The impact of molecular biology on the diagnosis and treatment of hemoglobin disorders

Identification and characterization of mechanistically distinct inducers of γ-globin transcription

Inhibition of HbS polymerization is a major target for therapeutic approaches in sickle cell anemia. Toward this goal, initial efforts at pharmacological elevation of fetal hemoglobin (HbF) has shown therapeutic efficacy. In order to identify well-tolerated, novel agents that induce HbF in patients, we developed a high-throughput screening approach based on induction of gamma-globin gene expression in erythroid cells. We measured gamma-globin transcription in K562 cells transfected with either gamma promoter elements fused with the locus control region hypersensitivity site 2 and luciferase reporter gene (HS2 gamma) or a beta-yeast artificial chromosome in which the luciferase reporter gene was recombined into the gamma-globin coding sequences (gamma YAC). Corresponding pharmacological increases in HbF protein were confirmed in both K562 cells and in human primary erythroid progenitor cells. Approximately 186,000 defined chemicals and fungal extracts were evaluated for their ability to increase gamma gene transcription in either HS2 gamma or gamma YAC models. Eleven distinct classes of compounds were identified, the majority of which were active within 24-48 hr. The short chain hydroxamate-containing class generally exhibited delayed maximal activity, which continued to increase transcription up to 120 hr. The cyclic tetrapeptide OSI-2040 and the hydroxamates were shown to have histone deacetylase inhibitory activity. In primary hematopoietic progenitor cell cultures, OSI-2040 increased HbF by 4.5-fold at a concentration of only 40 nM, comparable to the effects of hydroxyurea at 100 microM. This screening methodology successfully identifies active compounds for further mechanistic and preclinical evaluation as potential therapeutic agents for sickle cell anemia.

Modifications of RNA processing modulate the expression of hemoglobin genes

The developmental changes in hemoglobin gene expression known as "switching" involve both the sequential activation and silencing of the individual globin genes. We postulated that in addition to changes in transcription, posttranscriptional mechanisms may be involved in modulating globin gene expression. We studied globin RNA transcripts in human adult erythroid cells (hAEC to analyze the mechanism of silencing of the embryonic epsilon-globin gene in the adult stage and in K562 erythroleukemic cells to analyze the inactive state of their adult beta-globin genes. In hAEC, which express primarily the beta-globin gene, quantitative PCR analysis shows that beta-mRNA exon levels are high and comparable among the three exons; the RNA transcripts corresponding to exons of the gamma-globin gene are low, with slight differences among the three exons. Although epsilon-globin is not expressed, epsilon-globin RNA transcripts are detected, with exon I levels comparable to that of gamma-globin exon I and much higher than epsilon-exons II and III. As expected, in K562 cells that express high levels of epsilon- and gamma-globin, epsilon- and gamma-mRNA levels are high, with comparable levels of exons I, II, and III. In K562 cells beta-mRNA levels are very low but beta-exon I levels are much higher than that of exons II or III. Moreover, all or most of the globin transcripts for the highly expressed globin genes in both cell types (gamma and beta in hAEC, epsilon and gamma in K562 cells) found in the cytoplasm or nucleus are correctly processed. The globin transcripts that are detected both in the cytoplasm and nucleus of cells without expression of the corresponding protein are largely unspliced (containing one or two intervening sequences). These studies suggest that in addition to changes in transcription rates, changes in completion or processing of globin RNA transcripts may contribute to the developmental regulation of the hemoglobin phenotype.