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Sci Rep. 2017 Aug 3;7(1):7199. doi: 10.1038/s41598-017-07314-5.

Uracil Accumulation and Mutagenesis Dominated by Cytosine Deamination in CpG Dinucleotides in Mice Lacking UNG and SMUG1.

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Department of Clinical Molecular Biology, Ahus Campus, University of Oslo, Oslo, Norway.
Akershus University Hospital, Lørenskog, Norway.
Department of Cancer Research and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway.
The Liaison Committee for Education, Research and Innovation in Central Norway, Trondheim, Norway.
Department of Informatics, University of Oslo, PO Box 1080 Blindern, NO-0316, Oslo, Norway.
MRC Laboratory of Molecular Biology, Cambridge, UK.
LifeTechnologies AS, Ullernschauseen 52, 0379, Oslo, Norway.
Theoretical Biology and Biophysics (T-6), Los Alamos National Laboratory, Los Alamos, NM, 87545, USA.
Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA.
University of New Mexico Comprehensive Cancer Center, Albuquerque, NM, 87102, USA.
Department of Microbiology, Oslo University Hospital, Rikshospitalet, PO Box 4950 Nydalen, NO-0424, Oslo, Norway.
Department of Clinical Molecular Biology, Ahus Campus, University of Oslo, Oslo, Norway.
Akershus University Hospital, Lørenskog, Norway.


Both a DNA lesion and an intermediate for antibody maturation, uracil is primarily processed by base excision repair (BER), either initiated by uracil-DNA glycosylase (UNG) or by single-strand selective monofunctional uracil DNA glycosylase (SMUG1). The relative in vivo contributions of each glycosylase remain elusive. To assess the impact of SMUG1 deficiency, we measured uracil and 5-hydroxymethyluracil, another SMUG1 substrate, in Smug1 -/- mice. We found that 5-hydroxymethyluracil accumulated in Smug1 -/- tissues and correlated with 5-hydroxymethylcytosine levels. The highest increase was found in brain, which contained about 26-fold higher genomic 5-hydroxymethyluracil levels than the wild type. Smug1 -/- mice did not accumulate uracil in their genome and Ung -/- mice showed slightly elevated uracil levels. Contrastingly, Ung -/- Smug1 -/- mice showed a synergistic increase in uracil levels with up to 25-fold higher uracil levels than wild type. Whole genome sequencing of UNG/SMUG1-deficient tumours revealed that combined UNG and SMUG1 deficiency leads to the accumulation of mutations, primarily C to T transitions within CpG sequences. This unexpected sequence bias suggests that CpG dinucleotides are intrinsically more mutation prone. In conclusion, we showed that SMUG1 efficiently prevent genomic uracil accumulation, even in the presence of UNG, and identified mutational signatures associated with combined UNG and SMUG1 deficiency.

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