O17 Endogenous double-stranded RNA (dsRNA) in melanoma: an attack from within

Abstract Recent studies have linked endogenous double-stranded RNA (dsRNA) to the development of inflammatory and autoimmune diseases and cancer. However, the scale of dsRNA formation and its biological consequences is still unclear. Malignant melanoma is the most lethal skin cancer, with an increasing prevalence worldwide. Despite the development of immunotherapy and targeted therapies for melanoma, treatment options are still limited for most patients. dsRNA derived from repetitive DNA elements has the potential to stimulate pattern recognition receptors (PRRs). These PRRs can trigger antiviral signalling cascades and induce an interferon response that renders resistant melanoma sensitive to immunotherapy. Consequently, dsRNA can add a new approach to treating melanoma. Tissue culture and molecular biology techniques were used to induce dsRNA formation in melanoma cell lines (with different mitogen-activated protein kinas-activating mutations) using PRRs agonist poly (I:C), azacitidine (Aza) and hydrogen peroxide (H2O2) as stressors. The impact was measured by quantifying cytosolic dsRNA sensors, also known as PRRs (pPKR, MDA5, RIG1 and ADAR1), using reverse transcriptase quantitative polymerase chain reaction and Western blotting. Immunofluorescence and high-resolution microscopy were then applied to investigate dsRNA and its sensors in skin biopsy samples from patients with melanoma. Moreover, the nature of dsRNA in melanoma was studied by isolating native dsRNA from multiple melanoma cell lines by immune enrichment followed by RNA sequencing. Analysis revealed that inducing endogenous dsRNA significantly activates PRR transcripts and protein levels. However, this activation showed different trends. A375 (BRAFV600E) showed robust MDA5 and RIG1 stimulation to poly (I:C) and a mild ADAR1 and PKR response to Aza and H2O2. Meanwhile, C8161 [BRAF wild type (WT)] was resistant to poly (I:C) with a favourable ADAR1 and RIG1 response to Aza. Immunofluorescence analysis of early stages I and II melanoma skin biopsies showed a significant reduction in ADAR1, which melts dsRNA, with increased dsRNA within BRAF-mutant biopsies. RNAseq analysis showed that A375 (BRAFV600E) produced significantly more dsRNA than C8161 (BRAF WT) and primary dermal fibroblasts. The increase was predominantly due to the enhanced production of dsRNA from nuclear DNA. However, about 11% of the reads mapped to the mitochondrial genome vs. 14% and 19% in C8161 and fibroblasts, respectively. In addition, compared to primary dermal fibroblasts, the dsRNA transcriptome of melanoma cell lines shows unique characteristics and altered dsRNA signalling pathways. To summarize, our data support the hypothesis that endogenous dsRNA activates dsRNA sensors and triggers innate immune signalling. Hence, dsRNA signalling may be explored as a potential therapeutic strategy to treat resistant melanoma with an attack from within.