Embryonic inheritance of the chromatin organisation of the imprinted H19 domain in mouse spermatozoa

Insulin-like growth factor 2 (Igf 2) and H19 genes are oppositely imprinted and as such have been most extensively studied imprinted genes both genetically and at the molecular level. Imprints of the H19 gene, being established during spermatogenesis, are epigenetically transmitted to the somatic cells of the embryo. Current hypotheses attempting to explain the allele-specific silence of the H19 gene include DNA methylation and chromatin condensation. In order to understand the molecular basis of H19 epigenesis, it is crucial to identify the markings in the chromatin organising the imprinted domain in spermatozoa. Using Micrococcal nuclease (MNase), DNase I and Methidiumpropyl-EDTA. iron II (MPE.Fe(II)) as chromatin probes, we demonstrate that in mouse epididymal spermatozoa, at least 4kb DNA upstream of the H19 'cap' site, containing the imprinted and differentially methylated domain (DMD), is heterochromatic. The cleavage sites in this domain (-2 to -4kb) exhibit approximately 425bp periodicity. This structure is maintained in the paternal allele of normal embryos and is disrupted at -2.2, -2.65 and at -3.5kb in embryos maternally disomic for the distal end of chromosome 7 (MatDp 7). The hypersensitive sites in chromatin precisely register the MPE.Fe(II) cleavage sites in chromosomal DNA. Therefore, the DNA sequences in the imprinted domain constrain the chromatin structure in a way similar to that of 1.688g/cm(3) Drosophila satellite chromatin. In addition, we find that condensation of the paternal allele correlates with methylation-dependent alteration in the structure of DNA sequences in DMD. These results suggest that CpG-methylation induces localised changes in DNA conformation and these facilitate consequent remodelling of chromatin thereby allowing the paternal and maternal H19 alleles to be distinguished.