How I treat sickle cell disease with gene therapy

Abstract

In 2023, 2 different gene therapies were approved for individuals with severe sickle cell disease (SCD). The small number of patients treated on the pivotal clinical trials that led to these approvals have experienced dramatic short-term reductions in the occurrence of painful vaso-occlusive crises, but the long-term safety and efficacy of these genetic therapies are yet to be ascertained. Several challenges and treatment-related concerns have emerged in regard to administering these therapies in clinical practice. This article discusses the selection and preparation of individuals with SCD who wish to receive autologous gene therapy, as well as the salient features of the care needed to support them through a long and arduous treatment process. I specifically focus on postinfusion care, as it relates to immune monitoring and infection prevention. Compared with allogeneic hematopoietic cell transplantation, delivering autologous gene therapy to an individual with SCD has distinct nuances that require awareness and special interventions. Using clinical vignettes derived from real-life patients, I provide perspectives on the complex decision-making process for gene therapy for SCD based on currently available data and make recommendations for evaluating and supporting these patients.

Graphical abstract

Figure 1.

Molecular basis of SCD and various genetic approaches to treat it. (A) Five different paralogs of globin genes that sequentially turn on during human ontogeny are located on Chr11. These are HBE (coding for ε-globin), transcribed during early embryonic life; HBG1 and HBG2 (coding for γ-globin), transcribed during fetal life; and HBB and HBD (coding for β-globin and δ-globin, respectively), transcribed from infancy to adulthood. SCD is caused by a single point mutation in the sixth codon of the HBB gene (c.20A>T [p.Glu6Val]), which causes the production of sickle Hb (HbS), which polymerizes under hypoxic conditions, resulting in sickle-shaped RBCs. (B) Lentiviral vectors can be used to deliver antisickling globin genes, such as HBB^T87Q or HBG^G16D, which randomly integrate into the genome and produce a “normal” copy of a β-like globin in addition to sickle globin. (C) Around the time of birth, there is a shift in gene expression from γ-globin (HBG1 and HBG2) to β-globin (HBB), resulting in a switch from fetal Hb (HbF, α2γ2) to adult Hb (HbA, α2β2) or, in the case of SCD, to HbS (α2β^S2). This switch is mediated by the binding of repressor transcription factors, such as BCL11A, to the globin locus at the HBG1 and HBG2 promoters. HbF production can be restored in adult erythrocytes by eliminating the expression of BCL11A or by preventing the BCL11A protein from binding to its cognate site in the globin locus. (D) The expression of BCL11A can be knocked down in HSCs using short hairpin RNA, thereby derepressing HBG1 and HBG2 expression in the adult progeny of genetically modified HSCs. (E) Alternatively, the erythroid-specific intronic enhancer of BCL11A can be disrupted by using CRISPR/Cas9 or base editing in HSCs to selectively reduce its expression in the adult erythroid progeny. (F) Likewise, the binding of BCL11A at the HBG1 and HBG2 promoters can be disrupted by CRISPR/Cas9 or base editing, thereby derepressing the expression of these genes. (G) An important distinction is whether a genetic modification results in HbF expression in all or most of the RBCs, that is, pancellular distribution, or whether HbF is produced in some but not all cells, which is termed heterocellular distribution. Heterocellular distribution can result in sickling of those RBCs that have insufficient antisickling globin production, thereby limiting the therapeutic benefit of gene modification. Cas9, CRISPR associated protein 9; Chr, chromosome; ESE, erythroid specific enhancer; LCR, locus control region; mRNA, messanger RNA.

Figure 2.

Clinical course of an individual receiving autologous gene therapy. (A) The first step is to screen the candidate interested in pursuing autologous cellular therapy. This may involve reviewing the patient history, laboratory investigations, and imaging data to evaluate the patient’s organ function, infections, and fitness to receive myeloablative chemotherapy and autologous HSCs. (B) Selected candidates discontinue hydroxyurea treatment and begin simple or exchange transfusions, which are continued for at least 2 to 3 months before mobilization, and apheresis collection are attempted. (C) Mobilization and apheresis are generally performed as inpatient procedures but can be safely performed as outpatient procedures in selected individuals. Each apheresis cycle comprises 3 to 4 consecutive days of daily plerixafor dosing and collection. If it proves impossible to mobilize blood cells adequately or to collect enough CD34⁺ cells, another collection cycle may be performed. When multiple successive cycles of apheresis are required, they are separated by at least 4 to 6 weeks to enable the patient to recover from the previous procedure completely and to continue transfusions in the interim. (D) The harvested apheresis pack is shipped to the manufacturing site on the same day or the following day, in accordance with the manufacturer’s instructions. (E-G) At the manufacturing facility, the apheresis products undergo CD34⁺ cell isolation (E) followed by transduction with a lentivirus or electroporation with Cas9 and guide RNA (F-G), and the final manufactured product is cryopreserved while release testing and quality control studies are performed. (H) After the completion of the quality testing, if the product meets the specified minimum criteria, it is shipped to the clinical site that will be delivering the product to the patient. (I) At this point, the patient is admitted and begins myeloablative conditioning. Before this conditioning, organ function evaluations may be repeated to confirm the eligibility of the patient to receive the conditioning. Fertility preservation should also be performed before myeloablative chemotherapy exposure. (J) After a washout period (usually 24-48 hours after busulfan treatment), a genetically modified cellular product is infused into the patient via a central venous catheter. (K) Recipients usually remain inpatient until engraftment and are then followed regularly to evaluate them for hematopoietic recovery, any residual SCD-related complications, and late effects of chemotherapy exposure.

Figure 3.

There is an imbalance between α-globin chains and β-like globin chains after gene addition but not after gene editing. (A) Under normal circumstances, α-globin and β-like globin chains are maintained in a stoichiometric balance within RBCs. (B) In individuals with ≥1 α-globin chain deletions, the excess β-like globin chains precipitate in the erythroid precursors, leading to ineffective erythropoiesis. (C & D) Gene-addition approaches may exacerbate this chain imbalance, especially when there is a deletion of one or more α-globin chains. (E) HbF induction approaches cause switching from β-globin to γ-globin and, hence, do not exacerbate the imbalance between α-globin and β-like globin chains.