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Ikaros: a key regulator of haematopoiesis
The International Journal of Biochemistry & Cell Biology 34 (2002) 1304–1307
Ikaros: a key regulator of haematopoiesis Belinda J. Westman∗ , Joel P. Mackay, David Gell School of Molecular and Microbial Biosciences, University of Sydney, Sydney, NSW 2006, Australia Received 29 April 2002; received in revised form 3 May 2002; accepted 3 May 2002
Abstract Ikaros is an essential transcription factor for normal lymphocyte development. Because of its interaction with a number of closely related factors, Ikaros is required for correct regulation of differentiation and cell proliferation in T- and B-cell lineages. Interestingly, Ikaros appears to function both as a transcriptional repressor and as an activator through its ability to bind a large number of nuclear factors, including components of both histone deacetylase and ATP-dependent chromatin remodelling complexes. In addition, nuclear localisation is important for Ikaros function—unlike most transcription factors, Ikaros is localised to discrete nuclear foci in lymphoid cells, suggesting it employs novel mechanisms to regulate transcription. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ikaros; Haematopoiesis; Pericentromeric–heterochromatin; Transcription
1. Introduction
2. Structure
The differentiation and proliferation of pluripotent haemotopoietic stem cells to mature blood cells is tightly controlled throughout the lifetime of an organism. This control is achieved in part at the level of transcription, where a number of multiprotein complexes cooperatively regulate the expression of the appropriate genes. In 1992, the transcription factor Ikaros (also known as LyF-1), was discovered in mice [1], and later in humans [2], and it was found to be vital for normal lymphocyte development. Ikaros is highly conserved from the sea lamprey through to humans [3] and is the founding member of a family of highly homologous proteins, namely Aiolos, Helios, Eos, and Pegasus. However, this review will only focus on Ikaros and its role in regulating haematopoiesis.
The Ikaros gene normally encodes at least six alternatively-spliced isoforms (Ik 1–6; [4]), each consisting of two functionally distinct domains of Krüppel-like zinc fingers (ZnFs): an N-terminal sequence-specific DNA binding domain and a Cterminal oligomerisation domain (Fig. 1). The number of ZnFs (up to 4) in the N-terminal domain confers different DNA binding activities. Indeed, isoforms with less than three N-terminal ZnFs are unable to bind DNA with high affinity [4]. Ikaros binds to DNA sites that contain the core motif GGGA, and ZnFs 2 and 3 appear to make the critical DNA contacts. The roles of ZnFs 1 and 4 are less well understood [5]. The C-terminal ZnF domain is present and identical in all Ikaros isoforms and mediates homo- and hetero-interactions between Ikaros family proteins. This function is essential for high-affinity DNA binding of these proteins and concomitant transcriptional regulation [4]. Interestingly, it is becoming evident that these complexes may not be dimeric (in contrast
∗ Corresponding author. Tel.: +61-2-9351-4025; fax: +61-2-9351-4726. E-mail address: [email protected] (B.J. Westman).
1357-2725/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 7 - 2 7 2 5 ( 0 2 ) 0 0 0 7 0 - 5
B.J. Westman et al. / The International Journal of Biochemistry & Cell Biology 34 (2002) 1304–1307
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Fig. 1. Exon composition of Ikaros isoforms (Ik 1–6). Exons (black boxes) are numbered, ZnFs are represented by ellipsoids. DNA binding and oligomerisation domains are indicated.
to many transcription factor complexes) but rather tetrameric or even of a higher order [3].
3. Biological roles The first clues about the function of Ikaros came from its pattern of expression in mice, where it is widely distributed within the developing and adult haematopoietic systems. It is abundant in embryonic haematopoietic progenitors, developing thymocytes and mature T, B and natural killer cells and is present at lower levels in erythroid and myeloid precursors [4]. Moreover, Ikaros isoforms display different expression patterns, with Ik-1 and -2 being most abundant throughout lymphocyte development, Ik-3, -5 and -6 present at lower levels, and Ik-4 only detectable in early T-cell progenitors [5]. Mouse gene targeting studies have confirmed the central role of Ikaros in the regulation of haematopoiesis [4]. Ikaros “null” mice lack all B-cell populations and exhibit severely disrupted T-cell development. Interestingly, mouse mutants expressing only the C-terminal interaction domain of Ikaros exhibit a more severe phenotype, completely lacking all B- and T cells and dying a few weeks after birth. In these mutants, termed Ikaros “dominant negative” (IkarosDN), the C-terminal domain of Ikaros is free to bind to, and disrupt the function of, other Ikaros family members (such as Aiolos and Helios), indicat-
ing a role for such hetero-multimerisation in normal lymphocyte development [4]. Furthermore, mice heterozygous for the IkarosDN mutation develop T-cell leukemias and lymphomas, suggesting Ikaros may act as a tumour suppressor by setting a “barrier” to T-cell proliferation that must normally be overcome by signalling events in response to antigen stimulation [3]. Putative gene targets regulated by Ikaros have been identified by inspection of DNA sequences [5], or through analysis of expression patterns in gene targeting studies [3,6,7]. However, direct regulation of these genes in vivo remains to be established. Most are haematopoietic-specific, such as the TCR and CD3 genes [5], although some are expressed in other cell types, such as in the central nervous system (KOR; [8]), fetal brain (TID1; [9]) and placental trophoblasts (P-LAP; [10]). Perhaps the most intriguing feature of Ikaros is that it appears to act as both a repressor and an activator of transcription. In part, it is the array of proteins that Ikaros is able to interact with that provides this flexibility. In addition to contacting other family members, it appears to interact with the co-repressor CtBP, the viral oncoprotein E1A, the histone deacetylase repressor complexes NURD and SIN3, and the nucleosome–remodelling complex SWI/SNF [3]. Because of the nature of these binding partners, much of the influence of Ikaros upon transcription is thought to be mediated through chromatin reorganisation.
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B.J. Westman et al. / The International Journal of Biochemistry & Cell Biology 34 (2002) 1304–1307
Fig. 2. Models for Ikaros function as both a repressor and activator of transcription. (A) Ikaros (Ik, blue square) may act as a classical transcription factor by binding specific DNA sites (orange) as a dimer/multimer and recruiting repression (red octagons) or activation complexes (green circles) to target genes (yellow), (B) Ikaros could simultaneously bind target genes and pericentromeric–heterochromatin (PC–HC; black ring) and, via multimerisation, move genes to inaccessible regions, (C) Ikaros could potentiate gene expression by sequestering repressors (such as NURD) to PC–HC, thereby leaving DNA sites free for activators to bind.
Protein complexes such as NURD or SWI/SNF may simply be recruited by Ikaros bound to its cognate sites in the regulatory region of a target gene (Fig. 2A; [3]). However, it seems that sub-nuclear localisation of all the components of the system may constitute a further level of regulation. Notably, Ikaros localises to regions of pericentromeric–heterochromatin (PC–HC) in activated lymphocytes [3]. Inactivation of a number of genes containing Ikaros binding sites has been shown to coincide with relocation of these genes to Ikaros foci, suggesting Ikaros may physically recruit genes into transcriptionally-inaccessible nuclear regions (Fig. 2B; [3]). Additionally, Ikaros may directly contribute to the maintenance of heterochromatic
silencing by targeting the NURD complex via Mi-2 to PC–HC [3]. Formation of such large protein–DNA complexes may be facilitated by multimerisation of Ikaros. It has also been suggested that Ikaros may potentiate gene expression by titrating repression complexes (such as NURD) away from the promoters of target genes and into PC–HC, thereby providing access to transcriptional activators (Fig. 2C; [11]). In summary, it is likely that Ikaros utilises a combination of mechanisms to regulate transcription, dependent upon cell type and stage of development. In any case, it is clear that Ikaros has a vital and extensive role in biology, and is likely to exploit novel and interesting mechanisms to achieve its function.
B.J. Westman et al. / The International Journal of Biochemistry & Cell Biology 34 (2002) 1304–1307
4. Diseases associated with Ikaros Aberrant Ikaros expression has been linked with infant acute lymphoblastic leukemia [12], T-cell acute lymphoblastic leukemia [13] and blast crisis in chronic myelogenous leukemia [14]. In these diseases, either overexpression of dominant negative isoforms or a reduced level of Ikaros activity (as well as various insertions and deletions adjacent to the C-terminal domain) has been observed. Elucidating the molecular mechanisms of Ikaros function may lead to the development of agents capable of returning control over haematopoiesis to patients were this has been lost.
Acknowledgements This work is supported by a research grant to JPM from the University of Sydney Cancer Research Fund. DAG is a Wellcome International Prize Travelling Research Fellow and BJW is supported by an Australian Postgraduate Award. References [1] K. Georgopoulos, D.D. Moore, B. Derfler, Ikaros: an early lymphoid-specific transcription factor and a putative mediator for T-cell commitment, Science 258 (5083) (1992) 808– 812. [2] W. Nietfeld, A. Meyerhans, Cloning and sequencing of hIk-1, a cDNA encoding a human homologue of mouse Ikaros/ LyF-1, Immunol. Lett. 49 (1/2) (1996) 139–141. [3] K. Georgopoulos, Haematopoietic cell-fate decisions, chromatin regulation and Ikaros, Nat. Rev. Immunol. 2 (3) (2002) 162–174. [4] K. Georgopoulos, S. Winandy, N. Avitahl, The role of the Ikaros gene in lymphocyte development and homeostasis, Annu. Rev. Immunol. 15 (1997) 155–176.
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[5] A. Molnar, K. Georgopoulos, The Ikaros gene encodes a family of functionally diverse zinc finger DNA-binding proteins, Mol. Cell Biol. 14 (12) (1994) 8292–8303. [6] R.A. Lopez, S. Schoetz, K. DeAngelis, D. O’Neill, A. Bank, Multiple hematopoietic defects and delayed globin switching in Ikaros null mice, Proc. Natl. Acad. Sci. U.S.A. 99 (2) (2002) 602–607. [7] I. Christopherson, M. Piechoki, G. Liu, S. Ratner, A. Galy, Regulation of L-selectin expression by a dominant negative Ikaros protein, J. Leukoc. Biol. 69 (4) (2001) 675–683. [8] X.L. Hu, J. Bi, H.H. Loh, L.N. Wei, An intronic Ikarosbinding element mediates retinoic acid suppression of the kappa opioid receptor gene, accompanied by histone deacetylation on the promoters, J. Biol. Chem. 276 (7) (2001) 4597– 4603. [9] X. Yin, M. Rozakis-Adcock, Genomic organization and expression of the human tumorous imaginal disc (TID1) gene, Gene 278 (1/2) (2001) 201–210. [10] T. Ito, S. Nomura, M. Okada, Y. Katsumata, F. Kikkawa, T. Rogi, M. Tsujimoto, S. Mizutani, AP-2 and Ikaros regulate transcription of human placental leucine aminopeptidase/ oxytocinase gene, Biochem. Biophys. Res. Commun. 290 (3) (2002) 1048–1053. [11] J. Koipally, E.J. Heller, J.R. Seavitt, K. Georgopoulos, Unconventional potentiation of gene expression by Ikaros, J. Biol. Chem. 277 (15) (2002) 13007–13015. [12] L. Sun, N. Heerema, L. Crotty, X. Wu, C. Navara, A. Vassilev, M. Sensel, G.H. Reaman, F.M. Uckun, Expression of dominant-negative and mutant isoforms of the antileukemic transcription factor Ikaros in infant acute lymphoblastic leukemia, Proc. Natl. Acad. Sci. U.S.A. 96 (2) (1999) 680–685. [13] L. Sun, M.L. Crotty, M. Sensel, H. Sather, C. Navara, J. Nachman, P.G. Steinherz, P.S. Gaynon, N. Seibel, C. Mao, A. Vassilev, G.H. Reaman, F.M. Uckun, Expression of dominantnegative Ikaros isoforms in T-cell acute lymphoblastic leukemia, Clin. Cancer Res. 5 (1999) 2112–2120. [14] H. Nakayama, F. Ishimaru, N. Avitahl, N. Sezaki, N. Fujii, K. Nakase, Y. Ninomiya, A. Harashima, J. Minowada, J. Tsuchiyama, K. Imajoh, T. Tsubota, S. Fukuda, T. Sezaki, K. Kojima, M. Hara, H. Takimoto, S. Yorimitsu, I. Takahashi, A. Miyata, S. Taniguchi, Y. Tokunaga, H. Gondo, Y. Niho, M. Harada, Decreases in Ikaros activity correlate with blast crisis in patients with chronic myelogenous leukemia, Cancer Res. 59 (16) (1999) 3931–3934.
Ikaros: a key regulator of haematopoiesis Belinda J. Westman∗ , Joel P. Mackay, David Gell School of Molecular and Microbial Biosciences, University of Sydney, Sydney, NSW 2006, Australia Received 29 April 2002; received in revised form 3 May 2002; accepted 3 May 2002
Abstract Ikaros is an essential transcription factor for normal lymphocyte development. Because of its interaction with a number of closely related factors, Ikaros is required for correct regulation of differentiation and cell proliferation in T- and B-cell lineages. Interestingly, Ikaros appears to function both as a transcriptional repressor and as an activator through its ability to bind a large number of nuclear factors, including components of both histone deacetylase and ATP-dependent chromatin remodelling complexes. In addition, nuclear localisation is important for Ikaros function—unlike most transcription factors, Ikaros is localised to discrete nuclear foci in lymphoid cells, suggesting it employs novel mechanisms to regulate transcription. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ikaros; Haematopoiesis; Pericentromeric–heterochromatin; Transcription
1. Introduction
2. Structure
The differentiation and proliferation of pluripotent haemotopoietic stem cells to mature blood cells is tightly controlled throughout the lifetime of an organism. This control is achieved in part at the level of transcription, where a number of multiprotein complexes cooperatively regulate the expression of the appropriate genes. In 1992, the transcription factor Ikaros (also known as LyF-1), was discovered in mice [1], and later in humans [2], and it was found to be vital for normal lymphocyte development. Ikaros is highly conserved from the sea lamprey through to humans [3] and is the founding member of a family of highly homologous proteins, namely Aiolos, Helios, Eos, and Pegasus. However, this review will only focus on Ikaros and its role in regulating haematopoiesis.
The Ikaros gene normally encodes at least six alternatively-spliced isoforms (Ik 1–6; [4]), each consisting of two functionally distinct domains of Krüppel-like zinc fingers (ZnFs): an N-terminal sequence-specific DNA binding domain and a Cterminal oligomerisation domain (Fig. 1). The number of ZnFs (up to 4) in the N-terminal domain confers different DNA binding activities. Indeed, isoforms with less than three N-terminal ZnFs are unable to bind DNA with high affinity [4]. Ikaros binds to DNA sites that contain the core motif GGGA, and ZnFs 2 and 3 appear to make the critical DNA contacts. The roles of ZnFs 1 and 4 are less well understood [5]. The C-terminal ZnF domain is present and identical in all Ikaros isoforms and mediates homo- and hetero-interactions between Ikaros family proteins. This function is essential for high-affinity DNA binding of these proteins and concomitant transcriptional regulation [4]. Interestingly, it is becoming evident that these complexes may not be dimeric (in contrast
∗ Corresponding author. Tel.: +61-2-9351-4025; fax: +61-2-9351-4726. E-mail address: [email protected] (B.J. Westman).
1357-2725/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 7 - 2 7 2 5 ( 0 2 ) 0 0 0 7 0 - 5
B.J. Westman et al. / The International Journal of Biochemistry & Cell Biology 34 (2002) 1304–1307
1305
Fig. 1. Exon composition of Ikaros isoforms (Ik 1–6). Exons (black boxes) are numbered, ZnFs are represented by ellipsoids. DNA binding and oligomerisation domains are indicated.
to many transcription factor complexes) but rather tetrameric or even of a higher order [3].
3. Biological roles The first clues about the function of Ikaros came from its pattern of expression in mice, where it is widely distributed within the developing and adult haematopoietic systems. It is abundant in embryonic haematopoietic progenitors, developing thymocytes and mature T, B and natural killer cells and is present at lower levels in erythroid and myeloid precursors [4]. Moreover, Ikaros isoforms display different expression patterns, with Ik-1 and -2 being most abundant throughout lymphocyte development, Ik-3, -5 and -6 present at lower levels, and Ik-4 only detectable in early T-cell progenitors [5]. Mouse gene targeting studies have confirmed the central role of Ikaros in the regulation of haematopoiesis [4]. Ikaros “null” mice lack all B-cell populations and exhibit severely disrupted T-cell development. Interestingly, mouse mutants expressing only the C-terminal interaction domain of Ikaros exhibit a more severe phenotype, completely lacking all B- and T cells and dying a few weeks after birth. In these mutants, termed Ikaros “dominant negative” (IkarosDN), the C-terminal domain of Ikaros is free to bind to, and disrupt the function of, other Ikaros family members (such as Aiolos and Helios), indicat-
ing a role for such hetero-multimerisation in normal lymphocyte development [4]. Furthermore, mice heterozygous for the IkarosDN mutation develop T-cell leukemias and lymphomas, suggesting Ikaros may act as a tumour suppressor by setting a “barrier” to T-cell proliferation that must normally be overcome by signalling events in response to antigen stimulation [3]. Putative gene targets regulated by Ikaros have been identified by inspection of DNA sequences [5], or through analysis of expression patterns in gene targeting studies [3,6,7]. However, direct regulation of these genes in vivo remains to be established. Most are haematopoietic-specific, such as the TCR and CD3 genes [5], although some are expressed in other cell types, such as in the central nervous system (KOR; [8]), fetal brain (TID1; [9]) and placental trophoblasts (P-LAP; [10]). Perhaps the most intriguing feature of Ikaros is that it appears to act as both a repressor and an activator of transcription. In part, it is the array of proteins that Ikaros is able to interact with that provides this flexibility. In addition to contacting other family members, it appears to interact with the co-repressor CtBP, the viral oncoprotein E1A, the histone deacetylase repressor complexes NURD and SIN3, and the nucleosome–remodelling complex SWI/SNF [3]. Because of the nature of these binding partners, much of the influence of Ikaros upon transcription is thought to be mediated through chromatin reorganisation.
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B.J. Westman et al. / The International Journal of Biochemistry & Cell Biology 34 (2002) 1304–1307
Fig. 2. Models for Ikaros function as both a repressor and activator of transcription. (A) Ikaros (Ik, blue square) may act as a classical transcription factor by binding specific DNA sites (orange) as a dimer/multimer and recruiting repression (red octagons) or activation complexes (green circles) to target genes (yellow), (B) Ikaros could simultaneously bind target genes and pericentromeric–heterochromatin (PC–HC; black ring) and, via multimerisation, move genes to inaccessible regions, (C) Ikaros could potentiate gene expression by sequestering repressors (such as NURD) to PC–HC, thereby leaving DNA sites free for activators to bind.
Protein complexes such as NURD or SWI/SNF may simply be recruited by Ikaros bound to its cognate sites in the regulatory region of a target gene (Fig. 2A; [3]). However, it seems that sub-nuclear localisation of all the components of the system may constitute a further level of regulation. Notably, Ikaros localises to regions of pericentromeric–heterochromatin (PC–HC) in activated lymphocytes [3]. Inactivation of a number of genes containing Ikaros binding sites has been shown to coincide with relocation of these genes to Ikaros foci, suggesting Ikaros may physically recruit genes into transcriptionally-inaccessible nuclear regions (Fig. 2B; [3]). Additionally, Ikaros may directly contribute to the maintenance of heterochromatic
silencing by targeting the NURD complex via Mi-2 to PC–HC [3]. Formation of such large protein–DNA complexes may be facilitated by multimerisation of Ikaros. It has also been suggested that Ikaros may potentiate gene expression by titrating repression complexes (such as NURD) away from the promoters of target genes and into PC–HC, thereby providing access to transcriptional activators (Fig. 2C; [11]). In summary, it is likely that Ikaros utilises a combination of mechanisms to regulate transcription, dependent upon cell type and stage of development. In any case, it is clear that Ikaros has a vital and extensive role in biology, and is likely to exploit novel and interesting mechanisms to achieve its function.
B.J. Westman et al. / The International Journal of Biochemistry & Cell Biology 34 (2002) 1304–1307
4. Diseases associated with Ikaros Aberrant Ikaros expression has been linked with infant acute lymphoblastic leukemia [12], T-cell acute lymphoblastic leukemia [13] and blast crisis in chronic myelogenous leukemia [14]. In these diseases, either overexpression of dominant negative isoforms or a reduced level of Ikaros activity (as well as various insertions and deletions adjacent to the C-terminal domain) has been observed. Elucidating the molecular mechanisms of Ikaros function may lead to the development of agents capable of returning control over haematopoiesis to patients were this has been lost.
Acknowledgements This work is supported by a research grant to JPM from the University of Sydney Cancer Research Fund. DAG is a Wellcome International Prize Travelling Research Fellow and BJW is supported by an Australian Postgraduate Award. References [1] K. Georgopoulos, D.D. Moore, B. Derfler, Ikaros: an early lymphoid-specific transcription factor and a putative mediator for T-cell commitment, Science 258 (5083) (1992) 808– 812. [2] W. Nietfeld, A. Meyerhans, Cloning and sequencing of hIk-1, a cDNA encoding a human homologue of mouse Ikaros/ LyF-1, Immunol. Lett. 49 (1/2) (1996) 139–141. [3] K. Georgopoulos, Haematopoietic cell-fate decisions, chromatin regulation and Ikaros, Nat. Rev. Immunol. 2 (3) (2002) 162–174. [4] K. Georgopoulos, S. Winandy, N. Avitahl, The role of the Ikaros gene in lymphocyte development and homeostasis, Annu. Rev. Immunol. 15 (1997) 155–176.
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[5] A. Molnar, K. Georgopoulos, The Ikaros gene encodes a family of functionally diverse zinc finger DNA-binding proteins, Mol. Cell Biol. 14 (12) (1994) 8292–8303. [6] R.A. Lopez, S. Schoetz, K. DeAngelis, D. O’Neill, A. Bank, Multiple hematopoietic defects and delayed globin switching in Ikaros null mice, Proc. Natl. Acad. Sci. U.S.A. 99 (2) (2002) 602–607. [7] I. Christopherson, M. Piechoki, G. Liu, S. Ratner, A. Galy, Regulation of L-selectin expression by a dominant negative Ikaros protein, J. Leukoc. Biol. 69 (4) (2001) 675–683. [8] X.L. Hu, J. Bi, H.H. Loh, L.N. Wei, An intronic Ikarosbinding element mediates retinoic acid suppression of the kappa opioid receptor gene, accompanied by histone deacetylation on the promoters, J. Biol. Chem. 276 (7) (2001) 4597– 4603. [9] X. Yin, M. Rozakis-Adcock, Genomic organization and expression of the human tumorous imaginal disc (TID1) gene, Gene 278 (1/2) (2001) 201–210. [10] T. Ito, S. Nomura, M. Okada, Y. Katsumata, F. Kikkawa, T. Rogi, M. Tsujimoto, S. Mizutani, AP-2 and Ikaros regulate transcription of human placental leucine aminopeptidase/ oxytocinase gene, Biochem. Biophys. Res. Commun. 290 (3) (2002) 1048–1053. [11] J. Koipally, E.J. Heller, J.R. Seavitt, K. Georgopoulos, Unconventional potentiation of gene expression by Ikaros, J. Biol. Chem. 277 (15) (2002) 13007–13015. [12] L. Sun, N. Heerema, L. Crotty, X. Wu, C. Navara, A. Vassilev, M. Sensel, G.H. Reaman, F.M. Uckun, Expression of dominant-negative and mutant isoforms of the antileukemic transcription factor Ikaros in infant acute lymphoblastic leukemia, Proc. Natl. Acad. Sci. U.S.A. 96 (2) (1999) 680–685. [13] L. Sun, M.L. Crotty, M. Sensel, H. Sather, C. Navara, J. Nachman, P.G. Steinherz, P.S. Gaynon, N. Seibel, C. Mao, A. Vassilev, G.H. Reaman, F.M. Uckun, Expression of dominantnegative Ikaros isoforms in T-cell acute lymphoblastic leukemia, Clin. Cancer Res. 5 (1999) 2112–2120. [14] H. Nakayama, F. Ishimaru, N. Avitahl, N. Sezaki, N. Fujii, K. Nakase, Y. Ninomiya, A. Harashima, J. Minowada, J. Tsuchiyama, K. Imajoh, T. Tsubota, S. Fukuda, T. Sezaki, K. Kojima, M. Hara, H. Takimoto, S. Yorimitsu, I. Takahashi, A. Miyata, S. Taniguchi, Y. Tokunaga, H. Gondo, Y. Niho, M. Harada, Decreases in Ikaros activity correlate with blast crisis in patients with chronic myelogenous leukemia, Cancer Res. 59 (16) (1999) 3931–3934.