The rescue of epigenomic abnormalities in ICF1 patient iPSCs following DNMT3B correction is incomplete at a residual fraction of H3K4me3-enriched regions

Background Bi-allelic hypomorphic mutations in DNMT3B disrupt DNA methyltransferase activity and lead to Immunodeficiency, Centromeric instability, Facial anomalies syndrome, type 1 (ICF1). While several ICF1 phenotypes have been linked to abnormally hypomethylated repetitive regions, the unique genomic regions responsible for the remaining disease phenotypes remain largely uncharacterized. Here we explored two ICF1 patient-induced pluripotent stem cells (iPSCs) and their CRISPR/Cas9 corrected clones to determine whether gene correction can overcome DNA methylation defects and related/associated changes in the epigenome of non-repetitive regions. Results Hypomethylated regions throughout the genome are highly comparable between ICF1 iPSCs carrying different DNMT3B variants, and significantly overlap with those in ICF1-peripheral blood and lymphoblastoid cell lines. These regions include large CpG island domains, as well as promoters and enhancers of several lineage-specific genes, in particular immune-related, suggesting that they are pre- marked during early development. The gene corrected ICF1 iPSCs reveal that the majority of phenotype- related hypomethylated regions re-acquire normal DNA methylation levels following editing. However, at the most severely hypomethylated regions in ICF1 iPSCs, which also display the highest increased H3K4me3 levels and enrichment of CTCF-binding motifs, the epigenetic memory persisted, and hypomethylation was uncorrected. Conclusions Restoring the catalytic activity of DNMT3B rescues the majority of the aberrant ICF1 epigenome. However, a small fraction of the genome is resilient to this reversal, highlighting the challenge of reverting disease states that are due to genome-wide epigenetic perturbations. Uncovering the basis for the persistent epigenetic memory will promote the development of strategies to overcome this obstacle. ### Competing Interest Statement The authors have declared no competing interest.