Molecular mechanism of genomic imprinting in somatic cell cultures

The mouse insulin-like growth factor 2 (Igf-2) and the H19 genes, located on distal chromosome 7, are genomically imprinted such that the paternally derived Igf-2 and the maternally derived H19 are expressed. Igf-2 and H19 are reciprocally imprinted yet demonstrate a similar spatio-temporal pattern of expression during embryogenesis. Somatic cell cultures were derived from mouse embryos with maternal uniparental disomy for distal chromosome 7 (MatDi7) and normal littermates to examine the molecular basis of genomic imprinting.;MatDi7 cells maintained the Igf-2 imprint since these cells generally did not express Igf-2. Methylation analysis indicated that extensive de novo methylation of Igf-2 had not occurred during culturing. Expression of the maternal Igf-2 in MatDi7 cells was increased in a dose-dependent manner by treatment with 5-aza-2;Igf-2 and H19 were expressed in a coordinated manner in normal cells suggesting a common regulatory mechanism. Spontaneous activation of Igf-2 in a MatDi7 clone was associated with de novo methylation of the Igf-2 upstream region and the H19 promoter so that the maternal allele(s) had acquired a methylation pattern characteristic of the paternal allele. Expression of Igf-2 and H19 from different parental chromosomes may result from parental-specific epigenetic modifications.;The molecular mechanisms of the human Igf-2 and H19 imprints were studied using human-mouse somatic cell hybrids containing a single copy of human chromosome 11p where the Igf-2/H19 locus resides. Treatment of the hybrids with 5-bromodeoxyuridine resulted in a dose-dependent increase in expression of the human H19 and a dose-dependent decrease in Igf-2 expression. These results indicate that Igf-2 and H19 may compete for transcriptional regulatory element(s) allowing only Igf-2 or H19 to be transcribed from a single parental chromosome. Therefore, the genomic imprints of Igf-2 and H19 may be mechanistically and functionally related.