Hydroxyurea-Mediated Reactivation of Fetal Genes in Primary Erythroid Cell Cultures from Beta-Thalassemia Patients.

Fetal hemoglobin (HbF) is frequently increased in the hemoglobinopathies such as sickle cell anemia and b-thalassemia. Epidemiological studies have indicated that an increase in HbF ameliorates the clinical symptoms of these diseases (Rodgers and Rachmilewitz - Bri J. Haemat. 91: 263 -1995). In sickle cell anemia, HbF containing red blood cells have a lower concentration of sickle hemoglobin (HbS), and the HbF itself inhibits HbS polymerization, decreasing cell sickling process (Eaton and Hofrichter - Science 268:1142 -1995). In b-thalassemia patients, HbF partially compensate HbA deficiency and could potentially improve RBC survival resulting in an increase of hemoglobin levels. Hydroxyurea (HU) is one of the pharmacological agents currently used to stimulate HbF synthesis in patients with sickle cell anemia and more recently has been tested in clinical trials for b-thalassemia patients too (Olivieri et al. Ann. NY Acad Sci 850:100-1998; Rigano et al. Hemogl. 21(3): 219- 1997; Dixit et al. Ann. Haematol. 84: 441 -2005). The mechanism involved in the HU-mediated changes is still unclear. It may involve a selection of a minor pre-existing subpopulation of F-cells that has a growth and/or survival advantage (cellular mechanism). This mechanism may be particularly effective for cells derived from patients with hemoglobinopathies, where F-cells may be resistant to “ineffective erythropoiesis”. An alternative mechanism could involve stimulation of HbF in all or the majority of cell-population by direct induction of g genes (molecular mechanism). Here we report the analysis on thalassemia patients homozygoutes for Lepore genotype that present high levels of fetal hemoglobin. We combined the use of primary erythroid cell culture from peripheral blood stem cells of these patients, with primary transcript in situ hybridization (RNA-FISH) of the g and b globin genes to investigate the mechanism of action of hydroxyurea in adult erythroid cells. RNA-FISH on erythroid cell cultures from these patients reveals that the majority of cells express one g allele only (g: 75.2 %, g:g 19.6%). The analysis in hydroxyurea-treated cultures shows the increase of cells transcribing both g-alleles, indicating the reactivation of fetal genes (g: 58.1%, g:g 40%). This evidence suggests that the molecular mechanism is involved directly on fetal genes reactivation to increase fetal hemoglobin production in HU-treated patients.