Epigenetic Memory Marks Determine Epiallele Stability at Loci Targeted by De Novo DNA Methylation

It is generally assumed that DNA methylation changes at genomic regions targeted by the de novo RNA-directed DNA methylation (RdDM) pathway are unstable. Here, we show that RdDM targets in Arabidopsis can be classified into two groups on the basis of whether there is remethylation following the restoration of NRPD1 function in nrpd1 mutant plants—remethylable loci and non-remethylable loci. In contrast to the remethylable loci, the non-remethylable loci contain higher levels of the euchromatic marks of trimethylation at Lys 4 of histone H3 (H3K4me3), which interferes with the recruitment of the RdDM molecular machinery, and acetylation at Lys 18 of histone H3 (H3K18ac), which helps to recruit the DNA demethylase ROS1 to antagonize RdDM. Here, using targeted methylation erasure by CRISPR–dCas9–TET1, we demonstrate that methylated CG (mCG) and mCHG (where H represents A, C or T) are memory marks that are required for targeting the RdDM machinery to remethylable loci. Our results show that histone and DNA methylation marks are critical in determining the ability of RdDM target loci to form stable epialleles, and contribute to understanding the formation and transmission of epialleles.