MINI REVIEW published: 09 September 2019 doi: 10.3389/fonc.2019.00867 Towards a Better Understanding of Cohesin Mutations in AML Sergi Cuartero 1,2,3*, Andrew J. Innes 1,4 and Matthias Merkenschlager 1* 1 Faculty of Medicine, MRC London Institute of Medical Sciences, Institute of Clinical Sciences, Imperial College London, London, United Kingdom, 2 Centre for Genomic Regulation (CRG), The Barcelona Institute of Science and Technology, Barcelona, Spain, 3 Josep Carreras Leukaemia Research Institute (IJC), Barcelona, Spain, 4 Faculty of Medicine, Centre for Haematology, Imperial College London, London, United Kingdom Classical driver mutations in acute myeloid leukemia (AML) typically affect regulators of cell proliferation, differentiation, and survival. The selective advantage of increased proliferation, improved survival, and reduced differentiation on leukemia progression is immediately obvious. Recent large-scale sequencing efforts have uncovered numerous novel AML-associated mutations. Interestingly, a substantial fraction of the most frequently mutated genes encode general regulators of transcription and chromatin state. Understanding the selective advantage conferred by these mutations remains a major challenge. A striking example are mutations in genes of the cohesin complex, a major regulator of three-dimensional genome organization. Several landmark studies Edited by: have shown that cohesin mutations perturb the balance between self-renewal and Cyrus Khandanpour, differentiation of hematopoietic stem and progenitor cells (HSPC). Emerging data now University Hospital Münster, Germany begin to uncover the molecular mechanisms that underpin this phenotype. Among Reviewed by: these mechanisms is a role for cohesin in the control of inflammatory responses in Jisha Antony, University of Otago, New Zealand HSPCs and myeloid cells. Inflammatory signals limit HSPC self-renewal and drive Manja Wobus, HSPC differentiation. Consistent with this, cohesin mutations promote resistance to Dresden University of Technology, Germany inflammatory signals, and may provide a selective advantage for AML progression. *Correspondence: In this review, we discuss recent progress in understanding cohesin mutations in Sergi Cuartero AML, and speculate whether vulnerabilities associated with these mutations could be sergi.cuartero@crg.eu exploited therapeutically. Matthias Merkenschlager matthias.merkenschlager@ Keywords: cohesin, leukemia, interferon, inflammation, hematopoiesis, AML lms.mrc.ac.uk Specialty section: INTRODUCTION This article was submitted to Hematologic Malignancies, Hematopoietic homeostasis requires tight regulation to ensure production of sufficient numbers a section of the journal of blood cells at all stages of differentiation. This is achieved by a complex network of signaling Frontiers in Oncology pathways and gene regulatory mechanisms that control cell proliferation, differentiation, and Received: 29 March 2019 survival of hematopoietic stem and progenitor cells (HSPC) and their progeny. Skewing of this Accepted: 21 August 2019 balance in favor of excessive differentiation results in stem cell depletion, exhaustion and eventually, Published: 09 September 2019 inability to replenish mature blood cells. In contrast, uncontrolled self-renewal, increased survival, Citation: and failure to differentiate are hallmarks of leukemia. Cuartero S, Innes AJ and The homeostatic balance between self-renewal and differentiation of HSPC is sensitive to a Merkenschlager M (2019) Towards a Better Understanding of Cohesin broad range of perturbations. Mutations that disrupt it not only provide classifiers of clinical Mutations in AML. disease, but also offer insights into the molecular control of self-renewal, differentiation, and cell Front. Oncol. 9:867. proliferation. Many AML-associated mutations are clearly linked to one of these categories, such doi: 10.3389/fonc.2019.00867 as constitutive activation of RAS proteins or FLT3, that drive uncontrolled proliferation (1–3), Frontiers in Oncology | www.frontiersin.org 1 September 2019 | Volume 9 | Article 867 Cuartero et al. Understanding Cohesin Mutations in AML mutations that prevent cell cycle arrest and apoptosis such as proliferation (21). This explains how leukemic cells can tolerate TP53 (4), and mutations that hinder differentiation such as in cohesin mutations, but fails to explain why cohesin mutations the transcription factors RUNX1 or C/EBPα (5, 6). The clonal occur with high frequencies in AML. advantage conferred by such mutations is immediately obvious. In addition to essential functions in the cell cycle, cohesin Recent large-scale sequencing studies have shown that the has a major role in three-dimensional genome organization mutational landscape of AML is highly enriched for mutations (22). Cohesin cooperates with the DNA-binding protein CTCF in general transcriptional regulators and chromatin modifiers, in the formation of topologically associated domains (TADs), which are found in ∼70% of patients (7). Examples of this which facilitate preferential interactions between genes and group include mutations in proteins involved in chromatin enhancers within the same CTCF-demarcated domain (23–26). modifications (ASXL1, EZH2), DNA methylation (DNMT3A, Impaired formation of these structures randomizes the three- TET2), or transcriptional splicing (SRSF2, U2AF1) (7–9). dimensional topology across single cells (27), thus exposing genes Although we understand the biological functions of many and enhancers to illegitimate interactions (28). of these molecules and pathways in exquisite detail, their Here we review recent progress that links impaired cohesin selective advantage for AML cells remains largely unknown (10). function to the regulation of inflammatory gene expression, Understanding the link between these novel AML mutations and self-renewal, and differentiation of hematopoietic progenitors the molecular mechanisms of self-renewal, differentiation and (29–34), revealing potential explanations for why cohesin is cell survival is critical for understanding the pathophysiology of recurrently mutated in AML. We speculate about the role AML, and for the identification of new therapeutic approaches to of inflammatory gene expression in AML and its potential cancer (Figure 1). therapeutic implications. A striking example are mutations in the subunits of the cohesin protein complex (SMC1, SMC3, RAD21, and STAG1/2). Cohesin forms a ring-shaped structure that can encircle DNA COHESIN MUTATIONS IN AML and hold sister chromatids together. This function of cohesin is essential for DNA replication (11–13), DNA repair (14– Mutations in members of the cohesin complex are found 17), and chromosome segregation in mitosis (18–20). Despite in 6–13% of AML cases (7, 35–37). In addition to AML, this essential role in cell cycle progression, heterozygous cohesin mutations are also found in other myeloid or hypomorphic cohesin mutations are compatible with cell malignancies, including Down syndrome acute megakaryoblastic leukemia (DS-AMKL), myelodysplastic syndrome (MDS), myeloproliferative neoplasms (MPN), chronic myelomonocytic leukemia (CMML), and chronic myelogenous leukemia (CML) (38, 39). Cohesin mutations are frequent in myeloid but not in lymphoid malignancies, suggesting that the pro-leukemic effect of reduced cohesin function may be connected to myeloid-specific traits. The majority of RAD21 and STAG2 mutations cause non- sense, frame-shift, or splice-site changes, presumably leading to protein truncation or exon skipping. On the other hand, mutations in SMC1A and SMC3 are missense, causing amino acid substitutions in different protein domains (38). The effect of each of these mutations on the formation of the cohesin complex is still largely unexplored. Some of the mutant transcripts can give rise to dominant-negative proteins in cord blood progenitors (32) while others result in the degradation of the mutant transcript (38). Most cohesin mutations are heterozygous, consistent with the idea that complete loss of the complex is incompatible with cell cycle progression. This has been confirmed by studies showing that partial cohesin loss in AML cells is not linked to increased aneuploidy (29, 36, 38, 40). However, since the Stag2 and Smc1a genes are on the X chromosome, male cases with mutations in these genes are not heterozygous. In the case of STAG2 mutant cells it has been shown that STAG1 becomes essential FIGURE 1 | Classical and non-classical AML mutations. (A) Classical AML (41), suggesting that loss of STAG2 can be at least partially mutations deregulate proliferation, survival and differentiation pathways and compensated by STAG1. provide an obvious selective advantage to AML. (B) Novel AML mutations Cohesin mutations in patients appear to be mutually exclusive, include mutations in transcription and chromatin regulators and their selective advantage is less obvious. indicating that a mutation in just one member of the complex is sufficient to reduce cohesin activity to the point where it Frontiers in Oncology | www.frontiersin.org 2 September 2019 | Volume 9 | Article 867 Cuartero et al. Understanding Cohesin Mutations in AML provides a clonal advantage. Cohesin mutations often co-occur were decreased (31). In competitive repopulation assays, cohesin- with mutations in other genes, such as NPM1, TET2, ASXL1, deficient cells outcompeted wild-type cells. An important aspect and EZH2 (36, 38). Nonetheless, it is thought that the majority of this study was the demonstration that Smc3 heterozygosity of cohesin mutations are early events in leukemogenesis (38, 42). on its own is not sufficient to trigger leukemic transformation. The prognostic significance of cohesin mutations in myeloid However, the combination of Smc3 heterozygosity and an malignancies is not yet fully clear. In MDS, STAG2 mutations are internal duplication in the FLT3 receptor (one of the most associated with significantly reduced survival (37). However, no common mutations in AML) induced acute myeloid leukemia in significant association between cohesin mutations and survival in mice. This indicates that SMC3 mutations must cooperate with AML was found in an early study (36) while a more recent study other mutations to cause leukemia in this model. However, a reports a significant association with increased overall survival mouse model of conditional Stag2 deletion presented features and disease-free survival (43). of myeloid dysplasia (44). Also, these mice had increased frequencies of both long-term and short-term HSC, indicating that mutations in different cohesin subunits do not always cause THE ROLE OF COHESIN EARLY the same phenotypes. Mouse models of shRNA-mediated knock-down of different HEMATOPOIESIS cohesin subunits developed similar, but not identical alterations The frequency of cohesin mutations in AML prompted several in stem cell compartments (30). In the bone marrow there groups to investigate the contribution of cohesin to early was a marked increase in granulocyte-macrophage progenitors hematopoiesis and myeloid differentiation (Table 1). A mouse (GMP), accompanied by a decrease in long-term and short-term model of conditional Smc3 heterozygosity (31) presented an HSCs. These models of cohesin deficiency did not develop acute altered composition of the hematopoietic stem cell (HSC) myeloid leukemia. However, the mice displayed several features compartment. Short-term HSCs and multipotent progenitor resembling a myeloid disorder, including splenomegaly and populations (MPP) were increased, while in long-term HSCs myeloid hyperplasia. In addition, cohesin-mutated mouse cells TABLE 1 | Main phenotypes and transcriptional changes in models of cohesin depletion in hematopoiesis. Model Main phenotypes of cohesin depletion Transcriptional changes References Mouse model of conditional Increase in ST-HSC and MPP and decrease in LT-HSC Deregulation of lineage-specific transcription factors (31) Smc3 heterozygous deletion Increased bone marrow chimerism in competitive transplantation Global reduction in transcription Mx1-Cre; Smc3lox/WT assays Absence of leukemic transformation unless combined with Flt3 mutations Increased self-renewal in methylcellulose assays Mouse model of conditional Features of myelodysplasia Downregulation of lymphoid, myeloid and erythroid (44) Stag2 deletion Mx1-Cre; Increase in ST-HSC, LT-HSC, MPP, and GMP lineage commitment genes Stag2lox/lox Increased bone marrow chimerism in competitive transplantation Downregulation of genes involved in assays HSC quiescence Blockade in B-cell differentiation Mouse model of Decrease in ST and LT-HSC and increase in GMP Upregulation of myeloid differentiation genes (Fcgr3, (30) doxycycline-inducible shRNA Myeloid disorder features (splenomegaly and myeloid hyperplasia) Cebpa) expression for Rad21, Smc1a, Increased self-renewal in methylcellulose assays Downregulation of lymphoid development genes and Stag2 TRE-shRNA; (Blnk, Lax1, Cd86) ROSA26(M2rtTA/+) Human CD34+ cord blood cells Impaired sensitivity to cytokine-induced differentiation Upregulation of HSC genes (HOX genes, MEIS1) (32) transduced with mutant cohesin Increase in CD34+ progenitors after engraftment in NSG mice Downregulation of myeloid differentiation genes genes (RAD21E212* , Increased self-renewal in methylcellulose assays (MPO, CSF1R) RAD21Q592* , SMC1A R711G ) Human CD34+ cord blood cells Delayed differentiation and expansion of immature cells over time Upregulated genes enriched in HSC-specific genes (33) transduced with shRNASTAG2 after engraftment in NSG mice Increased self-renewal in methylcellulose assays Mouse HSPCs transduced with Increased self-renewal in methylcellulose assays Increased expression of the self-renewal genes (29) shRNARad21 HoxA7 and HoxA9 Rad21−/− mouse macrophages Defective inflammatory response Impaired inducible gene expression (34) and Rad21+/− HSPCs Decrease in LPS-induced differentiation Downregulation of tonic interferon expression Mouse HSPCs transduced with Decrease in LPS-induced differentiation Inhibited NFkB transcriptional response (45) shRNARad21 and mouse models Positive selection of Rad21-deficient HSCs during aging of conditional Rad21 deletion Mx-Cre; Rad21lox/+ * indicates nonsense mutations. Frontiers in Oncology | www.frontiersin.org 3 September 2019 | Volume 9 | Article 867 Cuartero et al. Understanding Cohesin Mutations in AML acquire increased self-renewal capacity in in vitro methylcellulose example is the transcriptional regulation of the lymphoid colony formation assays. transcription factor Ebf1, that has four STAG2 binding sites in Importantly, similar results were obtained with human cells hematopoietic progenitors. In Stag2−/− mice, cis-interactions at (32, 33). Cohesin-deficient cord blood progenitors or AML this locus are lost, leading to abrogation of Ebf1 expression and cell lines displayed reduced sensitivity to the differentiation- failure to differentiate into lymphoid progenitors (44). inducing effects of cytokines. The same effect was observed Another mechanism that has been proposed to explain the by over-expressing cohesin genes carrying mutations identified increased self-renewal capacity of cohesin-deficient HSPCs is in AML, indicating that these can act as dominant-negative the derepression of the self-renewal transcription factor HOXA9 mutants. These cells were also characterized by a higher (29, 57). HOXA9 is normally silenced by the Polycomb complex, frequency of CD34+ progenitors and increased self-renewal which represses Hox loci in HSPCs by H3K27 trimethylation. capacity in methylcellulose (32). In line with these findings, In cohesin-deficient mouse HSPCs, this repressive chromatin cohesin-deficient human blood progenitors have increased mark is lost and Hoxa9 is upregulated, leading to increased in vivo reconstitution capacity after transplantation into self-renewal. This finding suggests that cohesin cooperates with immunodeficient mice (33). Polycomb to silence Hox genes in HSPCs. Consistently, in mouse embryonic stem cells, cohesin complexes containing STAG2 (but not STAG1) contribute to the maintenance of chromatin TRANSCRIPTIONAL CONSEQUENCES OF interactions within Polycomb domains (58). COHESIN MUTATIONS IN HSPCS Given that AML-associated cohesin mutations do not affect COHESIN IN THE CONTROL OF THE genome integrity (38), the observed resistance to differentiation INFLAMMATORY RESPONSE has been ascribed to the gene regulatory role of cohesin. In all models tested, transcriptional changes were mild (31, 32). This is As discussed in the previous section, all studies that have expected, as even complete removal of cohesin only changes the compared wild-type and cohesin-deficient HSPCs found expression of 10% of genes (23). clear changes in chromatin accessibility and gene expression. Previous reports suggested a role for cohesin in facilitating These changes may indicate a direct effect of cohesin, or, chromatin remodeling in human and mouse cells (46, 47). alternatively, they may reflect the less mature state of cohesin- Consistent with this, cohesin-deficient HSPCs present genome- deficient progenitor populations. In order to rule out this wide alterations in chromatin accessibility (30–33, 45). These possibility and determine what genes are directly controlled changes broadly correlate with altered gene expression. by cohesin in myeloid cells, a recent study used terminally Therefore, it has been proposed that defective chromatin differentiated macrophages to allow a like-for-like comparison accessibility impacts on normal dynamics of transcription of transcriptional and chromatin state between wild-type and factor binding, which leads to transcriptional deregulation and cohesin-deficient cells (34). This strategy uncovered a key role abnormal differentiation. for cohesin in the regulation of inflammatory gene expression. An extreme case of chromatin alterations was observed in Consistent with a body of knowledge demonstrating that cohesin human cord blood cells, where dominant negative cohesin is required for interactions within topological domains (23–26), mutations reduced chromatin accessibility genome-wide (32). interactions between upstream key transcriptional regulators Interestingly, a minority of sites displayed increased accessibility, of the inflammatory response and their surrounding enhancers specifically binding sites for the transcription factors GATA2 and were decreased after acute cohesin depletion. As the organization RUNX1. This has been proposed to result in an upregulation of of the inflammatory response is hierarchical, reduced levels of HSPC transcriptional programs and obstruct differentiation. A upstream regulators impact on the network, and deregulation role for cohesin in regulating RUNX1 expression has also been spreads to the majority of inducible genes. Importantly, re- described in model organisms (48). analysis of HSPC gene expression data showed that cohesin also How is cohesin linked to chromatin accessibility? Cohesin controls inflammatory gene expression in progenitor cells (34). binding sites are highly accessible (49). In yeast, cohesin Inflammatory signals not only mediate cross-talk between cooperates with the chromatin structure remodeling complex immune cells to coordinate the immune response, but (RSC) to actively evict nucleosomes and generate nucleosome- also regulate the balance between HSPC self-renewal free DNA (50–52), which is required for cohesin loading (53). and differentiation. This function, known as emergency In mouse embryonic stem cells, depletion of a member of the hematopoiesis, is normally activated during infection in order PBAF complex (a vertebrate ortholog of RSC) results in sister- to regenerate mature myeloid cell populations (59). Several chromatid cohesion defects (54). In humans, cohesin is found inflammatory cytokines and ligands, including interferons, are in a complex with the ATP-dependent chromatin remodeling involved in the activation of emergency hematopoiesis. Type I enzyme SNF2H (55). SNF2H, but not cohesin, is required for interferon induces HSC exit from quiescence, entry into the cell the establishment of arrays of phased nucleosomes around CTCF cycle and differentiation. Importantly, chronic exposure to type I binding sites (56). interferon is detrimental to HSCs (60, 61). Type II interferon— Reduced cohesin dosage can also alter the frequency of or IFNγ–also regulates HSC activity both in homeostasis and chromatin interactions near transcription factor genes. One during infection (62). IFNγ acts on a subset of HSCs to induce Frontiers in Oncology | www.frontiersin.org 4 September 2019 | Volume 9 | Article 867 Cuartero et al. Understanding Cohesin Mutations in AML myeloid differentiation by activating transcription factors that the same mechanism that favors self-renewal in cohesin- like C/EBPβ (63). The interleukin IL-1 brings about myeloid deficient HSPCs through impaired sensitivity to inflammatory differentiation through activation of a NF-κB-PU.1 axis (64). signals may operate in cohesin-deficient AMLs. The implication Activation of Toll-like receptor (TLR) signaling in HSPCs, is that in settings with increased inflammatory signaling, cohesin which activates both the NF-κB and the interferon pathways, mutations could confer resistance to inflammatory signals and also promotes myeloid differentiation (65–69). As in the case of increased self-renewal and clonal expansion. chronic interferon exposure, sustained TLR activation becomes Constitutively increased inflammatory signals are a hallmark detrimental and impairs the repopulating capacity of HSPCs. of aging. Basal levels of pro-inflammatory cytokines such as IL6 As cohesin is required to induce expression of inflammatory or TNFα increase with age in healthy individuals (70). This response genes, cohesin-deficient HSPCs are less prone to leads to alterations in hematopoietic differentiation, which are differentiate in inflammatory conditions [(34, 45); Figure 2]. reminiscent of emergency hematopoiesis: myelopoiesis-biased This acquired resistance to differentiation allows increased differentiation and reduced HSC self-renewal (71, 72). Consistent proliferation of immature progenitors, providing a possible with its role in conferring resistance to inflammatory signals, explanation for some of the phenotypes displayed by cohesin- competitive assays show that cohesin mutant HSCs become deficient mice. Therefore, cohesin mutations in AML illustrate a dominant over wild-type HSCs in aged mice (45). As clonal mechanistic connection between the control of transcriptional hematopoiesis is a feature of aging (73), it has been suggested regulation and the responsiveness to differentiation-inducing that cohesin mutations could be positively selected during aging, stimuli in myeloid cells. The selective advantage of mutations eventually promoting a pre-leukemic state (45). This is in line in other transcriptional regulators may potentially be with a report showing that cohesin mutations are early events, explained by similar mechanisms involving the control of considered to be pre-leukemic (42). However, cohesin subunits inflammatory signaling. are not among the top frequently mutated genes in cases where clones of hematopoietic cells carrying somatic mutations are found in the absence of any hematologic dysplasia, known as INFLAMMATORY GENE EXPRESSION IN clonal hematopoiesis of indeterminate potential (CHIP) (73). AML Therefore, the emergence of cohesin mutations associated to aging may immediately lead to pre-leukemic dysplasias rather Consistent with the finding that cohesin regulates inflammatory gene expression in hematopoietic progenitors and myeloid cells, than CHIP. Further investigation is required to understand the AML patient cells with cohesin mutations show a striking role of cohesin mutations during aging. reduction of inflammatory and interferon pathways. This is the A number of previous studies have reported altered case when comparing AML with and without cohesin mutations expression of cytokines and other inflammatory mediators in across all samples in The Cancer Genome Atlas (TCGA), as well myeloid disorders (74–76). For example, FLT3-ITD+ AML show as within a specific histological subtype (34). These data suggest increased expression of microRNA miR-155, which is known FIGURE 2 | Cohesin regulates the balance between self-renewal and differentiation. Cohesin controls expression of pro-inflammatory genes that promote HSPC differentiation. In cohesin-mutant AML, inflammatory gene expression is downregulated, increasing resistance to differentiation and favoring HSPC self-renewal. Frontiers in Oncology | www.frontiersin.org 5 September 2019 | Volume 9 | Article 867 Cuartero et al. Understanding Cohesin Mutations in AML for its anti-inflammatory effects, its ability to inhibit interferon preparations are tolerated much better and can be used signaling, and to increase HSPC self-renewal in mouse models with feasible dosing regimens. There are currently established (77). Sensitivity of human AML cells to IFNγ is inversely related roles for interferon in the myeloproliferative disorders (89, to RIP1/3 signaling. High levels of RIP1/3 signaling stabilize 90), hypereosinophilic syndromes (91), and chronic myeloid SOCS1, and SOCS1 antagonizes IFNγ signaling, effectively leukemia (CML) (92). Intriguingly in CML, interferon treatment protecting AML cells from the differentiation-inducing effects appears to preferentially target the leukemic stem cell population, of IFNγ (78). On the other hand, reduced RIPK3 expression and can induce cytogenetic remissions, some of which are is thought to reduce differentiation and TNFR-driven death of durable upon treatment withdrawal, suggesting that it can cure AML cells (79). Interleukin-1 (IL-1) inhibits growth of normal some patients (92). Similar effects are observed in the JAK2 hematopoietic progenitors, but promotes expansion of AML cells myeloproliferative disorders, with reduction or clearance of the by increasing p38 MAPK phosphorylation. This effect can be mutant clones in up to 50% of patients (93). reversed by blocking IL-1 with p38 MAPK inhibitors (80). In acute leukemia, interferon treatment impairs proliferation As an illustration of the complexity of the pathways involved of AML cell lines in vitro, and has anti-leukemic effects in in the regulation of inflammation and differentiation, TNF patient-derived xenograft models (PDX) in a dose dependent activates NFκB via JNK in AML cells (81) and can dampen manner (94). This has been explained by cell intrinsic interferon signaling via SOCS1 (78). NFκB is constitutively effects of interferon on leukemic blasts (reduced proliferation, activated in CD34+ CD38− AML cells (82), promoting leukemia increased apoptosis, and reduced secretion of growth-promoting stem cell survival and proliferation (77). Although exogenous cytokines), increased immunogenicity of interferon-treated interferon reduces in vitro self-renewal induced by RUNX1- leukemic blasts, as well as immunomodulatory effects on the ETO and RUNX1-ETO9a, interferon and interferon-stimulated residual normal hematopoietic cells, and increased clearance by genes are elevated by RUNX1-ETO in human and in murine the host immune system. However, despite the encouraging pre- models (83). Finally, the chromatin modifier TET2 is required clinical data, the clinical outcomes in interferon trials in AML for emergency myelopoiesis (84), and TET2 mutants show have been disappointing, with durable responses seen in only greater fitness in inflammatory environments partially due to small percentage of patients. While patients with secondary AML increased resistance to TNF, which triggers IL6 overproduction arising from a myeloproliferative disorder seem most susceptible, and activation of an anti-apoptotic lncRNA (85, 86). Taken this is not exclusively the case. However, much of the clinical together, these studies link myeloid disorders with inflammation experience pre-dates the availability of current sequencing and indicate that AML may use a spectrum of different strategies technologies and so stratification of AML by mutation may reveal for managing inflammatory signals. genetic susceptibilities to interferon treatment. INTERFERON TREATMENT IN AML CONCLUSIONS The interferon pathway is central to the inflammatory gene Many recently identified AML mutations are in genes encoding expression network, and it is heavily deregulated in cohesin- regulators of transcription and chromatin state. Understanding deficient macrophages (34). The deregulation of upstream how these mutations are beneficial to cancer cell fitness is a interferon regulators like STAT1 and IRF7 disrupts basal major challenge (10). Regulators of transcription and chromatin interferon secretion, which maintains anti-viral transcriptional state usually regulate the expression of hundreds or thousands of responsiveness by auto- and paracrine feed-forward signaling genes, which complicates the task of pinpointing the target genes (87, 88). In the absence of cohesin, STAT and IRF-dependent that are responsible for the increased fitness of mutated cells. enhancers fail to be induced, and consequently most interferon- Mutations in subunits of the cohesin complex result in induced genes are deregulated. Importantly, both enhancer clear alterations in the hematopoietic stem cell compartment activation and constitutive interferon gene expression can and in HSPC function (29–33, 45). However, the specificity of be partially rescued with exogenous interferon (34). These cohesin control on HSPC gene expression has been difficult to findings provide grounds to speculate that cohesin-mutated accommodate with current models of cohesin function. AMLs could be particularly vulnerable to interferon treatment. The transcriptional control of inducible gene expression Mechanistically, supplying exogenous interferon could partially provides a possible explanation for the high frequency of cohesin rescue expression of upstream transcription factors and mutations in myeloid malignancies. Inflammatory signaling regulators of the pathway, enabling normal enhancer activation, promotes the differentiation of HSPCs toward a myeloid fate and transcription of downstream effectors. This would in turn (59), and cohesin-deficient cells show increased resistance to increase the inflammatory responsiveness of cohesin-mutant these differentiation-inducing stimuli (34, 45). In bone marrow AML cells, potentially restoring the balance between self-renewal microenvironments with alterations in cytokine levels such as and differentiation and restricting their selective advantage. those found in aging (70), myelodysplastic syndrome (MDS) There is a long history of using type I interferons to (95) or leukemia (75, 76), mutations that confer reduced treat hematological malignancies, with varying degrees of responsiveness to differentiation-inducing signals are likely to success. While early studies were hampered by treatment be positively selected and clonally expanded (45). This is limiting side-effects, more recent recombinant and pegylated consistent with observations that cohesin mutations appear early Frontiers in Oncology | www.frontiersin.org 6 September 2019 | Volume 9 | Article 867 Cuartero et al. Understanding Cohesin Mutations in AML in the history of AML (42), and that cohesin mutations by AUTHOR CONTRIBUTIONS themselves alter the composition of the HSPC compartment but are insufficient to trigger AML (31). All authors listed have made a substantial, direct and AML has an inherently poor prognosis, and even intellectual contribution to the work, and approved it with intensive chemotherapy or hematopoietic stem cell for publication. transplantation the risk of relapse remains high. AML is a highly heterogeneous disease by morphological, clinical, and genetic FUNDING criteria, underlining the need for targeted approaches. For a subset of recurrent mutations, such as FLT3, specific inhibitors This work was funded by Wellcome Investigator are in clinical use (2). For others, like cohesin mutations, Award 099276/Z/12/Z (MM). 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