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

Inappropriate activation of the TLX1 (T-cell leukemia homeobox 1) gene by chromosomal translocation is a recurrent event in human T-cell Acute Lymphoblastic Leukemia (T-ALL). Ectopic expression of TLX1 in murine bone marrow progenitor cells using a conventional retroviral vector efficiently yields immortalized cell lines and induces T-ALL-like tumors in mice after long latency.

To eliminate a potential contribution of retroviral insertional mutagenesis to TLX1 immortalizing and transforming function, we incorporated the TLX1 gene into an insulated self-inactivating retroviral vector.

Retrovirally transduced TLX1-expressing murine bone marrow progenitor cells had a growth/survival advantage and readily gave rise to immortalized cell lines. Extensive characterization of 15 newly established cell lines failed to reveal a common retroviral integration site. This comprehensive analysis greatly extends our previous study involving a limited number of cell lines, providing additional support for the view that constitutive TLX1 expression is sufficient to initiate the series of events culminating in hematopoietic progenitor cell immortalization. When TLX1-immortalized cells were co-cultured on OP9-DL1 monolayers under conditions permissive for T-cell differentiation, a latent T-lineage potential was revealed. However, the cells were unable to transit the DN2 myeloid-T (DN2mt)-DN2 T-lineage determined (DN2t) commitment step. The differentiation block coincided with failure to upregulate the zinc finger transcription factor gene Bcl11b, the human ortholog of which was shown to be a direct transcriptional target of TLX1 downregulated in the TLX1⁺ T-ALL cell line ALL-SIL. Other studies have described the ability of TLX1 to promote bypass of mitotic checkpoint arrest, leading to aneuploidy. We likewise found that diploid TLX1-expressing DN2mt cells treated with the mitotic inhibitor paclitaxel bypassed the mitotic checkpoint and displayed chromosomal instability. This was associated with elevated expression of TLX1 transcriptional targets involved in DNA replication and mitosis, including Ccna2 (cyclin A2), Ccnb1 (cyclin B1), Ccnb2 (cyclin B2) and Top2a (topoisomerase IIα). Notably, enforced expression of BCL11B in ALL-SIL T-ALL cells conferred resistance to the topoisomerase IIα poison etoposide.

Taken together with previous findings, the data reinforce a mechanism of TLX1 oncogenic activity linked to chromosomal instability resulting from dysregulated expression of target genes involved in mitotic processes. We speculate that repression of BCL11B expression may provide part of the explanation for the observation that aneuploid DNA content in TLX1⁺ leukemic T cells does not necessarily portend an unfavorable prognosis. This TLX1 hematopoietic progenitor cell immortalization/T-cell differentiation assay should help further our understanding of the mechanisms of TLX1-mediated evolution to malignancy and has the potential to be a useful predictor of disease response to novel therapeutic agents in TLX1⁺ T-ALL.