Multicolour Flow Cytometric Finetuning of the Classification of Myelodysplastic Syndromes.

The WHO classification of myeloid disorders contribute to a more refined classification and prognostication of myelodysplastic syndromes (MDS). The considerable differences in clinical behaviour of pure refractory anemia (RA) versus refractory cytopenia with multilineage dysplasia (RCMD) with or without ringsideroblasts are of importance in the management of patients with MDS. Flow cytometry may add additional diagnostic criteria to adequately discriminate RA from RCMD (+/− ringsideroblasts; (RS)) and may contribute in identifying Idiopathic Cytopenia of Undetermined Significance. We developed a 4-colour flow-cytometric procedure that comprises all differentiation stages of granulocytic, monocytic and erythroid lineages, instrumental for the recognition of various subpopulations within all three lineages in normal bone marrow samples. In 43 evaluable patients with MDS (RA, RARS, RCMD, RCMD-RS, MDS-U, RAEB-1 and 2), aberrant expression of differentiation antigens were demonstrated in 1 or more lineages. Flow-cytometry identified aberrancies in granulopoiesis and monocytopoiesis in 93% and 74% of the cases, respectively. In the majority of cases abnormal relations between CD13, CD16, CD11b, CD15 and HLA-DR were prominent in the granulopoiesis. In 34% of the cases a striking monocytopenia was detected, whereas in 59% abnormal surface expression of CD14, CD36 and CD33 indicating aberrant differentiation of monocytes. We defined aberrant myeloid blasts by a leukaemia associated phenotype (LAP) according to the definitions used in acute myeloid leukaemia. In 47% of the patients a LAP was detectable by demonstrating co-expression of CD5, CD7, CD19 and CD56 on CD34⁺ myeloid blasts. In all patients diagnosed as RA/RARS and MDS-U (n=12) according to WHO criteria, additional flow aberrancies were identified including a leukaemia associated phenotype of myeloid blasts in 41% of the cases. Only in 3 out of 28 cases with RCMD/RCMD-RS no erythroid aberrancies were detectable by flow-cytometry. In 9 normal control BM samples, no flow-cytometric abnormalities were present. It is concluded that flow-cytometry in MDS identifies aberrancies in the granulocytic and monocytic lineages and may classify patients with multi-lineage aberrancies not otherwise determined by cytology (WHO). Flow-cytometry may discriminate pure RA or MDS-U from RCMD. Since new drugs are emerging in low-risk MDS, the value of flow-cytometry might be of importance to further refine the classification in MDS. The exact role of these aberrant differentiation patterns on IPSS, clinical behaviour, impact on treatment decisions and as tool in disease monitoring have to be determined in future prospective studies.