Pigmented (melanotic) neurofibroma. Report of an unusual case with immunohistochemical, ultrastructural and cytogenetic analyses.

In the spectrum of neurofibromas, pigmented tumors are rare variants usually showing only faint, macroscopically obvious pigmentation. We report a case of a huge pigmented neurofibroma with extended, macroscopically striking pigmentation in a patient with stigmata of neurofibromatosis. The immunohistochemical and ultrastructural findings support a melanotic line of differentiation besides schwann cell differentiation and indicate a phenotypic neoplastic spectrum between tumorous schwann cells and melanocytes. Using comparative genomic hybridization, striking chromosomal aberrations were not detected. High level amplifications of the known chromosomal regions, including genes of major enzymes responsible for melanin synthesis, appear to be unlikely. However, smaller chromosomal defects might have been overlooked by the limited resolution of this screening method. Therefore, other mechanisms up-regulating melanogenesis, such as mutations in regulatory genes, have to be considered.