38. Intralesional molecular heterogeneity in a BRAF-mutant BRAF inhibitor resistant melanoma: a case illustrating the challenges for personalised medicine

Abstract

Background Mutations of the serine/threonine-protein kinase (BRAF) gene occur in approximately half of all patients with met-astatic melanoma. The BRAF inhibitors have proven highly active in treating BRAF mutant metastatic melanoma patients. Eventually, almost all patients develop resistance to BRAF inhibition and relapse. Aims We sought to determine whether the patient’s initial vemurafe nib resistance developed through the reactivation of the MAPK pathway due to additional mutations to the NRAS or BRAF genes. Methods Sanger sequencing was performed on the patient’s ve-murafenib progressing metastasis to detect somatic mutations in exon 1 and exon 2 of the NRAS gene and in exon 11 and exon 15 of the BRAF gene. Immunohistochemical staining, macrodissection and RT-PCR was performed to identify any mutational heterogeneity within the tumour. Results Sequencing revealed a G13R NRAS mutation in addition to the original BRAF V600E mutation within the same vemu-rafenib progressing metastasis. Immunohistochemical staining for p-ERK1/2 expression clearly identified two areas within the tumour with differential staining. Macrodissection of these areas revealed a NRAS wt and BRAF V600E mutant subpopulation, while the other subclone was NRAS G13R mutant and BRAF V600E mutant. Conclusion This study demonstrates the heterogeneous nature of melanoma, even at the intra-lesional level, highlighting that personalising or adjusting therapies based on genotyping of a portion of a single metastasis, might not accurately depict the drivers of mutagenesis across the entire patient’s metastatic melanoma tumour cell population.