H3K4 Methylation-Mediated Memory of an Active Transcriptional State Impairs Nuclear Reprogramming

Xenopus eggs can induce the reversal of differentiation processes of somatic cells. Yet, the egg is not fully efficient in reprogramming a differentiated nucleus, as certain genes retain a memory of gene expression of their somatic cell of origin. This is thought to be a reason for the low success rate of current cloning and reprogramming strategies. While previous studies addressed extensively the mechanisms that maintain an inactive state of genes (OFF-memory), we investigated the importance of memory of an active transcriptional state (ON-memory) in maintaining cell fate identity and on resistance to reprogramming. We find that donor cell-type specific ON-memory gene-expression in the wrong cell-type of nuclear transfer (NT)-embryos is as common as OFF-memory gene-expression. When compared to properly reprogrammed genes, we find that ON-memory genes show an elevated level of the active histone mark H3K4me3 in endoderm donor cells. Importantly, we show that a reduction of H3K4 methylation level in donor cells decreases the extent of ON-memory gene expression, globally improves transcriptional reprogramming, and enhances the development of NT-embryos. Therefore, our study reveals that H3K4 methylation safeguards endoderm cell identity and acts as a major barrier for efficient reprogramming in NT-embryos. Furthermore, our results suggest that efficient cell fate reprogramming not only relies on the erasure of epigenetic modifications conferring OFF-memory but also crucially depends on the removal of H3 lysine 4 methylation-mediated memory of an active state of gene expression. Overall design: 73 samples, single-ended RNA-seq libraries from neurula stage 18 or 21 endoderm and gastrula stage 11 ectoderm samples; 2 single-ended ChIP-seq libraries from endoderm cells of neurula (stage 21) embryos with antibody for H3K4me3, 2 replicates for each histone modification pull-down.