and now for something completely different

[bmanning () vacation karoshi com]

Control of ground-state pluripotency by allelic regulation of Nanog

Nature advance online publication 12 February 2012. doi:10.1038/nature10807

Authors: Yusuke Miyanari & Maria-Elena Torres-Padilla

Pluripotency is established through genome-wide reprogramming during mammalian pre-implantation development, resulting 
in the formation of the naive epiblast. Reprogramming involves both the resetting of epigenetic marks and the 
activation of pluripotent-cell-specific genes such as Nanog and Oct4 (also known as Pou5f1). The tight regulation of 
these genes is crucial for reprogramming, but the mechanisms that regulate their expression in vivo have not been 
uncovered. Here we show that Nanog—but not Oct4—is monoallelically expressed in early pre-implantation embryos. Nanog 
then undergoes a progressive switch to biallelic expression during the transition towards ground-state pluripotency in 
the naive epiblast of the late blastocyst. Embryonic stem (ES) cells grown in leukaemia inhibitory factor (LIF) and 
serum express Nanog mainly monoallelically and show asynchronous replication of the Nanog locus, a feature of 
monoallelically expressed genes, but ES cells activate both alleles when cultured under 2i conditions, which mimic the 
pluripotent ground state in vitro. Live-cell imaging with reporter ES cells confirmed the allelic expression of Nanog 
and revealed allelic switching. The allelic expression of Nanog is regulated through the fibroblast growth 
factor–extracellular signal-regulated kinase signalling pathway, and it is accompanied by chromatin changes at the 
proximal promoter but occurs independently of DNA methylation. Nanog-heterozygous blastocysts have fewer 
inner-cell-mass derivatives and delayed primitive endoderm formation, indicating a role for the biallelic expression of 
Nanog in the timely maturation of the inner cell mass into a fully reprogrammed pluripotent epiblast. We suggest that 
the tight regulation of Nanog dose at the chromosome level is necessary for the acquisition of ground-state 
pluripotency during development. Our data highlight an unexpected role for allelic expression in controlling the dose 
of pluripotency factors in vivo, adding an extra level to the regulation of reprogramming.