Somatic mutation of PIGA in hematopoietic stem cells causes deficiency of glycosyl phosphatidylinositol-anchored proteins in paroxysmal nocturnal hemoglobinuria (PNH) that underlies the intravascular hemolysis but does not account for expansion of the PNH clone. Immune mechanisms may mediate clonal selection but appear insufficient to account for the clonal dominance necessary for PNH to become clinically apparent. Herein, we report 2 patients with PNH whose PIGAmutant cells had a concurrent, acquired rearrangement of chromosome 12. In both cases, der(12) had a break within the 3 untranslated region of HMGA2, the architectural transcription factor gene deregulated in many benign mesenchymal tumors, that caused ectopic expression of HMGA2 in the bone marrow. These observations suggest that aberrant HMGA2 expression, in concert with mutant PIGA, accounts for clonal hematopoiesis in these 2 patients and suggest the concept of PNH as a benign tumor of the bone marrow. ( IntroductionParoxysmal nocturnal hemoglobinuria (PNH) is a consequence of nonmalignant clonal expansion of hematopoietic stem cells with somatic mutation of PIGA. 1 Mutant PIGA 2 explains the deficiency of glycosyl phosphatidylinositol-anchored proteins (GPI-APs) that underlies the intravascular hemolysis of PNH. 3 However, PIGAmutant stem cells have no intrinsic proliferative advantage, 4,5 suggesting a 2-step model of pathogenesis.Step 1 of this model, clonal selection, 6,7 is envisioned as a conditional survival advantage that depends on deficiency of 1 or more GPI-APs. The close association of PNH with aplastic anemia, suggests that the selection pressure is immune mediated. 6,7 But, although 60% to 70% of patients with aplastic anemia have small, subclinical populations of GPI-AP Ϫ hematopoietic cells at diagnosis, 8 only 10% to 15% subsequently develop clinically apparent PNH. 9 In the remainder, GPI-AP Ϫ cells persist subclinically or disappear, 8 suggesting that mutant PIGA (and the consequent deficiency of GPI-APs) is necessary for clonal selection but is insufficient to account for the clonal expansion required for clinical manifestations of PNH to become apparent.Clonal expansion, step 2 of the PNH pathogenesis model, is envisioned as a consequence of clonal evolution in which a second somatic mutation bestows on the PIGA-mutant stem cell a proliferative advantage. 10 Herein, we present evidence supporting this 2-step model by showing a concurrent, acquired genetic abnormality in the PIGAmutant cells of 2 patients that establishes a novel mechanism for the nonmalignant clonal hematopoieis characteristic of PNH. Patients, materials, and methods PatientsInformed consent was obtained from patients J20 and US1 according to protocols approved by the Institutional Review Boards of Osaka University Hospital (Osaka, Japan) and the University of Utah School of Medicine (Salt Lake City, UT), respectively. Hybrid cell linesMonocytes derived from J20 or US1 were fused with the hypoxanthine phosphoribosyltransferase-negative mouse myeloma cell line, P...