Comparison of mutation profiles in primary melanomas and corresponding nodal naevi using next-generation sequencing

Background: Nodal naevi (NN) represent aggregates of melanocytes within peripheral lymph nodes. NN are relatively often found in patients with malignant melanoma (MM), and may mimic metastatic disease.

Aim: To study mutation profiles in MM and NN to find out whether NN descend from a primary MM.

Methods: Next-generation sequencing was performed on formalin-fixed paraffin-embedded tissue of 26 pairs of primary MM and corresponding NN detected by sentinel lymph node biopsy, and 29 MM-characteristic genes were investigated.

Results: In this study, 90% of mutations were detected exclusively in either MM or NN, but not both, in the same patient; the percentage of identical NN and MM mutations in the same individual was only 10%. The most frequently discovered shared mutations were a C>G substitution in the CDKN2A gene and in-frame deletion in ARID1A. Oncogenic driver mutations were frequently observed in MM but only rarely in NN. About three-quarters of mutations in both MM and NN were characterized by C>T or G>A substitutions. The detected rate of ultraviolet (UV)-related C>T base changes was comparably high in both primary MM (35%) and NN (32%).

Conclusions: Based on our data, it seems that NN descend from previously UV-exposed BRAF wildtype cutaneous melanocytes, rather than from primary MM or arrested progenitor cells.