Differential expression and imprinting status of Ins1 and Ins2 genes in extraembryonic tissues of laboratory mice

There are two functional insulin genes in the mouse genome. The Ins2 gene is imprinted and expressed monoallelically from the paternal allele in the yolk sac. In the present study we have re-examined the imprinting status of Ins1. We found that Ins1 is not expressed in the yolk sac of several laboratory mouse strains. The asynchrony of replication at the wild type locus was significantly lower than at imprinted loci and was more similar to non-imprinted loci. Finally, we have taken the advantage of the Ins1(neo) allele created by homologous recombination to examine the allelic usage at this locus. We observed that the neo gene inserted at the Ins1 locus was expressed from both the paternally and the maternally transmitted allele. Therefore, the Ins1 gene does not share any of the basic properties of imprinted genes. On the basis of these data, we concluded that Ins1 locus is unlikely to be imprinted in common laboratory mice.

Targeted regulation of imprinted genes by synthetic zinc-finger transcription factors

Epigenetic control of transcription is essential for mammalian development and its deregulation causes human disease. For example, loss of proper imprinting control at the IGF2-H19 domain is a hallmark of cancer and Beckwith-Wiedemann syndrome, with no targeted therapeutic approaches available. To address this deficiency, we engineered zinc-finger transcription proteins (ZFPs) that specifically activate or repress the IGF2 and H19 genes in a domain-dependent manner. Importantly, we used these ZFPs successfully to reactivate the transcriptionally silent IGF2 and H19 alleles, thus overriding the natural mechanism of imprinting and validating an entirely novel avenue for 'transcription therapy' of human disease.

Biallelic transcription of Igf2 and H19 in individual cells suggests a post-transcriptional contribution to genomic imprinting

The H19 and insulin-like growth factor 2 (Igf2) genes in the mouse are models for genomic imprinting during development. The genes are located only 90 kb apart in the same transcriptional orientation [1], but are reciprocally imprinted: Igf2 is paternally expressed while H19 is maternally expressed. It has been suggested that expression of H19 and repression of Igf2 (or the converse) on a given chromosome are mechanistically linked and that the parental imprint operates at the level of transcription [2]. Although expression of Igf2 and H19 is thought to be monoallelic, the data have so far been obtained exclusively by looking at steady-state RNA levels using techniques that reflect the average activity of the genes in a cell population [3] [4]. Here, we have adapted a fluorescent in situ hybridisation (FISH) method to detect nascent RNA molecules of Igf2 and H19 at the initial transcription sites in the nuclei of wild-type mouse embryonic liver cells. Nine different transcription patterns were observed, reflecting a high heterogeneity of transcription at the single-cell level. Our observations suggest that regulation of Igf2 and H19 by parental imprinting is much more complex than previously proposed and acts at both transcriptional and post-transcriptional levels.