40 International Journal of HEMATOLOGY 1. Introduction Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal stem cell disorder characterized by the clini- cal triad of hemolytic anemia, bone marrow failure, and venous thrombosis [1]. Hemolysis is due to an intrinsic abnormality of red cells deficient in several membrane pro- teins that protect them against damage from the membrane attack complex of complement, including CD55 (decay Detection of CD55- and/or CD59-Deficient Red Cell Populations in Patients With Plasma Cell Dyscrasias John Meletis, a Evangelos Terpos, a Michalis Samarkos, a Christos Meletis, b Effie Apostolidou, a Veroniki Komninaka, a Konstantinos Korovesis, a Konstantinos Anargyrou, a Olga Benopoulou, a Despina Mavrogianni, a Eleni Variami, a Nora Viniou, a Konstantinos Konstantopoulos a a First Department of Internal Medicine, University of Athens School of Medicine, Laiko General Hospital, Athens; b Department of Electrical and Computer Engineering, National Technical University of Athens,Athens, Greece Received March 21, 2001; received in revised form August 13, 2001; accepted August 15, 2001 Abstract Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal disorder characterized by a decrease or absence of gly- cosylphosphatidylinositol (GPI)-anchored molecules such as CD55 and CD59 from the surface of affected cells, resulting in intravascular hemolysis, cytopenia, and venous thrombosis. A PNH-like phenotype has been detected in various hematologi- cal disorders, mainly in aplastic anemia and myelodysplastic syndromes, but also in lymphoproliferative syndromes (LPSs).To the best of our knowledge, CD55- or CD59-deficient red cells have not been detected in plasma cell dyscrasias (PCDs). The aim of this study was the detection of CD55- and/or CD59-deficient red cell populations in patients with PCD. Seventy-seven patients were evaluated; 62 with multiple myeloma (MM), 7 with Waldenström macroglobulinemia (WM), 6 with monoclonal gammopathy of undetermined significance (MGUS), and 2 with heavy chain disease (HCD). The sephacryl gel microtyping system was applied; Ham and sucrose lysis tests were also performed on all samples with CD55- or CD59-negative popula- tions. Red cells deficient in both molecules were detected in 10 (12.9%) of 77 patients with PCD: 2 (28.6%) of 7 with WM, 1 (16.6%) of 6 with MGUS, 6 (9.6%) of 62 with MM, and 1 of 2 patients with HCD. Isolated CD55 deficiency was found in 28.5% of all PCD patients, whereas isolated CD59 deficiency was not observed in any patients.These findings illustrate the existence of the PNH phenotype in the red cells of patients with PCD; further investigation is needed into the mechanisms and signifi- cance of this phenotype. Int J Hematol. 2002;75:40-44. ©2002 The Japanese Society of Hematology Key words: Paroxysmal nocturnal hemoglobinuria; CD55; CD59; Multiple myeloma; Waldenström macroglobulinemia Correspondence and reprint requests: Professor John Meletis, First Department of Internal Medicine, University of Athens School of Medicine, Laiko General Hospital, Athens 11527, Greece; telephone and fax: 301 77 71 161 (e-mail: imeletis@cc.uoa.gr). accelerating factor [DAF]) and CD59 (membrane inhibitor of reactive lysis [MIRL]) [2]. These proteins share a common phospholipid element, glycosylphosphatidylinositol (GPI), that forms a peptide bond with the C-terminal amino acid of certain proteins, serving to attach the protein to the mem- brane [3]. Patients with PNH have a somatic mutation in the X-linked gene PIG-A (phosphatidyl inositol glycan comple- mentation group A), encoding a protein required for an early step in the biosynthesis of the GPI molecule. This mutation occurs in an early hematopoietic stem cell, resulting in an absence or decrease in GPI-anchored proteins in all types of blood cells [4]. Several PNH clones, bearing different PIG-A mutations, frequently coexist in the same patient, which accounts for the concomitant existence of blood cells with a complete deficiency of GPI-anchored proteins (PNH III) and blood cells with a partial deficiency of these molecules (PNH II) [5].