Abstract 611: MYC DNA methylation in prostate tumor tissue is associated with tumor aggressiveness

Abstract

Abstract We previously reported an increased risk of aggressive, but not non-aggressive, prostate cancer associated with pre-diagnostic blood DNA methylation levels in exon 3 of the MYC oncogene among Caucasian men in the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial. In the present study, we aimed to investigate MYC DNA methylation in prostate tissue in relation to tumor aggressiveness (based on Gleason score). We also aimed to investigate whether MYC methylation in prostate tissue is associated with race or tumor/normal status. We accessed formalin-fixed, paraffin-embedded prostate tumor and normal tissue from 50 Caucasian and 50 African American prostate cancer patients that underwent radical prostatectomy at the University of Maryland; for each race group, we selected 25 patients with a Gleason score of 7 and 25 with a Gleason score of 6. We systematically selected the most recent samples, and year of surgery for our patients ranged from 2001-2017. Age at surgery ranged from 42-75 years. We used pyrosequencing to measure DNA methylation at six CpG sites in MYC exon 3 (Chr8:128753145 - Chr8:128753221). We excluded 11 patients for whom the assays failed, resulting in a final sample size of 43 African American and 46 Caucasian men. The six CpG sites demonstrated moderate correlations with one another in tumor tissue (Spearman rho: 0.30-0.60, p-value<0.01 for all pairings). Correlations between the CpG sites were less pronounced in the normal tissue samples (Spearman rho: -0.08-0.57) and five pairs were no longer significantly correlated. We used paired t-tests to evaluate differences in MYC DNA methylation between tumor and normal samples from the same patients, and unpaired t-tests to evaluate differences by Gleason score and race. We observed significantly higher MYC DNA methylation for all six CpG sites in the normal compared to the tumor samples (p<0.05). The difference in mean percent methylation between normal and tumor for the six sites ranged from 2.3 to 16.3 percentage points. In the tumor samples, we also observed significantly higher MYC DNA methylation for patients with a Gleason score of 6 compared to those with a Gleason score of 7 for three of the CpG sites. These included Chr8:128753145 (mean percent methylation for Gleason 6 group=89.0%; mean for Gleason 7=83.6%; t-test p-value=0.002), Chr8:128753154 (mean for Gleason 6=87.7%; mean for Gleason 7=81.7%; t-test p-value=0.004) and Chr8:128753200 (mean for Gleason 6=82.6%; mean for Gleason 7=78.8%; t-test p-value=0.02). Also in tumor tissue, we observed borderline significantly higher DNAm at Chr8:128753151 for African American than Caucasian men (mean for African American men=79.1%; mean for Caucasian men=73.7%; t-test p-value=0.06); no other CpG sites displayed differences by race. Our results provide further evidence for a role of altered MYC DNA methylation in aggressive prostate cancer. Citation Format: Kathryn Hughes Barry, Kareshma Mohanty, Gary Rose, Ashley Cellini, Nicholas Ambulos, Jing Yin, Liying Yan, Matthew Poulin, Ann Meyer, Yuji Zhang, Søren Bentzen, Allen Burke, Arif Hussain, Sonja I. Berndt. MYC DNA methylation in prostate tumor tissue is associated with tumor aggressiveness [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 611.