Genomic and transcriptomic analysis of Japanese melanoma reveals candidate biomarkers for immune checkpoint inhibitor responders.

Background Immune checkpoint inhibitor (ICI) has greatly improved the prognosis of advanced melanoma. Whereas the efficacy in Japanese patients has been found to be lower than Caucasian, the genomic and transcriptomic features associated with response to ICI of Japanese melanomas remain to be elucidated. Patients and methods Total 129 tumour samples from 78 melanoma patients, who received therapeutic regimens with or without ICI treatment, were collected at 13 institutions in Japan. We performed exome and RNA sequencing and investigated the association of genomic and transcriptomic factors with clinical efficacy of ICI therapy. Time-course data were also analysed. This is the first and largest genomic cohort study for Japanese melanoma, in which tumour samples were prospectively analysed. Results Number of somatic SNVs of Japanese melanomas is lower than that of TCGA Caucasian data due to the biased distribution of WHO subtypes. Driver subtypes of BRAF, NRAS and NF1 was less prevalent but Triple Wildtype predominantly existed in the Japanese cohort. Whereas exome-wide survey revealed no significant association of mutated genes with ICI response, by transcriptomic analysis we identified inflammation-associated genes including several chemokines and cytokines were highly expressed in responders. Follicular helper T cells estimated by immune-cell composition analysis were found significantly enriched in responders (p = 0.0422). Through time-course transcriptome analysis, in addition to several cytotoxic T-cell genes as previously reported, MARCO on tumour-associated macrophages was found induced by ICI treatment in responders (p = 0.0040). The protein expression of these genes was confirmed by immunohistochemical and multiplex immunofluorescent analyses. Some of these genes can be useful biomarkers for prediction of ICI response in the treatment of Japanese melanoma. Conclusions Through prospective genomic and transcriptomic analyses of Japanese melanoma samples, candidate biomarkers for ICI treatment were identified.