Int. J. Dev. Biol. 54: 803 - 813 (2010)
doi: 10.1387/ijdb.103081ll
© UBC Press

Chromatin states of developmentally-regulated genes revealed by DNA and histone methylation patterns in zebrafish embryos

Leif C. Lindeman1, Cecilia L. Winata2, Hvard Aanes3, Sinnakaruppan Mathavan4, Peter Alestrm3 and Philippe Collas*,1

1Institute of Basic Medical Sciences, Faculty of Medicine, University of Oslo, and Norwegian Center for Stem Cell Research, Oslo, Norway, 2Stem Cell and Developmental Biology, Genome Institute of Singapore, Biopolis, Singapore and 3BasAM, Norwegian School of Veterinary Science, Oslo, Norway

ABSTRACT Embryo development proceeds from a cascade of gene activation and repression events controlled by epigenetic modifications of DNA and histones. Little is known about epigenetic states in the developing zebrafish, despite its importance as a model organism. We report here DNA methylation and histone modification profiles of promoters of developmentally-regulated genes (pou5f1, sox2, sox3, klf4, nnr, otx1b, nes, vasa), as well as tert and bactin2, in zebrafish embryos at the mid-late blastula transition, shortly after embryonic genome activation. We identify four classes of promoters based on the following profiles: (i) those enriched in marks of active genes (H3K9ac, H4ac, H3K4me3) without transcriptionally repressing H3K9me3 or H3K27me3; (ii) those enriched in H3K9ac, H4ac and H3K27me3, without H3K9me3; one such gene was klf4, shown by in situ hybridization to be mosaically expressed, likely accounting for the detection of both activating and repressive marks on its promoter; (iii) those enriched in H3K4me3 and H3K27me3 without acetylation; and (iv) those enriched in all histone modifications examined. Culture of embryo-derived cells under differentiation conditions leads to H3K9 and H4 deacetylation and H3K9 and H3K27 trimethylation on genes that are inactivated, yielding an epigenetic profile similar to those of fibroblasts or muscle. All promoters however retain H3K4me3, indicating an uncoupling of H3K4me3 occupancy and gene expression. All non-CpG island developmentally-regulated promoters are DNA unmethylated in embryos, but hypermethylated in fibroblasts. Our results suggest that differentially expressed embryonic genes are regulated by various patterns of histone modifications on unmethylated DNA, which create a developmentally permissive chromatin state.

Keywords:

Danio rerio, DNA methylation, embryo, gene expression, histone modification, zebrafish

*Corresponding author e-mail: philippe.collas@medisin.uio.no