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Figure 4 | Epigenetics & Chromatin

Figure 4

From: Active demethylation in mouse zygotes involves cytosine deamination and base excision repair

Figure 4

Evidence supports AID mediated cytosine deamination with U-G mismatch long-patch BER in the zygote. (A) Schematic representation of the possible AID-mediated deamination scenarios. (1) AID deamination of a methylated cytosine in the context of the preferred binding motif (WRC) resulting in demethylation and creating a T-G mismatch. This would be recognised by TDG generating and apyrimidinic site that would subsequently be repaired by either short-patch (SP) (no further loss of methylation in neighbouring methylated cytosines) or long-patch (LP) (possible further demethylation by new incorporation of cytosines not followed by de novo methylation) BER. (2) AID deamination of a non-methylated cytosine in the context of the preferred binding motif (WRC) resulting in no loss of methylation and creating a U-G mismatch. This would be recognised by UNG2 generating and apurinic site that would subsequently be repaired by either SP (no loss of methylation in neighbouring methylated cytosines) or LP (resulting in demethylation by new incorporation of cytosines not followed by de novo methylation) BER. (B) Comparison of the changes of DNA methylation between wild-type control (B6xB6), TDG maternally deleted (TDG MAT KOxB6) and UNG2 null (UNG2 KOxB6) late zygotes. Box-and-whisker plots of the total immunofluorescence signal (3D imaging semi-quantification) ratio between the paternal and maternal pronuclei (male/female ratio) for 5mC showing there is only a significant increase in the levels of paternal DNA methylation in UNG2 deleted and not in TDG maternally deleted fertilised oocytes. This is compatible with AID-mediated cytosine deamination followed by LP BER with no evidence for direct 5mC deamination and T-G mismatch repair, which is in agreement with the reported reduced activity of AID on 5mC relative to cytosine, its canonical substrate. **(P < 0.05, ANOVA, Dunn’s multiple comparison test), ns (P > 0.05, ANOVA, Dunn’s multiple comparison test).

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