CD55⁻CD59⁻ Granulocytes in Patients with Aplastic Anemia Are Clonal Populations Derived from Single PIG-A Mutant Stem Cells without Any Proliferative Advantage.

Small populations of CD55⁻CD59⁻ blood cells are detectable in approximately 50% of all acquired aplastic anemia (AA) patients and the presence of such PNH-type cells are also associated with a good response to immunosuppressive therapy (Sugimori C, et al. Blood 2006). In most patients showing 0.1% to 1.0% PNH-type cells at the diagnosis of AA, the PNH-type cell proportion remains unchanged over 3 years even after successfully responding to immunosuppressive therapy (Mochizuki K, et al. ASH 2006). Although these findings suggest that small populations of PNH-type cells are derived from a limited number of PIG-A mutants without any proliferative advantage, this hypothesis has not yet been verified at the molecular level. To appropriately address this issue, we studied 3 patients with AA who showed 0.14 to 1.6% PNH-type granulocytes. The CD55⁻CD59⁻ granulocytes were sorted from these patients 2 different times at a minimum of 6 month intervals and then they were subjected to a PIG-A gene analysis. Five exons were amplified using 6 different primer sets and each amplified product was then subcloned into E. coli. At least 5 transformed clones for each amplified product were randomly plucked and subjected to sequencing. Single mutations were thereafter detected in all 3 patients as shown in Table 1. The same single mutations were then detected in the CD55⁻CD59⁻ granulocytes from the patient obtained 6 months after the first examination. Two patients were in a state of hematologic remission at from 1 to 7 years after the first examination of their peripheral blood and the proportion of PNH-type cells remained stable during this period in all patients. Response to immunosuppressive therapy was not evaluable in one patient because he rejected ATG therapy. These findings indicate that PNH-type granulocytes from patients with AA are therefore clonal populations derived from single hematopoietic stem cells (HSCs) with a PIG-A mutation. If an AA patient has many HSCs with PIG-A mutations before the development of AA, then the immune system attack against HSCs should allow for the survival of the PIG-A mutants leading to the generation of a polyclonal PNH-type cell population. The presence of clonal PNH-type cells at the time of AA diagnosis suggests that the number of HSCs with a PIG-A mutation in healthy individuals may therefore be much lower than we expected and the paucity of PIG-A mutant HSCs may therefore account for the absence of increased number of PNH-type cells in approximately 20% of all AA patients who apparently respond to immunosuppressive therapy.