Epigenetic control of gene expression is a complex process involving interactions between genomic DNA, a functionally diverse group of nuclear proteins, and small regulatory RNA molecules. These factors modify DNA and chromatin without changing the coding sequences of genes. The epigenetic processes of post-translational acetylation and deacetylation of histone proteins are important regulators of gene transcription. In general, histone acetyltransferases (HATs) activate gene transcription, whereas histone deacetylases (HDACs) are associated with transcription silencing. Epigenetic therapy holds much promise for the treatment of a variety of hematologic disease, and HDAC inhibitors (HDACis) are currently in clinical use for the management of hemoglobinopathies, as well as for hematologic malignancies and some enzymopathies. The therapeutic mechanism of action of HIDACis, however, is incompletely understood. HDACis cause a general increase in histone acetylation, but this effect is heterogeneous, both in terms of the genes that are affected and of the functional consequences of induced increase in acetylation, as expression of some genes is upregulated by this process, and expression of others is downregulated. Similar anomalies are observed with other epigenetic modifiers such as hypomethylating agents.
Disorders of red cell enzymes involved in glycolysis and the pentose phosphate pathway are caused by defects in the genes encoding these enzymes and are characterized by chronic hemolytic anemia. The underlying mutations are typically missense events that cause a decrease in enzyme activity (as opposed to nonsense mutations that cause complete loss of function). These observations prompted Dr. Kalliopi Makarona and colleagues in the laboratory of Dr. Anastasios Karadimitris of Hammersmith Hospital in London to investigate whether HDACis could be used to upregulate expression of genes involved in the glycolytic and pentose phosphate pathways and thereby ameliorate clinical symptoms. Initial analysis in B cells and proerythroblasts that had been treated with the HDACi sodium butyrate revealed that of the 16 enzymes involved in the glycolytic and pentose phosphate pathways, only glucose-6-phosphate dehydrogenase (G6PD) mRNA was increased. Further investigation revealed the mechanism underlying increased G6PD transcription. HDAC inhibition resulted in hyperacetylation of the G6PD gene promoter, which led to the recruitment of selected HAT and HDAC proteins and a change in the dynamic equilibrium between members of these two protein families. This change increased accessibility of the promoter, enabling the transcription factor Sp1 to bind and recruit RNA polymerase II, thereby initiating transcription of the G6PD gene. The next step was to investigate the effect of HDACi on erythroid precursor cells from patients with G6PD deficiency. Samples derived from five patients, each with a different missense mutation, were studied. In all five cases, there was an increase in G6PD mRNA from the defective allele located on the X chromosome and a concomitant increase in the amount of functional enzyme that was sufficient to restore activity to normal levels.
In Brief
These exciting findings provide proof of principle that G6PD deficiency can be overcome in nucleated erythroid precursors in vitro by epigenetic therapy, but they also highlight the gaps in our knowledge about the basis of the specificity of HDACis (i.e., why was G6PD the only one of the 16 genes invovled in the glycolytic and pentose phosphate pathways whose expression was enhanced by the HDACi). Nonetheless, the results of these preclinical experiments are encouraging, and suggest that further studies aimed at investigating in vivo efficacy are warranted. An effective therapeutic strategy would have important consequences not only for treating G6PD-deficient individuals with clinically significant hemolysis, but also for the management of malaria. G6PD deficiency is common in areas where malaria was or still is endemic, as it ameliorates disease severity and thus provides a selective advantage. However, acute hemolysis can be triggered in G6PD-deficient patients by primaquine, an antimalaria drug that targets hypnozoites of Plasmodium vivax and gametocytes of Plasmodium falciparum. Eradication of those parasite stages are of crucial importance in malaria elimination efforts. Therefore, a treatment option that would allow patients with G6PD deficiency to be safely treated with primaquine would be particularly relevant clinically.
Competing Interests
Dr. Coetzer declares no relevant conflicts of interest.
