Distinct genetic and phenotypic characteristics of not otherwise specified (NOS) melanoma

e21571 Background: We previously reported on the biological relevance of melanoma histologic subtypes (Lattanzi et al, JNCI 2019). However, a substantial percentage of primary melanoma is histologically subtyped as NOS. The impact of this designation on clinical outcome is untested but casually believed not to have an impact considering the increasing reliance of molecular subclassification of melanoma tumor that might eliminate the need for histologic characterization. We also reported our data that supported a role for melanoma germline prognostic factors independent of known prognostic criteria (Chat et al, Front Oncol 2023). In this study, we sought to determine if NOS represents phenotypically and genetically distinct subtype or its clinical behavior is determined by AJCC criteria. Methods: We studied primary melanoma patients prospectively accrued and followed up using an IRB approved protocol at NYULH. We tested the association between NOS and baseline characteristics (e.g. thickness, ulceration, stage, Mitotic Index, TIL) and its association with recurrence free survival (RFS), and overall survival (OS) using Kaplan-Meier and multivariable Cox regression analyses. We also performed Principal Component Analysis (PCA) to test the clustering of NOS compared to other subtypes. We then examined the association between germline genetic variants and the development of NOS melanoma. Analyzing imputed SNP data generated using global screening array (GSA) from blood DNA we compared NOS (cases) with other histologic melanomas (controls) controlling for clinical and demographic variables. Results: NOS were identified in 550/2,891 (19%) of patients (median follow up of 4.44 years). PCA revealed clustering of NOS melanoma and superficial spreading melanoma (SSM) with similar favorable prognostic criteria (thinner, less ulcerated, earlier stage) compared to other subtypes (P < 0.001) However, NOS patients had a significantly shorter RFS compared to SSM (median RFS 8.83 years compared to 17.5 years, respectively; HR = 1.79 (95% CI: 1.37-2.33), P < 0.001 in multivariate analysis Genetic analysis of 919 cases (NOS = 127 versus all others n = 792) revealed a cluster of four genetic variants defining a linkage disequilibrium (LD) block mapping in the region of annotated microRNA 924 (LD-MIR924; OR = 1.844, p = 2.72E-06, 95% CI = 1.40-2.43), associating with ~1.8-fold increased risk of developing NOS compared to other melanoma subtypes. Conclusions: Our data suggest NOS melanoma might be a distinct genetic subtype that require further analysis considering the significant association with worse clinical outcome. The genetic variants associated with the risk of developing NOS melanoma point to MIR924, a microRNA previously linked to cancer progression. Our findings support the integration of studying patients’ genetics in addition to tumor somatic mutations to better understand the heterogeneous behavior of melanoma.