Allele-Specific Histone Acetylation Accompanies Genomic Imprinting of the Insulin-Like Growth Factor II Receptor Gene**Supported by NIH Grant DK-36054 and by the Research Service of the Department of Veterans Affairs.

The mouse insulin-like growth factor II receptor (Igf2r) gene encodes two reciprocally imprinted RNA transcripts: paternally imprinted Igf2r sense and maternally imprinted Igf2r antisense. Although DNA methylation has been implicated in the initiation and maintenance of genomic imprinting, acetylation of core histones has recently been appreciated as another important factor that regulates gene expression. To determine whether histone acetylation participates in the regulation of Igf2r imprinting, we examined the relative abundance of acetylated histones in interspecific mice (M. spretus x C57BL/6). Oligonucleosomes derived from liver were immunoprecipitated with acetyl-histone antiserum and were analyzed for the allelic distribution of DNA from the region of the sense and antisense Igf2r promoters. In nucleosomes associated with the Igf2r sense promoter, histone acetylation was demonstrated on the maternal allele, which is transcriptionally active. There was much less histone acetylation on the suppressed paternal allele. In nucleosomes associated with the Igf2r antisense promoter, the active paternal allele was heavily acetylated, whereas the suppressed maternal allele was underacetylated. Treatment of cultured fibroblasts with the histone deacetylase inhibitor Trichostatin A induces partial relaxation of genomic imprinting as well as decreased DNA methylation of both Igf2r sense and antisense promoters. These results demonstrate that increases in histone acetylation can lead to decreased DNA methylation, thereby modulating the regulation of the imprinted expression of Igf2r sense and antisense transcripts.