Head and Neck Mucosal Melanomas are Characterized by Overexpression of the DNA Mutating Enzyme APOBEC3B

Introduction Primary head and neck mucosal melanomas (HNMMs) are rare and exhibit aggressive biologic behavior, dismal prognosis, and elevated mutational loads. The molecular mechanisms responsible for high genomic instability observed in HNMMs remain elusive. The DNA cytosine deaminase APOBEC3B constitutes a major endogenous source of mutation in a plethora of human cancers. APOBEC3B-related mutations are identified through C-to-T/-G base substitutions in 5'-TCA/T trinucleotide motifs. Herein, we provide immunohistochemical and genomic data strongly supportive of the role of APOBEC3B in HNMMs. Materials and Methods APOBEC3B protein levels were assessed in oral (N=16) and sinonasal (N=15) melanomas by immunohistochemistry using a custom rabbit monoclonal antibody (5210-87-13). Benign oral melanocytic nevi (N=13) served as a control group. Nuclear APOBEC3B immunoreactivity was visualized and quantified with the Aperio ScanScope XT platform. Statistical differences between groups were calculated using Kruskal-Wallis oneway non-parametric tests. Publicly available genomic data were further analyzed to assess the impact of APOBEC3B expression in mucosal melanomas. Results Focal to diffuse, nuclear APOBEC3B immunopositivity was observed in all oral melanomas (H-score range=9-72, median=39) and 12 of 15 sinonasal melanomas (H-score range=1-110, median=24). The intensity of immunohistochemical positivity demonstrated a wide range of inter- and intra-tumor heterogeneity. APOBEC3B protein levels were significantly higher in oral and sinonasal melanomas than oral nevi (p<0.0001 and p=0.0032, respectively), which were overall negative for APOBEC3B (H-score range=1-12, median=6). APOBEC3B levels, however, did not differ significantly between oral and sinonasal tumors (p>0.99). Genomic analysis showed that overall mutation load, number of C-to-T transitions, and proportion of APOBEC3B-related mutation signatures 2 and 13 were higher in HNMMs than melanomas developing in other mucosal sites, and cutaneous melanomas. Conclusions The mutagenic enzyme APOBEC3B is overexpressed in HNMMs but not benign mucosal melanotic neoplasms. APOBEC3B may be a novel biomarker and therapeutic target for this clinically aggressive type of human malignancy.