Cellular stress triggers TEL nuclear export via two genetically separable pathways

Germline ETV6 mutation promotes inflammation and disrupts lymphoid development of early hematopoietic progenitors

Germline mutations in ETV6 are associated with a syndrome of thrombocytopenia and leukemia predisposition, and ETV6 is among the most commonly mutated genes in leukemias, especially childhood B-cell acute lymphoblastic leukemia. However, the mechanisms underlying disease caused by ETV6 dysfunction are poorly understood. To address these gaps in knowledge, using CRISPR/Cas9, we developed a mouse model of the most common recurrent, disease-causing germline mutation in ETV6. We found defects in hematopoiesis related primarily to abnormalities of the multipotent progenitor population 4 (MPP4) subset of hematopoietic progenitor cells and evidence of sterile inflammation. Expression of ETV6 in Ba/F3 cells altered the expression of several cytokines, some of which were also detected at higher levels in the bone marrow of the mice with Etv6 mutation. Among these, interleukin-18 and interleukin-13 abrogated B-cell development of sorted MPP4 cells, but not common lymphoid progenitors, suggesting that inflammation contributes to abnormal hematopoiesis by impairing lymphoid development. These data, along with those from humans, support a model in which ETV6 dysfunction promotes inflammation, which adversely affects thrombopoiesis and promotes leukemogenesis.

Antileukemic effects of topoisomerase I inhibitors mediated by de-SUMOylase SENP1

SUMO-specific proteases (SENPs) play pivotal roles in maintaining the balance of SUMOylation/de-SUMOylation and in SUMO recycling. Deregulation of SENPs leads to cellular dysfunction and corresponding diseases. As a key member of the SENP family, SENP1 is highly correlated with various cancers. However, the potential role of SENP1 in leukemia, especially in acute lymphoblastic leukemia (ALL), is not clear. This study shows that ALL cells knocking down SENP1 display compromised growth rather than significant alterations in chemosensitivity, although ALL relapse samples have a relatively higher expression of SENP1 than the paired diagnosis samples. Camptothecin derivatives 7-ethylcamptothecin (7E-CPT, a monomer compound) and topotecan (TPT, an approved clinical drug) induce specific SENP1 reduction and severe apoptosis of ALL cells, showing strong anticancer effects against ALL. Conversely, SENP1 could attenuate this inhibitory effect by targeting DNA topoisomerase I (TOP1) for de-SUMOylation, indicating that specific reduction in SENP1 induced by 7E-CPT and/or topotecan inhibits the proliferation of ALL cells.

Novel insights into the impact of the SUMOylation pathway in hematological malignancies (Review)

The small ubiquitin-like modifier (SUMO) system serves an important role in the regulation of protein stability and function. SUMOylation sustains the homeostatic equilibrium of protein function in normal tissues and numerous types of tumor. Accumulating evidence has revealed that SUMO enzymes participate in carcinogenesis via a series of complex cellular or extracellular processes. The present review outlines the physiological characteristics of the SUMOylation pathway and provides examples of SUMOylation participation in different cancer types, including in hematological malignancies (leukemia, lymphoma and myeloma). It has been indicated that the SUMO pathway may influence chromosomal instability, cell cycle progression, apoptosis and chemical drug resistance. The present review also discussed the possible relationship between SUMOylation and carcinogenic mechanisms, and evaluated their potential as biomarkers and therapeutic targets in the diagnosis and treatment of hematological malignancies. Developing and investigating inhibitors of SUMO conjugation in the future may offer promising potential as novel therapeutic strategies.

The important roles of protein SUMOylation in the occurrence and development of leukemia and clinical implications

Leukemia is a severe malignancy of the hematopoietic system, which is characterized by uncontrolled proliferation and dedifferentiation of immature hematopoietic precursor cells in the lymphatic system and bone marrow. Leukemia is caused by alterations of the genetic and epigenetic regulation of processes underlying hematologic malignancies, including SUMO modification (SUMOylation). Small ubiquitin-like modifier (SUMO) proteins covalently or noncovalently conjugate and modify a large number of target proteins via lysine residues. SUMOylation is a small ubiquitin-like modification that is catalyzed by the SUMO-specific activating enzyme E1, the binding enzyme E2, and the ligating enzyme E3. SUMO is covalently linked to substrate proteins to regulate the cellular localization of target proteins and the interaction of target proteins with other biological macromolecules. SUMOylation has emerged as a critical regulatory mechanism for subcellular localization, protein stability, protein-protein interactions, and biological function and thus regulates normal life activities. If the SUMOylation process of proteins is affected, it will cause a cellular reaction and ultimately lead to various diseases, including leukemia. There is growing evidence showing that a large number of proteins are SUMOylated and that SUMOylated proteins play an important role in the occurrence and development of various types of leukemia. Targeting the SUMOylation of proteins alone or in combination with current treatments might provide powerful targeted therapeutic strategies for the clinical treatment of leukemia.

Functional characterization of a germline ETV6 variant associated with inherited thrombocytopenia, acute lymphoblastic leukemia, and salivary gland carcinoma in childhood

Germline pathogenic ETV6 variants have been discovered in families with inherited thrombocytopenia and predisposition to hematological and solid malignancies. We present a patient with short stature who was initially diagnosed with chronic immune thrombocytopenia. Subsequently, the patient developed acute lymphoblastic leukemia, followed by mammary analog secretory carcinoma. Sequencing analysis identified an ETV6 c.641C > T (p.Pro214Leu) germline variant. The variant protein exhibited attenuated nuclear localization, increased protein degradation, and reduced transcription repression function. Our findings suggest that the ETV6 gene should be sequenced in patients with inherited thrombocytopenia and malignancy, and emphasize the importance of careful follow-up to identify secondary cancer in patients with pathogenic ETV6 variants.

ETV6-related thrombocytopenia and leukemia predisposition

Germ line mutations in ETV6 are responsible for a familial thrombocytopenia and leukemia predisposition syndrome. Thrombocytopenia is almost completely penetrant and is usually mild. Leukemia is reported in ∼30% of carriers and is most often B-cell acute lymphoblastic leukemia. The mechanisms by which ETV6 dysfunction promotes thrombocytopenia and leukemia remain unclear. Care for individuals with ETV6-related thrombocytopenia and leukemia predisposition includes genetic counseling, treatment or prevention of excessive bleeding and surveillance for the development of hematologic malignancy.

ETV6 and ETV7: Siblings in hematopoiesis and its disruption in disease

ETV6 (TEL1) and ETV7 (TEL2) are closely-related members of the ETS family of transcriptional regulators. Both ETV6 and ETV7 have been demonstrated to play key roles in hematopoiesis, particularly with regard to maintenance of hematopoietic stem cells and control of lineage-specific differentiation, with evidence of functional interactions between both proteins. ETV6 has been strongly implicated in the molecular etiology of a number of hematopoietic diseases, including as a tumor suppressor, an oncogenic fusion partner, and an important regulator of thrombopoiesis, but recent evidence has also identified ETV7 as a potential oncogene in certain malignancies. This review provides an overview of ETV6 and ETV7 and their contribution to both normal and disrupted hematopoiesis. It also highlights the key clinical implications of the growing knowledge base regarding ETV6 abnormalities with respect to prognosis and treatment.

Functional heterogeneity of PAX5 chimeras reveals insight for leukemia development

PAX5, a transcription factor pivotal for B-cell commitment and maintenance, is one of the most frequent targets of somatic mutations in B-cell precursor acute lymphoblastic leukemia. A number of PAX5 rearrangements result in the expression of in-frame fusion genes encoding chimeric proteins, which at the N-terminus consistently retain the PAX5 DNA-binding paired domain fused to the C-terminal domains of a markedly heterogeneous group of fusion partners. PAX5 fusion proteins are thought to function as aberrant transcription factors, which antagonize wild-type PAX5 activity. To gain mechanistic insight into the role of PAX5 fusion proteins in leukemogenesis, the biochemical and functional properties of uncharacterized fusions: PAX5-DACH1, PAX5-DACH2, PAX5-ETV6, PAX5-HIPK1, and PAX5-POM121 were ascertained. Independent of the subcellular distribution of the wild-type partner proteins, ectopic expression of all PAX5 fusion proteins showed a predominant nuclear localization, and by chromatin immunoprecipitation all of the chimeric proteins exhibited binding to endogenous PAX5 target sequences. Furthermore, consistent with the presence of potential oligomerization motifs provided by the partner proteins, the self-interaction capability of several fusion proteins was confirmed. Remarkably, a subset of the PAX5 fusion proteins conferred CD79A promoter activity; however, in contrast with wild-type PAX5, the fusion proteins were unable to induce Cd79a transcription in a murine plasmacytoma cell line. These data show that leukemia-associated PAX5 fusion proteins share some dominating characteristics such as nuclear localization and DNA binding but also show distinctive features.

IMPLICATIONS

This comparative study of multiple PAX5 fusion proteins demonstrates both common and unique properties, which likely dictate their function and impact on leukemia development.

Autoinhibition of ETV6 (TEL) DNA binding: appended helices sterically block the ETS domain

ETV6 (or TEL), a transcriptional repressor belonging to the ETS family, is frequently involved in chromosomal translocations linked with human cancers. It displays a DNA-binding mode distinct from other ETS proteins due to the presence of a self-associating PNT domain. In this study, we used NMR spectroscopy to dissect the structural and dynamic bases for the autoinhibition of ETV6 DNA binding by sequences C-terminal to its ETS domain. The C-terminal inhibitory domain (CID) contains two helices, H4 and H5, which sterically block the DNA-binding interface of the ETS domain. Importantly, these appended helices are only marginally stable as revealed by amide hydrogen exchange and (15)N relaxation measurements. The CID is thus poised to undergo a facile conformational change as required for DNA binding. The CID also dampens millisecond timescale motions of the ETS domain hypothesized to be critical for the recognition of specific ETS target sequences. This work illustrates the use of appended sequences on conserved structural domains to generate biological diversity and complements previous studies of the allosteric mechanism of ETS1 autoinhibition to reveal both common and divergent features underlying the regulation of DNA binding by ETS transcription factors.

Proteomic analysis reveals a novel mechanism induced by the leukemic oncogene Tel/PDGFRβ in stem cells: activation of the interferon response pathways

Objective proteomic analysis offers opportunities for hypothesis generation on molecular events associated with pathogenesis in stem cells. Relative quantification mass spectrometry was employed to identify pathways affected by Tel/PDGFRβ, an oncogene associated with myeloproliferative neoplasia (MPN). Its effects on over 1800 proteins were quantified with high confidence. Of those up-regulated by Tel/PDGFRβ several were involved in the interferon gamma (IFNγ) response. To validate these observations we employed embryonic and myeloid stem cells models which revealed Tel/PDGFRβ-induced STAT1 up-regulation and activation was responsible for modulating the interferon response. A STAT1 target highly up-regulated was ICSBP, a transcriptional regulator of myeloid and eosinophilic differentiation. ICSBP interacts with CBP/p300 and Ets transcription factors, to promote transcription of additional genes, including the Egr family, key regulators of myelopoiesis. These interferon responses were recapitulated using IFNγ stimulation of stem cells. Thus Tel/PDGFRβ induces aberrant IFN signaling and downstream targets, which may ultimately impact the hematopoietic transcriptional factor network to bias myelomonocytic differentiation in this MPN.

A review of post-translational modifications and subcellular localization of Ets transcription factors: possible connection with cancer and involvement in the hypoxic response

Post-translational modifications and subcellular localizations modulate transcription factors, generating a code that is deciphered into an activity. We describe our current understanding of these processes for Ets factors, which have recently been recognized for their importance in various biological processes. We present the global picture for the family, and then focus on particular aspects related to cancer and hypoxia. The analysis of Post-translational modification and cellular localization is only beginning to enter the age of "omic," high content, systems biology. Our snap-shots of particularly active fields point to the directions in which new techniques will be needed, in our search for a more complete description of regulatory pathways.

Dynamic compartmentalization of base excision repair proteins in response to nuclear and mitochondrial oxidative stress

DNAs harbored in both nuclei and mitochondria of eukaryotic cells are subject to continuous oxidative damage resulting from normal metabolic activities or environmental insults. Oxidative DNA damage is primarily reversed by the base excision repair (BER) pathway, initiated by N-glycosylase apurinic/apyrimidinic (AP) lyase proteins. To execute an appropriate repair response, BER components must be distributed to accommodate levels of genotoxic stress that may vary considerably between nuclei and mitochondria, depending on the growth state and stress environment of the cell. Numerous examples exist where cells respond to signals, resulting in relocalization of proteins involved in key biological transactions. To address whether such dynamic localization contributes to efficient organelle-specific DNA repair, we determined the intracellular localization of the Saccharomyces cerevisiae N-glycosylase/AP lyases, Ntg1 and Ntg2, in response to nuclear and mitochondrial oxidative stress. Fluorescence microscopy revealed that Ntg1 is differentially localized to nuclei and mitochondria, likely in response to the oxidative DNA damage status of the organelle. Sumoylation is associated with targeting of Ntg1 to nuclei containing oxidative DNA damage. These studies demonstrate that trafficking of DNA repair proteins to organelles containing high levels of oxidative DNA damage may be a central point for regulating BER in response to oxidative stress.