Biosci. Rep. (2011) / 31 / 363–369 (Printed in Great Britain) / doi 10.1042/BSR20100108 RECQL5 is an important determinant for camptothecin tolerance in human colorectal cancer cells Xiaoqi WANG*†, Xincheng LU*, Guangjin ZHOU*, Hua LOU* and Guangbin LUO* 1 *Department of Genetics, Case Comprehensive Cancer Center and University Hospitals, Case Western Reserve University, Cleveland, OH 44106, U.S.A., and †The Central Laboratory, The First Teaching Hospital of Jilin University, Changchun 130021, People’s Republic of China ✬ ✫ ✩ ✪ Synopsis CPTs (camptothecins) are an important class of effective anticancer agents that target type I topoisomerase in humans. Irinotecan and topotecan are currently used to treat various types of cancers and many CPT derivatives are being developed. However, these drugs are only effective in a small percentage of each type of cancer and the molecular underpinning for this individualized response to the drug has remained elusive. Thus, identification of the main determinants for cell survival in response to this unique class of drug should help to improve their clinical applications. In the present study, we examined whether RECQL5 constitutes an important determinant of CPT resistance in colon cancer cells. Specifically, RECQL5-deficient derivatives of both DDL1 and HCT116 cells, two colorectal cancer cell lines were generated by adenovirus-based somatic gene-targeting experiments and the CPT sensitivity between the RECQL5-proficient parental lines and their corresponding RECQL5-deficient derivatives were examined. We found that deletion of RECQL5 from DDL1 and HCT116 cells both resulted in a significant enhancement in CPT sensitivity under in vitro culture conditions. More importantly, xenograft tumours derived from RECQL5-deficient HCT116 cells, but not those from the parental line, could be cured by a CPT-based therapy in nude mice. Thus, the present study has identified RECQL5 as a major determinant for CPT resistance in colorectal cancer cells and a potential candidate as a biomarker for irinotecan-based treatment for colon cancer. Key words: camptothecin (CPT), colorectal cancer, drug tolerance, RECQL5, topoisomerase I (Topo I) inhibitor INTRODUCTION CPT (camptothecin) and its derivatives are a class of anticancer agents collective known as Topo I (topoisomerase I) inhibitors [1,2]. These agents target type I topoisomerase and kill human cells by interfering with the function of Topo I during DNA replication [3–7]. Two of them, irinotecan and topotecan have been approved by the U.S. Food and Drug Administration for several types of human cancers, including non-small-cell lung cancer and advanced colorectal cancer [6,7]. Importantly, these drugs are currently being tested in many different multi-agent treatments for various types of cancers. For examples, irinotecan are being tested in multi-agents therapies for both colorectal cancers as well as other types of cancer with promising res- ults [8–10]. Thus, these anticancer drugs are among the most promising classes of anticancer agents [1]. In this regard, an im- proved understanding of the mechanisms of drug tolerance for ............................................................................................................................................................................................................................................................................................................ Abbreviations used: AAV, adeno-associated virus; CPT, camptothecin; DSB, double-stranded break; HR, homologous recombination; Topo I, topoisomerase I. 1 To whom correspondence should be addressed (email guangbin.luo@case.edu). this unique class of anticancer agents would be empirical in the rational design of multi-agent combinatorial therapies with these agents and/or the development of biomarkers for personalized therapies. We have reported recently that Recql5, a member of the mam- malian RecQ helicase family, functions to promote cell survival after CPT treatment in a homologous recombination-independent manner and that Recql5-deficient cells are uniquely hypersensit- ive to CPT but not many other DNA-damaging agents, including methyl methanesulfonate, cisplatinum, mitomycin C and etopos- ide [11–13]. These findings demonstrate that Recql5 plays an important role in a novel pathway of CPT resistance. Import- antly, as the mouse Recql5 and human RECQL5 proteins are also highly conserved, RECQL5 may represent an important de- terminant for resistance to Topo I inhibitors in human cells. The aim of the present work was to investigate whether RECQL5, the human homologue of mouse Recql5, also plays an important role in the cytotoxic response of human colorectal cancer cells to www.bioscirep.org / Volume 31 (5) / Pages 363–369 363 Bioscience Reports www.bioscirep.org