Transient Responses to NOTCH and TLX1/HOX11 Inhibition in T-Cell Acute Lymphoblastic Leukemia/Lymphoma

Abstract

To improve the treatment strategies of T-cell acute lymphoblastic leukemia/lymphoma (T-ALL), further efforts are needed to identify therapeutic targets. Dysregulated expression of HOX-type transcription factors occurs in 30–40% of cases of T-ALL. TLX1/HOX11 is the prototypical HOX-type transcription factor. TLX1 may be an attractive therapeutic target because mice that are deficient in TLX1 are healthy. To test this possibility, we developed a conditional doxycycline-regulated mouse model of TLX1-initiated T-ALL. TLX1 induced T-ALL after ∼5–7 months with penetrance of 15–60%. Similar to human TLX1-type T-ALLs, the TLX1-induced tumors were arrested at the cortical stage of T-cell development and acquired activating NOTCH1 mutations. Inhibition of NOTCH signaling abrogated growth of cell lines derived from the TLX1-induced tumors. NOTCH inhibition also transiently delayed leukemia progression in vivo. Suppression of TLX1 expression slowed the growth of TLX1 tumor cell lines. Suppression of TLX1 in vivo also transiently delayed leukemia progression. We have shown that TLX1 functions as a T-cell oncogene that is active during both the induction and the maintenance phases of leukemia. However, the effect of suppressing NOTCH or TLX1 was transient. The tumors eventually “escaped” from inhibition. These data imply that the biological pathways and gene sets impacted by TLX1 and NOTCH have largely lost their importance in the fully established tumor. They have been supplanted by stronger oncogenic pathways. Although TLX1 or NOTCH inhibitors may not be effective as single agents, they may still contribute to combination therapy for TLX1-driven acute leukemia.

Characterization of the genome-wide TLX1 binding profile in T-cell acute lymphoblastic leukemia

“…TLX1 driven T-ALLs show a unique gene expression signature related to early cortical thymocytes with corresponding expression of CD1a, CD4 and CD8 surface marker proteins. Initial studies showed that TLX1 could immortalize murine hematopoietic precursors 2,3 , but its leukemic potential has only been fully established using TLX1 transgenic mouse models that develop clonal T-cell malignancies with a long latency 4,5 . These TLX1 induced murine T-cell tumors share common features with human TLX1 positive leukemia, including activation of Notch signaling, loss of the Bcl11b tumor suppressor gene and a transcriptional program that disrupts the mitotic checkpoint and induces aneuploidy during T-cell transformation 4 .…”

“…Moreover, Notch1 mutations are almost uniformly identified in murine T-cell tumors that developed from TLX1 transgenic mice 4,5 . Therefore, NOTCH1 activation and aberrant expression of TLX1 are considered collaborative events in the multi-step pathogenesis of T-ALL.…”

The DN2 Myeloid-T (DN2mt) Progenitor is a Target Cell for Leukemic Transformation by the TLX1 Oncogene

“…Murine models of TLX1 + T-ALL have provided support for this hypothesis. However, the long latency of T-cell tumor formation and clonal nature of the resulting malignancies indicated the requirement for additional mutations [10-13]. Multiple lines of investigation have implicated dysregulated expression of TLX1 target genes involved in the mitotic spindle assembly checkpoint that predispose to Chromosomal Instability (CIN) and aneuploidy, as contributing to TLX1-mediated leukemogenesis [12,14-16].…”

TLX1-Induced T-cell Acute Lymphoblastic Leukemia

“…However, until recently, very little was known about the specific mechanisms that mediate T-cell transformation downstream of TLX1. Moreover, the generation of a mouse model of TLX1-induced T-ALL eluded the efforts of several groups over the past decade (15,16). In this context, the generation of an LCK-TLX1 transgenic mouse model that recapitulates the histologic, transcriptional, and genetic features of TLX1-induced T-ALL has provided a muchneeded tool for the analysis of the molecular and cellular mechanisms of TLX1-induced transformation (16).…”

“…The 6-month latency before the development of leukemia in LCK-TLX1 transgenic mice (15,16) and the observation that these tumors show clonal expression of the T-cell receptor b suggest that TLX1 overexpression may not be sufficient for T-cell transformation and that full leukemia development may depend on the acquisition of secondary cooperating mutations. The NOTCH1 signaling pathway plays a prominent role in T-ALL pathogenesis, with more than 50% of the patients harboring activating NOTCH1 mutations (17).…”