CHROMATIN MODIFICATION OF IMPRINTED H19 GENE IN MAMMALIAN SPERMATOZOA

The allele-specific epigenetic markings of endogenously imprinted gene s in placental mammals occur during gametogenesis. The identification of the molecular nature of gametic imprints is the first step towards understanding the mechanistic basis of epigenesis in embryonic and adu lt somatic tissues. The specific question addressed in this work is wh ether the closely positioned but oppositely imprinted insulinlike grow th factor 2 (IGF 2) and H19 genes, which have similar temporal regulat ion during development, differ in chromatin structure in mammalian spe rmatozoa. During terminal differentiation of mammalian spermatozoa, ab out 3-15% of the haploid genome retains a quasisomatic-type chromatin structure, whereas the remaining genomes interact with protamines that are further cross-linked by -S-S- bridges. Micrococcal nuclease (MNas e) and DNase I digestions of human (HSN) and porcine sperm nuclei (PSN ) showed that the IGF 2 gene in both types of nuclei retained somatic- type nucleosomes that were close-packed with a periodicity of 150 bp. However, the H19 gene in both species was predominantly organised by u nique structural repeats, which were 650-674 bp in PSN and 438-522 bp in HSN, condensing at least 20 kb of chromatin. These results, togethe r with previous studies, suggest that epigenetic chromatin modificatio n leading to preferential condensation of the paternal H19 allele in e mbryonic tissues is already present in the germ cells. Mol. Reprod. De v. 50:474-484, 1998. (C) 1998 Wiley-Liss, Inc.