A let-7 microRNA-binding site polymorphism in KRAS predicts improved outcome in patients with metastatic colorectal cancer treated with salvage cetuximab/panitumumab monotherapy

Abstract

Purpose: An inherited mutation in KRAS (LCS6-variant or rs61764370) results in altered control of the KRAS oncogene. We studied this biomarker's correlation to anti-EGFR monoclonal antibody (mAb) therapy response in patients with metastatic colorectal cancer.

Experimental design: LCS6-variant and KRAS/BRAF mutational status was determined in 512 patients with metastatic colorectal cancer treated with salvage anti-EGFR mAb therapy, and findings correlated with outcome. Reporters were tested in colon cancer cell lines to evaluate the differential response of the LCS6-variant allele to therapy exposure.

Results: In this study, 21.2% (109 of 512) of patients with metastatic colorectal cancer had the LCS6-variant (TG/GG), which was found twice as frequently in the BRAF-mutated versus the wild-type (WT) group (P=0.03). LCS6-variant patients had significantly longer progression-free survival (PFS) with anti-EGFR mAb monotherapy treatment in the whole cohort (16.85 vs. 7.85 weeks; P=0.019) and in the double WT (KRAS and BRAF) patient population (18 vs. 10.4 weeks; P=0.039). Combination therapy (mAbs plus chemotherapy) led to improved PFS and overall survival (OS) for nonvariant patients, and brought their outcome to levels comparable with LCS6-variant patients receiving anti-EGFR mAb monotherapy. Combination therapy did not lead to improved PFS or OS for LCS6-variant patients. Cell line studies confirmed a unique response of the LCS6-variant allele to both anti-EGFR mAb monotherapy and chemotherapy.

Conclusions: LCS6-variant patients with metastatic colorectal cancer have an excellent response to anti-EGFR mAb monotherapy, without any benefit from the addition of chemotherapy. These findings further confirm the importance of this mutation as a biomarker of anti-EGFR mAb response in patients with metastatic colorectal cancer, and warrant further prospective confirmation.

Figure 1. LCS6-variant patients have improved PFS vs non-variant patients in response to EGFR moAbs monotherapy for all patients, with no benefit of additional chemotherapy

(A) Median progression-free survival (PFS) by KRAS LCS6 genotype in all patients treated with anti-EGFR moAbs monotherapy as salvage treatment. (B) Median PFS according to the KRAS 3′-UTR LCS6 SNP genotype status in all patients treated with anti-EGFR moAbs based combination chemotherapy as salvage treatment. (C) Median PFS according to type of therapy in all KRAS 3′-UTR LCS6 SNP carriers. (D) Median PFS according to type of therapy in all non-KRAS 3′-UTR LCS6 SNP carriers.

Figure 2. LCS6-variant patients have improved PFS vs non-variant patients in response to EGFR moAbs monotherapy for double wt patients, with no benefit of additional chemotherapy

(A) Median PFS according to the KRAS 3′-UTR LCS6 SNP genotype status in the double (KRAS and BRAF) wt patients’ population treated with anti-EGFR moAbs monotherapy as salvage treatment. (B) Median PFS according to the KRAS 3′-UTR LCS6 SNP genotype status in the double (KRAS and BRAF) wt patients’ population treated with anti-EGFR moAbs based combination chemotherapy as salvage treatment. (C) Median PFS according to type of therapy in the double (KRAS and BRAF) wt KRAS 3′-UTR LCS6 SNP carriers. (D) Median PFS according to type of therapy in the double (KRAS and BRAF) wt non-KRAS 3′-UTR LCS6 SNP carriers.

Figure 3. LCS6-variant patients do not have improved OS with the addition of chemotherapy for all patients

(A) Median overall survival (OS) according to the KRAS 3′-UTR LCS6 SNP genotype status in all patients treated with anti-EGFR moAbs monotherapy as salvage treatment. (B) Median OS according to the KRAS 3′-UTR LCS6 SNP genotype status in all patients treated with anti-EGFR moAbs based combination chemotherapy as salvage treatment. (C) Median OS according to type of therapy in all KRAS 3′-UTR LCS6 SNP carriers. (D) Median OS according to type of therapy in all non-KRAS 3′-UTR LCS6 SNP carriers.

Figure 4. LCS6-variant patients do not have improved OS with the addition of chemotherapy for double wt patients

(A) Median OS according to the KRAS 3′-UTR LCS6 SNP genotype status in the double (KRAS and BRAF) wt patients’ population treated with anti-EGFR moAbs monotherapy as salvage treatment. (B) Median OS according to the KRAS 3′-UTR LCS6 SNP genotype status in the double (KRAS and BRAF) wt patients’ population treated with anti-EGFR moAbs based combination chemotherapy as salvage treatment. (C) Median OS according to type of therapy in the double (KRAS and BRAF) wt KRAS 3′-UTR LCS6 SNP carriers. (D) Median OS according to type of therapy in the double (KRAS and BRAF) wt non-KRAS 3′-UTR LCS6 SNP carriers.

Figure 5. The KRAS LCS6-variant causes over-expression of a KRAS reporter

(A) HCT-116 colon cancer cells were transfected with dual-luciferase reporters harboring either the full-length KRAS 3′UTR T-allele or G-allele (LCS6-variant), as indicated. Dual-luciferase activities were measured and renilla was normalized to firefly. Results are graphed as the mean and standard deviation of the mean of four independent experiments preformed in duplicate. The p-value was calculated using the students t-test. (B) The KRAS LCS6-variant exhibits altered gene expression in response to anti-cancer agents. HCT-116 colon cancer cells transfected with either the KRAS LCS6-variant or non-variant 3′UTR reporters were exposed to various concentrations of anti-cancer compounds (as indicated). The results are expressed as the relative expression of the LCS-variant versus the non-variant allele. Graphed is the mean and standard deviation of the mean for two experiments performed in duplicate.