Beyond nest size: the clinicopathological spectrum of large nested melanocytic tumours and the value of comparative genomic hybridisation and messenger RNA expression analysis

Large nested melanomas (LNMs) are a rare subtype of naevoid melanoma consisting of large junctional melanocytic nests that are more common in older individuals and/or associated with sun damage. However, the presence of large melanocytic nests alone does not lead to a diagnosis of malignancy, ​as they can also be found in melanocytic naevi. LNMs are challenging because they lack most classic histological features of malignancy and require thorough clinicopathological evaluation. This ambiguity calls for a critical reassessment of the current diagnostic criteria for the subclassification of benign or malignant within the spectrum of large nested melanocytic tumours (LNMTs). Eighteen LNMTs and six special-site melanocytic naevi (SSMNs) ​were studied using different approaches: clinical features, dermoscopy, histopathology, immunohistochemistry, array comparative genomic hybridisation (aCGH) and messenger RNA (mRNA) sequencing analysis, with the aim of identifying novel and reproducible criteria for the recognition of LNMs.Careful clinicopathological evaluation of the 18 LNMTs led to the diagnosis of seven LNMs and 11 large nested melanocytic naevi (LNMNs). Lentiginous spread and nest bridging were significantly associated with LNMs after Holm-Bonferroni correction. Asymmetry, largest nest size, poor lateral demarcation, the number of colours and dermoscopic structures, and preferentially expressed antigen in melanoma (PRAME) immunostaining were more common in LNMs but did not reach statistical significance. Four of seven LNMs and nine of 11 LNMNs lacked the BRAF V600E mutation. Regarding aCGH, no LNMN or SSMN cases had ≥3 copy number variations (CNVs), in contrast to 50% of LNM cases. Importantly, LNM and LNMN could be distinguished by differential mRNA expression of nine genes. Our study demonstrates that there is a spectrum of LNMTs and that the clinicopathological diagnosis of LNM, for which we support the term 'late-onset nested naevoid melanomas', can be significantly strengthened by the presence of lentiginous pattern, nest bridging, gene CNV and differential mRNA expression.