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Targeting epigenetic pathways is a promising approach for cancer therapy. Here, we report on the unexpected finding that targeting calcium signaling can reverse epigenetic silencing of tumor suppressor genes (TSGs). In a screen for drugs that reactivate silenced gene expression in colon cancer cells, we found three classical epigenetic targeted drugs (DNA methylation and histone deacetylase inhibitors) and 11 other drugs that induce methylated and silenced CpG island promoters driving a reporter gene (GFP) as well as endogenous TSGs in multiple cancer cell lines. These newly identified drugs, most prominently cardiac glycosides, did not change DNA methylation locally or histone modifications globally. Instead, all 11 drugs altered calcium signaling and triggered calcium-calmodulin kinase (CamK) activity leading to MeCP2 nuclear exclusion. Blocking CamK activity abolished gene reactivation and cancer cell killing by these drugs, showing that triggering calcium fluxes is an essential component of their epigenetic mechanism of action. Our data identify calcium signaling as a new pathway that can be targeted to reactivate TSGs in cancer.

Understanding the heterogeneous genetic mechanisms of tumor initiation in lymphoid leukemias (LL) will lead to improvements in prognostic classification and treatment regimens. In previous studies of mouse leukemias, we showed that retroviral insertion at the Evi32 locus leads to increased expression of Prdm14, a pluripotency gene implicated in the self-renewal capacity of embryonic stem cells and the early stages of breast cancer. Here we show that PRDM14 is also overexpressed in ~25% of human lymphoid neoplasms, with increased frequencies in T-cell acute LL (T-ALL) and hyperdiploid precursor B-cell acute LL (pre-B ALL). To test if Prdm14 overexpression could initiate leukemia, mice were transduced with bone marrow (BM) cells transfected with a Prdm14 expression vector. Lymphoid leukemias developed in 96% of female mice and 42% of male mice. Prior to the onset of leukemia, differentiation of transduced cells was biased up to 1000-fold towards cells with features of common lymphoid progenitors (CLP), and lymphoid differentiation showed a relative block at the pro-B stage. Microarray gene expression analysis of expanded CLP-like cells prior to the onset of leukemia demonstrated upregulation of genes involved in pluripotency, tumor initiation, early B-lineage commitment, Wnt/Ras signaling, and the epithelial-to-mesenchymal transition. Among the dysregulated genes were imprinted genes and non-coding RNAs including Dlk1 and Meg3, which are also key pluripotency mediators. Heightened expression of the estrogen-dependent oncogene, Myb, in tumors suggests a basis for the increased frequency of cancer in female mice. These data provide the first direct evidence for the association of Prdm14 with cancer initiation in an in vivo mouse model and in human lymphoid malignancies, while suggesting mechanisms for Prdm14’s mode of action.

BackgroundAKXD recombinant inbred strains of mice have proven to be very useful in the identification of potential oncogenes and tumor suppressors involved in the development of lymphoid and myeloid malignancies. In these tumors, the hematopoietic insertion of an active AKV murine leukemia virus (MuLV) is associated with the onset of disease. Common sites of retroviral insertion (CIS) identify genes causally associated with the development or initiation of lymphoma.MethodologyIn the present study, we analyzed a previously uncharacterized CIS, Ecotropic Viral Integration Site 32 (Evi32), which is located on mouse chromosome 1. We analyzed candidate genes in the region to identify those involved in Evi32 mediated oncogenesis.ResultsHere we show that proviral insertion at Evi32 correlates with significant overexpression of a putative transcription factor, PR-domain containing 14 (Prdm14). Tumors with insertions at Evi32 are consistently lymphoid in nature. Prdm14 is normally expressed early in embryonic development with the highest expression in undifferentiated embryonic stem (ES) cells. This study implicates Prdm14 as a proto-oncogene involved in lymphoblastic lymphoma formation.

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