The human Dek
oncogene was first discovered as a fusion with the gene encoding the CAN nucleoporin protein, NUP214, in a subset of acute myeloid leukemia (AML) patients (1
). Apart from its original association with cancer as a fusion protein, upregulated DEK expression has also been linked to various solid human tumors, as well as AML types that do not exhibit the above translocation (2–10
). We previously reported that DEK overexpression promotes the transformation of human keratinocytes, and that Dek
knockout mice are partially resistant to chemically induced papilloma formation (11
). The overexpression of DEK inhibits cell death, and its knockdown results in apoptosis in HeLa cells and primary keratinocytes, in part through the stabilization and transcriptional activation of p53 (12
). Other reports similarly are consistent with a role for DEK in cellular survival (13
). Together, these findings support a role for DEK
as an oncogene. The underlying molecular mechanisms by which it functions, however, remain poorly understood.
DEK is a highly abundant, evolutionarily conserved and ubiquitous nuclear protein that can be regulated at the level of transcription and post-translational modifications (15–21
). Evidence suggests that it may function as a nuclear architectural protein. Similar to the classical chromatin architectural high mobility group (HMG) proteins, DEK contains regions enriched in acidic amino acids (22
) and preferentially binds to supercoiled and cruciform DNA (24
). Perhaps related to such a role, the literature supports distinct intracellular functions for DEK in DNA replication (25
), positive and negative regulation of gene transcription (26–30
), histone acetylation (31
), mRNA splicing (20
) and nucleosome assembly (34
). Several reports have already suggested that DEK may modulate genome stability. First, a C-terminal fragment of DEK could partially rescue Ataxia–Telangiectasia (A–T) fibroblast mutagen sensitivity, high spontaneous recombination rates, and radio-resistant DNA synthesis (35
). Second, DEK knockdown sensitized HeLa cells to genotoxic stress (18
) and resulted in chemosensitivity to doxorubicin in melanoma cells (14
). Accordingly, we hypothesized that DEK expression is important for DNA damage sensing and/or repair.
Sustaining DNA integrity is a continuous cellular challenge in the face of endogenous and exogenous DNA damage insults. Sensor kinases detect damage and signal the appropriate cellular responses. Proper repair of a lesion is followed by the resumption of normal cellular proliferation and continued growth, whereas incomplete repair results in prolonged cell-cycle arrest and ultimately, cell death. The protein kinase ataxia telangiectasia mutated (ATM) undergoes autophosphorylation on Ser1981 in response to DNA double-strand breaks (DSBs), and then phosphorylates and activates downstream targets including structural maintenance of chromosomes 1 (SMC1) (36
). The DNA-dependent protein kinase (DNA-PK), which is composed of the Ku70/80 heterodimer and the DNA-PK catalytic subunit (DNA-PKcs), responds to DSBs by the initial recruitment of Ku70/80, binding and autophosphorylation of DNA-PKcs, and subsequent recruitment of the DNA repair machinery. The most extensively studied types of DSB DNA repair are homologous recombination (HR) and non-homologous end joining (NHEJ). HR utilizes the ATM pathway proteins and homologous DNA sequences as a template for DNA repair, while NHEJ fuses broken ends of DNA with little regard for homology (38
Here we demonstrate that the loss of DEK expression, either by knock-down in human cells or by genetic deletion in murine cells, is sufficient to activate the DNA damage response and to exaggerate phenotypic responses to exogenous stress. DEK loss results in stimulated ATM activity, attenuated Ku70/80 recruitment, and repressed DNA-PK activity, in correlation with a decrease in repair by NHEJ. Together, these data suggest that DEK expression modulates ATM and DNA-PK signaling and the efficiency of DNA DSB repair.