Monitoring BRAF and NRAS mutations with cell-free circulating tumor DNA from metastatic melanoma patients

Abstract

The mutation status of the BRAF and NRAS genes in tumor tissue is used to select patients with metastatic melanoma for targeted therapy. Cell-free circulating DNA (cfDNA) represents an accessible, non-invasive surrogate sample that could provide a snapshot of the BRAF and NRAS genotype in these patients. We investigated the feasibility of the Idylla™ assay for detection of BRAF and NRAS mutations in cfDNA of 19 patients with metastatic melanoma at baseline and during the course of treatment. The cfDNA genotype obtained with Idylla was compared to the results obtained with matched-tumor tissue and to clinical outcome. At baseline, 47% of patients harbored a BRAFV600 mutation in their cfDNA. Two months after targeted treatment the BRAFV600 mutant cfDNA was undetectable in all patients and 3 were disease-free. Moreover, 15% of patients harbored a NRAS mutation that was detected with plasma before treatment. The sensitivity and specificity were 80% and 89% for the BRAF status, and 79% and 100% for the NRAS status in pretreatment cfDNA compared to results obtained with a tissue test. Due to the small size of the population, no significant correlation was observed between the presence of BRAF or NRAS mutations in cfDNA and the metastatic tumor load or overall survival. In conclusion, this study demonstrated that evaluation with the Idylla system of the BRAF and NRAS mutation status in cfDNA may be a surrogate for determination of the BRAF and NRAS status in tumor tissue.

Keywords: BRAF; IDYLLA™; NRAS; cfDNA; metastatic melanoma.

Figure 1. Overview of the results and clinical data

Figure 2. Boxplot of cfDNA concentration (measured with Qubit, ng/μl) and the presence of plasmatic mutations WT versus mutated

Horizontal line indicates median = 4 ng/μL (range: 1–6) for WT cfDNA [range 0.6 to 390.0 ng/μL] and median = 9 ng/μL (range: 3–14) for mutated cfDNA. Squares indicate the value of cfDNA. P-value of the Student T-test indicates a significant difference of cfDNA mean concentration between WT and mutated.

Figure 3. Boxplot of cfDNA concentration (measured with Qubit, ng/μl) and the number of metastatic sites

Horizontal lines indicate median for one, two, three or four metastatic sites. Squares indicate value of cfDNA. Spearman rho test indicates correlation between cfDNA and number of metastatic sites. The regression line show a non-significant positive correlation (p = 0.12) between the number of metastatic sites and cfDNA concentration.