Reduced H3K27me3 Expression Is Common in Nodular Melanomas of Childhood Associated With Congenital Melanocytic Nevi But Not in Proliferative Nodules

The formation of a nodule within a congenital melanocytic nevus (CMN) raises concerns about possible melanoma. Most new nodular growths that develop during childhood, however, are benign proliferative nodules (PN); melanoma is very rare. The distinction of melanoma from PN can at times be difficult clinically and histopathologically, requiring ancillary molecular tests for diagnosis. Although the application of molecular methods has revealed new insights into the mutational and genomic landscape of childhood melanomas, little is known about epigenetic events that may drive the growth of a melanoma or PN in a CMN. In this study we compared the expression of H3K27me3, a key regulator in chromatin remodelling-controlled transcription, in PNs and pediatric nodular melanomas arising within medium-sized to large CMN by immunohistochemistry. Significant loss of H3K27me3 expression was seen in 4 of 5 melanomas, but not in any of the 20 PNs. This observation suggests that epigenetic events likely play a role in the pathogenesis of melanoma developing in the dermis or subcutis of CMN. Furthermore, assessing for H3K27me3 expression by immunohistochemistry may be diagnostically useful for problematic cases.