Regulation of Imprinting in Cancer

Andrew Hoffman
Department of Medicine; Stanford University

Imprinting of the insulin-like growth factor II gene (IGF2) is controlled by epigenetic factors including DNA methylation, histone modifications and intra-chromosome looping. The close juxtaposition of chromosome territories and shared sites of transcription suggest that inter-chromosomal associations may also exist. IGF2 imprinting is often lost in a variety of pediatric and adult malignancies, theoretically providing a growth advantage by increasing the levels of this fetal mitogen. To test whether abnormal IGF2 imprinting in tumor cells can be corrected or normalized by epigenetic reprogramming, we isolated nuclei from four tumor cell lines, WTCL and SKNEP (both from Wilms' tumor), HRT18 (colorectal cancer), H522 (lung cancer), all of which showed loss of IGF2 imprinting (LOI), and fused them to enucleated mouse or human fibroblast cell lines (MBW2 and HFB1) in which IGF2 is normally imprinted. After nuclear transfer, we detected monoallelic expression of IGF2 in the reconstructed cell clones. In H522/MBW2 reconstructed cell clones, the aberrant H19 imprinting was also corrected. In self-constructed control clones, where the tumor nuclei were transferred into enucleated tumor cells, there was no correction of aberrant IGF2 imprinting. Thus, normal fibroblasts are capable of employing their intact imprinting machinery to epigenetically correct aberrant IGF2 imprinting in tumor cells. These data will contribute a more complete understanding of the mechanisms underlying the loss of IGF2 imprinting in tumors and may provide clues to new cancer therapies that target IGF2 epigenetic mechanisms. (Supported by NIH grant DK36054, the March of Dimes, and the Research Service of the Department of Veterans Affairs.)