CRISPR Exploits Vulnerability of Sickle Cell Disease

    Sickle Cell Disease (SCD), also known as sickle cell anaemia (SCA) and drepanocytosis, is a hereditary blood disorder, characterized by an abnormality in the oxygen-carrying haemoglobin molecule in red blood cells.  This leads to a propensity for the cells to assume an abnormal, rigid, sickle-like shape under certain circumstances.

    A direct consequence of natural selection among Homo sapiens, sickle cell disease (SCD) has afflicted a large segment of the population for several millennia. However, with the rapid rise of new genomic editing techniques, scientists may have found the proverbial Achilles heel for this disorder.

    Researchers from Dana-Farber/Boston Children’s Cancer and Blood Disorders Center believe that changes to a small patch of DNA within the enhancer region of the BCL11A gene may circumvent the genetic defect that underlies SCD and possibly other blood disorders such as thalassemia.

    Previous research has shown that BCL11A controls whether red blood cells produce the adult form of hemoglobin—which in SCD is mutated—or a fetal form that is unaffected by and counteracts the effects of the sickle mutation. Moreover, additional studies have indicated that sickle cell patients with elevated levels of fetal hemoglobin have a milder form of the disease.

    The investigators found that many naturally occurring variations within the enhancer region of BCL11A led to positive outcomes for SCD patients and only affected red blood cells, even though the gene is active in immune and brain cells.

    The findings from this study were published recently in Nature through an article entitled “BCL11A enhancer dissection by Cas9-mediated in situ saturating mutagenesis.”

    In an attempt to mimic and improve upon the natural variations, the researchers developed CRISPR-based gene editing tools to systematically cut out tiny sections of DNA step-by-step along the entire length of the enhancer in blood stem cells from human donors. The team then allowed the cells to mature into red blood cells and found that the amount of fetal hemoglobin the cells produced had increased dramatically. Additionally, the scientists discovered a specific location in the enhancer that when cut leads to the production of high levels of fetal hemoglobin.

    “There was no efficient way of conducting this kind of experiment until now,” explained co-senior author Daniel Bauer, M.D., pediatric hematologist/oncologist at Dana-Farber/Boston Children’s Hospital “Our goal was to break the enhancer, rather than fix the hemoglobin mutation, but to do so in very precise ways that are only possible since gene editing technologies like CRISPR became available.”

    When the researchers carried out their experiments in animal models of SCD, they found that removal of this part of the enhancer affected BCL11A’s expression was restricted to only red blood cells, leaving other cell types unaffected.

    “These experiments may have revealed the genetic Achilles heel of sickle cell disease,” noted Stuart Orkin, M.D., co-senior author and chairman of pediatric oncology at Dana-Farber Cancer Institute. “Alterations to these specific portions of the enhancer have the same effect as knocking the whole enhancer out altogether, suggesting that this could be a promising strategy to translate into the clinic.”

    The researchers were excited by their findings and feel that their data provides proof of principle that targeted edits to BCL11A’s enhancer in blood stem cells could be an attractive approach for curing SCD and related conditions.

    “Although fixing the sickle mutation itself would seem the most straightforward approach, it turns out that blood stem cells, the ultimate targets for this kind of therapy, are much more resistant to genetic repair than to genetic disruption,” Dr. Bauer added. “Therefore, making a single DNA cut that breaks the enhancer solely in blood stem cells could be a much more feasible strategy.”