Management of paroxysmal nocturnal hemoglobinuria in CALR mutated post‐essential thrombocythemia myelofibrosis: A case report

Abstract Paroxysmal nocturnal hemoglobinuria (PNH) results from the loss of erythrocyte surface proteins, leading to complement activation and its spectrum of effects. We explore this case of a 57‐year‐old man with post‐essential thrombocythemia (ET) myelofibrosis (MF) who developed symptomatic anemia with evidence of hemolysis on lab work. While hemolysis was localized to be intramedullary based on workup, the exact diagnosis was undetermined, leading to a prolonged course of steroid therapy to control anemia. The hemolysis was eventually attributed to PNH diagnosed on flow cytometry and the patient was treated with complement inhibitors with eventual failure of therapy. He ultimately underwent a successful hematopoietic cell transplant (HCT) with post‐transplantation flow cytometry showing complete resolution of PNH. While PNH has been identified as a progression of myelodysplastic syndromes, the mechanisms of its rare development in myeloproliferative neoplasms are poorly elucidated. Furthermore, its rarity and often vague symptoms make diagnosis and treatment a challenge. This is the second reported case of a JAK2‐negative, CALR‐positive post‐ET MF and the first reported case to be treated with HCT. This case probes for further insight into the clinical significance between MF and PNH, its impact on management, and further consideration for HCT as curative therapy in such patients who fail complement inhibitor therapy.