Cross-reactivity of NRASQ61R antibody in a subset of Spitz nevi with 11p gain: a potential confounding factor in the era of pathway-based diagnostic approach.

The most recent World Health Organization classification for skin tumors (2018) categorizes melanomas and their precursor lesions, benign or intermediate, into nine pathways based not only on their clinical and histomorphologic characteristics but also on their molecular profile and genetic fingerprint. In an index case of a partially sampled atypical spitzoid lesion, which proved to be an 11p-amplified Spitz nevus with HRASQ61R mutation, we observed cross-reactivity with the NRASQ61R antibody (clone SP174). Overall, we assessed the status of HRAS and NRAS genes and their immunoreaction to NRASQ61R antibody in 16 cases of 11p-amplified Spitz nevi/atypical Spitz tumors. We also assessed the immunoexpression of NRASQ61R antibody in various malignancies with proven BRAFV600E, NRASQ61R, L or K, KRASQ61R and HRASQ61R, and HRASQ61R mutations and ALK+ Spitz lesions. Finally, we assessed the expression of PReferentially expressed Antigen in MElanoma (PRAME) immunohistochemistry in our 11p Spitz cohort. Three of 16 cases (3/16) harbored the HRASQ61R mutation and exhibited diffuse immunoreaction with the NRASQ61R antibody. All the cases in our cohort were negative for the NRASQ61R mutation. All NRASQ61R-, KRASQ61R-, and HRASQ61R-mutated neoplasms were positive for the antibody, further supporting the cross-reactivity between the RAS proteins. All the cases of our cohort were essentially negative for PRAME immunohistochemistry. In the era of pathway-based approach in the diagnosis of melanocytic neoplasms, the cross-reactivity between the NRASQ61R- and HRASQ61R-mutated proteins can lead to a diagnostic pitfall in the assessment of lesions with spitzoid characteristics.