Date Published: May 14, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Levi Makala, Salvatore Di Maro, Tzu-Fang Lou, Sharanya Sivanand, Jung-Mo Ahn, Betty S. Pace.
Fetal hemoglobin (HbF) improves the clinical severity of sickle cell disease (SCD), therefore, research to identify HbF-inducing agents for treatment purposes is desirable. The focus of our study is to investigate the ability of FK228 analogues to induce HbF using a novel KU812 dual-luciferase reporter system. Molecular modeling studies showed that the structure of twenty FK228 analogues with isosteric substitutions did not disturb the global structure of the molecule. Using the dual-luciferase system, a subgroup of FK228 analogues was shown to be inducers of HbF at nanomolar concentrations. To determine the physiological relevance of these compounds, studies in primary erythroid progenitors confirmed that JMA26 and JMA33 activated HbF synthesis at levels comparable to FK228 with low cellular toxicity. These data support our lead compounds as potential therapeutic agents for further development in the treatment of SCD.
Several classes of pharmacological compounds that reactivate γ-globin gene transcription have been identified. They include cytotoxic agents, DNA methyl transferase, and histone deacetylase (HDAC) inhibitors. Cytotoxic compounds terminate actively cycling progenitors and perturb cellular growth to trigger rapid erythroid regeneration and γ-globin gene activation. S-stage cytotoxic drugs, such as cytosine arabinoside , myleran , vinblastine , and hydroxyurea [4, 5], induce HbF production in primates and humans [4, 6, 7]. The Multicenter Study of Hydroxyurea established this agent as the first FDA-approved treatment for SCD . Hydroxyurea was shown to reduce vaso-occlusive episodes in the majority of sickle-cell patients treated. However, limitations to using hydroxyurea such as bone marrow suppression , concerns over long-term carcinogenic complications, and a 30% non-response rate [7, 9], make the development of alternative therapies desirable.
Drug-mediated HbF induction remains the best treatment approach to ameliorate the symptoms and complications of SCD due to its ability to inhibit hemoglobin S polymerization. In addition, HbF provides an effective treatment for β-thalassemia by correcting globin chain imbalance . Other therapies aimed at the underlying molecular causes of the β-hemoglobinopathies include hematopoietic stem cell transplantation  and gene therapy involving the transfer of normal γ- or β-globin genes into hematopoietic stem cells. Despite promising results and ongoing research, the option for stem cell transplantation is limited by the lack of suitable donors for the majority of SCD patients. On the other hand, gene therapy offers a universal cure but there are concerns about mutagenesis of target genes due to random vector integration and the effects of viral sequences on nearby gene expression . Therefore, pharmacologic HbF induction remains a viable choice for the development of additional therapeutic options for treating SCD.
The current drug treatment options for SCD are limited with hydroxyurea being the only FDA-approved drug. The key finding of this study is the high-efficiency synthesis of FK228 analogues with structural modifications which did not disturb the global chemical structure of the parent compound. The analogues exhibited HbF induction at nanomolar concentrations in primary erythroid progenitors demonstrating physiological relevance. These data support the FK228 analogues as potential therapeutic agents and also validates the KU812 dual-luciferase stable cell lines as an efficacious screening system to identify γ-globin activators. Long-term our goal is to establish a group of HbF inducers that selectively inhibit Class I HDACs to expand our understanding of epigenetic mechanisms of γ-globin gene regulation and to facilitate the development of drug therapy for SCD.