title: Normal Sequence and Activity but Reduced Levels of DNA-Pkcs in Human Lymphoblastic Cells Implicate Impaired Protein Stability with Radiosensitive Phenotype.
abstract: BACKGROUND
  Non-homologous end joining (NHEJ) is the main repair pathway for DNA double strand breaks (DSBs) induced by ionizing radiation in mammalian cells. Subsets of cancer patients are hypersensitive to radiotherapy after standard doses. We sought to determine the radiosensitivity of human lymphoblastic cells (LB0005) for the abnormality in NHEJ components.
  
  
  METHODS
  Lymphoblastic (LB0005) cells are derived from an adult cancer patient with late radionecrosis. A low magnesium in vitro DNA-end joining assay was performed to examine for any defect in NHEJ activity. Single-nucleotide polymorphism (SNP) and sequence analysis were performed to examine for abnormality if any, in the genetic sequence of known NHEJ components.
  
  
  RESULTS
  LB0005 cells showed a gain of functional abnormality in the NHEJ pathway. While genetic sequence analysis showed no apparent mutational variations in the known classical NHEJ components, DNA-PKcs (DNA-dependent protein kinase catalytic subunit) protein is reduced in quantity compared to normal control, in spite of higher transcript levels.
  
  
  CONCLUSIONS
  Taken together cells derived from a radiosensitive patient showed an abnormality in NHEJ activity. Proteins other than the classical NHEJ factors may regulate the NHEJ activity. Furthermore, the defect in theses regulatory proteins may have an impact on the stability of DNA-PKcs.
authors.affiliation: Department of Radiation Oncology, Division of Cellular and Molecular Research, National Cancer Centre, Singapore.
authors.forename: Seow Fong
authors.initials: SF
authors.lastname: Yap
authors.forename: Cynthia Sk
authors.initials: CS
authors.lastname: Boo
authors.forename: Susan LE
authors.initials: SL
authors.lastname: Loong
authors.forename: Rajamanickam
authors.initials: R
authors.lastname: Baskar
keywords: DNA-PKcs; NHEJ; Non-homologous end joining