Post-Transcriptional Genetic Silencing of BCL11A to Treat Sickle Cell Disease

Abstract

Background: Sickle cell disease is characterized by hemolytic anemia, pain, and progressive organ damage. A high level of erythrocyte fetal hemoglobin (HbF) comprising α- and γ-globins may ameliorate these manifestations by mitigating sickle hemoglobin polymerization and erythrocyte sickling. BCL11A is a repressor of γ-globin expression and HbF production in adult erythrocytes. Its down-regulation is a promising therapeutic strategy for induction of HbF.

Methods: We enrolled patients with sickle cell disease in a single-center, open-label pilot study. The investigational therapy involved infusion of autologous CD34+ cells transduced with the BCH-BB694 lentiviral vector, which encodes a short hairpin RNA (shRNA) targeting BCL11A mRNA embedded in a microRNA (shmiR), allowing erythroid lineage-specific knockdown. Patients were assessed for primary end points of engraftment and safety and for hematologic and clinical responses to treatment.

Results: As of October 2020, six patients had been followed for at least 6 months after receiving BCH-BB694 gene therapy; median follow-up was 18 months (range, 7 to 29). All patients had engraftment, and adverse events were consistent with effects of the preparative chemotherapy. All the patients who could be fully evaluated achieved robust and stable HbF induction (percentage HbF/(F+S) at most recent follow-up, 20.4 to 41.3%), with HbF broadly distributed in red cells (F-cells 58.9 to 93.6% of untransfused red cells) and HbF per F-cell of 9.0 to 18.6 pg per cell. Clinical manifestations of sickle cell disease were reduced or absent during the follow-up period.

Conclusions: This study validates BCL11A inhibition as an effective target for HbF induction and provides preliminary evidence that shmiR-based gene knockdown offers a favorable risk-benefit profile in sickle cell disease. (Funded by the National Institutes of Health; ClinicalTrials.gov number, NCT03282656).

Figure 1.. Fetal Hemoglobin Induction and Gene Marking after Gene Therapy.

Panel A shows hemoglobin (Hb), Panel B absolute reticulocyte count (ARC), and Panel C lactate dehydrogenase over time after infusion. In Panels A, B, and C, circled dots indicate the presence of transfused red cells at the time of the baseline sample. In these three panels, Patient 3 is not shown owing to reinitiation of regular red-cell transfusions. Panel D shows HbF as a percent of HbF + HbS at various times after infusion. Panel E shows the percent of F-cells (circulating untransfused red cells that express HbF) at various times after infusion. In Panels A through F, the baseline values shown indicate the values before gene therapy, which are the latest samples collected before cell infusion for which relevant data are available. Patients 2, 3, and 7 at the baseline time point were receiving regular transfusions; for Patient 4, the baseline time point is 2 months after hydroxyurea was discontinued and after 3 months of pre–gene therapy exchange transfusions; for Patient 6, the baseline time point is 2 weeks after hydroxyurea was discontinued and 1 month after a transfusion given for clinical indications; and Patient 8 at the baseline time point was receiving hydroxyurea before starting pre–gene therapy transfusions. Panel G shows the whole-blood vector copy number in peripheral blood for each of the six patients at various times after infusion.