Tumor mutational burden (TMB), defi ned as the number of somatic mutations per megabase of interrogated genomic sequence, varies across malignancies. Panel sequencing-based estimates of TMB have largely replaced whole-exome sequencing-derived TMB in the clinic. Retrospective evidence suggests that TMB can predict the effi cacy of immune checkpoint inhibitors, and data from KEYNOTE-158 led to the recent FDA approval of pembrolizumab for the TMBhigh tumor subgroup. Unmet needs include prospective validation of TMB cutoffs in relationship to tumor type and patient outcomes. Furthermore, standardization and harmonization of TMB measurement across test platforms are important to the successful implementation of TMB in clinical practice.Signifi cance: Evaluation of TMB as a predictive biomarker creates the need to harmonize panel-based TMB estimation and standardize its reporting. TMB can improve the predictive accuracy for immunotherapy outcomes, and has the potential to expand the candidate pool of patients for treatment with immune checkpoint inhibitors. Responses were observed across tumor types, and microsatellite instability-high (MSI-H) status did not account for all of the increased clinical benefi t in the TMB-high subgroup ( 24 ).On the basis of these data, the FDA approved pembrolizumab monotherapy for the subgroup of solid-tumor patients with TMB ≥10 mut/Mb who are treatment-refractory and lack satisfactory alternative treatment options. Although data from KEYNOTE-158 demonstrated a role for TMB in selection of patients for cancer treatment, important issues remain,

Purpose A let-7 microRNA-complementary site (LCS6) polymorphism in the 3’UTR of the KRAS gene has been shown to disrupt let-7 binding and upregulate KRAS expression. We evaluated the LCS6 genotype and its association with KRAS mutation status, clinicopathological features, and disease-free survival (DFS) in stage III colon cancer patients enrolled in a phase III clinical trial (NCCTG N0147). Experimental Design The LCS6 genotype was assayed by RT-PCR in DNA extracted from whole blood (n=2834) and compared to paired tumor tissue (n=977). Chi-squared and two-sample t tests were used to compare baseline factors and KRAS mutation status between patients defined by LCS6 variant status. Log-rank tests and multivariate Cox models assessed associations between LCS6 status and DFS, respectively. Results We identified 432 (15.2%) blood samples and 143 (14.6%) tumor samples heterozygous or homozygous for the LCS6 G-allele, and 2402 of 2834 (84.8%) blood samples and 834 of 977(85.4%) tumor samples homozygous for the LCS6 T-allele. Genotype results were highly concordant (99.8%) in cases with paired blood and tumor tissue (n=977). G-allele carriers were significantly more frequent in Caucasians vs other races (chi-squared test, P <0.0001). The LCS6 genotype was not associated with KRAS mutation status, clinicopathological features (all P > 0.2) or DFS (log-rank P=0.49; HR 0.929; 95% CI: 0.76–1.14), even after combining LCS6 genotype with KRAS mutation status. Conclusions In the largest association study investigating the LCS6 polymorphism in colon cancers, the germline LCS6 genotype was not associated with KRAS mutation status or with clinical outcome in patients with stage III tumors.