DNA damage represents a persistent threat to
DNA damage represents a persistent threat to genomic stability. A critical link exists between DNA mutation, chromosomal rearrangement and cancer development. In myeloid malignancies, various chromosomal translocations and/or mutations increased cellular reactive oxygen species (ROS), followed by DSBs . The majority of tumor ONC201 have defects in maintaining genomic stability due to the loss of an appropriate response to DNA damage. Mutations in the genes that encode DNA damage response proteins are responsible for a variety of genomic instability syndromes. Thus, DNA repair is an important mechanism for maintaining genetic stability.
Non-homologous end-joining (NHEJ) is the most important DNA repair mechanism in mammalian cells. NHEJ repairs DSBs by ligating two free DNA ends with little or no homology. Several components involved in NHEJ have been reported, such as Ku70, Ku80, DNA-dependent protein kinase (DNA-PK), and DNA ligase IV. A DSB is first recognized by Ku70/Ku80 complex and DNA-PK catalytic subunit (DNA-PKcs). DNA-PKcs is a 470kDa serine/threonine protein kinase catalytic subunit (DNA-PKcs), and Ku proteins are its DNA-binding regulatory components , . Finally, DNA ligase IV heterodimerized with XRCC4, a nuclear phosphoprotein, is recruited to the DSB site and executes the final rejoining step.
DNA-PKcs mutant mice exhibit congenital bone marrow failure associated with deficiencies in DNA repair . DNA-PKcs activation is an essential step in the repair process of DSB , and it has been proposed that DNA-PKcs is a molecular sensor for DNA damage that enhances cellular response signaling via phosphorylation of many downstream targets. Compared with another error-free DSB repair mechanism, homologous recombination (HR) repair, NHEJ, is error-prone, leading to more complicated genetic mutations and more malignant phenotypes, including anti-cancer drug resistance , .
The modulation of NHEJ components is expected to affect anti-cancer drug sensitivity. The relationship between DNA ligase IV expression and etoposide resistance has been reported in a human leukemia cell line, CEM . In the case of DNA PKcs, increased expression of DNA ligase IV confers resistance to adriamycin , and inhibition of DNA-PKcs by wortmannin, an inhibitor of phosphatidylinositol 3-kinase, has been shown to potentiate the chemosensitivity of multidrug-resistant human leukemia, CEM cells . However, the mechanism of DNA-PKcs expression remains to be elucidated, especially its overexpression in malignant tumors or leukemia cells. Furthermore, the relationship between DNA ligase IV and the MDR phenotype has not been fully disclosed.
Materials and methods
Discussion It has been reported that a correlation between DNA-PK and radio-sensitivity exists in lung carcinoma cell lines and cervical cancer cells , . Cytotoxic drug resistance such as that of daunorubicin (DNR) depends on the loss or recovery from DNA damage and it at least partially results from overexpression of DNA-PKcs , , . It has also been reported that DNA-PK is a therapeutic target and an indicator of poor prognosis in B-cell chronic lymphocytic leukemia . The tumor/normal tissue expression ratio of ATM and DNA-PKcs is useful for evaluating non-small cell lung cancer patients . Moreover, down-regulation of DNA-PKcs and Ku70 increased chemo-sensitization of Hela cells . It has been reported that CML stem cell and progenitor cells exhibit higher DNA-PKcs expression  than their normal counterparts, suggesting that they have better error-prone DNA repair. Rapid repair from DNA damage by error-prone NHEJ as shown in Fig. 2 is expected to make these cells more resistant to anti-cancer drugs and also to induce further genomic changes such as complex chromosomal abnormality. The promoter of human DNA-PKcs has not been fully analyzed. Although the presence of a GC-rich region was reported, the direct analysis of DNA-PKcs promoter activity has not been performed , . Our promoter analysis revealed that various factors and promoter areas are involved in its transcription and that the proximal short GC-rich promoter region plays an important role in DNR-resistance. Although it has been reported that CML cells have altered DNA repair processes, including error-prone HR and different NHEJ mechanisms , overexpression of NHEJ components was also observed in HL60/DNR, suggesting that this overexpression could be induced regardless of initial genomic changes. Our analysis using p210 (BCR/ABL) and T315I mutated p210-overexpressed BaF3 cells (Supplementary Fig. 2) supports this interpretation.