The acetyllysine reader BRD3R promotes human nuclear reprogramming and regulates mitosis

It is well known that both recipient cells and donor nuclei demonstrate a mitotic advantage as observed in the traditional reprogramming with somatic cell nuclear transfer (SCNT). However, It is not known whether a specific mitotic factor plays a critical role in reprogramming. Here we identify an isoform of human bromodomain-containing 3 (BRD3), BRD3R (BRD3 with Reprogramming activity), as a reprogramming factor. BRD3R positively regulates mitosis during reprogramming, upregulates a large set of mitotic genes at early stages of reprogramming, and associates with mitotic chromatin. Interestingly, a set of the mitotic genes upregulated by BRD3R constitutes a pluripotent molecular signature. The two BRD3 isoforms display differential binding to acetylated histones. Our results suggest a molecular interpretation for the mitotic advantage in reprogramming, and show that mitosis may be a driving force of reprogramming. Human BJ cells transduced with lentiviral particles of the conventional reprogramming factors (OCT3/4, SOX2 and KLF4) were used as controls. Two types of controls were used: 1) BJ transduced with OSK (OCT4, SOX2 and KFL4) viruses; 2) BJ cells transduced with OSK plus GFP viruses. Experimental treatment was BJ cells transduced with OSK plus BRD3R viruses. RNA was extracted from cells at day 3 of reprogramming because the reprogramming cells are still homogeneous and transgenes are well expressed at this time point.