The homeoprotein Dlx5 drives murine T-cell lymphomagenesis by directly transactivating Notch and upregulating Akt signaling

Abstract

Homeobox genes play a critical role in embryonic development, but they have also been implicated in cancer through mechanisms that are largely unknown. While not expressed during normal T-cell development, homeobox transcription factor genes can be reactivated via recurrent chromosomal rearrangements in human T-cell acute leukemia/lymphoma (T-ALL), a malignancy often associated with activated Notch and Akt signaling. To address how epigenetic reprogramming via an activated homeobox gene might contribute to T-lymphomagenesis, we investigated a transgenic mouse model with thymocyte-specific overexpression of the Dlx5 homeobox gene. We demonstrate for the first time that Dlx5 induces T-cell lymphomas with high penetrance. Integrated ChIP-seq and mRNA microarray analyses identified Notch1/3 and Irs2 as direct transcriptional targets of Dlx5, a gene signature unique to lymphomas from Lck-Dlx5 mice as compared to T-cell lymphomas from Lck-MyrAkt2 mice, which were previously reported by our group. Moreover, promoter/enhancer studies confirmed that Dlx5 directly transactivates Notch expression. Notch1/3 expression and Irs2-induced Akt signaling were upregulated throughout early stages of T-cell development, which promoted cell survival during β-selection of T lymphocytes. Dlx5 was required for tumor maintenance via its activation of Notch and Akt, as tumor cells were highly sensitive to Notch and Akt inhibitors. Together, these findings provide unbiased genetic and mechanistic evidence that Dlx5 acts as an oncogene when aberrantly expressed in T cells, and that it is a novel discovery that Notch is a direct target of Dlx5. These experimental findings provide mechanistic insights about how reactivation of the Dlx5 gene can drive T-ALL by aberrant epigenetic reprogramming of the T-cell genome.

Wnt signaling mediates oncogenic synergy between Akt and Dlx5 in T-cell lymphomagenesis by enhancing cholesterol synthesis

“…2 A. Among the up-regulated genes observed in lymphomas from Lck-Dlx5;Lck-MyrAkt2 mice, Notch , Myc , and Ccnd1 were previously reported to be upregulated in lymphomas from Lck-Dlx5 mice, and Myc was previously shown to be upregulated in Lck-MyrAkt2 mice 22 . However, upregulation of genes involved in the Wnt pathway and cholesterol synthesis were unique to the lymphomas from the Lck-Dlx5;Lck-MyrAkt2 mice.…”

“…We previously implicated Dlx5 as an oncogene when it was found to be aberrantly expressed due to a recurrent clonal chromosomal rearrangement that placed the intact Dlx5 locus beside the Tcrb enhancer in a transgenic Lck-MyrAkt2 mouse model of T-ALL 17 . Subsequently, transgenic mice specifically overexpressing Dlx5 in immature thymic T-cells ( Lck-Dlx5 mice) were shown to develop T-ALL by directly transactivating Notch and upregulating Akt signaling 22 . We thus hypothesized that constitutively active Akt2 and overexpression of Dlx5 cooperate to promote T-cell lymphomagenesis.…”

“…The transgenic mouse models Lck-MyrAkt2 and Lck-Dlx5-Myc-tag were previously reported 17 , 22 . Mice were sacrificed upon evidence of labored breathing, hunched posture, distended abdomen, or upon a 10% change in body weight, under Fox Chase Cancer Center Institutional Animal Care and Use Committee protocol #96-36.…”

“…Mouse models. The transgenic mouse models Lck-MyrAkt2 and Lck-Dlx5-Myc-tag were previously reported 17,22 . Mice were sacrificed upon evidence of labored breathing, hunched posture, distended abdomen, www.nature.com/scientificreports/ Figure 8.…”

“…Moreover, the resulting tumors frequently acquired Notch1 mutations and were sensitive to Notch inhibitors. Additionally, Pten was frequently inactivated in these tumors, which suggests that Notch activation and Pten loss cooperate tumorigenically in these T-ALLs 22 . To address whether the Akt pathway cooperates with the Dlx5-Notch pathway in murine T-ALL development, we crossed Lck-MyrAkt2 mice to Lck-Dlx5 mice.…”