A cohort study of the nature of paroxysmal nocturnal hemoglobinuria clones and PIG-A mutations in patients with aplastic anemia

Background: Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by the clonal expansion of blood cells, which are deficient in glycosylphosphatidylinositol anchored proteins (GPI-APs). As PNH frequently occurs during the clinical course of acquired aplastic anemia (AA), it is likely that a process inducing bone marrow failure in AA is responsible for the selection of GPI-AP deficient blood cells or PNH clone. Objective: To explore the nature and mutation of a PNH clone in AA. Methods: We performed regular repeated flow cytometric analyses of CD59 expression on peripheral blood cells from a cohort of 32 patients with AA. Mutation of phosphatidylinositol glycan class A (PIG-A) was also studied. Results: Fifty-one episodes of occurrences of CD59 negative granulocytes out of a total cohort 167 flow cytometric analyses (31%) were observed in 22 patients (69%). CD59 negative erythrocytes were less apparent than the granulocytes. Repeated occurrences of PNH clones were observed in 16 patients. Most of the emerging PNH clones were transient in nature. They were more frequently detected during episodes of lower white blood cell and platelet counts. Persistence and expansion of the GPI-AP deficient blood cell populations to the level of clinical PNH were seen in only four patients (12.5%). Analysis of PIG-A gene demonstrated eight mutations among the four patients, with two and four independent mutations in two patients. Conclusions: Our study indicates that PIG-A mutations of hematopoietic stem cells with resultant PNH clones, are relatively common among AA patients. It also supports the hypothesis of selection of the PNH clone by a process or condition associated with or responsible for bone marrow failure in AA. However, there must be an additional factor favoring expansion or growth of the clone to the level of clinical or florid PNH.