Alterations in the promoter-specific imprinting of the insulin-like growth factor-II gene in Wilms' tumor

The human IGP2 gene, which encodes a mitogenic peptide required for normal fetal development, is overexpressed in many types of tumors. IGF2 is transcribed from four promoters (P1-P4), and in most tissues, the gene is imprinted. In this study, we have analyzed IGF2 promoter usage and determined the allelic expression from each promoter in 19 ApaI- and 22 AluI-heterozygous Wilms' tumors. Loss of IGF2 imprinting (LOI) was observed in 8 ApaI-informative tumors. In these tumors, each parental allele was expressed in equal abundance, indicating that there was complete relaxation of IGF2 imprinting. In each LOI tumor, expression from promoter P1 as well as from the normally imprinted promoters P2-P4 was biallelic. In the 11 ApaI-informative tumors which maintained IGF2 imprinting (maintenance of imprinting), transcription from promoters P2-P4 was always monoallelic, while transcripts from P1 were derived from either one or both alleles, The lack of consistency of IGF2 imprinting of promoter P1 in maintenance of imprinting tumors was also observed in normal fetal tissues of 6-12 weeks gestation, suggesting a similarity in IGF2 regulation between Wilms' tumors and embryonic tissue development. These data suggest that the increased expression of IGF2 in Wilms' tumor may be caused either by biallelic gene expression in LOI tumors from promoters P2-P4 and/or by a reversion to an earlier stage of development which is characterized by increased synthesis of this fetal growth factor.