Neurofibromin protein loss in desmoplastic melanoma subtypes: implicating NF1 allelic loss as a distinct genetic driver?

Alexander Kadokura, Noah Frydenlund, Dominick A. Leone, Shi Yang, Mai P. Hoang, April Deng, Marier Hernandez-Perez, Asok Biswas, Rajendra Singh, Ron Yaar, Meera Mahalingam

Loss of the NF1 allele, coding for the protein neurofibromin, and polymorphism in the proto-oncogene RET (RETp) are purportedly common in desmoplastic melanoma (DM). DM is categorized into pure (PDM) and mixed (MDM) subtypes which differ in prognosis. Most NF1 mutations result in a truncated/absent protein, making immunohistochemical screening for neurofibromin an ideal surrogate for NF1 allelic loss. Using anti-neurofibromin, our aims were to ascertain the incidence of neurofibromin loss in DM subtypes and to evaluate the relationship with RET, perineural invasion (PNI), and established histopathologic prognosticators. A total of 78 archival samples of DM met criteria for inclusion (54 cases of non-DM serving as controls). Immunohistochemistry was performed for neurofibromin, while direct DNA sequencing was used for RETp and BRAF mutation status. Statistical analyses included χ(2)-test as well as Fischer's exact test. Neurofibromin loss was more common in DM than non-DM (69% vs. 54%, P = 0.02). In DM, significant differences in neurofibromin loss were noted in the following: non- head and neck (H&N) vs. H&N biopsy site (88% vs. 55%) and PDM vs. MDM variants (80% vs. 56%). No significant associations were noted with gender, presence of a junctional component, Breslow depth, ulceration, mitoses, host response, RETp, BRAF status, or PNI. RETp was marginally associated with PNI-positive DM versus PNI-negative DM (36 vs. 18%, P = 0.08). Our findings, the largest to date investigating neurofibromin in DM, validate the incidence of NF1 mutations/allelic loss in DM, and suggest that the DM subtypes have distinct genetic drivers.