Abstract 283: Identification of a high-risk subgroup in primary prostate cancers presenting with targetable immune biology

Abstract

Abstract Background Recent studies have demonstrated limited success of immune checkpoint therapies in unselected prostate cancer. We therefore assessed an immune based DNA Damage Repair Deficiency (DDRD) assay, that we have previously reported represents activation of the cGAS STING pathway in the TCGA dataset of primary prostate cancers, to investigate the presence of targetable immune biology in prostate cancer. In addition we applied a second assay (the prostate cancer metastatic signature-PCM) that predicts risk of metastatic recurrence for early prostate cancer to assess if immune therapy could have a role in treating high risk disease. Methods 498 samples in the TCGA dataset with RNA sequencing data were scored with the PCM and DDRD assays. Integrative analysis was performed on 488 of those samples with matched RNA sequencing, promoter site methylation, somatic mutation and somatic copy number variation. Gene expression of n=6 immune checkpoint targets was investigated with the subgroups identified using T-tests. The prevalence of immune infiltration in each subgroup was tested by applying a cut off to the leukocyte fraction. The viability of reproducing those subgroups with RNA sequencing alone was tested in the TCGA dataset and an independent validation dataset of 321 resected primary prostate cancers. Cox proportional hazards regression analysis was performed for biochemical recurrence and metastatic events in both datasets. Results Integrative analysis of the TCGA dataset identified four patient subgroups characterised primarily by variances in copy number and genomic mutation. One of these subgroups ‘Metastatic-like DDRD' had significantly higher PCM scores and DDRD immune scores compared to the other subgroups (p < 2E-12). This subgroup of patients showed elevated leukocyte fraction and expression of immune checkpoint genes: CD274 (PDL1), CTLA4, ICOS, IDO1, HAVCR2 (TIM3) & LAG3 (p < 2E-6). Genomic instability with amplification of 8q and a larger prevalence of somatic mutations including that of TP53 was also detected in this subgroup. The ‘Metastatic-like DDRD' subgroup was found to have a significant association with poor survival outcome in TCGA (multivariable: p < 0.008), a result that has also been replicated in an independent dataset in both univariate (p < 0.001) and multivariable analysis (p < 0.01). Conclusions We have identified and validated a poor prognostic subgroup, representing 10-20% of early prostate cancer patients that are at increased risk of developing metastatic disease and present with targetable immune biology. These patients may represent a viable target population for immune checkpoint and DNA damaging therapies in prostate cancer. Citation Format: Emma Reilly, Andrena McCavigan, Steven M. Walker, Nuala McCabe, Eileen Parkes, Denis P. Harkin, Richard D. Kennedy, Laura A. Knight. Identification of a high-risk subgroup in primary prostate cancers presenting with targetable immune biology [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 283.