Differential chromatin packaging of genomic imprinted regions between expressed and non-expressed alleles

Chromosomal regions subject to genomic imprinting comprise a functional domain exhibiting parental-specific expression of genes and hence may take a unique chromatin structure. Here we have examined the chromatin packaging state of allelic sites in the Zfp127/Snrpn locus on mouse chromosome 7 and in the Igf2r locus on mouse chromosome 17 with an assay consisting of chromatin fractionation and allele-specific detection. The results showed that non-transcribed alleles of Igf2r are packaged more compactly than transcribed alleles in F(1) hybrid mice of both types of cross between C57BL/6 and MSM strains, whereas a non-imprinted gene, Sod-2, in the vicinity of Igf2r does not show such a difference. This indicates a close correlation between imprinting and the differential packaging of chromatin. On the other hand, the Zfp127/Snrpn locus showed such an allele-specific fractionation pattern only in F(1) hybrid mice of a cross but not in those of the reciprocal cross. Analysis of the congenic mice produced for this locus did not provide any difference. These results suggest that chromatin of imprinted domains in different compaction levels is affected by distinct blueprints in homologous chromosomes that are heritable through the germ line.