title: Generation of bivalent chromatin domains during cell fate decisions.
abstract: BACKGROUND
  In self-renewing, pluripotent cells, bivalent chromatin modification is thought to silence (H3K27me3) lineage control genes while 'poising' (H3K4me3) them for subsequent activation during differentiation, implying an important role for epigenetic modification in directing cell fate decisions. However, rather than representing an equivalently balanced epigenetic mark, the patterns and levels of histone modifications at bivalent genes can vary widely and the criteria for identifying this chromatin signature are poorly defined.
  
  
  RESULTS
  Here, we initially show how chromatin status alters during lineage commitment and differentiation at a single well characterised bivalent locus. In addition we have determined how chromatin modifications at this locus change with gene expression in both ensemble and single cell analyses. We also show, on a global scale, how mRNA expression may be reflected in the ratio of H3K4me3/H3K27me3.
  
  
  CONCLUSIONS
  While truly 'poised' bivalently modified genes may exist, the original hypothesis that all bivalent genes are epigenetically premarked for subsequent expression might be oversimplistic. In fact, from the data presented in the present work, it is equally possible that many genes that appear to be bivalent in pluripotent and multipotent cells may simply be stochastically expressed at low levels in the process of multilineage priming. Although both situations could be considered to be forms of 'poising', the underlying mechanisms and the associated implications are clearly different.
authors.affiliation: MRC Molecular Haematology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, Oxford, OX3 9DS, UK. doug.higgs@imm.ox.ac.uk.
authors.forename: Marco
authors.initials: M
authors.lastname: De Gobbi
authors.forename: David
authors.initials: D
authors.lastname: Garrick
authors.forename: Magnus
authors.initials: M
authors.lastname: Lynch
authors.forename: Douglas
authors.initials: D
authors.lastname: Vernimmen
authors.forename: Jim R
authors.initials: JR
authors.lastname: Hughes
authors.forename: Nicolas
authors.initials: N
authors.lastname: Goardon
authors.forename: Sidinh
authors.initials: S
authors.lastname: Luc
authors.forename: Karen M
authors.initials: KM
authors.lastname: Lower
authors.forename: Jacqueline A
authors.initials: JA
authors.lastname: Sloane-Stanley
authors.forename: Cristina
authors.initials: C
authors.lastname: Pina
authors.forename: Shamit
authors.initials: S
authors.lastname: Soneji
authors.forename: Raffaele
authors.initials: R
authors.lastname: Renella
authors.forename: Tariq
authors.initials: T
authors.lastname: Enver
authors.forename: Stephen
authors.initials: S
authors.lastname: Taylor
authors.forename: Sten Eirik W
authors.initials: SE
authors.lastname: Jacobsen
authors.forename: Paresh
authors.initials: P
authors.lastname: Vyas
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Gibbons
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Higgs