Targeting The PRMT5 Enzyme To Induce γ-Globin Gene Derepression In Sickle Cell Anemia

Sickle Cell Disease (SCD) is an autosomal recessive disorder that affects thousands of people across the United States. Although it is not a disease exclusive to the African American population, it is most common among persons from this ethnic group with an estimated 1 out of every 500 individuals affected. The disease is caused by a point mutation in the sixth codon of the beta-globin gene which is found on chromosome 11. This missense mutation involves the replacement of the hydrophilic glutamic acid with a hydrophobic valine that affects the structure of the b-GLOBIN protein leading to altered erythrocyte elasticity and oxygen carrying capacity. As a result, less oxygen is delivered to the tissue and the deformed, rigid cells often become trapped in the microvasculature causing local tissue ischemia, inflammation and the pain crisis that is characteristic of the disease. Early research into the understanding of this devastating disease has shown that a modulator of SCD severity is the persistence of fetal hemoglobin (HbF or γ-GLOBIN). Indeed SCD patients carrying a genetic mutation leading to hereditary persistence of HbF are able to maintain a near crisis-free life. This is so because HbF is made up of 2 alpha and 2 gamma subunits and lacks the beta subunits found in adult hemoglobin. As a result, HbF lacks the ability to sickle when there are significant changes in the oxygen levels. Several genetic, biochemical and clinical observations indicate that even modest induction of γ-GLOBIN may be sufficient to ameliorate the SCD phenotype. In the past decade efforts have focused on identifying regulatory complexes that modulate fetal-γ-GLOBIN gene expression that could be targeted for pharmacological intervention to improve and to monitor the outcome of this incurable disease. Physiologic transcriptional silencing of the γ-GLOBIN locus during development could be reversed for therapeutic benefit. To date, compounds such as 5-aza-2-deoxycytidine, hydroxyurea, short-chain fatty acids, and histone deacetylase (HDAC) inhibitors, have been used to increase HbF, however, effects are variable and mechanism of action is poorly understood. Thus, it is imperative that additional therapies be developed to enhance HbF expression in patients not responding well to the drugs available on the market. Recent work has identified key components of the repressor complex that silences fetal-γ-GLOBIN including protein arginine methyltransferase 5 enzyme (PRMT5) and histone deacetylase protein 1 and 2 (HDAC1/2). Studies are ongoing to characterize the regulatory complex that is recruited to the γ-GLOBIN gene promoter and develop a lead optimized PRMT5 inhibitor drug for clinical translation. Pretreatment of the erythroleukemia cell line K562 with three of our newly developed PRMT5 inhibitors (CMP5, HLCL7 and CMP12) leads to re-expression of g-GLOBIN gene product HbF at mRNA and protein levels in a time and dose dependent fashion. As PRMT5, LSD1 and HDAC1/2 have been shown to be all involved in the regulation of expression of HbF gene we further evaluated the effect of specific inhibitors targeting each of these proteins alone or in combination. Our data strongly indicate that while each agent alone led to re-expression of HbF, combination treatments led to an enhanced protein expression. Studies are ongoing to caracterize the regulatory complex that is recruited to the γ-GLOBIN gene promoter and develop a lead optimized PRMT5 inhibitor drug for clinical translation. We believe that our results will be the key for developing of more effective and specific therapies for this incurable disease.