title: TDP1/TOP1 Ratio as a Promising Indicator for the Response of Small Cell Lung Cancer to Topotecan.
abstract: OBJECTIVE
  Small cell lung cancer (SCLC) is one of the most challenging tumors to treat due to high proliferation rate, early metastatic dissemination and rapid development of chemotherapy resistance. The current treatment protocols involve the use of topoisomerase 1 (TOP1) poisons such as irinotecan and topotecan in combination with platinum-based compounds. TOP1 poisons kill cancer cells by trapping TOP1 on DNA, generating lethal DNA double-strand breaks. A potential mechanism employed by cancer cells to resist killing by TOP1 poisons is to overexpress enzymes involved in the repair of TOP1-DNA breaks. Tyrosyl DNA phosphodiesterase 1 (TDP1) is a key player in this process and despite its importance, no data is currently available to correlate TDP1 protein and mRNA levels with catalytic activity in SCLC. In addition, it is not known if TDP1 and TOP1 protein levels correlate with the cellular response of SCLC to TOP1 based therapies.
  
  
  RESULTS
  We report a remarkable variation in TDP1 and TOP1 protein levels in a panel of SCLC cell lines. TDP1 protein level correlates well with TDP1 mRNA and TDP1 catalytic activity, as measured by two newly developed independent activity assays, suggesting the potential utility of immunohistochemistry in assessing TDP1 levels in SCLC tissues. We further demonstrate that whilst TDP1 protein level alone does not correlate with topotecan sensitivity, TDP1/TOP1 ratio correlates well with sensitivity in 8 out of 10 cell lines examined.
  
  
  CONCLUSIONS
  This study provides the first cellular analyses of TDP1 and TOP1 in SCLC and suggests the potential utility of TDP1/TOP1 ratio to assess the response of SCLC to topotecan. The establishment and validation of an easy-to-use TDP1 enzymatic assay in cell extracts could be exploited as a diagnostic tool in the clinic. These findings may help in stratifying patients that are likely to benefit from TOP1 poisons and TDP1 inhibitors currently under development.
authors.affiliation: The Wellcome Trust DNA Repair Group, Genome Damage and Stability Centre, University of Sussex, Brighton BN1 9RQ, UK.
authors.forename: Cornelia
authors.initials: C
authors.lastname: Meisenberg
authors.affiliation: The Sussex Translational Drug Discovery Group, University of Sussex, Brighton, UK.
authors.forename: Simon E
authors.initials: SE
authors.lastname: Ward
authors.affiliation: Brighton and Sussex Medical School, University of Sussex, Brighton, UK (former affiliation) ; Centre of Experimental Cancer Medicine, Barts Cancer Institute, Queen Mary University London, UK.
authors.forename: Peter
authors.initials: P
authors.lastname: Schmid
authors.affiliation: The Wellcome Trust DNA Repair Group, Genome Damage and Stability Centre, University of Sussex, Brighton BN1 9RQ, UK ; Krebs Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, UK ; Center of Genomics, Helmy Institute for Medical Sciences, Zewail City of Science and Technology, Giza, Egypt.
authors.forename: Sherif F
authors.initials: SF
authors.lastname: El-Khamisy
keywords: Enzymatic assay; Small cell lung cancer; TDP1 protein level; TOP1-DNA; Topotecan