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Isolation of camptothecin-sensitive chinese hamster cell mutants: phenotypic heterogeneity within the ataxia
M A Johnson1, P E Bryant, N J Jones
1School of Biological Sciences, Donnan Laboratories, University of Liverpool, Liverpool L69 7ZD, UK.
Abstract:
Using a replica microwell method, four Chinese hamster lines which exhibit hypersensitivity to the topoisomerase I inhibitor camptothecin, designated CM1, CM2, CM3 and CM6, have been isolated. Their sensitivity towards camptothecin varied from 3.5- to 8.2-fold with relative sensitivity as follows: CM2 < CM3 < CM6 < CM1. Genetic analysis of the CM mutants has established that CM1, CM3 and CM6 fail to complement each other and can each be assigned to the irs2 (XRCC8) complementation group. The mutant CM2 could not be definitively assigned to a complementation group because it presented a semi-dominant phenotype. In contrast to their sensitivity to camptothecin, the four CM mutants were less sensitive (1.1- to 2.2-fold) to the topoisomerase II inhibitors etoposide and adriamycin, although CM1, CM3 and CM6 were more sensitive (2.5- to 3. 8-fold) to streptonigrin (a free radical generator and a topoisomerase II inhibitor). All four mutant lines displayed an increased sensitivity to the bifunctional alkylating agent mitomycin C (2.4- to 5.1-fold). Surprisingly, given their assignment to the irs2 (XRCC8) complementation group, CM1, CM3 and CM6 displayed only a minor increase in sensitivity to ionizing radiation (1.6-fold or less). Similar sensitivity of these CM mutants was observed for the radiomimetic compound bleomycin (1.7-fold sensitive or less). This study indicates that XRCC8 mutants are isolated at high frequency from the parent line V79 and that phenotypic heterogeneity amongst the irs2 (XRCC8) complementation group is greater than previously encountered. Mutations in different regions of the XRCC8 gene may be responsible for the differing cellular phenotypes. Hamster XRCC8 mutants show phenotypic similarities to cultured cells from ataxia telangiectasia and Nijmegen break syndrome (NBS) patients and are likely to be defective in the same pathway in which the ATM (ataxia telangiectasia-mutated) and the NBS genes operate.
Insights
Researchers isolated four Chinese hamster cell lines hypersensitive to camptothecin, a topoisomerase I inhibitor. These mutants, primarily in the XRCC8 complementation group, show varied sensitivities to DNA-damaging agents, suggesting diverse mutations within the XRCC8 gene.
Area of Science:
- Cell biology
- Genetics
- DNA repair mechanisms
Background:
- Topoisomerase I inhibitors like camptothecin are crucial in cancer therapy.
- Understanding DNA repair pathways is essential for drug development and disease research.
- Chinese hamster cell lines are widely used models for genetic and cellular studies.
Purpose of the Study:
- To isolate and characterize Chinese hamster cell lines with hypersensitivity to camptothecin.
- To genetically analyze these mutants and determine their complementation groups.
- To investigate their sensitivity profiles to various DNA-damaging agents.
Main Methods:
- Replica microwell method for isolating mutant cell lines.
- Sensitivity assays using camptothecin, etoposide, adriamycin, streptonigrin, mitomycin C, ionizing radiation, and bleomycin.
- Genetic complementation analysis to assign mutants to complementation groups.
Main Results:
- Four camptothecin-hypersensitive mutants (CM1, CM2, CM3, CM6) were isolated.
- Mutants CM1, CM3, and CM6 belong to the irs2 (XRCC8) complementation group; CM2 showed a semi-dominant phenotype.
- Mutants exhibited varied sensitivities to topoisomerase II inhibitors, streptonigrin, and mitomycin C, with less sensitivity to ionizing radiation and bleomycin than expected for XRCC8 mutants.
Conclusions:
- XRCC8 mutants can be isolated at high frequency and display significant phenotypic heterogeneity.
- Differential mutations within the XRCC8 gene likely cause the observed phenotypic variations.
- These hamster XRCC8 mutants share phenotypic similarities with human ataxia telangiectasia and Nijmegen break syndrome cells, implicating a conserved DNA repair pathway.