PCM1::JAK2 fusion associates with an atypical form of mycosis fungoides.

Deregulation of JAK-STAT pathway seems to be relevant in mycosis fungoides (MFs). We report the case of a 23-year-old woman diagnosed of atypical MF carrying isolated PCM1::JAK2 fusion and eosinophilia. The disease was refractory to common treatments and progressed increasing the number of large CD30 positive T-cells. After progression, treatment with brentuximab vedotin was decided and decreased the proportion of large cells, but the low-grade component persisted, and the skin lesions worsened. Immunohistochemical expression of p-STAT3 detected in most tumor cells demonstrated the abnormal activation of JAK-STAT pathway. Very few cases of mature T-cell lymphomas carrying PCM1::JAK2 gene fusion have been reported to date, and we review previous cases described with this alteration. Described cases shared similar clinicopathological features and low genetic complexity, and the presence of PCM1::JAK2 fusion associates with a distinctive form of the disease.