Promoter-dependent tissue-specific expressive nature of imprinting gene, insulin-like growth factor II, in human tissues.

The insulin-like growth factor II (IGF2) is a polypeptide with structure homology to insulin which possesses mitogen activity, and imprinted with paternal allele. In order to elucidate the distribution of imprinting pattern and relationship between allele- and tissue-specific expression of IGF2 in growth and maturation of human tissues, we investigated allele-specific expression of IGF2 in a wide spectrum of normal maturated human tissues by a PCR-based assay and found monoallelic expression in all eight-type tissues tested except human adult liver. Moreover, when a RT-PCR based sensitive allele-specific primer extension for an Apa I polymorphism within exon 9 of IGF2 was used, the analysis revealed the gene was normally imprinted in placenta; in contrast to the finding with placenta, IGF2 transcripts were biallelically expressed in human adult liver. Our results have clearly demonstrated preferential paternal expression and tissue-specific imprinting pattern of IGF2 in all human tissues tested in this study. Collectively, since IGF2 expression in developing fetal and adult liver is specified by distinct promoters, these extensive observations definitively indicate that transcriptional imprinting of IGF2 is more likely a promoter dependent manner.