Evolution of GPI-Deficient Clones Predicts Clinical Course in Paroxysmal Nocturnal Haemoglobinuria.
Authors: Richards Stephen J. , Cullen Matthew J. , Dickinson Anita J. , Hall Claire , Hill Anita , Hillmen Peter 
Published: October 2019

Abstract
Flow cytometric analysis of GPI-linked antigens has had a major impact on the diagnosis of PNH. Significant numbers of patients with aplastic anemia have small PNH clones, and due to the precision in clone size measurement, reliable serial monitoring can now be undertaken although the clinical value of this is not proven. From our series of 234 PNH patients, we analysed clinical correlates between disease type and red cell and granulocyte peripheral blood clone sizes as determined by flow cytometry at presentation. For hemolytic patients (n = 99) the mean PNH clone sizes were: granulocytes 84.8%; red cells 45.3% (type III cells 33.6%). For aplastic patients (no macroscopic hemolysis) the mean clone sizes were: granulocytes 18.5%; red cells 6.4% (type III cells 4.5%). The two groups were statistically different (Mann Whitney U; P<0.001). Monitoring of PNH clones in 86 of these patients who had at least 3 samples over a minimum of 12 months (mean 55 months; range 15–174) not only showed distinct groups of patients with highly characteristic patterns of disease but also provided insights into the incidence of spontaneous remission, progression from aplastic to hemolytic disease, and development of leukemia. Firstly, hemolytic patients that present with >90% granulocyte clones (n = 30; mean follow up 48 months) with virtually all their hematopoiesis maintained from PNH stem cells have clone sizes that remain stable and their clinical behavior suggests that their PNH can persist for up to 40 years. The second group of patients (n = 16) were those with hemolytic PNH with granulocyte clones of <90%. Mean granulocyte clone size at presentation was 68.4% (range 34.7– 90%) with a mean follow-up of 66 months (range 24–164). Of these, 6 showed stable clone sizes, 2 increasing clone size, and 8 showing reductions in granulocyte clone size. The third group were those presenting with aplastic anemia (n = 34). This group showed the most significant variation in clone size and clinical behavior. Of the 12 patients with persistent aplastic anemia, the majority had slowly increasing clone sizes with 5 patients progressing to hemolytic PNH after a variable time period ranging from 26 to 79 months. Only 3 patients developed MDS or AML. Two of these were from the >90% granulocyte clone group (2/30) and developed as a terminal event, one with GPI-MDS, and the second with a rapid emergence of GPI+MDS. One patient in the aplastic group showed progression to AML (1/34). 27% of patients had an improvement in cytopenias with concurrent decrease in PNH clone size. For hemolytic patients with granulocyte clones of <90%, the 8 patients with falling clone sizes had improving blood counts. The PNH granulocyte clone halved in a mean of 74 months. Of the patients with aplastic anemia, 15 showed resolution of anemia with normalization of counts and all but one had an associated fall in granulocyte PNH clone sizes. Eleven patients have been treated in clinical trials of the anti-complement antibody, eculizumab, for a period of up to 2 years and over this period the proportion of PNH granulocytes has remained stable. This data demonstrates that the size and type of granulocyte and red cell PNH clones at presentation predicts the clinical course for individual patients assisting long term clinical management planning. Moreover, regular clone size monitoring predicts the likelihood of spontaneous reduction in the PNH clone and potentially for spontaneous remission.