Conference Coverage

Sickle cell disease phenotype reversed by gene therapy



SAN DIEGO – An adult with sickle cell disease has had significant remissions in symptoms and a near elimination of transfusion requirements after receiving an infusion of autologous stem cells genetically modified to simultaneously induce the fetal form of hemoglobin and decrease sickle hemoglobin.

In a first-in-human, proof-of-concept study, transduction of hematopoietic stem cells with a lentiviral vector targeted against the gamma globin repressor BCL11A in erythroid cells led to rapid induction of fetal hemoglobin and a reversal of the sickle cell disease (SCD) phenotype in the early phase of stem cell reconstitution, reported Erica B. Esrick, MD, from the Dana-Farber/Boston Children’s Cancer and Blood Disorders Center in Boston.

“The potential advantage of this approach over the gene-addition strategy of gene therapy is that we can harness the physiologic switch machinery that exists in the cell to simultaneously increase fetal hemoglobin and decrease sickle hemoglobin,” she said at a briefing prior to her presentation at the annual meeting of the American Society of Hematology.

Several research groups are developing autologous gene therapy for beta-hemoglobinopathies, including the use of CRISPR-Cas9 technology to mimic a rare, naturally occurring mutation that causes the fetal type of hemoglobin to persist into adulthood in some patients with SCD and beta-thalassemia.

Dr. Esrick and her colleagues are trying a different approach: Using gene therapy to knock down BCL11A expression to induce gamma globin expression.

For the treatment, autologous hematopoietic stem cells are collected from patients following mobilization with plerixafor. The cells are then transduced with a lentiviral vector consisting of a novel short hairpin RNA embedded in an endogenous micro-RNA. The investigators refer to the construct as a shmiR (“schmeer”). The construct is designed to be erythroid specific, with BCL11A knocked down only in the red cell lineage, to avoid potential off-target effects of the therapy.

Following stem cell collection and transduction, patients undergo conditioning with busulfan prior to infusion of the modified stem cells.

In three patients treated thus far, the process has been shown to be highly efficient, with approximately 96% of treated cells transduced.

In the patient mentioned before, neutrophil engraftment was confirmed on day 22 after transfusion of the modified cells. He experienced adverse events that were consistent with myeloablative conditioning, but no adverse events associated with the modified cells.

During 6 months of follow-up the patient did not experience SCD-related pain, respiratory events, or neurologic events, and did not have anemia, with a total hemoglobin of 11 g/dL at 6 months. He has not required any transfusions since engraftment.

Patients in the trial will be followed for 2 years, and then will be enrolled in a 15-year follow-up study designed to evaluate the safety and the durability of therapy.

Dr. Esrick reported receiving honoraria from Bluebird Bio, maker of the short hairpin RNA construct used in the trial.

SOURCE: Esrick EB et al. ASH 2018, Abstract 1023.

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