Analysis of telomere maintenance in artemis defective human cell lines

Abstract

Telomeres are physical ends of chromosomes consisting of (TTAGGG)n DNA sequence and a specialized set of proteins that protect chromosomal ends from degradation and from eliciting DNA damage response. These specialized set of proteins, known as shelterin, directly bind to telomeric DNA. In addition, some DNA double-strand break (DSB) repair proteins such as, DNA-PKcs and KU70/80, play active roles in telomere maintenance. Mouse knock-out experiments have revealed that deletion of either DNA-PKcs or Ku70/80 resulted in elevated levels of telomeric fusion, indicative of dysfunctional telomeres. Artemis protein is involved in DNA DSB repair through non-homologous end joining (NHEJ) and it is phosphorylated by DNAPKcs. Human cells defective in Artemis have been identified and shown to be radiosensitive and patients with an Artemis defective gene suffer from radiosensitive severe-combined immune deficiency syndrome (RS-SCID). Mouse cells defective in Artemis have elevated levels of telomeric fusion. We have demonstrated in this thesis that Artemis defective human cell lines show a mild telomeric dysfunction phenotype detectable at the cytological level. The nature of telomere dysfunction phenotype appears to be similar to that observed in DNAPKcs defective cells as exemplified by the presence of IR induced chromatid telomeric fusions. We have also shown that (a) DNA damage occurring within the telomeric DNA is difficult to repair or irreparable in older cells and that (b) Artemis defective older cells show higher proportion of DNA damage at telomeres than their normal counterparts. Finally, we have demonstrated that inhibition of DNA-PKcs causes (a) an increase in telomeric fusions in Artemis defective cell lines relative to both normal cell lines after inhibition and Artemis cell lines before inhibition and (b)elevated levels of DNA damage at telomeres following exposure of cells to radiation relative to both irradiated normal cells exposed to a DNA-PKcs inhibitor and irradiated Artemis defective cells but not exposed to the DNA-PKcs inhibitor. These results suggest that the effects of Artemis and DNA-PKcs on telomeres are cumulative. We have also performed (a) experiments to examine telomere function in Artemis defective cell lines after knocking down DNA-PKcs levels by RNAi and b) preliminary experiments to knock-down Artemis in DNA-PKcs defective cells. Taken together, our results suggest that the Artemis defect causes mild telomere dysfunction phenotype in human cells.