RARalpha), causes a refractory form of acute promyelocytic leukemia. The highly conserved N-terminal BTB (bric a brac, tramtrack, broad complex)/POZ domain of PLZF plays a critical role in this disease, since it is required for transcriptional repression by the PLZF-RARalpha fusion protein. The crystal structure of the PLZF BTB domain revealed an obligate homodimer with a highly conserved charged pocket formed by apposition of the two monomers. An extensive structure-function analysis showed that the charged pocket motif plays a major role in transcriptional repression by PLZF. We found that mutations of the BTB domain that neutralize key charged pocket residues did not disrupt dimerization, yet abrogated the ability of PLZF to repress transcription and led to the loss of interaction with N-CoR, SMRT, and histone deacetylases (HDACs). We extended these studies to the Bcl-6 protein, which is linked to the pathogenesis of non-Hodgkin's lymphomas. In this case, neutralizing the charged pocket also resulted in loss of repression and corepressor binding. Experiments with purified protein showed that corepressor-BTB interactions were direct. A comparison of the PLZF, Bcl-6, and the FAZF (Fanconi anemia zinc finger)/ROG protein shows that variations in the BTB pocket result in differential affinity for corepressors, which predicts the potency of transcriptional repression. Thus, the BTB pocket represents a molecular structure involved in recruitment of transcriptional repression complexes to target promoters.

Histone deacetylase inhibitors induce remission in transgenic models of therapy-resistant acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is associated with chromosomal translocations, invariably involving the retinoic acid receptor alpha (RAR alpha) gene fused to one of several distinct loci, including the PML or PLZF genes, involved in t(15;17) or t(11;17), respectively. Patients with t(15;17) APL respond well to retinoic acid (RA) and other treatments, whereas those with t(11;17) APL do not. The PML-RAR alpha and PLZF-RAR alpha fusion oncoproteins function as aberrant transcriptional repressors, in part by recruiting nuclear receptor-transcriptional corepressors and histone deacetylases (HDACs). Transgenic mice harboring the RAR alpha fusion genes develop forms of leukemia that faithfully recapitulate both the clinical features and the response to RA observed in humans with the corresponding translocations. Here, we investigated the effects of HDAC inhibitors (HDACIs) in vitro and in these animal models. In cells from PLZF-RAR alpha/RAR alpha-PLZF transgenic mice and cells harboring t(15;17), HDACIs induced apoptosis and dramatic growth inhibition, effects that could be potentiated by RA. HDACIs also increased RA-induced differentiation. HDACIs, but not RA, induced accumulation of acetylated histones. Using microarray analysis, we identified genes induced by RA, HDACIs, or both together. In combination with RA, all HDACIs tested overcame the transcriptional repression exerted by the RAR alpha fusion oncoproteins. In vivo, HDACIs induced accumulation of acetylated histones in target organs. Strikingly, this combination of agents induced leukemia remission and prolonged survival, without apparent toxic side effects.