Fluctuations in histone H4 isoforms during cellular reprogramming monitored by middle-down proteomics

Cellular reprogramming remodels the gene expression program by re-setting the epigenome of somatic cells into an embryonic-like state. Post-translational modifications of histones play an important role in this process. We previously used ChIP-seq to profile the distribution of specific histone H3 marks during the entire reprogramming process and found widespread loss of H3K27me3 in the initial stages[1]. Here, using an unbiased middle-down proteomics approach we have identified 74 unique isoforms of histone H4 and quantified them across nine stages of reprogramming. Our data showed substantial differences between the precursor cells and late-phase reprogramming cells. Also, ESCs and iPSCs displayed higher levels of H4 acetylation and tri-methylation concomitantly with lower levels of mono- and di-methylation when compared to cells undergoing reprogramming. These results suggest that during reprogramming, the histone H4 “PTM code” experiences dynamic changes in multiple waves, with a final reset taking place during the transition to an ESC-like state, in perfect agreement with the in-parallel generated multi-omics data[1]. Together, these data represent an invaluable resource to study PTMs of H4 and their potential cross-talk and extend our knowledge on the epigenetic mechanisms involved in acquisition of induced pluripotency.