In this issue of Haematologica, Vervoort et al.1 demonstrate that deletion of IKZF1 (IKZF1-del) in B-cell precursor acute lymphoblastic leukemia (BCP-ALL) promotes resistance to cytarabine (AraC). They further show that inhibition of either cyclin-dependent kinases 8/19 (CDK8/19) or casein kinase 2 (CK2) restores the sensitivity of IKZF1-del BCP-ALL to AraC treatment.
BCP-ALL is the most common childhood malignancy. Despite improved outcomes, more children die from BCP-ALL than from any other type of malignancy. In adults, BCP-ALL is an aggressive disease with high mortality and requires very toxic treatment. Advances in targeted chemotherapy along with immunotherapy have greatly improved the overall prognosis of BCP-ALL in both children and adults.2,3 However, relapsed BCP-ALL remains a clinical challenge, and is associated with high mortality.4 IKZF1 encodes IKAROS, a master regulator of hematopoiesis and a tumor suppressor protein.5,6 Deletion of a single copy of IKZF1 results in high-risk BCP-ALL, which very often has a gene expression profile that is characteristic of Philadelphia chromosome-like BCP-ALL, which is associated with a high relapse rate and a poor outcome.7 Resistance to chemotherapy is commonly observed in BCP-ALL with IKZF1-del.8 Several mechanisms have been proposed to explain these observations, along with therapeutic strategies to overcome resistance to particular chemotherapeutics.9 To understand the mechanism of chemoresistance in BCP-ALL due to IKZF1 deletion, Vervoort et al. used both a genetic (by CRISPR/Cas9) and pharmacological (administration of iberdomide) approach to target Ikaros. They tested IKAROS-null BCP-ALL cells for sensitivity to common chemotherapeutics and showed that the absence of IKZF1 is strongly associated with increased resistance to AraC. The causative role of IKZF1 deletion in increased resistance to AraC was demonstrated both in vitro and in vivo using patient-derived xenografts and a retrospective analysis of minimal residual disease in patients. Using a functional approach, the authors identified two mechanisms through which loss of Ikaros drives resistance to AraC treatment (Figure 1).
The first mechanism is decreased incorporation of AraC due to lower expression of the solute carrier human equilibrative nucleoside transporter 1 (hENT1). The expression of this transporter is critical for AraC influx, at the doses of AraC used for the treatment of BCP-ALL. The authors found that IKZF1-del cells have reduced AraCTP incorporation, which can explain the mechanism of resistance to AraC in those cells. Reduced expression of hENT1 has been associated with AraC sensitivity in both acute myeloid leukemia (AML) and BCP-ALL.
The second mechanism is increased expression of the Evi1 oncogene. Elevated expression of Evi1 has been associated with resistance to AraC and poor prognosis in AML. In addition, Evi1 functions as a regulator of hematopoietic stem cell self-renewal. Although the function of Evi1 has been studied extensively in AML and AML stem cells, alterations in Evi1 have not been associated with BCP-ALL. Based on the data presented by Vervoort et al., it is likely that Evi1 repression is also important for normal B-cell differentiation and that increased Evi1 expression can promote stemness and drug resistance in BCP-ALL. Since IKZF1 is located at chromosome 7p, and IKZF1 mutations have been observed in pediatric AML, these data suggest a potential mechanism of IKAROS tumor suppressor activity in AML – via repression of Evi1 expression.
The authors searched for a way to restore sensitivity to AraC in IKZF1-null BCP-ALL (Figure 1). They used a CRISPR/ Cas9-based loss-of-function screen that targets all human kinases. They identified that the loss of homologous CDK8/19 restores sensitivity of IKZF1-null BCP-ALL to AraC. The use of the small molecule inhibitor CCT251921, which specifically targets CDK8 and 19, in combination with AraC, showed a strong synergistic effect in both IKZF1-wildtype and IKZF1-null BCP-ALL cells. This suggests that this novel combination can be a potent treatment for different types of high-risk BCP-ALL, regardless of IKZF1 status. Additional findings showed that loss of CK2 restores sensitivity to AraC in IKZF1-null BCP-ALL. Combination treatment with the CK2 inhibitor silmitasertib and AraC showed strong synergistic cytotoxicity in both IKZF1-wildtype and IKZF1-null BCP-ALL. It has been shown previously that CK2 inhibition can sensitize IKZF1 haploinsufficient BCP-ALL via enhancement of IKAROS protein function produced from the remaining allele.10 The data presented in the paper by Vervoort et al. suggest that CK2 inhibition can induce chemosensitivity of BCP-ALL cells via other mechanisms. It is intriguing that CK2 enhances Evi1 activity through direct phosphorylation, which could be an additional mechanism of CK2 inhibition in leukemia.
Figure 1.Mechanisms of resistance to cytarabine treatment. Vervoort et al. demonstrate that IKZF1 deletion results in resistance to cytarabine (AraC) via reduced expression of hENT1 and increased expression of EVI1. Treatment with CDK8/19 or CK2 inhibitors restores sensitivity to AraC treatment.
This research has both basic/translational and clinical impacts on the BCP-ALL field in many different ways. First, it identifies a novel mechanism of drug resistance to chemotherapy in high-risk BCP-ALL. As the authors mentioned in the Discussion, awareness that IKZF1-del BCP-ALL is resistant to AraC treatment due to hENT1 repression suggests that the use of different nucleoside analogs (e.g., fludarabine and clofarabine) would be beneficial for patients since these analogs are imported by other transporters. Secondly, the research indicates a potential, novel targeted treatment for BCP-ALL, with the data providing a rationale for preclinical testing of silmitastetib and AraC combination therapy for BCP-ALL. Silmitasertib has been tested in phase I/II trials for other malignancies, which could further accelerate testing of this combination treatment in clinical trials. Thirdly, the role of Evi1 in the pathogenesis of BCP-ALL is examined, with the results suggesting a potential role of Evi1 in BCP-ALL drug resistance. Further testing of the utility of Evi1 expression as a biomarker for drug-resistant BCP-ALL is warranted. Finally, the role of IKZF1/IKAROS is considered. Most of the research concerning the tumor suppressive role of IKAROS is focused on BCP-ALL, despite evidence for a role of IKZF1 in the pathogenesis of AML. The data presented suggest that IKZF1 can regulate the expression of Evi1, a known oncogene in AML, which provides further support for studying the role of IKZF1 in AML. Overall, the studies by Vervoort et al. allow a better understanding of the role of IKZF1 in the chemoresistance of BCP-ALL and also provide a rationale for preclinical and clinical testing of these novel combination treatments for this disease.
Footnotes
- Received June 21, 2024
- Accepted July 5, 2024
Correspondence
Disclosures
No conflicts of interest to disclose.
Funding
The work in the laboratory of SD was supported by grants from the National Institute of Health (R01 CA278226), Hyundai Hope on Wheels Foundation and Four Diamonds Foundation. JS has been supported by Hyundai Hope on Wheels Foundation and Four Diamonds Foundation.
References
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