Reprogramming somatic cells to induced pluripotent stem cells entails the erasure of somatic cell identity followed by the establishment of a new pluripotent state. This process requires the somatic cell to reset its epigenome, as it shifts to a pluripotent stem cell fate. An enormous barrier faced by the reprogramming cell is the closed compacted chromatin of the somatic cell, which needs to be converted to open active chromatin to allow expression of the pluripotency network of genes. An open chromatin state requires acetylation of histone residues. The histone acetyltransferase MYST4 has a role in maintaining stemness of neural stem cells,^1,2,3 and is active within embryonic stem cells.⁴ However, its role in regulating chromatin structure, and thus gene expression in induced pluripotent stem cells (iPSCs) has yet to be elucidated.

To explore the role of MYST4 in reprogramming, we used mouse embryonic fibroblast (MEFs) isolated from Myst4^–/– mutant, Myst4-Tg BAC transgenic overexpressing and wild type control mice. Reprogramming experiments were performed using a single polycistronic lentiviral vector, constitutively expressing Oct4, Sox2, Klf4 and cMyc.⁵ Although all genotypes gave rise to iPSC colonies, we found the reprogramming efficiency was markedly different between Myst4^–/– and Myst-Tg cells compared to wild type cells. Remarkably, in the absence of MYST4, approximately 4-fold fewer iPSC colonies were generated, while MYST4 overexpression resulted in an approximately 3-fold increase in iPSC colonies. These differences in reprogramming efficiency were not related to effects of MYST4 on proliferation of MEFs, viral infection efficiency, or transgenic and endogenous Oct4, Klf4, Sox2, and c-Myc expression levels. Alkaline phosphatase staining performed on reprogramming cells at key time points revealed that MYST4 was required during the early-middle stages of reprogramming from day 6 onwards. RNA-seq and ChIP-seq experiments are currently underway to identify the genomic target genes of MYST4 during the reprogramming process.

Overall our results show that the absence and overexpression of MYST4 has significant effects on the reprogramming process, indicating an important, and as yet unexplored, role of MYST4 directed histone acetylation in reprogramming somatic cells to pluripotency. Investigation of this mechanism will lead to a better understanding of how chromatin modifications regulate reprogramming, and will also provide insight into the role of chromatin modifiers in controlling the pluripotent state.