Chromatin structure and imprinting: developmental control of DNase-I sensitivity in the mouse insulin-like growth factor 2 gene.

摘要

The insulin-like growth factor 2 (Igf2) gene on distal mouse chromosome 7 is expressed predominantly from the paternal allele. Two regions of paternal allele-specific methylation were previously identified; one _3 kb upstream of promoter 1, and another in the 3'-coding portion of the gene. The 3'-region is methylated in an expressing tissue (fetal liver), whereas in a non-expressing tissue (fetal brain), it is not methylated. By contrast, in the 5' region, the paternal allele is highly methylated in all tissues. Here, another characteristic of chromatin, namely, sensitivity to DNase-I was studied and developmental analysis was focused on the 2 differentially methylated regions of Igf2. In the upstream region, 4 clustered DNase-I hypersensitive sites (HSS) were detected in embryonic stem (ES) cells and in mid-gestation embryos, but not in neonatal liver or brain. In promoter 1 [P1], at _0.3 kb upstream of exon 1, a tissue-specific HSS was found that was present in neonatal liver, in which P1 is active, but was absent in ES cells, the embryo, and in neonatal brain. No DNase-I HSS were detected in the 3' differentially methylated region of Igf2. In all these regions, no differences were detected in DNase-I sensitivity between the parental chromosomes. These results establish major developmental and tissue-specific control of chromatin in the Igf2 locus. The presence of the HSS upstream of Igf2 precedes transcriptional activation of the Igf2 gene and may be indicative of a promoter for another transcript that is transcribed in the opposite direction. The HSS in P1 is largely liver-specific. This promoter therefore is differently regulated than the more general fetal promoters P2 and P3. Whereas methylation can be allele-specific, presumably reflecting the gene imprint, the nuclease sensitivity, as detected by our assay, is not. These results, taken together with previous observations, reveal developmental and tissue-specific complexity in the expression of the parental imprint at the level of chromatin and transcription. It is proposed that epigenetic features of tissue-specific control and of the control of allelic expression are intricately linked.