GENOMIC DELETION OF AN IMPRINT MAINTENANCE ELEMENT ABOLISHES IMPRINTING OF BOTH INSULIN-LIKE-GROWTH-FACTOR-II AND H19

Insulin-like growth factor II (Igf2) is maternally imprinted in normal tissues with only the paternal copy of the gene being transcribed, wh ereas the contiguous gene H19 is paternally imprinted. Dysregulation o f IGF2 imprinting is commonly observed in Wilns' tumor and other human tumors. Previous work comparing promoter-specific imprinting of human and mouse Igf2 suggested the presence of a cis element upstream of Ig f2 that regulates or maintains the imprinting of three downstream prom oters, To explore the molecular mechanism of maintenance of genomic im printing, we targeted the region between insulin 2 and Igf2, where the cis imprint maintenance element (IME) resides in mouse fibroblasts, I n those clones in which the targeting vector was randomly integrated i nto the genome, mouse Igf2 remained imprinted. However, when the targe ted region containing the IME was deleted by homologous recombination, whether from the paternal or maternal allele, activation of the impri nted maternal allele of Igf2 was observed. In addition, there was a lo ss of H19 imprinting when the IME was deleted, The requirement of IME from both parental alleles for the maintenance of genomic imprinting t hus suggests the importance of a spatial structure of DNA around Igf2 and H19, Modifications in the IME, like abnormal methylation in Wilms' tumors, may represent a novel mechanism for loss of genomic imprintin g.