A role for DNA methylation in gastrulation and somite patterning

DNA methylation constitutes an important epigenetic factor in the control o f genetic information. In this study, we analyzed expression of the DNA met hyltransferase gene and examined DNA methylation patterns during early deve lopment of the zebrafish. Maternal transcripts of the zebrafish DNA methylt ransferase gene (MTase) are ubiquitously present at high levels in early em bryos with overall levels decreasing after the blastula stage. At 24 h, met hyltransferase mRNA is predominantly found in the brain, neural tube, eyes, and differentiating somites. Expression of MTase in the somites is highest in the anterior cells of the somites. Despite the high levels of MTase mRN A in blastula-stage embryos, we observe DNA hypomethylation at the blastula and gastrula stages compared to sperm or older embryos. Zebrafish embryos treated with 5-azacytidine (5-azaC) and 5-aza-2-deoxycytidine (5-azadC), nu cleotide analogs known to induce cellular differentiation and DNA hypomethy lation in mammalian cells, exhibit DNA hypomethylation and developmental pe rturbations. These defects are specifically observed in embryos treated at the beginning of the blastula period, just prior to midblastula transition. The most common phenotype is the loss of tail and abnormal patterning of s omites. Head development is also affected in some embryos. Histological and in situ hybridization analyses reveal whole or partial loss of a different iated notochord and midline muscle in treated embryos. When examined during gastrulation, 5-azaC-treated embryos have a shortened and thickened axial mesoderm, We propose that DNA methylation is required for normal gastrulati on and subsequent patterning of the dorsal mesoderm. (C) 1999 Academic Pres s.