CHROMATIN STRUCTURE AND IMPRINTING - DEVELOPMENTAL CONTROL OF DNASE-ISENSITIVITY IN THE MOUSE INSULIN-LIKE-GROWTH-FACTOR-2 GENE

The insulin-like growth factor 2 (lgf2) gene on distal mouse chromosom e 7 is expressed predominantly from the paternal allele. In previous s tudies we identified two regions of paternal allele-specific methylati on; one at similar to 3 kb upstream of promoter 1, and a second in the 3', coding portion of the gene. The 3' region is methylated in an exp ressing tissue (fetal liver), whereas in a non-expressing tissue (feta l brain), it is not methylated. By contrast, in the 5' region, the pat ernal allele is highly methylated in all tissues. Here, we have studie d another characteristic of chromatin, namely, sensitivity to DNase-l and have focused our developmental analysis on the two differentially methylated regions of lgf2. In the upstream region, four clustered DNa se-l hypersensitive sites (HSS) were detected in embryonic stem (ES) c ells and in midgestation embryos, but not in neonatal liver or brain. In promoter 1 (Pi), at similar to 0.3 kb upstream of exon 1, we detect ed a tissue-specific HSS 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-l HSS were detected in the 3' differentially methylate d region of lgf2. In all these regions, we did not detect differences in DNase-l sensitivity between the parental chromosomes. These results establish major developmental and tissue-specific control of chromati n in the lgf2 locus. The presence of the HSS upstream of lgf2 precedes transcriptional activation of the lgf2 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 ther efore is differently regulated than the more general fetal promoters P 2 and P3. Whereas methylation can be allele-specific, presumably refle cting the gene imprint, the nuclease sensitivity, as detected by our a ssay, is not. These results, taken together with previous observations , reveal developmental and tissue-specific complexity in the expressio n of the parental imprint at the level of chromatin and transcription. We propose that epigenetic features of tissue-specific control and of the control of allelic expression are intricately linked. (C) 1995 Wi ley-Liss, Inc.