Role of Nucleophosmin 1 immunostain in detecting leukemia cutis of acute myeloid leukemia with NPM1 mutation.

Summary: Background: The role of NPM1 immunostain as a surrogate marker for acute myeloid leukemia (AML) with Nucleophosmin (NPM1) mutation in leukemia cutis has not been investigated. Method: NPM1 immunostain was performed on the leukemia cutis diagnosed in 2017-2024 of patients with and without NPM1 mutation. Targeted NGS assays were performed on the initial bone marrow biopsies. Results: There were 18 skin biopsies from 17 patients (13 males, 4 females, age range of 33-87 years and median 57) with AML-NPM1. Except for one patient, all leukemia cutis presented as multiple lesions, often on the trunk and extremities. The time interval between AML-NPM1 diagnosis and the detection of leukemia cutis was 0.03 to 79.5 months (median 2.3 months). NPM1 immunostain was positive in 15/16 (94%) skin biopsies with leukemic infiltrate. In two skin biopsies showing inflammatory dermatosis, the leukemic infiltrate was not present and NMP1 immunostain was negative. NPM1 immunostain was negative in 13/14 leukemia cutis with other molecular alterations but not NMP1. The sensitivity and specificity of NPM1 immunostain in detecting cutaneous leukemic infiltrate are 94% and 94%, respectively. Conclusion: Though small in number, our study shows that NPM1 immunostain is sensitive and specific in detecting AML-NMP1 mutated cells in skin.