Transient Differentiation-State Plasticity Occurs During Acute Lymphoblastic Leukemia Initiation.

Leukemia is characterized by oncogenic lesions that result in a block of differentiation while phenotypic plasticity is retained. A better understanding of how these two phenomena arise during leukemogenesis in humans could help inform diagnosis and treatment strategies. Here, we leveraged the well-defined differentiation states during T-cell development to pinpoint the initiation of T-cell acute lymphoblastic leukemia (T-ALL), an aggressive form of childhood leukemia, and study the emergence of phenotypic plasticity. Single-cell whole genome sequencing of leukemic blasts was combined with multiparameter flow cytometry to couple cell identity and clonal lineages. Irrespective of genetic events, leukemia-initiating cells altered their phenotypes by differentiation and de-differentiation. Construction of phylogenies of individual leukemias using somatic mutations revealed that phenotypic diversity is reflected by the clonal structure of the cancer. The analysis also indicated that the acquired phenotypes are heritable and stable. Together, these results demonstrate a transient period of plasticity during leukemia initiation where phenotypic switches appear unidirectional.

STAT5 does not drive steroid resistance in T-cell acute lymphoblastic leukemia despite the activation of BCL2 and BCLXL following glucocorticoid treatment.

Physiological and pathogenic interleukin-7-receptor (IL7R)-induced signaling provokes glucocorticoid resistance in a subset of patients with pediatric T-cell acute lymphoblastic leukemia (T-ALL). Activation of downstream STAT5 has been suggested to cause steroid resistance through upregulation of anti-apoptotic BCL2, one of its downstream target genes. Here we demonstrate that isolated STAT5 signaling in various T-ALL cell models is insufficient to raise cellular steroid resistance despite upregulation of BCL2 and BCL-XL. Upregulation of anti-apoptotic BCL2 and BCLXL in STAT5-activated T-ALL cells requires steroid-induced activation of NR3C1. For the BCLXL locus, this is facilitated by a concerted action of NR3C1 and activated STAT5 molecules at two STAT5 regulatory sites, whereas for the BCL2 locus this is facilitated by binding of NR3C1 at a STAT5 binding motif. In contrast, STAT5 occupancy at glucocorticoid response elements does not affect the expression of NR3C1 target genes. Strong upregulation of BIM, a NR3C1 pro-apoptotic target gene, upon prednisolone treatment can counterbalance NR3C1/STAT5-induced BCL2 and BCL-XL expression downstream of IL7- induced or pathogenic IL7R signaling. This explains why isolated STAT5 activation does not directly impair the steroid response. Our study suggests that STAT5 activation only contributes to steroid resistance in combination with cellular defects or alternative signaling routes that disable the pro-apoptotic and steroid-induced BIM response.

Phosphoproteomic profiling of T cell acute lymphoblastic leukemia reveals targetable kinases and combination treatment strategies.

Protein kinase inhibitors are amongst the most successful cancer treatments, but targetable kinases activated by genomic abnormalities are rare in T cell acute lymphoblastic leukemia. Nevertheless, kinases can be activated in the absence of genetic defects. Thus, phosphoproteomics can provide information on pathway activation and signaling networks that offer opportunities for targeted therapy. Here, we describe a mass spectrometry-based global phosphoproteomic profiling of 11 T cell acute lymphoblastic leukemia cell lines to identify targetable kinases. We report a comprehensive dataset consisting of 21,000 phosphosites on 4,896 phosphoproteins, including 217 kinases. We identify active Src-family kinases signaling as well as active cyclin-dependent kinases. We validate putative targets for therapy ex vivo and identify potential combination treatments, such as the inhibition of the INSR/IGF-1R axis to increase the sensitivity to dasatinib treatment. Ex vivo validation of selected drug combinations using patient-derived xenografts provides a proof-of-concept for phosphoproteomics-guided design of personalized treatments.

T-cell lymphoblastic lymphoma and leukemia: different diseases from a common premalignant progenitor?

T-cell lymphoblastic lymphoma (T-LBL) and lymphoblastic leukemia (T-ALL) represent malignancies that arise from the transformation of immature precursor T cells. Similarities in T-LBL and T-ALL have raised the question whether these entities represent 1 disease or reflect 2 different diseases. The genetic profiles of T-ALL have been thoroughly investigated over the last 2 decades, whereas fairly little is known about genetic driver mutations in T-LBL. Nevertheless, the comparison of clinical, immunophenotypic, and molecular observations from independent T-LBL and T-ALL studies lent strength to the theory that T-LBL and T-ALL reflect different presentations of the same disease. Alternatively, T-LBL and T-ALL may simultaneously evolve from a common malignant precursor cell, each having their own specific pathogenic requirements or cellular dependencies that differ among stroma-embedded blasts in lymphoid tissues compared with solitary leukemia cells. This review aims to cluster recent findings with regard to clinical presentation, genetic predisposition, and the acquisition of additional mutations that may give rise to differences in gene expression signatures among T-LBL and T-ALL patients. Improved insight in T-LBL in relation to T-ALL may further help to apply confirmed T-ALL therapies to T-LBL patients.