The Amount of Melanin Influences p16 Loss in Spitzoid Melanocytic Lesions: Correlation With CDKN2A Status by FISH and MLPA.

The risk assessment of spitzoid lesions is one of the most difficult challenges in dermatopathology practice. In this regard, the loss of p16 expression and the homozygous deletion of CDKN2A, have been pointed in the literature as reliable indicators of high risk. However, these findings are poorly reproducible, and the molecular bases underlying the loss of p16 expression remain unclear. We aimed to identify the underlying events causing loss of CDKN2A/p16 in spitzoid tumors. We evaluated the immunohistochemical expression of p16, and the presence of CDKN2A genetic alterations detected through fluorescence in situ hybridization (FISH) and multiplex ligation-dependent probe amplification (MLPA), in a series of 130 Spitz nevi, 20 atypical spitzoid tumors, and 11 spitzoid melanoma. We found a significant loss of p16 expression in cases with high amount of melanin content in the 3 groups (P<0.000001) and a similar proportion of p16-negative cases in the group of Spitz nevi and atypical spitzoid tumors. MLPA allowed the recognition of CDKN2A microdeletions, which correlated with p16 loss (P=0.01). MLPA and FISH were more accurate than immunohistochemistry to detect CDKN2A alterations; although contrary to MLPA, FISH fails to recognize CDKN2A microdeletions. According to our results, p16 expression may be useful in the study of cases with atypical features and low melanin content, but it has no value in highly pigmented spitzoid lesions.