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

“The standard curative treatment for sickle cell disease is allogeneic hematopoietic stem-cell transplantation. Matched sibling donor transplantation is curative in more than 90% of patients, but limitations include a higher risk of complications in older patients, a risk of severe graft-versus-host disease (GVHD), and lack of an available matched sibling in approximately 80% of cases.

[..] In utero and during infancy, the abnormal HbS protein is produced at very low levels because the erythrocytes have not yet shifted from expression of the γ-globin gene (HBG), which encodes the developmentally regulated component of HbF, to expression of the HBB gene, a phenomenon known as hemoglobin switching. Thus, infants with sickle cell disease are typically free of clinical symptoms, owing to the potent antisickling properties of HbF combined with the lower levels of HbS.

[..] We have previously described a lentiviral vector that mediates potent erythroid-specific knockdown of BCL11A through RNA interference (RNAi), using a microRNA (miRNA)-adapted short hairpin RNA (shRNAmiR). To further optimize the vector titer, this therapeutic shmiR, under transcriptional control of regulatory elements derived from the β-globin locus, was inserted in a lentiviral vector backbone previously used in clinical trials, resulting in the self-inactivating third-generation BCH-BB694 lentiviral vector. In preclinical studies using this vector, lentiviral vector–transduced CD34+ cells derived from donors with sickle cell disease achieved a potent induction of HbF. Here we report treatment of patients with sickle cell disease using autologous cells transduced with a shmiR vector, as well as validation of BCL11A inactivation as a strategy for HbF induction.

The primary aim of this pilot study is to assess the safety and feasibility of gene therapy with the BCH-BB694 BCL11A shmiR-encoding lentiviral vector in patients with severe sickle cell disease. [..] Patients were enrolled after providing written informed consent or, if the patient was younger than 18 years old, after a guardian provided consent. Patients 12 to 17 years old provided assent. Eligibility was restricted to patients who had a genotype of HbSS, HbS/β0, HbSD, or HbSO and had clinically severe sickle cell disease, defined, as in many allogeneic transplantation sickle cell disease studies, by two or more episodes of acute chest syndrome during the preceding 2 years, three or more severe pain events during the preceding 2 years, recurrent priapism, red-cell alloimmunization in patients requiring transfusions, or an indication for regular transfusions for primary or secondary stroke prophylaxis. Patients were eligible for enrollment if they met these severity criteria despite receiving hydroxyurea therapy. Patients with an HLA-matched sibling who could serve as donor for transplantation were excluded from the study.

As of October 2020, a total of 11 patients had provided informed consent: one did not meet eligibility criteria, one had complications of underlying disease after consent and withdrew from the study, two had less than 6 months follow-up, and one had not yet been infused, leaving six patients with at least 6 months of follow-up [..].

[..] Patients received fully myeloablative conditioning with intravenous busulfan for 4 consecutive days, with pharmacokinetic dose adjustment to target a daily area under the curve of approximately 5500 μM per minute. The transduced CD34+ cells were infused at least 24 hours after the last dose of busulfan. Patients are followed for 2 years before being offered enrollment in a 13-year long-term follow-up study.

[..] Patient 8 received a new diagnosis of type 1 diabetes mellitus 2 weeks after infusion, while the patient had a severe respiratory infection. The presence of autoantibodies suggests that the patient had an underlying predisposition to type 1 diabetes mellitus that was unmasked by intercurrent illness or conditioning.

[..] Hemolysis continued in all patients but was reduced from baseline, with the absolute reticulocyte count falling from 175,000–680,000 per microliter to 160,000–355,000 per microliter and with lactate dehydrogenase falling from 167–726 U per liter to 217–485 U per liter.

[..] No patient has had a vaso-occlusive crisis, acute chest syndrome, or stroke since the gene therapy infusion. Patient 4 (whose infusion had been performed 20 months ago as of October 2020) had required frequent hospitalizations for priapism before gene therapy. Between 4 and 8 months after gene therapy, he had several severe episodes of priapism. He has not had a priapism-related emergency department visit or hospitalization since month 8, although intermittent episodes of priapism have recurred. Patient 6 had left hip avascular necrosis before study enrollment. At 9 months after gene therapy, symptoms consistent with avascular necrosis of the right hip had developed in this patient.

Unlike earlier trials of gene therapy in which enhancer-containing γ retroviruses caused leukemia in some patients owing to insertional mutagenesis, the current generation of lentiviral vectors has not so far been associated with insertional mutagenesis, with up to 12 years of follow-up. However myeloablative busulfan conditioning also carries a small risk of secondary cancer, and in one patient with sickle cell disease who underwent gene therapy in another trial, myelodysplastic syndrome and subsequent acute myeloid leukemia developed that were unrelated to the lentiviral vector. In this trial and other autologous genetic treatment studies, it will be important to evaluate the risk of post-alkylator myelodysplasia and to determine whether factors such as busulfan exposure affect this risk.”

Full article, Esrick EB, Lehmann LE, Biffi A et al. New England Journal of Medicine, 2020.12.5