726 INSULIN-LIKE GROWTH FACTOR-2 (IGF2) LOSS OF IMPRINTING MARKS A FIELD DEFECT WITHIN HUMAN PROSTATES CONTAINING CANCER

Abstract

INTRODUCTION AND OBJECTIVES: Loss of imprinting (LOI) is an epigenetic alteration involving loss of parental origin-specific expression at normally imprinted genes. A LOI for IGF2, a paracrine growth factor, has been implicated in cancer development and may be an early molecular event in this process. We hypothesize that an IGF2 LOI may be part of a widespread field defect that occurs in the histologically normal prostate tissue of men who have prostate cancer. METHODS: To evaluate IGF2 LOI as part of a prostate field defect, we microdissected normal adjacent (2mm) and distant (10mm) tissues surrounding tumor foci from 20 radical prostatectomy specimens. IGF2 imprinting in informative prostate tissue sets was quantitated using a fluorescent primer extension assay and expression analyzed. As a mechanism controlling expression and imprinting, DNA methylation analyses were performed using quantitative Pyrosequencing. RESULTS: A marked IGF2 LOI was demonstrated in adjacent (39%) and distant (38%) normal tissues. No significant difference in IGF2 LOI in prostate tumors (45%) was noted when compared to these tissues (p 0.02). The expression of IGF2 and its adjacent imprinted gene H19 were increased in adjacent and distant tissues compared to tumors (p 0.05). Hypomethylation within the promoter DMR0 was demonstrated in tumor tissues when compared to distant normal tissues. IGF2 LOI in tumors was not associated with differences in methylation at the intergenic ICR containing CTCF binding site 6 suggesting other factors underlie LOI in prostate tumors. In normal prostate tissues containing a LOI, hypermethylation at the ICR was demonstrated. CONCLUSIONS: LOI of IGF2 occurs not only in prostate tumor foci, but is widely prevalent throughout the peripheral zone of the prostate. This provides evidence for a widespread epigenetic field defect that may predispose to the development of this age-related cancer. The enhancer-competition model of IGF2-H19 imprinting utilizing methylation at the ICR does not apply to human prostate cancers. Furthermore, LOI provides a molecular marker for the presence of prostate cancer in histologically normal appearing tissues.