Clinical significance of early T-cell precursor acute lymphoblastic leukaemia: results of the Tokyo Children's Cancer Study Group Study L99-15

Targeting T cell malignancies using CAR-based immunotherapy: challenges and potential solutions

Chimeric antigen receptor (CAR) T cell therapy has been successful in treating B cell malignancies in clinical trials; however, fewer studies have evaluated CAR T cell therapy for the treatment of T cell malignancies. There are many challenges in translating this therapy for T cell disease, including fratricide, T cell aplasia, and product contamination. To the best of our knowledge, no tumor-specific antigen has been identified with universal expression on cancerous T cells, hindering CAR T cell therapy for these malignancies. Numerous approaches have been assessed to address each of these challenges, such as (i) disrupting target antigen expression on CAR-modified T cells, (ii) targeting antigens with limited expression on T cells, and (iii) using third party donor cells that are either non-alloreactive or have been genome edited at the T cell receptor α constant (TRAC) locus. In this review, we discuss CAR approaches that have been explored both in preclinical and clinical studies targeting T cell antigens, as well as examine other potential strategies that can be used to successfully translate this therapy for T cell disease.

The effect of co-occurring lesions on leukaemogenesis and drug response in T-ALL and ETP-ALL

Despite advances in the management of acute lymphoblastic leukaemia (ALL), current regimens fail to significantly transform outcomes for patients with high-risk subtypes. Advances in genomic analyses have identified novel lesions including mutations in genes that encode chromatin modifiers and those that influence cytokine and kinase signalling, rendering many of these alterations potentially targetable by tyrosine kinase and epigenetic inhibitors currently in clinical use. Although specific genomic lesions, gene expression patterns, and immunophenotypic profiles have been associated with specific clinical outcomes in some cancers, the application of precision medicine approaches based on these data has been slow. This approach is complicated by the reality that patients often harbour multiple mutations, and in many cases, the precise functional significance and interaction of these mutations in driving leukaemia and drug responsiveness/resistance remains unknown. Given that signalling pathways driving leukaemic pathogenesis could plausibly result from the co-existence of specific lesions and the resultant perturbation of protein interactions, the use of combined therapeutics that target multiple aberrant pathways, according to an individual’s mutational profile, might improve outcomes and lower a patient’s risk of relapse. Here we outline the genomic alterations that occur in T cell ALL (T-ALL) and early T cell precursor (ETP)-ALL and review studies highlighting the possible effects of co-occurring lesions on leukaemogenesis and drug response.

Multi-omic approaches to improve outcome for T-cell acute lymphoblastic leukemia patients

In the last decade, tremendous progress in curative treatment has been made for T-ALL patients using high-intensive, risk-adapted multi-agent chemotherapy. Further treatment intensification to improve the cure rate is not feasible as it will increase the number of toxic deaths. Hence, about 20% of pediatric patients relapse and often die due to acquired therapy resistance. Personalized medicine is of utmost importance to further increase cure rates and is achieved by targeting specific initiation, maintenance or resistance mechanisms of the disease. Genomic sequencing has revealed mutations that characterize genetic subtypes of many cancers including T-ALL. However, leukemia may have various activated pathways that are not accompanied by the presence of mutations. Therefore, screening for mutations alone is not sufficient to identify all molecular targets and leukemic dependencies for therapeutic inhibition. We review the extent of the driving type A and the secondary type B genomic mutations in pediatric T-ALL that may be targeted by specific inhibitors. Additionally, we review the need for additional screening methods on the transcriptional and protein levels. An integrated 'multi-omic' screening will identify potential targets and biomarkers to establish significant progress in future individualized treatment of T-ALL patients.

Targeting steroid resistance in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is characterized by a variable response to steroids during induction and/or consolidation therapy. Notably, recent work suggested that these differences in glucocorticoid sensitivity might, at least in part, be mediated by hyperactivation of specific oncogenic pathways such as RAS/MEK/ERK, PI3K/AKT and IL7R/JAK/STAT. In this review, we elaborate on putative associations between aberrant signaling, therapy resistance, incidence of relapse and clinical outcome in human T-ALL. Furthermore, we emphasize that this potential association with clinical parameters might also be mediated by the tumor microenvironment as a result of increased sensitivity of leukemic T-cells towards cytokine induced signaling pathway activation. With this in mind, we provide an overview of small molecule inhibitors that might have clinical potential for the treatment of human T-ALL in the near future as a result of their ability to overcome steroid resistance thereby potentially increasing survival rates in this aggressive hematological neoplasm.

Early T precursor acute lymphoblastic leukaemia/lymphoma shows differential immunophenotypic characteristics including frequent CD33 expression and in vitro response to targeted CD33 therapy

The differential immunophenotypic characteristics of early T precursor (ETP) acute lymphoblastic leukaemia/lymphoma (ALL) remain incompletely characterized. The study group (n = 142) included 106 (74·7%) men and 36 (25·3%) women with a median age of 34·9 years (range, 2-79) at diagnosis. Patients were subtyped by flow cytometry immunophenotyping as follows: 33 (23·2%) ETP; 32 (22·5%) early non-ETP; 60 (42·2%) thymic; and 17 (12·1%) mature. Excepting definitional markers, there was a significant differential expression of the markers CD2, CD10, CD33 and TdT between ETP-ALL and non-ETP-ALL. Positive CD33 expression (≥20% of leukaemic blasts) was detected in 21/33 (63%) ETP-ALL compared with 17/95 (17·9%) non-ETP-ALL (P < 0·001). Notably, targeted anti-CD33 therapy with IMGN779 resulted in significant growth inhibition and increased apoptosis in ETP-ALL cells in vitro. An 11-marker T-ALL immunophenotype score discriminated reliably between ETP and non-ETP ALL. Longitudinal analysis of ETP-ALL cases in this study demonstrated that the immunophenotype may be occasionally dynamic but is largely stable over the disease course. In summary, identification of ETP-ALL might be enhanced by using an 11-marker T-ALL immunophenotype score. CD33 expression is frequent in ETP-ALL, and in vitro data suggest that exploring anti-CD33 therapy in ETP-ALL is warranted.

T-lymphoid/myeloid mixed phenotype acute leukemia and early T-cell precursor lymphoblastic leukemia similarities with NOTCH1 mutation as a good prognostic factor

Purpose: T-lymphoid/Myeloid Mixed phenotype acute leukemia (T/M-MPAL) is ambiguous leukemia which overlaps with early T-cell precursor lymphoblastic leukemia (ETP-ALL). We have revisited the immunophenotyping profile of T/M-MPAL and ETP-ALL to identify differences and/or similarities, as these entities represent a therapeutic challenge in clinical practice. Patients and methods: A total of 26 ETP-ALL and 10 T/M-MPAL cases were identified among 857 cases of childhood leukemia (T-ALL, n=266 and AML, n=591) before any treatment decisions. The variables analyzed were age strata, sex, clinical features, immunophenotyping, and molecular aberrations. Immunophenotyping was performed in all samples using a panel of cytoplasm and membrane antibodies to identify the lineage and blast differentiation. The mutational status of STIL-TAL1, TLX3, RUNX1, NOTCH1, FBXW7, FLT3, IL7R, RAS, KTM2A, and CDKN2A/B was tested using RT-PCR, FISH, and PCR sequencing methods. The outcomes were assessed in terms of overall survival (OS). Results: The immunophenotypes were similar in ETP-ALL and T/M-MPAL, regarding the cellular expression of CD34, CD117, CD13/CD33, and CD11b, although CD2 and HLA-DR were more frequent in T/M-MPAL (p<0.01). aMPO positivity and myelomonocyte differentiation were definitive in separating both entities. NOTCH1, FLT3-ITD, and N/KRAS mutations as well as TLX3 and KMT2A rearrangements were found in both ETP-ALL and T/M-MPAL. Thirty-one patients received ALL protocol whereas five had AML therapy. The overall 5-year survival rate (pOS) was 56.4% for patients treated using ALL protocols. No differences were observed between T/M-MPAL (pOS of 57%) and ETP-ALL (pOS of 56%) patients. The prognostic value of NOTCH1mut was associated with significantly better OS (pOS 90%) than NOTCH1 wt (pOS 37%) (p=0.017). Conclusion: This research can potentially contribute to NOTCH1 as targeted therapy and prognostic assessment of T-cell mixed phenotype leukemia.

The Profile of Immunophenotype and Genotype Aberrations in Subsets of Pediatric T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a biologically heterogeneous malignancy, which reflects distinctive stages of T-cell differentiation arrest. We have revisited a cohort of pediatric T-ALL, in order to test if immunophenotypes associated with molecular alterations would predict the patient's outcome. Genetic mutations, translocations and copy number alterations were identified through Sanger sequencing, RT-PCR, FISH and multiplex ligation-dependent probe amplification (MLPA). We defined 8 immunophenotypic T-ALL subtypes through multiparametric flow cytometry: early T-cell precursor (ETP, n = 27), immature (n = 38), early cortical (n = 15), cortical (n = 50), late cortical (n = 53), CD4/CD8 double negative mature (n = 31), double positive mature (n = 35) and simple positive mature (n = 31) T-ALL. Deletions (del) or amplifications (amp) in at least one gene were observed in 87% of cases. The most frequent gene alterations were CDKN2A/B^del (71.4%), NOTCH1^mut (47.6%) and FBXW7^mut (17%). ETP-ALL had frequent FLT3^mut (22.2%) and SUZ12^del (16.7%) (p < 0.001), while CDKN2A/B^del were rarely found in this subtype (p < 0.001). The early cortical T-ALL subtype had high frequencies of NOTCH1^mut and IL7R^mut (71%, 28.6%, respectively), whereas, mature T-ALL with double positive CD4/CD8 had the highest frequencies of STIL-TAL1 (36.7%), LEF1^del (27.3%) and CASP8AP2^del (22.7%). The co-existence of two groups of T-ALL with NOTCH1^mut/IL7R^mut, and with TLX3/SUZ12^del/NF1^del/IL7R^mut, were characterized with statistical significance (p < 0.05) but only STIL-TAL1 (pOS 47.5%) and NOTCH1^WT/FBXW7^WT (pOS 55.3%) are predictors of poor T-ALL outcomes. In conclusion, we have observed that 8 T-ALL subgroups are characterized by distinct molecular profiles. The mutations in NOTCH1/FBXW7 and STIL-TAL1 rearrangement had a prognostic impact, independent of immunophenotype.

Recent Advances in the Biology and Treatment of T Cell Acute Lymphoblastic Leukemia

PURPOSE OF REVIEW

This article provides an overview of the current knowledge regarding the biology and treatment of T cell acute lymphoblastic leukemia (T-ALL) and highlights the most recent findings in this field over the past 5 years.

RECENT FINDINGS

Remarkable progress has been made in the genomic landscape of T-ALL over the past few years. The discovery of activating mutations of NOTCH1 and FBXW7 in a majority of patients has been a seminal observation, with several early phase clinical trials currently exploring these as potential therapeutic targets. Characterization of early T cell precursor ALL, incorporation of minimal residual disease assessment into therapeutic protocols, and use of pediatric-intensive regimens along with judicious use of allogeneic HCT have significantly improved risk stratification and treatment outcomes. Improved risk stratification and the use of novel targeted therapies based on recent genomic discoveries are expected to change the therapeutic landscape of T-ALL and hopefully improve the outcomes of this historically poor prognosis disease.

CD44 Expression Profile Varies According to Maturational Subtypes and Molecular Profiles of Pediatric T-Cell Lymphoblastic Leukemia

CD44 is a glycoprotein expressed in leucocytes and a marker of leukemia-initiating cells, being shown to be important in the pathogenesis of T cell acute lymphoblastic leukemia (T-ALL). In this study, we have (i) identified the aberrant antigenic pattern of CD44 and its isoform CD44v6 in T-ALL; (ii) tested the association with different T-cell subtypes and genomic alterations; (iii) identified the impact of CD44 status in T-ALL outcome. Samples from 184 patients (123 T-ALL and 61 AML; <19 years) were analyzed throughout multiparametric flow cytometry. Mutations in N/KRAS, NOTCH1, FBXW7 as well as STIL-TAL1 and TLX3 rearrangements were detected using standard molecular techniques. CD44 expression was characterized in all T-ALL and AML cases. Compared with AML samples in which the median fluorescence intensity (MFI) was 79.1 (1–1272), T-ALL was relatively low, with MFI 43.2 (1.9–1239); CD44v6 expression was rarely found, MFI 1 (0.3-3.7). T-ALL immature subtypes (mCD3/CD1a^neg) had a lower CD44 expression, MFI 57.5 (2.7–866.3), whereas mCD3/TCRγδ^pos cases had higher expressions, MFI 99.9 (16.4–866.3). NOTCH1^mut and STIL-TAL1 were associated with low CD44 expression, whereas N/KRAS^mut and FBXW7^mut cases had intermediate expression. In relation to clinical features, CD44 expression was associated with tumor infiltrations (p = 0.065). However, no association was found with initial treatment responses and overall survival prediction. Our results indicate that CD44 is aberrantly expressed in T-ALL being influenced by different genomic alterations. Unraveling this intricate mechanism is required to place CD44 as a therapeutic target in T-ALL.

Expression of CD56 defines a distinct subgroup in childhood T-ALL with inferior outcome. Results of the ALL-BFM 2000 trial

This study reports the prognostic impact of the expression of the natural killer cell marker CD56 in a large series of risk-adapted paediatric patients with T cell acute lymphoblastic leukaemia (T-ALL; n = 493) treated within the ALL-Berlin-Frankfurt-Münster (BFM) 2000 protocol. The immunophenotype was analysed centrally at diagnosis using flow cytometry and correlated with clinical parameters and outcome. CD56 expression was detected in 7·1% and early T-cell precursor (ETP) phenotype in 6·7% of all T-ALL patients. The percentage of ETP in the CD56+ T-ALL cohort was 4-fold higher than in the whole cohort. CD56+ T-ALL frequently expressed the progenitor marker CD34 and myeloid antigens CD13 and CD33. The 5-year event-free survival (EFS) rates for the European Group for the Immunological classification of Leukaemias/World Health Organization subgroups and the ETP phenotype were not statistically different. By contrast, patients with CD56 expression had a significantly reduced EFS (60 ± 8%) and overall survival (60 ± 8%) at 5 years, with a hazard ratio of 2·46 (P = 0·002) and 2·99 (P < 0·001), respectively. Moreover, CD56 expression in combination with the minimal residual disease (MRD)-based high risk assignment defined a population with a 'very-high' risk probability of relapse in the ALL-BFM 2000 trial. The CD56 marker has the potential to augment MRD-based risk stratification and may serve as a molecular target for antibody-based treatment strategies in childhood T-ALL.

Clinical Features and Prognosis According to Immunophenotypic Subtypes Including the Early T-Cell Precursor Subtype of T-Lymphoblastic Lymphoma in the Japanese Pediatric Leukemia/Lymphoma Study Group ALB-NHL03 Study

We reviewed the immunophenotypic subtypes of pediatric T-cell lymphoblastic lymphoma in the Japanese Pediatric Leukemia/Lymphoma Study Group ALB-NHL03 study. Of the 104 patients, 40 patients each had sufficient data to evaluate the immunophenotypes and early T-cell precursor (ETP) subtype. Pro-T, pre-T, intermediate T, and mature T cells were observed in 1, 9, 21, and 9 cases, respectively. The 3-year event-free survival (EFS) rates of those with pro-T/pre-T, intermediate T, and mature T cells were 80.0±12.6%, 71.4±9.9%, and 88.9±10.5%, respectively (P=0.546). There were 8 and 32 cases of ETP and non-ETP subtypes, with 3-year EFS rates of 75.0±15.3% and 71.9±8.0%, respectively (P=0.828), indicating that the immunophenotypic subtype was not predictive of EFS in this study.

Blockade of CD7 expression in T cells for effective chimeric antigen receptor targeting of T-cell malignancies

Effective immunotherapies for T-cell malignancies are lacking. We devised a novel approach based on chimeric antigen receptor (CAR)-redirected T lymphocytes. We selected CD7 as a target because of its consistent expression in T-cell acute lymphoblastic leukemia (T-ALL), including the most aggressive subtype, early T-cell precursor (ETP)-ALL. In 49 diagnostic T-ALL samples (including 14 ETP-ALL samples), median CD7 expression was >99%; CD7 expression remained high at relapse (n = 14), and during chemotherapy (n = 54). We targeted CD7 with a second-generation CAR (anti-CD7-41BB-CD3ζ), but CAR expression in T lymphocytes caused fratricide due to the presence of CD7 in the T cells themselves. To downregulate CD7 and control fratricide, we applied a new method (protein expression blocker [PEBL]), based on an anti-CD7 single-chain variable fragment coupled with an intracellular retention domain. Transduction of anti-CD7 PEBL resulted in virtually instantaneous abrogation of surface CD7 expression in all transduced T cells; 2.0% ± 1.7% were CD7⁺ vs 98.1% ± 1.5% of mock-transduced T cells (n = 5; P < .0001). PEBL expression did not impair T-cell proliferation, interferon-γ and tumor necrosis factor-α secretion, or cytotoxicity, and eliminated CAR-mediated fratricide. PEBL-CAR T cells were highly cytotoxic against CD7⁺ leukemic cells in vitro and were consistently more potent than CD7⁺ T cells spared by fratricide. They also showed strong anti-leukemic activity in cell line- and patient-derived T-ALL xenografts. The strategy described in this study fits well with existing clinical-grade cell manufacturing processes and can be rapidly implemented for the treatment of patients with high-risk T-cell malignancies.

Aberrant Signaling Pathways in T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive disease caused by the malignant transformation of immature progenitors primed towards T-cell development. Clinically, T-ALL patients present with diffuse infiltration of the bone marrow by immature T-cell blasts high blood cell counts, mediastinal involvement, and diffusion to the central nervous system. In the past decade, the genomic landscape of T-ALL has been the target of intense research. The identification of specific genomic alterations has contributed to identify strong oncogenic drivers and signaling pathways regulating leukemia growth. Notwithstanding, T-ALL patients are still treated with high-dose multiagent chemotherapy, potentially exposing these patients to considerable acute and long-term side effects. This review summarizes recent advances in our understanding of the signaling pathways relevant for the pathogenesis of T-ALL and the opportunities offered for targeted therapy.

T cell acute lymphoblastic leukemia (T-ALL): New insights into the cellular origins and infiltration mechanisms common and unique among hematologic malignancies

T-cell acute lymphoblastic leukemia (T-ALL) accounts for 15% and 25% of total childhood and adult ALL cases, respectively. During T-ALL, patients are at risk of organ infiltration by leukemic T-cells. Infiltration is a major consequence of disease relapse and correlates with poor prognosis. Transendothelial migration of leukemic cells is required to exit the blood stream into target organs. While mechanisms of normal T-cell transmigration are well known, the mechanisms of leukemic T-cell extravasation remain elusive; but involvement of chemokines, integrins and Notch signaling play critical roles. Here, we summarize current knowledge about molecular mechanisms of leukemic T-cell infiltration with special emphasis on the newly identified subtype early T-cell-progenitor (ETP)-ALL. Furthermore, we compare the extravasation potential of T-ALL cells with that of other hematologic malignancies such as B-ALL and acute myeloid leukemia (AML).

Immunophenotypic analysis of adult patients with T-cell lymphoblastic lymphoma treated with hyper-CVAD

OBJECTIVES

Immunophenotype is an important prognostic factor for childhood and adult T-cell acute lymphoblastic leukemia. However, immunophenotypic data from adult patients with T-cell lymphoblastic lymphoma (T-LBL) are scarcely available.

METHODS

Subjects were unselected adult patients with T-LBL who were treated with intensive chemotherapy. Immunophenotyping of tumor cells was performed according to standard techniques.

RESULTS

A total of eight patients with a median age of 31 years were analyzed who received hyper-CVAD treatment for LBL. Immunophenotypic analysis showed that the most common tumor type was cortical T-cell type [early T (n = 2), cortical T (n = 4), and medullary T (n = 2)]. Two patients diagnosed with early T-cell type had early disease progression.

CONCLUSIONS

Assessment of T-cell differentiation stages in malignant T lymphoblasts would be important in choosing treatment strategies for adult patients with T-LBL.

The genomic landscape of pediatric and young adult T-lineage acute lymphoblastic leukemia

Genetic alterations that activate NOTCH1 signaling and T cell transcription factors, coupled with inactivation of the INK4/ARF tumor suppressors, are hallmarks of T-lineage acute lymphoblastic leukemia (T-ALL), but detailed genome-wide sequencing of large T-ALL cohorts has not been carried out. Using integrated genomic analysis of 264 T-ALL cases, we identified 106 putative driver genes, half of which had not previously been described in childhood T-ALL (for example, CCND3, CTCF, MYB, SMARCA4, ZFP36L2 and MYCN). We describe new mechanisms of coding and noncoding alteration and identify ten recurrently altered pathways, with associations between mutated genes and pathways, and stage or subtype of T-ALL. For example, NRAS/FLT3 mutations were associated with immature T-ALL, JAK3/STAT5B mutations in HOXA1 deregulated ALL, PTPN2 mutations in TLX1 deregulated T-ALL, and PIK3R1/PTEN mutations in TAL1 deregulated ALL, which suggests that different signaling pathways have distinct roles according to maturational stage. This genomic landscape provides a logical framework for the development of faithful genetic models and new therapeutic approaches.

The genomic landscape of pediatric and young adult T-lineage acute lymphoblastic leukemia

Genetic alterations that activate NOTCH1 signaling and T cell transcription factors, coupled with inactivation of the INK4/ARF tumor suppressors, are hallmarks of T-lineage acute lymphoblastic leukemia (T-ALL), but detailed genome-wide sequencing of large T-ALL cohorts has not been carried out. Using integrated genomic analysis of 264 T-ALL cases, we identified 106 putative driver genes, half of which had not previously been described in childhood T-ALL (for example, CCND3, CTCF, MYB, SMARCA4, ZFP36L2 and MYCN). We describe new mechanisms of coding and noncoding alteration and identify ten recurrently altered pathways, with associations between mutated genes and pathways, and stage or subtype of T-ALL. For example, NRAS/FLT3 mutations were associated with immature T-ALL, JAK3/STAT5B mutations in HOXA1 deregulated ALL, PTPN2 mutations in TLX1 deregulated T-ALL, and PIK3R1/PTEN mutations in TAL1 deregulated ALL, which suggests that different signaling pathways have distinct roles according to maturational stage. This genomic landscape provides a logical framework for the development of faithful genetic models and new therapeutic approaches.

Early Response-Based Therapy Stratification Improves Survival in Adult Early Thymic Precursor Acute Lymphoblastic Leukemia: A Group for Research on Adult Acute Lymphoblastic Leukemia Study

Purpose Early thymic precursor (ETP) acute lymphoblastic leukemia (ALL) is an immunophenotypically defined subgroup of T-cell ALL (T-ALL) associated with high rates of intrinsic treatment resistance. Studies in children have shown that the negative prognostic impact of chemotherapy resistance is abrogated by the implementation of early response-based intensification strategies. Comparable data in adults are lacking. Patients and Methods We performed comprehensive clinicobiologic, genetic, and survival analyses of a large cohort of 213 adult patients with T-ALL, including 47 patients with ETP-ALL, treated in the GRAALL (Group for Research on Adult Acute Lymphoblastic Leukemia) -2003 and -2005 studies. Results Targeted next-generation sequencing revealed that the genotype of immunophenotypically defined adult T-ALL is similar to the pediatric equivalent, with high rates of mutations in factors involved in cytokine receptor and RAS signaling (62.2%), hematopoietic development (29.7%), and chemical modification of histones (48.6%). In contrast to pediatric cases, mutations in DNA methylation factor genes were also common (32.4%). We found that despite expected high levels of early bone marrow chemotherapy resistance (87%), the overall prognosis for adults with ETP-ALL treated using the GRAALL protocols was not inferior to that of the non-ETP-ALL group (5-year overall survival: ETP, 59.6%; 95% CI, 44.2% to 72.0% v non-ETP, 66.5%; 95% CI, 58.7% to 73.2%; P = 0.33) and that allogeneic stem-cell transplantation had a beneficial effect in a large proportion of patients with ETP-ALL. Conclusion Our results suggest that the use of response-based risk stratification and therapy intensification abrogates the poor prognosis of adult ETP-ALL.

Outcome and Clinical Significance of Immunophenotypic Markers Expressed in Different Treatment Protocols of Pediatric Patients With T-ALL in Developing Countries

BACKGROUND

T-cell acute lymphoblastic leukemia (T-ALL) accounts for about 15% of pediatric ALL. With wider use of intensive chemotherapy, the prognosis for childhood T-ALL has improved. Further gains in treatment outcome will likely require methods to identify patients who continue to fail on contemporary protocols. This study aimed to evaluate pediatric patients with T-ALL at 2 different Arabic cancer centers regarding their clinicopathologic, immunophenotypic, and cytogenetic features and outcome.

PATIENTS AND METHODS

This retrospective study included all children with T-ALL treated between 2003 and 2013 at 2 oncology centers in the Middle East. Patients were divided into (group I) treated with Berlin-Frankfurt-Münster (BFM)-90 treatment protocol between February 2003 and June 2007 and (group II) includes all patients treated thereafter by the Total Therapy Study XIII protocol for high-risk ALL.

RESULTS

This study included 103 patients with a median age of 8.9 years. The male to female ratio was 2.6:1. The median initial white blood cell count was 123 × 10⁹/L. Central nervous system leukemia was detected in 15%. The early T-cell precursor (ETP)-ALL phenotype was found in 16.5%. The 5-year overall survival was 20.7% ± 67.5% and 72.9% ± 5.7% (P < .01); the 5-year disease-free survival was 47.1% ± 13.8% and 77.3% ± 6.0% (P = .023); and the 5-year event-free survival was 28.6% ± 12.1% and 71.1% ± 6.2% (P = .003) for group I and II, respectively.

CONCLUSION

The outcome of patients with T-ALL significantly improved in patients who received the treatment protocol of ALL with high-risk criteria. This protocol eliminates the bad outcomes effect of several clinical and immunophenotypic markers. Patient with the ETP-ALL phenotype had a nonsignificant inferior outcome compared with the non-ETP-ALL group.

T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is biologically distinct from its B lymphoblastic (B-ALL) counterpart and shows different kinetic patterns of disease response. Although very similar regimens are used to treat T-ALL and B-ALL, distinctions in response to different elements of therapy have been observed. Similar to B-ALL, the key prognostic determinant in T-ALL is minimal residual disease (MRD) response. Unlike B-ALL, other factors including age, white blood cell count at diagnosis, and genetics of the ALL blasts are not independently prognostic when MRD response is included. Recent insights into T-ALL biology, using modern genomic techniques, have identified a number of recurrent lesions that can be grouped into several targetable pathways, including Notch, Jak/Stat, PI3K/Akt/mTOR, and MAPK. With contemporary chemotherapy, outcomes for de novo T-ALL have steadily improved and now approach those observed in B-ALL, with approximately 85% 5-year event-free survival. Unfortunately, salvage has remained poor, with less than 25% event-free and overall survival rates for relapsed disease. Thus, current efforts are focused on preventing relapse by augmenting therapy for high-risk patients, sparing toxicity in favorable subsets and developing new approaches for the treatment of recurrent disease.

Early T-Cell Precursor Acute Lymphoblastic Leukemia in an Infant With an NRAS Q61R Mutation and Clinical Features of Juvenile Myelomonocytic Leukemia

Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is a subtype of T-acute lymphoblastic leukemia (T-ALL) arising from a primitive precursor. We present a unique case of an infant with ETP-ALL with a missense NRAS mutation in codon 61 (c.182A>G, p.Q61R). The patient also had a minor population of non-ETP T-ALL blasts and clinical features typically associated with juvenile myelomonocytic leukemia (JMML), namely, absolute monocytosis, splenomegaly, and elevated hemoglobin F. The treatment was initiated with chemotherapy, followed by cord blood transplantation. The patient achieved remission, but unfortunately died from transplant-related complications. This case highlights an NRAS mutation in ETP-ALL with JMML-like phenotype.

The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia

The World Health Organization (WHO) classification of tumors of the hematopoietic and lymphoid tissues was last updated in 2008. Since then, there have been numerous advances in the identification of unique biomarkers associated with some myeloid neoplasms and acute leukemias, largely derived from gene expression analysis and next-generation sequencing that can significantly improve the diagnostic criteria as well as the prognostic relevance of entities currently included in the WHO classification and that also suggest new entities that should be added. Therefore, there is a clear need for a revision to the current classification. The revisions to the categories of myeloid neoplasms and acute leukemia will be published in a monograph in 2016 and reflect a consensus of opinion of hematopathologists, hematologists, oncologists, and geneticists. The 2016 edition represents a revision of the prior classification rather than an entirely new classification and attempts to incorporate new clinical, prognostic, morphologic, immunophenotypic, and genetic data that have emerged since the last edition. The major changes in the classification and their rationale are presented here.

An early thymic precursor phenotype predicts outcome exclusively in HOXA-overexpressing adult T-cell acute lymphoblastic leukemia: a Group for Research in Adult Acute Lymphoblastic Leukemia study

Gene expression studies have consistently identified a HOXA-overexpressing cluster of T-cell acute lymphoblastic leukemias, but it is unclear whether these constitute a homogeneous clinical entity, and the biological consequences of HOXA overexpression have not been systematically examined. We characterized the biology and outcome of 55 HOXA-positive cases among 209 patients with adult T-cell acute lymphoblastic leukemia uniformly treated during the Group for Research on Adult Acute Lymphoblastic Leukemia (GRAALL)-2003 and -2005 studies. HOXA-positive patients had markedly higher rates of an early thymic precursor-like immunophenotype (40.8% versus 14.5%, P=0.0004), chemoresistance (59.3% versus 40.8%, P=0.026) and positivity for minimal residual disease (48.5% versus 23.5%, P=0.01) than the HOXA-negative group. These differences were due to particularly high frequencies of chemoresistant early thymic precursor-like acute lymphoblastic leukemia in HOXA-positive cases harboring fusion oncoproteins that transactivate HOXA Strikingly, the presence of an early thymic precursor-like immunophenotype was associated with marked outcome differences within the HOXA-positive group (5-year overall survival 31.2% in HOXA-positive early thymic precursor versus 66.7% in HOXA-positive non-early thymic precursor, P=0.03), but not in HOXA-negative cases (5-year overall survival 74.2% in HOXA-negative early thymic precursor versus 57.2% in HOXA-negative non-early thymic precursor, P=0.44). Multivariate analysis further revealed that HOXA positivity independently affected event-free survival (P=0.053) and relapse risk (P=0.039) of chemoresistant T-cell acute lymphoblastic leukemia. These results show that the underlying mechanism of HOXA deregulation dictates the clinico-biological phenotype, and that the negative prognosis of early thymic precursor acute lymphoblastic leukemia is exclusive to HOXA-positive patients, suggesting that early treatment intensification is currently suboptimal for therapeutic rescue of HOXA-positive chemoresistant adult early thymic precursor acute lymphoblastic leukemia.

TRIAL REGISTRATION

The GRAALL-2003 and -2005 studies were registered at http://www.clinicaltrials.gov as #NCT00222027 and #NCT00327678, respectively.

Early T-cell precursor acute lymphoblastic leukaemia in children treated in AIEOP centres with AIEOP-BFM protocols: a retrospective analysis

BACKGROUND

Early T-cell precursor acute lymphoblastic leukaemia was recently recognised as a distinct leukaemia and reported as associated with poor outcomes. We aimed to assess the outcome of early T-cell precursor acute lymphoblastic leukaemia in patients from the Italian Association of Pediatric Hematology Oncology (AIEOP) centres treated with AIEOP-Berlin-Frankfurt-Münster (AIEOP-BFM) protocols.

METHODS

In this retrospective analysis, we included all children aged from 1 to less than 18 years with early T-cell precursor acute lymphoblastic leukaemia immunophenotype diagnosed between Jan 1, 2008, and Oct 31, 2014, from AIEOP centres. Early T-cell precursors were defined as being CD1a and CD8 negative, CD5 weak positive or negative, and positive for at least one of the following antigens: CD34, CD117, HLADR, CD13, CD33, CD11b, or CD65. Treatment was based on AIEOP-BFM acute lymphoblastic leukaemia 2000 (NCT00613457) or AIEOP-BFM acute lymphoblastic leukaemia 2009 protocols (European Clinical Trials Database 2007-004270-43). The main differences in treatment and stratification of T-cell acute lymphoblastic leukaemia between the two protocols were that in the 2009 protocol only, pegylated L-asparaginase was substituted for Escherichia coli L-asparaginase, patients with prednisone poor response received an additional dose of cyclophosphamide at day 10 of phase IA, and high minimal residual disease at day 15 assessed by flow cytometry was used as a high-risk criterion. Outcomes were assessed in terms of event-free survival, disease-free survival, and overall survival.

FINDINGS

Early T-cell precursor acute lymphoblastic leukaemia was diagnosed in 49 patients. Compared with overall T-cell acute lymphoblastic leukaemia, it was associated with absence of molecular markers for PCR detection of minimal residual disease in 25 (56%) of 45 patients; prednisone poor response in 27 (55%) of 49 patients; high minimal residual disease at day 15 after starting therapy in 25 (64%) of 39 patients (bone marrow blasts ≥ 10%, by flow cytometry); no complete remission after phase IA in 7 (15%) of 46 patients (bone marrow blasts ≥ 5%, morphologically); and high PCR minimal residual disease (≥ 5 × 10(-4)) at day 33 after starting therapy in 17 (85%) of 20 patients with markers available. Overall, 38 (78%) of 49 patients are in continuous complete remission, including 13 of 18 after haemopoietic stem cell transplantation, with three deaths in induction, five deaths after haemopoietic stem cell transplantation, and three relapses. Severe adverse events in the 2009 study were reported in 10 (30%) of 33 patients with early T-cell precursor acute lymphoblastic leukaemia versus 24 (15%) of 164 patients without early T-cell precursor acute lymphoblastic leukaemia and life-threatening events in induction phase IA occurred in 4 (12%) of 33 patients with early T-cell precursor acute lymphoblastic leukaemia versus 7 (4%) of 164 patients without early T-cell precursor acute lymphoblastic leukaemia. No difference was seen in the subsequent consolidation phase IB of protocol I.

INTERPRETATION

Early T-cell precursor acute lymphoblastic leukaemia is characterised by poor early response to conventional induction treatment. Consolidation phase IB, based on cyclophosphamide, 6-mercaptopurine, and ara-C at conventional (non-high) doses is effective in reducing minimal residual disease. Although the number of patients and observational time are limited, patients with early T-cell precursor acute lymphoblastic leukaemia treated with current BFM stratification and treatment strategy have a favourable outcome compared with earlier reports. The role of innovative therapies and haemopoietic stem cell therapy in early T-cell precursor acute lymphoblastic leukaemia needs to be assessed.

FUNDING

None.

Early T-cell precursor acute lymphoblastic leukemia/lymphoma (ETP-ALL/LBL) in adolescents and adults: a high-risk subtype

Early T-cell precursor (ETP) acute lymphoblastic leukemia/lymphoma (ALL/LBL) is a recently recognized high-risk T lymphoblastic leukemia/lymphoma (T-ALL/LBL) subgroup. The optimal therapeutic approaches to adult patients with ETP-ALL/LBL are poorly characterized. In this study, we compared the outcomes of adults with ETP-ALL/LBL who received treatment on frontline regimens with those of patients with other T-ALL/LBL immunophenotypic subtypes. Patients with newly diagnosed T-ALL/LBL who received frontline chemotherapy between the years 2000 and 2014 at The University of Texas MD Anderson Cancer Center were identified and immunophenotypically categorized into early, thymic, and mature per the World Health Organization (WHO) classification using CD1a and surface CD3 status. Patients with ETP-ALL/LBL were identified on the basis of the following immunophenotypes: CD1a(-), CD8(-), CD5(-)(dim), and positivity for 1 or more stem cell or myeloid antigens. A total of 111 patients with T-ALL/LBL (68% T-ALL; 32% T-LBL) with adequate immunophenotype data were identified. The median age was 30 years (range, 13-79). There was no difference in the outcomes of patients based on the WHO subtypes. Nineteen patients (17%) had ETP-ALL/LBL. The complete remission rate /complete remission with incomplete platelet recovery rate in patients with ETP-ALL/LBL was significantly lower than that of non-ETP-ALL/LBL patients (73% vs 91%;P= .03). The median overall survival for patients with ETP-ALL/LBL was 20 months vs not reached for the non-ETP-ALL/LBL patients (P= .008). ETP-ALL/LBL represents a high-risk disease subtype of adult ALL. Novel treatment strategies are needed to improve treatment outcomes in this T-ALL/LBL subset.

The Functional Role of PRC2 in Early T-cell Precursor Acute Lymphoblastic Leukemia (ETP-ALL) - Mechanisms and Opportunities

Early T-Cell precursor acute lymphoblastic leukemia (ETP-ALL) is a relatively newly identified subset of T-lineage ALL. There are conflicting results regarding prognosis, and the genetic basis of this condition is variable. Here, we summarize the current status of the field and discuss the role of mutations in the Polycomb Repressive Complex 2 frequently identified in ETP-ALL patients.

High-Level Expression, Purification and Large-Scale Production of l-Methionine γ-Lyase from Idiomarina as a Novel Anti-Leukemic Drug

l-Methionine γ-lyase (MGL), a pyridoxal 5'-phosphate-dependent enzyme, possesses anti-tumor activity. However, the low activity of MGL blocks the anti-tumor effect. This study describes an efficient production process for the recombinant MGL (rMGL) from Idiomarina constructed using the overexpression plasmid in Escherichia coli BL21 (DE3), purification, and large-scale production. The enzyme produced by the transformants accounted for 53% of the total proteins and accumulated at 1.95 mg/mL using a 500 L fermentor. The enzyme was purified to approximately 99% purity using a high-pressure mechanical homogenizer and nickel (Ni) Sepharose 6 Fast Flow (FF) chromatography. Then, the enzyme was polished by gel filtration, the endotoxins were removed using diethyl-aminoethanol (DEAE) Sepharose FF, and the final product was lyophilized with a vacuum freeze dryer at -35 °C. The specific activity of rMGL in the lyophilized powder was up to 108 U/mg. Compared to the control, the enzyme significantly inhibited cellular proliferation in a concentration-dependent manner as tested using the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) assay and induced cellular apoptosis as analyzed by Annexin V-fluorescein isothiocyanate (FITC) with fluorescence-activated cell sorting (FACS) in leukemia cells. This paper demonstrated the cloning, overexpression, and large-scale production protocols for rMGL, which enabled rMGL to be used as a novel anti-leukemic drug.

Novel biological insights in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive type of blood cancer that accounts for about 15% of pediatric and 25% of adult acute lymphoblastic leukemia (ALL) cases. It is considered as a paradigm for the multistep nature of cancer initiation and progression. Genetic and epigenetic reprogramming events, which transform T-cell precursors into malignant T-ALL lymphoblasts, have been extensively characterized over the past decade. Despite our comprehensive understanding of the genomic landscape of human T-ALL, leukemia patients are still treated by high-dose multiagent chemotherapy, potentially followed by hematopoietic stem cell transplantation. Even with such aggressive treatment regimens, which are often associated with considerable acute and long-term side effects, about 15% of pediatric and 40% of adult T-ALL patients still relapse, owing to acquired therapy resistance, and present with very dismal survival perspectives. Unfortunately, the molecular mechanisms by which residual T-ALL tumor cells survive chemotherapy and act as a reservoir for leukemic progression and hematologic relapse remain poorly understood. Nevertheless, it is expected that enhanced molecular understanding of T-ALL disease biology will ultimately facilitate a targeted therapy driven approach that can reduce chemotherapy-associated toxicities and improve survival of refractory T-ALL patients through personalized salvage therapy. In this review, we summarize recent biological insights into the molecular pathogenesis of T-ALL and speculate how the genetic landscape of T-ALL could trigger the development of novel therapeutic strategies for the treatment of human T-ALL.

Redefining ALL classification: toward detecting high-risk ALL and implementing precision medicine

Acute lymphoblastic leukemia (ALL) is the commonest childhood tumor and remains a leading cause of cancer death in the young. In the last decade, microarray and sequencing analysis of large ALL cohorts has revolutionized our understanding of the genetic basis of this disease. These studies have identified new ALL subtypes, each characterized by constellations of structural and sequence alterations that perturb key cellular pathways, including lymphoid development, cell-cycle regulation, and tumor suppression; cytokine receptor, kinase, and Ras signaling; and chromatin modifications. Several of these pathways, particularly kinase-activating lesions and epigenetic alterations, are logical targets for new precision medicine therapies. Genomic profiling has also identified important interactions between inherited genetic variants that influence the risk of leukemia development and the somatic genetic alterations that are required to establish the leukemic clone. Moreover, sequential sequencing studies at diagnosis, remission, and relapse have provided important insights into the relationship among genetic variants, clonal heterogeneity, and the risk of relapse. Ongoing studies are extending our understanding of coding and noncoding genetic alterations in B-progenitor and T-lineage ALL and using these insights to inform the development of faithful experimental models to test the efficacy of new treatment approaches.

How I treat T-cell acute lymphoblastic leukemia in adults

T-cell immunophenotype of acute lymphoblastic leukemia (T-ALL) is an uncommon aggressive leukemia that can present with leukemic and/or lymphomatous manifestations. Molecular studies are enhancing our understanding of the pathogenesis of T-ALL, and the discovery of activating mutations of NOTCH1 and FBXW7 in a majority of patients has been a seminal observation. The use of pediatric intensive combination chemotherapy regimens in adolescents and young adults has significantly improved the outcome of patients with T-ALL. The use of nelarabine for relapsed and refractory T-ALL results in responses in a substantial minority of patients. Allogeneic hematopoietic cell transplantation (HCT) still plays a key role in patients with high-risk or relapsed/refractory disease. γ-Secretase inhibitors hold promise for the treatment of patients with NOTCH1 mutations, and the results of clinical trials with these agents are eagerly awaited. It is recommended that younger patients receive a pediatric-intensive regimen. Older and unfit patients can receive suitable multiagent chemotherapy and be allocated to HCT based on their response, risk factors, and comorbidities. Although advances in the treatment of T-ALL have lagged behind those of B-cell ALL, it is hoped that the molecular revolution will enhance our understanding of the pathogenesis and treatment of this aggressive lymphoid malignancy.

Genomics in acute lymphoblastic leukaemia: insights and treatment implications

Acute lymphoblastic leukaemia (ALL) is the commonest childhood cancer and an important cause of morbidity from haematological malignancies in adults. In the past several years, we have witnessed major advances in the understanding of the genetic basis of ALL. Genome-wide profiling studies, including microarray analysis and genome sequencing, have helped identify multiple key cellular pathways that are frequently mutated in ALL such as lymphoid development, tumour suppression, cytokine receptors, kinase and Ras signalling, and chromatin remodeling. These studies have characterized new subtypes of ALL, notably Philadelphia chromosome-like ALL, which is a high-risk subtype characterized by a diverse range of alterations that activate cytokine receptors or tyrosine kinases amenable to inhibition with approved tyrosine kinase inhibitors. Genomic profiling has also enabled the identification of inherited genetic variants of ALL that influence the risk of leukaemia development, and characterization of the relationship between genetic variants, clonal heterogeneity and the risk of relapse. Many of these findings are of direct clinical relevance and ongoing studies implementing clinical sequencing in leukaemia diagnosis and management have great potential to improve the outcome of patients with high-risk ALL.

Efficacy of JAK/STAT pathway inhibition in murine xenograft models of early T-cell precursor (ETP) acute lymphoblastic leukemia

Early T-cell precursor (ETP) acute lymphoblastic leukemia (ALL) is a recently described subtype of T-ALL characterized by a unique immunophenotype and genomic profile, as well as a high rate of induction failure. Frequent mutations in cytokine receptor and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathways led us to hypothesize that ETP-ALL is dependent on JAK/STAT signaling. Here we demonstrate aberrant activation of the JAK/STAT pathway in ETP-ALL blasts relative to non-ETP T-ALL. Moreover, ETP-ALL showed hyperactivation of STAT5 in response to interleukin-7, an effect that was abrogated by the JAK1/2 inhibitor ruxolitinib. In vivo, ruxolitinib displayed activity in 6 of 6 patient-derived murine xenograft models of ETP-ALL, with profound single-agent efficacy in 5 models. Ruxolitinib treatment decreased peripheral blast counts relative to pretreatment levels and compared with control (P < .01) in 5 of 6 ETP-ALL xenografts, with marked reduction in mean splenic blast counts (P < .01) in 6 of 6 samples. Surprisingly, both JAK/STAT pathway activation and ruxolitinib efficacy were independent of the presence of JAK/STAT pathway mutations, raising the possibility that the therapeutic potential of ruxolitinib in ETP-ALL extends beyond those cases with JAK mutations. These findings establish the preclinical in vivo efficacy of ruxolitinib in ETP-ALL, a biologically distinct subtype for which novel therapies are needed.

ABT-199 mediated inhibition of BCL-2 as a novel therapeutic strategy in T-cell acute lymphoblastic leukemia

High levels of the anti-apoptotic factor BCL-2 can be therapeutically exploited by the BH3 mimetic ABT-199 in human T-ALL.

Clinically defining and managing high-risk pediatric patients with acute lymphoblastic leukemia

For children with acute lymphoblastic leukemia, the identification of those at higher risk of disease recurrence and modifying therapy based on this risk is a critical component to the provision of optimal care. The specific definitions of high-risk ALL vary across cooperative groups, but the themes are consistent, being largely based on leukemia biology and disease response. Intensification of conventional chemotherapy for those with high-risk disease has led to improved outcomes. It is anticipated that the development of rational targeted therapy for specific biologically unique subsets of children with leukemia will contribute to ongoing progress in improving the outcomes for children with acute lymphoblastic anemia.

Clinically defining and managing high-risk pediatric patients with acute lymphoblastic leukemia

For children with acute lymphoblastic leukemia, the identification of those at higher risk of disease recurrence and modifying therapy based on this risk is a critical component to the provision of optimal care. The specific definitions of high-risk ALL vary across cooperative groups, but the themes are consistent, being largely based on leukemia biology and disease response. Intensification of conventional chemotherapy for those with high-risk disease has led to improved outcomes. It is anticipated that the development of rational targeted therapy for specific biologically unique subsets of children with leukemia will contribute to ongoing progress in improving the outcomes for children with acute lymphoblastic anemia.

MLLT10 and IL3 rearrangement together with a complex four-way translocation and trisomy 4 in a patient with early T-cell precursor acute lymphoblastic leukemia: A case report

Cytogenetic classification of acute lymphoblastic leukemia (ALL) is primarily based on numerical and structural chromosomal abnormalities. In T-cell ALL (T-ALL), chromosomal rearrangements are identified in up to 70% of the patients while the remaining patients show a normal karyotype. In the present study, a 16-year-old male was diagnosed with T-precursor cell ALL and a normal karyotype after standard GTG-banding, was studied retrospectively (>10 years after diagnosis) in frame of a research project by molecular approaches. In addition to molecular cytogenetics, multiplex ligation-dependent probe amplification (MLPA) and high resolution array-comparative genomic hybridization (aCGH) were also applied. Thus, the following yet unrecognized balanced chromosomal aberrations were detected: der(3)t(3;5)(p23;q31.1), der(5)t(3;5)(p23;q35.3), der(5)t(5;10)(q31.1;p12.3) and der(10)t(5;10)(q35.3;p12.3). The oncogene MLLT10 was involved in this rearrangement as was the IL3 gene; in addition, trisomy 4 was present. All of these clonal aberrations were found in 40% of the cells. Even if this complex karyotype would have been identified at the time of diagnosis, most likely no other protocol of anticancer therapy (ALL-BFM 95) would have been applied. Three months after the end of a successful 2-year treatment, the patient suffered from isolated bone marrow relapse and died of sepsis during ALL-REZ-BFM protocol treatment.

Hhex regulates Kit to promote radioresistance of self-renewing thymocytes in Lmo2-transgenic mice

Lmo2 is an oncogenic transcription factor that is frequently overexpressed in T-cell acute leukemias, in particular poor prognosis early T-cell precursor-like (ETP-) acute lymphoblastic leukemia (ALL). The primary effect of Lmo2 is to cause self-renewal of developing CD4(-)CD8(-) (double negative, DN) T cells in the thymus, leading to serially transplantable thymocytes that eventually give rise to leukemia. These self-renewing thymocytes are intrinsically radioresistant implying that they may be a source of leukemia relapse after therapy. The homeobox transcription factor, Hhex, is highly upregulated in Lmo2-transgenic thymocytes and can phenocopy Lmo2 in inducing thymocyte self-renewal, implying that Hhex may be a key component of the Lmo2-induced self-renewal program. To test this, we conditionally deleted Hhex in the thymi of Lmo2-transgenic mice. Surprisingly, this did not prevent accumulation of DN thymocytes, nor alter the rate of overt leukemia development. However, deletion of Hhex abolished the transplantation capacity of Lmo2-transgenic thymocytes and overcame their radioresistance. We found that Hhex regulates Kit expression in Lmo2-transgenic thymocytes and that abrogation of Kit signaling phenocopied loss of Hhex in abolishing the transplantation capacity and radioresistance of these cells. Thus, targeting the Kit signaling pathway may facilitate the eradication of leukemia-initiating cells in immature T-cell leukemias in which it is expressed.