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Non-B DNA-forming Sequences and WRN Deficiency Independently Increase the Frequency of Base Substitution in Human Cells

Bacolla, A., Wang, G., Jain, A., Chuzhanova, N. A., Cer, R. Z., Collins, J. R., Cooper, David Neil, Bohr, V. A. and Vasquez, K. M. 2011. Non-B DNA-forming Sequences and WRN Deficiency Independently Increase the Frequency of Base Substitution in Human Cells. Journal of Biological Chemistry 286 (12) , pp. 10017-10026. 10.1074/jbc.M110.176636

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Although alternative DNA secondary structures (non-B DNA) can induce genomic rearrangements, their associated mutational spectra remain largely unknown. The helicase activity of WRN, which is absent in the human progeroid Werner syndrome, is thought to counteract this genomic instability. We determined non-B DNA-induced mutation frequencies and spectra in human U2OS osteosarcoma cells and assessed the role of WRN in isogenic knockdown (WRN-KD) cells using a supF gene mutation reporter system flanked by triplex- or Z-DNA-forming sequences. Although both non-B DNA and WRN-KD served to increase the mutation frequency, the increase afforded by WRN-KD was independent of DNA structure despite the fact that purified WRN helicase was found to resolve these structures in vitro. In U2OS cells, ∼70% of mutations comprised single-base substitutions, mostly at G·C base-pairs, with the remaining ∼30% being microdeletions. The number of mutations at G·C base-pairs in the context of NGNN/NNCN sequences correlated well with predicted free energies of base stacking and ionization potentials, suggesting a possible origin via oxidation reactions involving electron loss and subsequent electron transfer (hole migration) between neighboring bases. A set of ∼40,000 somatic mutations at G·C base pairs identified in a lung cancer genome exhibited similar correlations, implying that hole migration may also be involved. We conclude that alternative DNA conformations, WRN deficiency and lung tumorigenesis may all serve to increase the mutation rate by promoting, through diverse pathways, oxidation reactions that perturb the electron orbitals of neighboring bases. It follows that such “hole migration” is likely to play a much more widespread role in mutagenesis than previously anticipated.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Medicine
Subjects: Q Science > QH Natural history > QH426 Genetics
R Medicine > R Medicine (General)
Uncontrolled Keywords: DNA Helicase; DNA Structure; Gene Silencing; Mutagenesis Mechanisms; Oxidative Stress; Charge Transfer; DNA Secondary Structure; Mutation Frequency; U2OS WRN Knockdown; WRN Helicase Activity
Publisher: American Society for Biochemistry and Molecular Biology
ISSN: 0021-9258
Last Modified: 04 Jun 2017 03:43

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