Biological Predictive Factors of Response to ESA in Low Risk MDS Patients

In this trial, the investigators would like to understand why a small percentage of patients will be refractory to ESA (independently of International prognostic scoring system (IPSS) and % of blasts). In a retrospective study of the "Groupe Francophone des Myélodysplasies (GFM)" , the investigators showed that about 43% of patients are refractory or will relapse after initial response to ESA and it has been shown that these patients have a poorer survival. The investigators plan to give a 12-week treatment of Epoetin alfa or zeta in low risk MDS patients and measure different biological factors to predict response to ESA: * evaluation by flow cytometry before and after treatment of the degree of dyserythropoiesis and dysgranulopoiesis which could explain the primary resistance or loss of response of a subset of patients, * screening by molecular biology of predictive factors of response to ESA, * Iron homeostasis will be measured via hepcidin, GDF-15 and ferritin levels.