Parental imprinting of rat insulin-like growth factor II gene promoters is coordinately regulated.

The insulin-like growth factor II (IGF-II) gene is parentally imprinted in the mouse and human species. By following the inheritance of natural polymorphisms of IGF-II mRNA, we demonstrated that the tissue-specific parental imprinting of the IGF-II gene is conserved in the rat. The expression of the paternal IGF-II allele exceeded by more than 3 orders of magnitude that of the maternal allele in livers of 3-day-old Wistar x Fisher interstrain rat crosses. In contrast, the two alleles were both expressed in the rat central nervous system, which is also the only district of the organism where this gene is active in adult rodents. We also analyzed the allelic usage of the three IGF-II promoters, which generate alternatively spliced transcripts, and showed that parental imprinting of all transcription starts sites is coordinately regulated since P1, P2, and P3 are all repressed on the maternal allele in neonatal rat liver, and all of them are activated on both alleles in the choroid plexus of the central nervous system. RNase protection assays demonstrated that the activity ratio of the three IGF-II promoters can be different in tissues that show the same imprinting mode.