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.

Automated summary

Deregulation of JAK-STAT pathway is relevant in mycosis fungoides (MFs). We present a case of a 23-year-old woman with atypical MF carrying isolated PCM1::JAK2 fusion and eosinophilia, which was refractory to common treatments. Treatment with brentuximab vedotin decreased the proportion of large cells but did not improve the low-grade component and skin lesions. Abnormal activation of JAK-STAT pathway was demonstrated by immunohistochemical expression of p-STAT3 in most tumor cells. Previous cases of mature T-cell lymphomas with PCM1::JAK2 fusion shared similar clinicopathological features and low genetic complexity.