Molecular basis of clonal expansion of hematopoiesis in 2 patients with paroxysmal nocturnal hemoglobinuria (PNH)

Abstract

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...

Aberrant expression of microRNA in polycythemia vera

“…The target of let-7a is HMGA2, 37 an architectural transcription factor, whose aberrant expression contributes to clonal hematopoiesis in some cases of paroxysmal nocturnal hemoglobinuria. 38 Compared to levels in normal controls, we detected a significantly higher HMGA2 mRNA level in PMF granulocytes but not in granulocytes from patients with PV and ET. Similar findings have been reported by others.…”

The Role of Hmga2 in the Proliferation and Expansion of a Hematopoietic Cell in Myeloproliferative Neoplasms

“…In these studies, the virus was relatively often inserted into the HMGA2 locus, leading to removal of binding sites for let -7 miRNA and clonal outgrowth of hematopoietic cells, which brought a long -term effect including continuous independence of blood transfusion in severe thalassemia 70) . Overexpression and/or truncation of HMGA2 has also been found in patients with PNH and MDS, in which bone marrow failure rather than proliferative hematopoiesis is a common feature 26,68,72) . Bone marrow failure in these disorders is partly due to immunologic HSC injury by autoreactive cytotoxic T lymphocytes (CTLs) that produce tumor necrosis factor (TNF) -α and interferon (IFN) -γ 73 -75) .…”

“…Interestingly, it has been reported that HMGA2 mRNA expression was significantly higher in PMF patients with JAK2V617F mutation than patients without the mutation 25) . Moreover, in two patients with paroxysmal nocturnal hemoglobinuria (PNH), a chromosomal rearrangement causing a truncation of the 3´UTR of the HMGA2 gene have been also found particularly in abnormal clone without cell surface glycosyl phosphatidylinositol proteins (PNH clone), leading to the overexpression of HMGA2 in PNH clones 68) . These findings suggested the hypothesis that overexpression of HMGA2 may confer a clonal growth advantage to an abnormal progenitor cell, thus contributing to pathogenesis in MPN or other clonal hematologic disorders (Fig.…”

The small population of PIG-A mutant cells in myelodysplastic syndromes do not arise from multipotent hematopoietic stem cells

“…Preferential immune attack on normal HSC that spares the PIG-A mutant HSC is the most attractive hypothesis to explain the close relationship between AA and PNH. 32,33 However, other hypotheses such as secondary mutations, 34 resistance to apoptosis 35.36 or even neutral evolution have been proposed. 37 Regardless of the mechanism, our data suggest that PIG-A mutations arise from a multipotent HSC in both PNH and AA.…”