Acute mast cell leukemia associated with t(4;5)(q21;q33).

To the best of our knowledge, this manuscript describes clinical and pathologic findings of the first case of acute mast cell leukemia harboring t(4;5)(q21;q33), compatible with fusion of the PDGFRB gene to a rare partner, PRKG2. Translocation involving the PDGFRB gene is confirmed by fluorescence in situ hybridization study. This case presented a relatively fulminant clinical course with acute mast cell leukemia and "C" findings (cytopenia, hepatosplenomegaly, and weight loss), mast cell sarcoma, and severe basophilia. Despite aggressive presentation initially, the patient responded well to tyrosine kinase inhibitor treatment and is currently in complete remission 33 months after diagnosis. This case significantly extends the disease spectrum associated with PRKG2/PDGFRB fusion gene. Recognizing the whole spectrum of diseases associated with this fusion is critical because tyrosine kinase inhibitor treatment has been exceedingly effective in these patients.