Activating BRAF mutations in eruptive melanocytic naevi

Summary Background Eruptive melanocytic naevi (EMN) are melanocytic proliferations developing rapidly on previously unaffected skin in association with various clinical scenarios, most commonly systemic immunosuppression. However, the exact mechanism leading to development of EMN is not understood. In particular, it is not known whether EMN harbour the BRAF mutations which occur frequently in melanoma and most common naevi. Objectives To evaluate whether activating BRAF mutations may play a role in genesis of EMN. Methods Genomic DNA was isolated from 20 EMN from a patient treated with 6‐mercaptopurine (6‐MP). Primary BRAF genotyping was performed by allele‐specific polymerase chain reaction, followed by validation using direct sequencing. Results The BRAF V600E mutation was identified in 85% of EMN examined. Conclusions Our results implicate mutational activation of the BRAF–MAPK pathway as a factor in development of EMN in the setting of 6‐MP treatment. The mechanism leading to development of EMN in this, and potentially other patients, may relate to synergistic mutagenic effects of thioguanines and ultraviolet (UV) A. Together with the documented importance of BRAF mutations in melanoma development and maintenance, these findings highlight the importance of UVA protection, especially in patients treated with thiopurines such as 6‐MP.