Abstract 413: Neo-epitope detection and immune infiltrate analysis of colorectal cancer samples
Authors: Lo Fang Yin , Mandloi Nitin , Yeatman Timothy , Paul Kiran , Patil Ashwini , Anderson Steven , Gupta Ravi , Madan Anup 
Published: July 2017

Abstract
Colorectal cancer (CRC) is the third most common type of cancer in the United States. Targeted therapies that use monoclonal antibodies (mAbs) to EGFR have been shown to benefit some CRC patients. Until recently, KRAS has been the only predictive biomarker for anti-EGFR therapy for metastatic CRC. However, 40% to 60% of patients with wild-type KRAS do not respond to anti-EGFR therapy. Previously we have stratified 55 CRC samples by applying a RAS gene signature score which measures MEK pathway functional output independent of tumor genotype. We showed that samples that have RAS activating mutations such as KRAS and BRAF have significant higher RAS scores (p&lt;0.001). Here, we further investigate the potential immune reactivity in these CRC samples, and thereby the potential benefit of immunotherapy, by evaluating the tumor neo-epitope burden, and the quality of immune cell infiltration based on exome-seq and RNA-seq analysis. In the 53 samples that were sequenced, 779 unique non-synonymous mutations were detected by exome-seq. These 779 mutations spanned across 263 genes. The majority of these mutations are not shared between samples (~ 5% of the mutations were shared by more than 2 samples). Several driver gene mutations were identified in this study, including KRAS, TP53, PIK3CA, APC and HER2. HLA prediction based on Exome-Seq and RNA-Seq data shows that ~86.7% of the alleles predicted to be present in 53 samples were concordant between the two RNA-seq datasets. The predicted alleles based on exome-seq and RNA-seq results have 67-69% concordance. Prediction of neo-epitopes show that HLA-binding neo-epitopes are more frequent than TCR-binding ones, and that most neo-epitopes are private and not shared between samples. A more in-depth analysis of the tumor microenvironment was performed using the RNA-seq data. The epithelial, stromal and immune content of the tumors was comparable to tumors from TCGA CRC data. Next, the immune cell compartment was further stratified into 7 different immune cell types using signatures specific to CD8 andCD4 T, T-regulatory, NK, B-cells, Macrophages and Myeloid derived suppressor cells (MDSC). The immune make up of colorectal cancer is dominated by macrophages and MDSCs. Interestingly, both granulocytic G- and monocytic M-MDSCs are present together, supporting the idea that MDSCs confer an immune suppressive microenvironment in this cancer. Significantly, high MDSC infiltrated tumors showed upregulated expression of pro-tumorigenic insulin-like growth factor pathway genes. Additionally, tumors with lower burden of MDSC showed signature of complement activation suggesting innate cell-mediated anti-tumorigenic mechanisms of tumor control in CRC. These analyses provide potential biomarkers to stratify CRC patients based on their immune reactivity and predict response to cancer immunotherapy drugs.
Citation Format: Fang Yin Lo, Nitin Mandloi, Timothy Yeatman, Kiran Paul, Ashwini Patil, Steven Anderson, Ravi Gupta, Anup Madan. Neo-epitope detection and immune infiltrate analysis of colorectal cancer samples [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 413. doi:10.1158/1538-7445.AM2017-413