DT40 knock-out and knock-in studies determine the regions necessary and sufficient for transcription and epigenetic conversion of the chicken Ig-ß gene.

The chicken Ig-ß locus is organized by three cell-type-specific genes and two ubiquitously expressed genes. B-cell-specific DNase I hypersensitive sites (DHS) in that locus, including three present inside the flanking gene, were grouped into six regions and deleted. The deletions decreased Ig-ß mRNA content to <0.1% of that of normal DT40 cells and converted epigenetic parameters such as histone modifications, CG methylation and DNase I hypersensitivity into inactive states. Knocked-in DHS regions into knock-out cells reactivated both transcription of the Ig-ß gene and epigenetic parameters. Thus, the collaboration of the scattered regulatory regions was essential and sufficient not only for B-cell-specific transcription of the Ig-ß gene, but also for the conversion of epigenetic parameters. On the basis of the knock-in studies, we determined the regions involved in the conversion and maintenance of the epigenetic parameters. These scattered regulatory regions were limited in vicinity such as in an intron of the gene, in the intergenic regions and in the introns of a flanking gene.

Activation of the chicken Ig-beta locus by the collaboration of scattered regulatory regions through changes in chromatin structure.

A total of 10 B-lymphocyte-specific DNase I hypersensitive sites located in the chicken Ig-beta locus were divided into four regions and combinations of deletions of these regions were carried out. A decrease in transcription of the Ig-beta gene to <3% was demonstrated in cells with deletions in all four regions. The Ig-beta chromatin was resistant to DNase I digestion in these cells. Thus, the collaboration is shown to convert the Ig-beta chromatin from the condensed state to a relaxed state. H3 and H4 acetylation decreased to <8% but H3K4 hypermethylation was observed at the Ig-beta promoter and exon 3. The collaboration of four regions had virtually no effect on CG hypomethylation in the region upstream the transcriptional start site. Accordingly, neither the DNase I general sensitive state in the Ig-beta chromatin nor hyperacetylation of H3 and H4 histones in the promoter proximal region causes H3K4 di-methylation or CG hypomethylation in the promoter. From these analyses, a chromatin situation was found in which both an active state, such as enhanced H3K4 methylation, or CG hypomethylation, and an inactive state, such as DNase I resistance in the Ig-beta chromatin or hypoacetylation of H3 and H4 histones in the Ig-beta locus, coexist.