Genomic characterization of rectal cancer and molecular determinants of response to neoadjuvant chemoradiotherapy.

Abstract

192 Background: Rectal cancers are clinically different from colon cancers and have not yet been molecularly characterized due to the scarcity of pretreatment specimens. Discovery of molecular determinants of response to chemotherapy and radiation in patients with locally advanced rectal cancer (LARC, stage II and III) are needed to select those who can avoid surgery and benefit from watch-and-wait strategies. Methods: We profiled 371 pre-treatment specimens using targeted-exome sequencing of 468 cancer genes (n = 325), whole-exome sequencing (n = 100), and RNA-sequencing (n = 113). The targeted-sequencing cohort included patients with stage I (n = 44), II (n = 41), III (n = 176), and IV (n = 64) disease. Primary tumors were divided into lower (LR: 0-4 cm to anal verge, n = 62), middle (MR: 4-8 cm, n = 115), and upper rectum (UR: 8-12 cm, n = 107). We examined molecular determinants of complete response (CR) and relapse free survival (RFS) in LARC patients treated with chemoradiotherapy only (CRT: n = 39), induction chemotherapy + CRT (INCT: n = 87) and consolidation chemotherapy after CRT (CCNT: n = 63). Results: Among MSS cases, oncogenic gene and signaling pathway alterations did not vary by clinical stage. WNT pathway alterations, driven by APC mutations, were more frequent in the UR (89% UR v 86% MR v 60% LR, p < 0.001) while RTK/RAS alterations were more frequent in the LR (54% UR v 69% MR v 72% LR, p < 0.03). A set of genes enriched in mTOR signaling, G2M checkpoint, EMT transition, and DNA repair were overexpressed in the UR (FDR < 0.1). The 5-yr RFS rate for LARC was 75% (CI: 68%-82%) and 24% of the cases had a CR (n = 45). MSI cases had a higher rate of CR compared to MSS cases (50% v 23%, p = 0.07) and none relapsed (n = 8). KRAS-altered MSS tumors exhibited worse RFS in cases treated with CNCT (5-yr: 74% v 97%, p = 0.01), but not in cases treated with INCT (5-yr: 67% v 72%, p = 0.7). Conclusions: WNT alterations are more frequent in the UR while RTK/RAS alterations are more frequent in the LR, suggesting differences in tumor biology between proximal and distal rectal cancer. Further, we report correlations between distinct molecular profiles and response to treatment paradigms that could guide the design of future clinical trials.