Combining venetoclax, a selective BCL2 inhibitor, with low-dose navitoclax, a BCL-XL/BCL2 inhibitor, may allow targeting of both BCL2 and BCL-XL without dose-limiting thrombocytopenia associated with navitoclax monotherapy. The safety and preliminary efficacy of venetoclax with low-dose navitoclax and chemotherapy was assessed in this phase I dose-escalation study (NCT03181126) in pediatric and adult patients with relapsed/refractory (R/R) acute lymphoblastic leukemia or lymphoblastic lymphoma. Forty-seven patients received treatment. A recommended phase II dose of 50 mg navitoclax for adults and 25 mg for patients <45 kg with 400 mg adult-equivalent venetoclax was identified. Delayed hematopoietic recovery was the primary safety finding. The complete remission rate was 60%, including responses in patients who had previously received hematopoietic cell transplantation or immunotherapy. Thirteen patients (28%) proceeded to transplantation or CAR T-cell therapy on study. Venetoclax with navitoclax and chemotherapy was well tolerated and had promising efficacy in this heavily pretreated patient population. Significance: In this phase I study, venetoclax with low-dose navitoclax and chemotherapy was well tolerated and had promising efficacy in patients with relapsed/refractory acute lymphoblastic leukemia or lymphoblastic lymphoma. Responses were observed in patients across histologic and genomic subtypes and in those who failed available therapies including stem cell transplant. See related commentary by Larkin and Byrd, p. 1324. This article is highlighted in the In This Issue feature, p. 1307

Lineage-ambiguous leukemias are high-risk malignancies of poorly understood genetic basis. Here, we describe a distinct subgroup of acute leukemia with expression of myeloid, T lymphoid, and stem cell markers driven by aberrant allele-specific deregulation of BCL11B, a master transcription factor responsible for thymic T-lineage commitment and specification. Mechanistically, this deregulation was driven by chromosomal rearrangements that juxtapose BCL11B to superenhancers active in hematopoietic progenitors, or focal amplifications that generate a superenhancer from a noncoding element distal to BCL11B. Chromatin conformation analyses demonstrated long-range interactions of rearranged enhancers with the expressed BCL11B allele and association of BCL11B with activated hematopoietic progenitor cell cis-regulatory elements, suggesting BCL11B is aberrantly co-opted into a gene regulatory network that drives transformation by maintaining a progenitor state. These data support a role for ectopic BCL11B expression in primitive hematopoietic cells mediated by enhancer hijacking as an oncogenic driver of human lineage-ambiguous leukemia. Significance: Lineage-ambiguous leukemias pose significant diagnostic and therapeutic challenges due to a poorly understood molecular and cellular basis. We identify oncogenic deregulation of BCL11B driven by diverse structural alterations, including de novo superenhancer generation, as the driving feature of a subset of lineage-ambiguous leukemias that transcend current diagnostic boundaries. This article is highlighted in the In This Issue feature, p. 2659

We evaluated clinical significance of recently identified subtypes of acute lymphoblastic leukemia (ALL) in 598 children treated with minimal residual disease (MRD)–directed therapy. Among the 16 B-cell ALL (B-ALL) and 8 T-cell ALL subtypes identified by next-generation sequencing, ETV6–RUNX1, high-hyperdiploid, and DUX4-rearranged B-ALL had the best 5-year event-free survival rates (95.0%–98.4%); TCF3–PBX1, PAX5-altered (PAX5alt), T-cell, early T-cell precursor (ETP), intrachromosomal amplification of chromosome 21 (iAMP21), and hypodiploid ALL intermediate rates (80.0%–88.2%); and BCR–ABL1, BCR–ABL1-like, ETV6–RUNX1-like, and KMT2A-rearranged ALL the worst rates (64.1%–76.2%). All but 3 of the 142 patients with day 8 blood MRD <0.01% remained in remission. Among new subtypes, intensified therapy based on day 15 MRD ≥1% improved outcome of DUX4-rearranged, BCR–ABL1-like, and ZNF384-rearranged ALL, and achievement of day 42 MRD <0.01% did not preclude relapse of PAX5alt, MEF2D-rearranged, and ETV6–RUNX1-like ALL. Thus, new subtypes including DUX4-rearranged, PAX5alt, BCR–ABL1-like, ETV6–RUNX1-like, MEF2D-rearranged, and ZNF384-rearranged ALL have important prognostic and therapeutic implications. Significance: Genomic analyses and MRD should be used together for risk-directed treatment of childhood ALL. Six recently described subtypes—DUX4-rearranged, PAX5alt, BCR–ABL1-like, ETV6–RUNX1-like, MEF2D-rearranged, and ZNF384-rearranged ALL—had prognostic and therapeutic significance with contemporary risk-directed treatment. See related commentary by Segers and Cools, p. 294. See related video from the AACR Annual Meeting 2021: https://vimeo.com/558556916

Blinatumomab, a bispecific antibody that directs CD3+ T cells to CD19+ tumor cells, shows variable efficacy in B-progenitor acute lymphoblastic leukemia (B-ALL). To determine tumor-intrinsic and -extrinsic determinants of response, we studied 44 adults with relapsed or refractory B-ALL (including 2 minimal residual disease positive) treated with blinatumomab using bulk tumor and single-cell sequencing. The overall response rate in patients with hematological disease was 55%, with a high response rate in those with CRLF2-rearranged Philadelphia chromosome–like ALL (12 [75%] of 16). Pretreatment samples of responders exhibited a tumor-intrinsic transcriptomic signature of heightened immune response. Multiple mechanisms resulted in loss of CD19 expression, including CD19 mutations, CD19-mutant allele-specific expression, low CD19 RNA expression, and mutations in CD19 signaling complex member CD81. Patients with low hypodiploid ALL were prone to CD19− relapse resulting from aneuploidy-mediated loss of the nonmutated CD19 allele. Increased expression of a CD19 isoform with intraexonic splicing of exon 2, CD19 ex2part, at baseline or during therapy was associated with treatment failure. These analyses demonstrate both tumor-intrinsic and -extrinsic factors influence blinatumomab response. We show that CD19 mutations are commonly detected in CD19− relapse during blinatumomab treatment. Identification of the CD19 ex2part splice variant represents a new biomarker predictive of blinatumomab therapy failure.