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. Author manuscript; available in PMC: 2018 Feb 1.
Published in final edited form as: Exp Hematol. 2016 Nov 16;46:1–8. doi: 10.1016/j.exphem.2016.11.002

Ikaros: exploiting and targeting the hematopoietic stem cell niche in B-progenitor acute lymphoblastic leukemia

Michelle L Churchman 1, Charles G Mullighan 1
PMCID: PMC5241204  NIHMSID: NIHMS830551  PMID: 27865806

Abstract

Genetic alterations of IKZF1 encoding the lymphoid transcription factor IKAROS are a hallmark of high risk B-progenitor ALL such as BCR-ABL1 positive (Ph+) and Ph-like ALL, and are associated with poor outcome, even in the era of contemporary chemotherapy incorporating tyrosine kinase inhibitors in the treatment of Ph+ ALL. Recent experimental mouse modeling of B-progenitor ALL has shown that IKZF1 alterations have multiple effects, including arresting differentiating, skewing lineage of leukemia from myeloid to lymphoid, and in Ph+ leukemia, conferring resistance to TKI therapy without abrogating ABL1 inhibition. These effects are in part mediated by acquisition of an aberrant hematopoietic stem cell like program accompanied by induction of cell surface expression of stem cell and adhesion molecules that mediate extravascular invasion and residence in the niche, and activation of integrin signaling pathways. These effects can be exploited therapeutically using several approaches. IKZF1 alterations also result in upregulation of RXRA that encodes part of the heterodimeric retinoic acid X receptor. Rexinoids, a synthetic class of retinoids that specifically bind to retinoid “X” receptors, such as bexarotene potently reverse aberrant adhesion and niche mislocalization in vivo, and induce differentiation and cell cycle arrest. Focal adhesion kinase inhibitors block the downstream integrin-mediated signaling, and also reverse adhesion and niche mislocalization. Both agents act synergistically with TKI to prolong survival of Ph+ ALL in mouse and human xenograft model, with long term remission induced by FAK inhibitors. Thus, these findings provide important new conceptual insights into the mechanisms by which IKZF1 alterations result in drug resistance, and indicate that therapeutic strategies directed against the pathways deregulated by mutation, rather than attempting to restore IKZF1 expression directly, represent promising therapeutic approaches in this disease.

The problem of high risk B-progenitor ALL

Acute lymphoblastic leukemia (ALL) is the commonest childhood tumor, accounting for approximately 4000 newly diagnosed cases in North America each year (1). ALL is more commonly of B-progenitor than T-lineage, and is an expansion of immature lymphoid progenitors that results in effacement of normal hematopoiesis in the bone marrow with resulting pancytopenia, and invasion of extramedullary sites. Both B-progenitor and T-lineage ALL comprise multiple subtypes with constellations of structural chromosomal alterations such as aneuploidy, rearrangements or deletions, and sequence mutations (2). While the number of genetic alterations per case is lower than many other tumors, each ALL genome typically harbors 10–30 non-silent genetic changes that perturb key cellular pathways including lymphoid maturation, tumor suppression, cell cycle regulation, cytokine receptor, tyrosine kinase and/or Ras signaling, transcriptional regulation and chromatin modification (3). The nature of each alteration, and the temporal order of acquisition vary between ALL subtypes, but these pathways are perturbed in most B-ALL cases.

Common forms of B-ALL in children are ALL with high hyperdiploidy (gains of at least 5 chromosomes), hypodiploid with less than 44 chromosomes, t(12;21) encoding ETV6-RUNX1, t(1;19) encoding TCF3-PBX1 (and variants), rearrangement of KMT2A (MLL) to multiple partners, t(9;22) encoding BCR-ABL1, rearrangement of the cytokine receptor gene CRLF2 by IGH-CRLF2 rearrangement or an interstitial deletion on pseudoautosomal region 1 of chromosome X/Y resulting in P2RY8-CRLF2 expression. A recently described subtype of ALL termed Ph-like, or BCR-ABL1-like ALL, exhibits a gene expression profile similar to that of Ph+ ALL, but lacks BCR-ABL1 and instead harbors a range of rearrangements, deletions and mutations activating at least 15 different cytokine receptor and kinase genes (4–9). These alterations are typically early initiating lesions in leukemogenesis, and influence the likelihood of treatment success or failure: hyperdiploidy, ETV6-RUNX1 and TCF3-PBX1 are associated with high cure rates, whereas MLL-rearrangement, Ph+ and Ph-like leukemia are associated with poor outcome (7). Moreover, Ph and Ph-like leukemia become more frequent with increasing age.

A central role for IKZF1 alterations in the pathogenesis of high-risk B progenitor ALL

Much effort has been dedicated to defining the cooperating lesions that act with founding chromosomal rearrangements to drive leukemogenesis, and to understand the nature of genetic alterations that influence treatment outcome. Alterations in genes encoding transcriptional regulators of B-lymphoid development are a hallmark of B-progenitor ALL, occurring in over two-thirds of cases. The prevalence and type of these alterations is significantly associated with ALL subtype. PAX5 encodes paired box 5, and is targeted by deletions, point mutations or translocations in approximately 30% of B-ALL cases. PAX5 is required for B-lineage commitment and maturation (10), and the alterations observed in ALL result in loss-of-function or dominant negative effects (11), and promote B-ALL development in mouse models (12–14). However, while these alterations clearly contribute to leukemogenesis, they are not associated with outcome in most studies of human leukemia (15).

In contrast, IKZF1 alterations are less common in B-ALL, but are enriched in high-risk subtypes and associated with poor outcome. IKZF1 (encoding the founding member of the Ikaros family of zinc finger transcription factors) is mutated in approximately 15% of childhood B-ALL cases (11, 16) but is altered in over 70% of cases of Ph+ positive lymphoid leukemia: either de novo childhood or adult Ph+ ALL, or chronic myeloid leukemia (CML) at progression to lymphoid blast crisis (16–19). IKZF1 alterations are also common in Ph-like ALL(9, 20). In both diseases, deletion of CDKN2A/CDKN2B (encoding the tumor suppressors and cell cycle regulators ARF (p14), INK4A (p16) and INK4B (p15) are also observed in approximately 50% of cases. The majority of studies have shown that IKZF1 alterations are associated with poor outcome, both in ALL cohorts comprising multiple subtypes, and in BCR-ABL1 ALL, including cases treated with TKI (19, 21–36). An exception is a subtype of ALL characterized by deregulation of the double homeobox gene DUX4 and the ETS family transcription factor ERG, in which IKZF1 alterations are common but not associated with poor outcome (37, 38).

The role of IKZF1 alterations in the pathogenesis of B-ALL

Thus, a critical question in the field of B-ALL has been why IKZF1 alterations are selectively associated with poor outcome, particularly in forms of ALL driven by activated kinase signaling, and how this poor outcome might be mitigated. This remains true in the era of tyrosine kinase therapy, which has substantially improved outcomes for Ph+ ALL (39, 40), and is showing promise in Ph-like ALL (7, 41).

Ikaros is a zinc finger transcription factor required for the development of all lymphoid lineages. It has 8 exons, 7 of which are coding (Figure 1A). Exons 4–6 harbor four zinc fingers that bind DNA, and the C-terminus of exon 8 contains two zinc fingers that mediate homodimerization of Ikaros and heterodimerization with other Ikaros family members such as Aiolos (IKZF3). The genetic alterations of IKZF1 in ALL are most commonly deletions. In approximately two-thirds of cases these result in loss-of-function. In the remaining cases, focal deletions result in expression of an internally deleted isoform that lacks exons 4–7, and thus all DNA-binding zinc fingers that mislocalizes Ikaros and acts as a dominant negative (Figure 1A–B). Missense mutations in the DNA binding domain have similar effects (42).

Figure 1. Ikaros architecture and effects of perturbation.

Figure 1

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(A) Ikaros protein structure comprised of coding exons 2 through 8, including four N-terminal zinc fingers that comprise the DNA-binding domain and two C-terminal zinc fingers that are responsible for hetero- and homo-dimerization. (B) Immunofluorescence using an N-terminal specific anti-Ikaros antibody (green) and a DNA-binding stain, 4′,6-diamidino-2-phenylindole (DAPI; blue) showing punctate, nuclear staining of endogenous Ikaros in primary mouse Arf−/− BCR-ABL1+ pre-B cells transduced with empty vector (left panel) or IK6 (right panel, which lacks the DNA-binding domain and grossly mislocalizes to the cytoplasm. Scale bar = 5μm. (C) Schematic of Ikaros dimerization, DNA-binding, and association with the NuRD complex to repress the transcription of adhesion molecules and genes that restrict differentiation to allow proper developmental progression of lymphoid progenitors (top), thus loss of DNA-binding capability with the retention of the ability to dimerize results in potent dominant-negative effects of IK6, resulting in inappropriate expression of genes that affect a block in differentiation and confer an adhesive phenotype to typically non-adherent lymphoid progenitors. (D) Schematic summary of the effects of Ikaros alterations in various engineered mouse models of BCR-ABL1+ B-ALL in vivo. Transduction of BCR-ABL1 into unmanipulated whole bone marrow of Ikzf1+/− or wild type mice and transplantation directly back into syngeneic wild type mice results in marked acceleration of B-ALL in an Ikzf1+/− background (left arm). IK6 cooperates with Arf loss in driving lymphoid leukemia as demonstrated by transduction and transplantation of lineage negative bone marrow cells from either wild type of Arf−/− mice with BCR-ABL1 and empty vector or IK6 (middle arm). BCR-ABL1-transformed pre-B cells from the indicated genotypes in combination with IK6 have varying degrees of sensitivity to tyrosine kinase inhibition with dasatinib (right arm).

Ikaros has multiple roles in transcriptional regulation that are likely cell type and stage dependent, including transcriptional activation and repression, which are in part mediated by chromatin remodeling (Figure 1C). Ikaros, with histone deacetylases and the Ikaros family member Aiolos is part of the nucleosome remodeling and deacetylase (NuRD) complex that mediates histone deacetylation in a site-specific fashion dependent on the presence of Ikaros in the NuRD complex (43, 44). Ikaros also associates with polycomb repressor complex 2 (PRC2) independently of the NuRD complex (45). PRC2 mediates histone 3 lysine 27 (H3K27) trimethylation and transcriptional repression, and loss of Ikaros in mouse thymocytes results in transcriptional derepression of hematopoietic stem cell and NOTCH1 target genes (45, 46).

Prior mouse models of germline Ikzf1mutations showed that loss-of-function alleles resulted in lack of lymphoid development, and N-terminal zinc finger point mutations that perturb DNA binding resulted in an aggressive T-lineage lymphoproliferative disease (47). The propensity of Ikzf1-mutant mouse models to develop T-lineage malignancies may be related to the loss of normal transcriptional repression of NOTCH1 target genes in the T cell lineage (48). However, these germline Ikzf1-mutant models do not develop B-cell neoplasms, and conversely, IKZF1 alterations are uncommon in human T-lineage acute lymphoblastic leukemia.

Thus, these studies clearly show that Ikaros is a critical regulator of normal lymphoid development, and exerts complex transcriptional regulatory roles requiring chromatin remodeling, at least in mouse T cell development. However, until recently there were few data examining the role of Ikaros in the pathogenesis of pre-B ALL. In 2010, Virely et al. (49) demonstrated in a transgenic model of BCR-ABL1 ALL that Ikzf1 haploinsufficiency accelerated the onset of leukemia, but without mechanistic insights or examination of the role of Ikzf1 alterations in responsiveness to therapy. To address these questions, we performed a series of experiments modeling the role of IKZF1 alterations in BCR-ABL1 ALL (Figure 1D), with the goal of faithfully recapitulating the genetic alterations in human leukemia, examining the role in treatment response, and ultimately, to identify new therapeutic approaches in this form of leukemia.

Modeling IKZF1 alterations in BCR-ABL1 ALL

Prior studies using retroviral or transgenic expression of BCR-ABL1 in hematopoietic progenitors have shown that this fusion oncoprotein promotes the development of a myeloproliferative neoplasm similar to CML, that retroviral expression in Arf−/− mouse pre-B cells results in a highly penetrant pre-B ALL, and that the latency of pre-B ALL in a transgenic model is reduced in the setting of Ikzf1 haploinsufficiency. To faithfully model the most common genetic lesions in BCR-ABL1 ALL – IKZF1 deletion or the presence of the dominant negative isoform, IK6, and CDKN2A/B deletion – we used multiple retroviral bone marrow transplant approaches. These utilized unmanipulated or lineage-negative bone marrow from a range of genetic backgrounds including loss of Ikzf1 and Arf (50, 51), which was cotransduction with retroviruses expressing BCR-ABL1 together with IK6 or empty retroviral vector. These constructs coexpressed either GFP or RFP to enable purification of cells for transplantation, and/or luciferase to facilitate bioluminescent quantitation of disease burden.

Using unmanipulated donor wild type or Ikzf1+/− bone marrow, we confirmed that Ikaros haploinsufficiency resulted in acceleration of onset of pre-B (B220+ CD19+ BP1+) ALL (Figure 1D), without detectable expansion of lymphoid precursor populations in Ikzf1+/− bone marrow, indicating that Ikaros acts as a tumor suppressor, rather than expanding the pool of progenitor cells amenable to transformation. In addition, genomic analysis of the arising tumors using array-based comparative genomic hybridization showed that several of the tumors acquired secondary gene deletions that recapitulate those seen in ALL, such as deletion of Ebf1 and Cdkn2a, indicating loss of function of these genes promotes leukemogenesis.

In a complementary approach, lineage negative bone marrow from wild type or Arf−/− donor mice was cotransduced with BCR-ABL1 and IK6/empty vector, and contransduced cells transplanted into lethally irradiated recipients. This showed that either Arf loss or IK6 shifted disease lineage from myeloid to B-lymphoid, with all BCR-ABL;Arf−/−;IK6 tumors being of lymphoid lineage, demonstrating the interaction of these cooperating alterations in determining disease lineage (Figure 1D).

We next sought to examine the role of each alteration in determining responsiveness to therapy. Luciferase-marked BCR-ABL1 pre-B ALLs of Arf−/−, Ikzf1+/− and or IK6 genotype were established, and treatment with the tyrosine kinase inhibitor (TKI) dasatinib commenced at equivalent levels of tumor burden. This showed a reduction in responsiveness, as evidenced by time to death for tumors with Arf loss or Ikaros alterations, with the poorest response in Arf−/−;IK6 tumors (Figure 1D). There were no differences in ABL inhibition (as evidenced by CRKL phosphorylation) or emergence of ABL mutations to explain the poor response.

Ikaros alterations promote aberrant adhesion and cellular mislocalization in vivo

Several observations suggested that Ikaros alterations promote resistance, at least in part, by promoting abnormal adhesion between leukemic cells, and between leukemic cells and the bone marrow microenvironment (Figure 2). Mouse BCR-ABL1 pre-B leukemic cells typically grow in single cell suspension in vitro, but cells with alterations of Ikaros – haploinsufficiency, IK6 expression, knockdown of Ikzf1 or the NuRD cofactor Ikzf3, or expression of Ikaros point mutant alleles – exhibited strikingly abnormal growth with large leukemic cell aggregates, and enhanced adhesion to stromal/fibronectin monolayers. In a series of experiments incorporating immunophenotyping, transcriptome sequencing, proteomic sequencing and interrogation of Ikaros chromatin immunoprecipitation and sequencing (ChIP-seq) datasets, we observed that both mouse and human Ikaros mutant leukemias exhibit deregulated expression of adhesion molecules such as THY1 (CD90), the THY1 ligand ITGA5 (integrin alpha 5, part of the VLA-5 heterodimer), CD28 and L-selectin. Enforced expression of these molecules, knockdown by RNA interference and antibody-based THY1 neutralization demonstrated that the central role of these adhesion molecules, particularly THY1 and ITGA5 in mediating adhesion. This deregulated adhesion is likely due to loss of the normal transcriptional repressive effects of IKZF1, which binds at these gene loci and activation integrin-mediated signaling pathways (Figure 2).

Figure 2. Targetable pathways in Ikaros-altered leukemic cells affecting cell-to-cell and cell-to-stroma interactions within the bone marrow niche.

Figure 2

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Overview of key signaling molecules aberrantly upregulated in IKZF1-deficient leukemic B-ALL cells that mediate cell adhesion, survival, cytoskeletal reorganization, and migration. Integrin signaling can be activated by interactions with the extracellular matrix or cell-to-cell interactions via binding of THY1 at the cell surface. L-selectin (SELL) plays a role in tethering and rolling of lymphocytes along endothelial cells of the vasculature. FAK activation is central to many processes that promote adhesion, migration, proliferation and survival, thus providing an amenable therapeutic target for inhibition. In BCR-ABL1 ALL, FAK is activated by integrin signaling, and by BCR-ABL1-mediated Src activation. Together, aberrant overexpression of these signaling molecules and activation of their signaling pathways results in leukemic cells that are poised to extravasate, infiltrate and adhere to the extravascular HSC niche. These pathways can be attenuated by rexinoid receptor agonists, which directly upregulate Ikaros expression in leukemic cells that retain an intact wild type IKZF1 allele. In leukemic cells lacking functional Ikaros it is important to note that rexinoids can also induce broad changes in expression in an IKZF1-independent manner to induce differentiation thereby providing another attractive therapeutic intervention for refractory B-ALL.

Similar effects were observed in vivo. Imaging of GFP-marked BCR-ABL1 leukemic cells three days after transplantation showed abnormal, spindle-shaped morphology of leukemic cells in mouse calvaria, suggesting abnormal adherence to the bone marrow vasculature. To further explore this, we performed similar imaging experiments following transplantation of leukemic cells into Prrx1-Cre;Ai9 mice that express tdTomato in osteoblasts and perivascular CAR (CXCR12-associated reticulum cells) cells, showing aberrant adherence of IK6-expressing leukemic cells in these HSC niche compartments. In addition, imaging following marking of the arteriolar endothelium with an anti-Sca1 antibody demonstrated extravasation and adhesion of leukemic cells, which enveloped the vasculature. In parallel, we observed that Ikaros alterations result in acquisition of stem cell-like features, including a gene expression profile similar to embryonic/hematopoietic stem cells, and the ability of single leukemic cells to propagate colonies in single cell fluorescence activated cell sorting approaches. Thus, Ikaros alterations promote the acquisition of a stem cell phenotype and deregulation of adhesion, which results in aberrant bone marrow microenvironment mislocalization and activation of downstream signaling pathways.

Identification of rexinoids as a synthetic lethal approach in Ikaros mutated leukemia

The association between Ikzf1 alterations, adherence, activation of integrin signaling pathways, cellular mislocalization in the bone marrow microenvironment and resistance to therapy suggested that reversal of the adherent phenotype may represent a therapeutic approach to overcome drug resistance. As antibody-based neutralization of THY1 reversed the adherent phenotype in vitro, we hypothesized that a small molecule screen may also identify agents that reversed adhesion, and thus resistance. To investigate this, we developed a screening approach in which high content microscopy was used to enumerate and measure the size of cellular aggregates in 96 well plates. We then subjected cultures of BCR-ABL1 Ikaros wild type or IK6 cells to a panel of US Food and Drug Administration approved agents, including agents approved or in clinical development for the treatment of cancer to identify compounds that reverse adhesion independent of leukemic cell killing. This identified the rexinoid bexarotene as potently reversing this adherence. Bexarotene is an agonist of the rexinoid X receptor, a heterodimer of RXRA and PPARG (Peroxisome Proliferator-Activated Receptor gamma). A secondary screen with a collection of nuclear hormone receptor agonists and antagonists showed all retinoid and rexinoid receptor agonists tested, including 9 cis retinoic acid, 13 cis retinoic acid (isotretinoin) and all-trans-retinoic acid (ATRA) also attenuated adhesion, and a PPARγ agonist, N-oleoylethanolamine accentuated adhesion. As these agents exhibit incomplete selectivity for the various retinoid/rexinoid receptors, we tested specific RARA and RXRA agonists and showed that only a selective RXRA agonist abrogated adhesion. Moreover, transcriptome sequencing showed selective upregulation of Rxra expression in IK6 leukemic cells, and ChIP-seq showed that Ikaros bound to this gene.

In vitro, bexarotene and other retinoid/rexinoid receptor agonists induced cell cycle arrest and inhibited cell proliferation without inducing apoptosis or direct cytotoxicity, reversal of the HSC-like transcriptional signature an induction of a B cell differentiation program, and attenuation of single cell colony formation. In vivo, bexarotene reversed aberrant adhesion in the bone marrow niche and synergized with dasatinib resulting in near doubling of the survival of Ikaros mutant mouse leukemia. Similar effects were observed in bexarotene/dasatinib treated human BCR-ABL1;IKZF1 mutant leukemic cells treated ex vivo.

A key question is how retinoids/rexinoids abrogate the deleterious effects of Ikaros alterations and restore TKI sensitivity. These agents are broadly active in inducing differentiation in diverse tumors, notably in acute promyelocytic leukemia, where they induce degradation of the causal PML-RARA (and variant) fusion oncoproteins (52). In BCR-ABL1 ALL, we observed striking induction of differentiation on transcriptional profiling, but modest immunophenotypic maturation – leukemic cells remained at the pre-B cell stage of maturation, with modest induction of expression of markers of maturation. These effects are partly due to induction of expression of wild type Ikaros. Retinoid/rexinoid treated mouse and human BCR-ABL1;Ikaros mutant cells showed induction of expression of Ikaros, and on immunofluorescence, relocalization of Ikaros to the nucleus, without induction of the key Ikaros mutant IK6. In vivo, these agents completely reverse the aberrant mislocalization of Ikaros mutated leukemic cells.

How might these agents selectively induce expression of wild type Ikaros? ChIP-PCR for RARA and RXRA showed selective binding of RXRA to the IKZF1 locus that was upregulated on treatment with rexinoids. Moreover, ChIP-sequencing of chromatin modifications in BCR-ABL1 expressing K562 cells demonstrated the presence of an enhancer in the region of IKZF1 deleted in IK6 expressing cells. ChIA-PET analysis has shown that this enhancer interacts with the IKZF1 promoter. Thus, the IK6 allele lacks key regulatory elements required for induction of IKZF1 following treatment with rexinoids. Expression of RXRA is normally silenced by Ikaros, and loss of Ikaros function results in RXR overexpression, creating a therapeutic vulnerability, or “Achilles heel” in Ikaros mutant leukemia. Rexinoids such as bexarotene result in binding of RXRA to the IKZF1 locus, induction of Ikaros expression and relocalization of Ikaros to nuclear binding sites. This is likely only one mechanism by which rexinoids exert therapeutic effects in BCR-ABL1 ALL, and consistent with this, arrest of cellular proliferation was also observed in BCR-ABL1 leukemic cells lacking Ikaros alterations, although this was less marked than in Ikaros-mutant cells.

Targeting adhesion molecule signaling in Ikaros mutant ALL: FAK inhibition

Bexarotene is in clinical use for other malignant and non-malignant conditions, including cutaneous lymphoma (53), albeit without any relationship to Ikaros alterations. A challenge will be design and implementation of clinical trials in BCR-ABL1 ALL incorporating an additional targeted/novel agent in addition to TKIs such as imatinib/dasatinib, although such approaches clearly merit testing in view of the persisting inferior outcome associated with IKZF1 mutations in this form of the disease.

A second attractive therapeutic approach is targeting the adhesion molecule signaling pathways activated upon Ikaros alterations. We and others have shown that the focal adhesion kinase (FAK) pathway is activated downstream of integrin signaling, and is activated in Ikaros-mutated BCR-ABL1 ALL (42, 54, 55). FAK serves to integrate a variety of cell surface signaling inputs, leading to downstream activation of gene expression and reorganization of cytoskeletal architecture. FAK inhibition has been pursued to disrupt tumor cell-microenvironment interactions, particularly in solid tumors. Our initial studies documented the efficacy of tool compound FAK inhibitors in ex vivo studies of human BCR-ABL1;Ikzf1-mutant cells (42).

We systematically examined activation of FAK signaling pathways and the potential of FAK inhibition in a series of mouse and human cell models (54). Using phosphoproteomic analysis, we showed increased phosphorylation of FAK1, also known as Protein Tyrosine Kinase 2 (PTK2), and FAK2 (PTK2b) in Ikaros mutant tumors, and activation of FAK signaling as shown by elevated levels of p130Cas, a downstream target of FAK. We observed potent activity of the clinical FAK inhibitor VS-4718 in vitro and in vivo. This agent reversed leukemic cell-stromal adhesion and colony forming activity, and significantly prolonged survival of mice engrafted with human BCR-ABL1;IKZF1-mutant cells, with long term disease free survival observed in a subset of mice. As observed with bexarotene, this agent potently reversed the leukemic cell mislocalization characteristic of Ikaros-mutant leukemia. FAK inhibition was also effective in reversing adherence and inducing cell cycle arrest in other IKZF1-mutated cells such as Ph-like ALL cells harboring other fusions of ABL1 or other cytokine receptor alterations such as EPOR (54).

Conclusions

Loss-of-function genetic alterations in hematopoietic transcription factors such as Ikaros are a hallmark of lymphoid leukemia, resulting in therapeutic nihilism that approaches attempting to directly restore transcriptional activity are likely to be met with limited success. These studies have dissected a new mechanistic and therapeutic paradigm, in which such alterations do indeed result in arrest in maturation, but also, via transcriptional derepression, unexpectedly activate cellular pathways that allow leukemic cells to evade therapeutic eradication, in part due to adhesion within the bone marrow microenvironment and mislocalization to the HSC niche. These studies also highlight the power of unbiased high content screening approaches to identify new therapeutic vulnerabilities arising from deregulation of this regulatory network that involves derepression of adhesion molecule and retinoid X receptor expression, that may be agonized directly with rexinoids) or downstream with FAK inhibitors. These findings are now being pursued in early phase clinical trials. A broader question is the generalizability of this mechanism of treatment resistance. While the notion of the bone marrow niche supporting leukemic cell proliferation via contact or paracrine mechanisms is long-studied, the notion of specific leukemic cell genetic alterations leading to hijacking of a protective bone marrow niche is new – and warrants detailed exploration across the spectrum of high risk ALL.

Footnotes

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