A case report of atypical Spitz tumor harboring a novel MLPH-ALK gene fusion with discordant ALK immunohistochemistry results

Frequent kinase fusions have been reported in spitzoid neoplasms, approximately 10% of which involve ALK rearrangements. Herein, we report a case of atypical Spitz tumor with a novel MLPH-ALK fusion, which has not been previously reported to contribute to cancer development. The tumor was detected in the right arm of a 40-year-old woman. The novel ALK fusion was identified by a 5′-rapid amplification of cDNA ends-based system optimized for formalin-fixed, paraffin-embedded tissue. Initially, ALK expression was detected by immunohistochemistry using 5A4 antibodies for both sensitive and conventional polymer detection methods. However, the anti-ALK1 antibody, which is commonly used for the diagnosis of ALK-positive anaplastic large cell lymphoma, failed to confirm ALK expression. These results indicated that ALK immunohistochemistry results in ALK-rearranged atypical Spitz tumor may differ based on the type of primary antibody clone, which can be a potential diagnostic pitfall.