The prognostic role of microsatellite status, tumor mutational burden, and protein expression in CRC.

Abstract

572 Background: Comprehensive molecular profiling of CRC can inform treatment decisions by identifying patient subgroups at varying risks of death. Microsatellite instability (MSI) is prognostic in CRC and is used to select patients for immunotherapy. High tumor mutational burden (TMB) is associated with genomic instability and is prognostic in melanoma. Expression of p16 protein is prognostic in many tumor types. We used proteomic and genomic profiling to measure MSI, TMB and p16 in CRC tumors and to assess associations with patient survival. Methods: In archived clinical samples of CRC, 76 proteins were quantitated with mass spectrometry-based proteomics. MSI was measured by WGS and RNA-seq; unstable loci were quantified in tumor and normal samples. Cutoffs were derived via ROC analysis: high TMB was defined as > 4.5 somatic mutations per megabase; p16 as > 108 amol/ug. Patients were grouped by microsatellite status (MSI vs. microsatellite stable [MSS]), TMB (high vs. low), and p16 protein expression level. Survival curves were compared with the Mantel-Cox log-rank test. Results: Of 145 samples, 39 (27%) had high TMB and 29 (20%) had MSI. Patients with MSI tumors had longer OS than patients with MSS tumors (HR: 0.096; p = 0.003). Similarly, patients with high TMB had longer OS than those with low TMB (HR: 0.076; p < 0.001). High p16 expression was prognostic of poor survival (HR: 2.874; p = 0.019). Among patients with MSS tumors or low TMB, those with low p16 levels had longer OS than patients with high p16 (HR: 0.257; p = 0.002 and HR: 0.249; p = 0.002, for MSS and low TMB, respectively). A combination of MSS, low TMB, and low p16 also differentiated between long and short survivors (HR: 0.249; p = 0.002). These associations remained after adjustment for tumor sidedness. Further analyses of clinical correlates will be presented. Conclusions: A combination of MSS, low TMB and low p16 expression characterized a subset of patients with longer survival. This is important because patients with MSS tumors have limited treatment options but may respond to CDK4/6 inhibitors due to low p16 expression. Molecular profiling of CRC may identify patient subgroups with a relatively poor prognosis who could benefit from personalized therapy.