A key signaling protein, known as heme-regulated inhibitor (HRI), has been identified as a potential therapeutic target for the development of drugs to treat sickle cell disease. By using a screening tool that employed CRISPR gene-editing approaches, the researchers were able to determine that HRI played a key role in the regulation of hemoglobin production. The research was published online July 19 in the journal Science.
In a recent press release, study co-leader Gerd A. Blobel commented: "We have found a protein with activity specifically in red blood cells that could be a 'druggable' target, possibly with a small molecule – a pill that patients could take to treat sickle cell disease.”
What is Sickle Cell Disease?
Sickle cell disease (SCD) is an inherited blood disorder that affects hemoglobin, the protein responsible for transporting oxygen around the body. It is estimated that SCD affects roughly 100,000 Americans. Patients with the condition have an abnormal version of hemoglobin (Hgb S) which distorts the shape of red blood cells (RBCs), making them adopt a ‘sickle’ or crescent shape. The ‘sickled’ RBCs are more fragile in comparison to normal RBCs and are prone to rupture (hemolysis), which can lead to anemia. Their abnormal shape also means they can block blood vessels and damage organs.
Heme-Regulated Inhibitor and Hemoglobin Switching
HRI’s role is regulating hemoglobin production is well established, however, until now HRI’s role in “hemoglobin switching” was unknown. Fetal-to-adult hemoglobin switching occurs in newborns – whereby they switch from producing fetal hemoglobin (using the γ gene) to adult hemoglobin (using the β gene). SCD is caused by mutations in the β gene therefore it only affects patients after birth.
Increasing Levels of Fetal Hemoglobin
SCD patients with higher levels of fetal hemoglobin have a less severe form of the disease and it is associated with longer survival. Previous research has revealed that loss of a transcription factor called BCL11A can increase fetal hemoglobin levels. To identify kinases that may regulate BCL11A, and that had potential to be therapeutically targeted, Grevet et al. used a “protein kinase domain–focused CRISPR-Cas9–based genetic screen”.
This screen revealed HRI – a heme regulated inhibitor that represses the production of fetal hemoglobin. The team were able to reduce the levels of HRI which resulted in an increase in the production of fetal hemoglobin – this reduced sickling in cultured RBCs taken from patients with SCD. Results obtained from additional studies using a murine (mouse) model further supported the results obtained using the human cell cultures.
In a proof of principle experiment, the researchers investigated whether HRI’s effect on fetal hemoglobin could be enhanced by using it in combination with a pharmacologic fetal hemoglobin inducer. Pomalidomide (not currently indicated for treatment of SCD) has been shown to increase the level of fetal hemoglobin (in part by reducing transcription of BCL11A). The team’s results showed that when HRI was used in combination with pomalidomide, there was an enhanced effect.
Touching on next steps, Blobel commented: "Our long-term goal is to carry out follow-up studies to evaluate whether this approach improves clinical outcomes in patients,"
"At this point, our results suggest that HRI is a potential target for a new treatment for disorders of hemoglobin."
Jeremy D. Grevet, et al. (2018) Domain-focused CRISPR-screen identifies HRI as a fetal hemoglobin regulator in human erythroid cells. Science. Available at: http://doi.org/10.1126/science.aao0932