The use of selective BRAF inhibitors (BRAFi) has produced remarkable outcomes for patients with advanced cutaneous melanoma harboring a BRAF V600E mutation. Unfortunately, the majority of patients eventually develop drug-resistant disease. We employed a genetic screening approach to identify gain-of-function mechanisms of BRAFi resistance in two independent melanoma cell lines. Our screens identified both known and unappreciated drivers of BRAFi resistance, including multiple members of the DBL family. Mechanistic studies identified a DBL/RAC1/PAK signaling axis capable of driving resistance to both current and next-generation BRAFis. However, we show that the SRC inhibitor, saracatinib, can block the DBL-driven resistance. Our work highlights the utility of our straightforward genetic screening method in identifying new drug combinations to combat acquired BRAFi resistance.Significance: A simple, rapid, and flexible genetic screening approach identifies genes that drive resistance to MAPK inhibitors when overexpressed in human melanoma cells.

The use of selective BRAF inhibitors (BRAFi) has produced remarkable outcomes for patients with advanced cutaneous melanoma harboring a BRAF V600E mutation. Unfortunately, the majority of patients eventually develop drug-resistant disease. We employed a genetic screening approach to identify gain-of-function mechanisms of BRAFi resistance in two independent melanoma cell lines. Our screens identified both known and unappreciated drivers of BRAFi resistance, including multiple members of the DBL family. Mechanistic studies identified a DBL/Rac1/Pak signaling axis capable of driving resistance to both current and next-generation BRAF inhibitors. However, we show that the Src inhibitor, saracatinib, can block the DBLdriven resistance. Our work highlights the utility of our straightforward genetic screening method in identifying new drug combinations to combat acquired BRAFi resistance. KEYWORDSBRAF inhibitor resistance, cutaneous melanoma, VAV1, MCF2, RAC1, vemurafenib, saracatinib Melanoma is the deadliest form of skin cancer, with around 90,000 diagnoses of invasive disease and ~10,000 deaths per year(1). Patients had few treatment options until the development of vemurafenib, a highly selective kinase inhibitor that specifically targets the BRAF V600E mutant protein present in ~50% of all melanoma cases(2). Initially, vemurafenib provided complete or partial response in over 50% of patients and increased progression-free survival (3). Unfortunately, most patients relapse once tumors acquire resistance to vemurafenib.Genetic analysis of progression samples has identified resistance mechanisms, including amplification of BRAF V600E , expression of truncated BRAF V600E , and RAS mutation (4-6). However, these mechanisms explain only ~60% of cases of BRAF inhibitor (BRAFi) resistance (5,7,8). Drug resistance can be delayed by combining vemurafenib with cobimetinib, a MEK inhibitor (MEKi), but most patients eventually develop progressive disease via resistance mechanisms that have not been well characterized (7). Thus, unexplained cases of resistance to MAPK inhibition (MAPKi) in human melanoma represent an important unmet clinical need.Mechanisms of vemurafenib resistance have been studied in BRAF V600E mutant human melanoma cell lines using genome-wide shRNA and CRISPR loss-of-function screens (9-11). Overall, these studies showed little overlap in candidate mechanisms. Two screens have been reported that attempted to identify drivers of vemurafenib resistance by high throughput overexpression of genes via lentiviral libraries (12,13). Importantly, these screens failed to identify known mechanism of vemurafenib resistance (e.g. BRAF V600E amplification or N-terminal truncation)(4, 6). These observations led us to develop a simple insertional mutagenesis screening approach using the Sleeping Beauty (SB) transposon system to identify novel drivers of vemurafenib resistance in an unbiased forward genetic screen.The SB system is a well-established tool for developing mouse models of spontaneous cancer in which transposo...

Brain metastases commonly develop in melanoma and are associated with poor overall survival of about five to nine months. Fortunately, new therapies, including immune checkpoint inhibitors and BRAF/MEK inhibitors, have been developed. The aim of this study was to identify outcomes of different treatment strategies in patients with melanoma brain metastases in the era of checkpoint inhibitors. Patients with brain metastases secondary to melanoma were identified at a single institution. Univariate and multivariable analyses were performed to identify baseline and treatment factors, which correlated with progression-free and overall survival. A total of 209 patients with melanoma brain metastases were identified. The median overall survival of the cohort was 5.3 months. On multivariable analysis, the presence of non-cranial metastatic disease, poor performance status (ECOG 2–4), whole-brain radiation therapy, and older age at diagnosis of brain metastasis were associated with poorer overall survival. Craniotomy (HR 0.66, 95% CI 0.45–0.97) and treatment with a CTLA-4 checkpoint inhibitor (HR 0.55, 95% CI 0.32–0.94) were the only interventions associated with improved overall survival. Further studies with novel agents are needed to extend lifespan in patients with brain metastases in melanoma.

A 55-year-old woman presented with a 3-month history of right groin swelling, discomfort and impaired mobility. On examination, a palpable mass was noted both to the right of midline in the lower abdomen and in the right groin. MRI of the pelvis showed two masses involving the anterior abdominal wall and right groin, as well as lymph node involvement. CT imaging revealed multiple bilateral pulmonary metastases. Pathology demonstrated a myxohayline stroma morphology. Tumour was also notable for NR4A3 gene region rearrangement and mutation in KIT exon 11 at position c.1669 T>G. Based on these findings, she was diagnosed with extraskeletal myxoid chondrosarcoma (EMC). The patient has been on imatinib, a tyrosine kinase inhibitor with activity against KIT, for 3 years with stable disease. Metastatic EMC is generally treated with surgical resection and perioperative radiation therapy with adjuvant chemotherapy and is associated with poor prognosis.