Hotspot <i>KRAS</i> mutations in brain metastases at the first metastatic recurrence of cutaneous melanoma

ABSTRACT IMPORTANCE Brain metastases occur in 60% of patients with advanced melanoma and are a major cause of melanoma-related mortality and morbidity. Although our understanding of the molecular alterations associated with melanoma progression is improving, there are currently no validated biomarkers which might help identify those patients at highest risk of developing brain metastases. OBJECTIVE To examine the somatic mutational and copy-number landscape of brain metastases that develop as the isolated first visceral site of recurrence – “early brain-metastasis” compared to extracranial melanoma metastases. DESIGN, SETTING AND PARTICIPANTS Whole-exome sequencing of 50 tumors from patients undergoing surgical resection of one or more brain metastasis occurring as the first site of visceral relapse were identified from prospectively maintained databases in Sydney, Wellington, New York and Cambridge. Whole exome sequencing analyses allowed mutational profiles to be compared to cutaneous melanomas in The Cancer Genome Atlas (SKCM-TCGA; n=358) and the Memorial Sloan Kettering (SKCM-MSK-IMPACT; n=186) datasets. An external dataset comprising a further 18 patients with surgically resected early brain metastasis from two additional academic centers served as an independent validation cohort. MAIN OUTCOMES AND MEASURES To assess the frequency of driver mutations in early brain metastasis and their influence on survival. RESULTS In concordance with the landmark melanoma sequencing studies, we identified mutations in BRAF (21/50, 42%), NRAS (14/50, 28%) and NF1 (11/50, 22%) as the most frequently mutated melanoma driver genes. When compared to the mutational landscape of cutaneous melanomas in TCGA (SKCM-TCGA), KRAS was the most significantly enriched driver gene, with 5/50 (10%) of brain metastases harboring non-synonymous mutations, of which 4/5 (80%) were in the hotspot positions of codons 12 and 61. This was significantly higher than the corresponding frequency of KRAS -mutations within the entire SKCM-TCGA (2% (7/358), p=0.009, Fisher’s Exact Test) as well as the SKCM-MSK-IMPACT cohort (1.6% (3/186), p=0.016). Variants in KRAS were mutually exclusive from BRAF V600 , NRAS and HRAS mutations and were associated with a significantly reduced overall survival from resection of brain metastasis (relative to KRAS -wild type brain metastases) in multivariate Cox proportional hazard models (HR 1.80, 95% CI 1.46-24.89, p=0.013). Mutations in KRAS were also clonal and concordant with extracranial disease, which suggests these mutations are present within the primary tumor CONCLUSIONS AND RELEVANCE Our analysis, the largest to date, suggests that early metastases to the brain (presenting as the first site of visceral relapse) are characterized by significant enrichment of hotspot KRAS mutations, potentially implicating constitutive RAS-driven cellular programs in neurotropic metastatic behavior in these cases. Based on these data, we suggest that screening for KRAS mutations might help identify those patients with primary melanoma at higher risk of brain metastases or poor survival, and could help inform future surveillance strategies. Key Points Question What is the frequency of driver mutations in early melanoma brain metastases? Findings In this study of 50 patients with melanoma metastasizing first to the brain, KRAS mutations were the most significantly enriched driver gene (n=5, 10% of patients) when compared to landmark cutaneous melanoma studies. The high KRAS mutation frequency was also observed in an external validation cohort of 18 patients with early brain metastases. Mutations in KRAS were mutually exclusive from mutations in the key RAS signaling genes and conferred a worse overall survival from resection of brain metastasis. Meaning Hotspot KRAS mutations could help identify those patients with primary melanoma at higher risk of brain metastases that may benefit from more intensive, protracted surveillance as well as earlier use of adjuvant therapy.