MERIT Award Recipient: David Chen, Ph.D.
Roles of DNA PK in DNA Double Strand Break Repair
Deficiencies in DNA-damage signaling and repair pathways are fundamental to the etiology of most human cancers. Of the many types of DNA damage that occur within the cell, DNA double-strand breaks (DSBs) are particularly dangerous. An inability to respond properly to DSBs or to repair them correctly can lead to cell death or promote tumorigenesis. During the past 15 years, Dr. Chen and co-workers have worked on the mechanisms by which cells detect DSBs and signal the presence of these lesions to the DNA repair and cell cycle machineries. They have also investigated how some of the proteins involved in these events control other processes, such as telomere length. In recent years, Dr. Chen's group has focused on the DNA dependent protein kinase (DNA-PK). DNA-PK is the key component of the non-homologous end-joining (NHEJ) pathway of DSB repair in mammalian cells. DNA-PK consists of a heterodimeric DNA-binding subunit (Ku70/80) and an approximately 465 kDa catalytic subunit (DNA-PKcs). DNA-PKcs is a serine/threonine protein kinase whose activity is greatly stimulated by its recruitment to DNA breaks by the Ku heterodimer.
Utilizing transgenic mouse models, Dr. Chen's group demonstrated that the kinase activity of DNA-PKcs is absolutely essential for the repair of DNA DSBs. Though it was known for several years that the kinase activity of DNA-PK is stimulated by DNA damage, the in vivo substrates of DNA-PK, whether itself or other proteins, remained elusive. Very recently, Dr. Chen and co-workers demonstrated that DNA-PKcs is autophosphorylated in response to DNA damage at specific serine/threonine residues. This very early event is essential for the repair of DSBs. They also demonstrated, for the first time, the localization of DNA-PKcs at the sites of DNA damage in vivo. Additional phosphorylation targets of DNA-PKcs, such as the protein deficient in Werner's syndrome, were also identified. Ongoing research in the Chen laboratory aims to dissect the molecular functions of the kinase activity of DNA-PKcs and the biological consequences of DNA-PKcs-mediated auto and trans-phosphorylation in mammalian cells. Research by Dr. Chen's group should provide insights into how deficiencies in DSB repair lead to the development of cancer and should contribute to the development of more effective anti-cancer therapies.