Quantitative, spatial resolution of the epigenetic field effect in prostate cancer

BACKGROUND. Although a field effect in which transformed cells extend beyond morphologically evident tumor has been proposed in cancer, little direct evidence exists as to its magnitude and spatial resolution. We tested this hypothesis using molecular techniques to detect epigenetic changes in the primary tumor and surrounding tissues. METHODS. Ex vivo core biopsies, each spaced approximately I mm. apart, were generated from 37 unique prostatectomy samples. The first core biopsy was confirmed to be histologically positive for cancer, and the subsequent biopsies were confirmed to be histologically negative. The methylation ratio of GSTP1, APC, RAR beta 2, and RASSF1A were measured for all of the 159 cores. RESULTS. No field effect, defined as absence of epigenetically transformed cells, for GSTP1 was observed whereas APC, RAR beta 2, and RASSF1A showed a field effect up to 3 mm from the malignant core in three prostatectomy samples. Furthermore, for each case, different patterns of the field effect were observed. The field effect appeared most pronounced with RAR beta 2. In 11 prostatectomy samples in which a second focus of cancer was identified, cells harboring RAR beta 2 methylation extended a large distance away from the primary tumor in one sample. Bisulfite sequencing of RAR beta 2 confirmed the presence of epigenetic aberrations. CONCLUSIONS. This study quantifies previous observations of methylation in histologically negative samples and provides important assessment of field effects based on epigenetic events in cancer. These molecular approaches set the stage for consideration of such data in prospective trials for assessment of surgical margins and prediction of recurrence.