Abstract B036: Comprehensive transcriptomic profiling challenges the robustness of prostate cancer prognostic signatures to multifocality

Abstract

Abstract Purpose: Imaging and tissue-based biomarkers are increasingly utilized in men diagnosed with low-grade prostate cancer (PCa) to guide definitive management vs. active surveillance. PCa is uniquely multifocal, with multiple clonally distinct tumor foci present in the majority of men. A single tumor focus may also be composed of both low- and high-grade components. Hence, an ideal prognostic biomarker should be robust to both undersampling of a high-grade component, as well as a not sampled multifocal high-grade tumor focus. Materials and Methods: To assess the robustness of prognostic biomarkers to multifocality, we designed a comprehensive multiplexed targeted RNA sequencing assay (mxRNAseq) capable of assessing multiple classes of transcriptional alterations and deriving available prognostic signature scores (e.g., Prolaris CCP and OncotypeDX GPS). We applied this assay to a retrospective, multi-institution cohort of over 150 formalin-fixed, paraffin-embedded tissue samples representing the range of prostate cancer progression. Single candidate biomarkers and derived prognostic signatures were compared between low-grade foci in cases with and without high-grade foci, all-low-grade foci vs. all-high-grade foci, and cases with extremes of grade differences in multifocal tumors. Frequency of clinically relevant extreme multifocality was determined by identifying patients in a single-institution consecutive prostatectomy database over 9 years who had exclusively low-grade cancer on biopsy but very-high-grade disease on prostatectomy. Results: Our mxRNAseq assay robustly detected known coding gene/lncRNA expression, gene fusions and splice variants, and expressed mutations and germline variants. Supervised hierarchical clustering of target gene expression confirmed expected transcriptional module deregulation and derived prognostic signatures across prostate cancer progression. Prognostic biomarkers (including derived signatures) showed no significant differences in expression between low-grade foci from prostates with and without high-grade multifocal tumors and were uniformly higher in high-grade foci vs. low-grade foci from the same case. In four cases of extreme multifocality (Gleason score 6 vs. >=8 foci), prognostic signatures were significantly lower in low-grade foci vs. high-grade foci. In 1,418 men with biopsy Gleason score 3+3=6 or 3+4=7, 21 (1.5%) had Gleason score ≥ 4+4=8, where the biopsy almost certainly did not sample the most clinically relevant focus. Conclusions: Using a novel comprehensive mxRNAseq assay, our results challenge the robustness of prognostic biomarkers between multifocal low- and high-grade prostate cancer foci. Tissue-based prostate cancer prognostic biomarkers should be specifically validated using an extreme multifocality design to support utility in predicting the presence of an unsampled, aggressive multifocal tumor focus. Citation Format: Simpa Salami, Daniel H. Hovelson, Jeremy B. Kaplan, Romain Mathieu, Aaron M. Udager, Nicole Curci, Matthew Lee, Lorena Lazo de la Vega, Martin Susani, Nathalie Rioux-Leclercq, Daniel E. Spratt, Todd M. Morgan, Matthew S. Davenport, Mark A. Rubin, Shariat F. Shahrokh, Scott A. Tomlins, Ganesh S. Palapattu. Comprehensive transcriptomic profiling challenges the robustness of prostate cancer prognostic signatures to multifocality [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr B036.