AVROBIO Gene Therapy Education

AVROBIO Gene Therapy Reference

04 Understanding HSC Gene Therapy

Section 1

The HSC Approach to Gene Therapy

Learning Goal

To understand what HSC gene therapy is designed to do.

As noted in the Genetics and Your Body section of this website, most cells in the body contain DNA. Segments of the DNA code for genes, which are the instructions the body needs to make proteins. Changes in a gene, called variants or mutations, can lead to alterations in their protein instructions. As a result, the body may produce the protein incorrectly, insufficiently, or not at all, potentially resulting in disease.1-3

Overview of HSC Gene Therapy2,4,5

  1. Patient’s HSCs are collected
  2. Genetic material is introduced (transduced) using a vector.
  3. Modified HSCs are infused back into the patient.

HSC gene therapy is a technique that scientists developed to address genetic variants that cause genetic disorders. It combines advances in cell therapy and gene therapy. The aim of HSC gene therapy is to modify a person’s hematopoietic stem cells outside of the body (called ex vivo) by adding a copy of the gene that codes for a functional protein and returning the person’s own genetically modified stem cells to their body, where they divide and generate new cells.4-6 HSC gene therapy, like most gene therapy, is a relatively new technology. There have been several recent regulatory approvals for gene therapies; the majority remain investigational and are being studied in clinical trials.     

HSC gene therapy can use lentiviral vectors as the delivery vehicles designed to carry potentially therapeutic genetic material into a person’s own stem cells. Using a lentiviral vector potentially allows the therapeutic gene to integrate into the genetic material of the cells long-term.4,7

In this section, we focus on one approach to lentiviral vectors that involves a single delivery of genetically modified hematopoietic stem and progenitor cells (HSPCs). However, it is important to note that there are other uses of lentiviral vectors, such as chimeric antigen receptor (CAR)-based therapy, which involves delivery of genetically modified white blood cells such as T cells (known as CAR-T cell therapy) or natural killer (NK) cells (CAR-NK therapy) for treatment of certain kinds of cancer.4,8

HSPCs are a mixture of progenitor (early) cells and a small population of long-term stem cells, which repopulate the bone marrow. The progenitor cells, which constitute the majority of HSPCs, are not true stem cells; they become exhausted and no longer divide but rely on the stem cells to make more copies of themselves and their daughter cells, which potentially last for the person’s lifetime.9,10

Once a person’s stem cells are genetically modified and infused back into the person, the goal is that the modified stem cells work in two ways: 

  • The modified stem cells develop and produce mature cells that carry the therapeutic gene and produce the functional protein. These modified, mature cells can directly replace the disease-causing cells that contained the gene variant.11
  • In some cases, the modified mature cells can secrete (form and release) the functional protein. The protein can be taken up by other cells, including those in different organs like the kidney, liver, heart, and brain, where it potentially provides a long-term benefit. This process is often referred to as “cross-correction.”12
Once administered, HSC gene therapy relies on natural processes and biology to carry the genetically modified stem cells throughout the body. The ability of HSPCs to differentiate into many cell types potentially allows them to enter hard-to-reach tissues including muscle, bone, and the brain.1,2,13,14 Additionally, certain configurations of HSC gene therapy enable expression of therapeutic genes at higher levels than those normally present in HSPCs and their daughter cells, a factor that in some instances is necessary for clinical benefit.7

HSC Gene Therapy2,4,5,15-17

  1. Genetically modified stem cells are infused into the body.
  2. Modified stem cells are designed to engraft in the bone marrow and mature into a wide variety of immune and blood cells. Each of these daughter cells is expected to carry copies of the therapeutic gene required to produce the functional protein the patient needs.
  3. These daughter cells distribute the functional protein to the tissues from the heart to muscle to bone, and even the brain.

Prior to reinfusion, the manufacturer determines the vector copy number (VCN), which is the average number of copies of an integrated therapeutic gene per cell.18,19

  • The target VCN can differ based on the genetic disorder.
  • It is important to have VCN values that support a potential therapeutic effect, but that also balance that effect against any potential risk, such as a significant change to cell biology. For example, a very high VCN may stress the cell, while a very low VCN may not be enough to provide a benefit.
  • VCN is made possible by balancing the ratio between the lentiviral vector particles and the target cells during the transduction process prior to reinfusion of the genetically modified stem cells.

HSC gene therapy has been studied for almost 20 years, though most studies are still underway to evaluate safety and efficacy required for regulatory approval.20,21 The potential therapeutic benefits of HSC gene therapy are designed to be long-term. Participation in gene therapy studies does have associated risks, some serious. Consult your doctor for more information.

As of May 2022, HSC gene therapy has been approved in Europe broadly for the treatment of transfusion-dependent β-thalassemia (TDT) (conditionally approved),22 metachromatic leukodystrophy (MLD),23 and early cerebral adrenoleukodystrophy (CALD),24 although the manufacturer of the TDT and CALD gene therapies has decided to withdraw both therapies from the European market.1 For specific information related to the use of these gene therapies, please refer to their product labels.25-27 

Research on HSC gene therapy is underway for many genetic conditions including sickle cell disease, adenosine deaminase deficiency with severe combined immunodeficiency (ADA-SCID), Wiskott-Aldrich syndrome, and lysosomal disorders14,28 but it has not been proven safe and effective for use in these conditions, nor approved by any regulatory authority, and no HSC gene therapy using lentiviral vectors has been approved by the U.S. Food and Drug Administration (FDA).

Additional Interesting Fact

The first clinical trial involving a lentiviral vector was reported in 2003, in patients with HIV/AIDs.29

Key Learnings

Hematopoietic stem cell (HSC) gene therapy can use a lentiviral vector to introduce therapeutic genetic material into a person’s own HSPCs outside of the person’s body. The modified stem cells are then infused back into the person, where they are expected to develop and produce mature cells that carry the therapeutic gene and produce the functional protein.

In a process known as “cross-correction,” the genetically modified mature cells may secrete the functional protein, which can be taken up by other cells, including those in different organs, where it provides a therapeutic benefit.

HSC gene therapy may involve the genetic modification of HSPCs or white blood cells, such as T-cells or natural killer cells.

HSPCs can differentiate into many cell types, allowing them to enter hard-to-reach tissues including muscle, bone, and the brain. Additionally, under certain circumstances, HSC gene therapy may enable expression of therapeutic genes at higher levels than those normally present in HSPCs and their daughter cells.

Prior to reinfusion, the manufacturer determines the vector copy number (VCN), which is the average number of copies of an integrated therapeutic gene per cell. It is important to have VCN values that support a potential therapeutic effect, but that also balance that effect against any potential risk, such as a significant change to cell biology. For example, a very high VCN may stress the cell, while a very low VCN may not be enough to provide a benefit.

Continue learning about HSC gene therapy in the next section

Section 2

How is Hematopoietic Stem Cell Gene Therapy Made?

To understand the process by which lentiviral vectors used in hematopoietic stem cell (HSC) gene therapy are manufactured.

Next Section ››

References

  1. Milone MC, O’Doherty U. Clinical use of lentiviral vectors. Leukemia. 2018;32:1529. [PubMed]
  2. What Is Gene Therapy? How Does It Work? U.S. Food & Drug Administration; 2017. https://www.fda.gov/ForConsumers/ConsumerUpdates/ucm589197.htm. Accessed December 22, 2021.
  3. DNA is constantly changing through the process of mutation. In: Essentials of Genetics, Unit 2.5; A Brief History of Genetics: Defining Experiments in Genetics, Unit 6.5. Nature Education; 2014. https://www.nature.com/scitable/topicpage/dna-is-constantly-changing-through-the-process-6524898/. Accessed December 22, 2021.
  4. Bulcha JT, Wang Y, Ma H, Tai PWL, Gao G. Viral vector platforms within the gene therapy landscape. Signal Transduct Target Ther. 2021;6(1):53. [PubMed]
  5. How does gene therapy work? Help Me Understand Genetics: Gene Therapy and Other Medical Advances. MedlinePlus Genetics. U.S. National Library of Medicine, National Institutes of Health, U.S. Department of Health and Human Services; 2021. https://medlineplus.gov/genetics/understanding/therapy/procedures/. Accessed December 22, 2021.
  6. Goodman MA, Malik P. The potential of gene therapy approaches for the treatment of hemoglobinopathies: achievements and challenges. Ther Adv Hematol. 2016;7(5):302-315. [PubMed]
  7. Morgan RA, Gray D, Lomova A, Kohn DB. Hematopoietic stem cell gene therapy: progress and lessons- learned. Cell Stem Cell. 2017;21(5):574-590. [PubMed]
  8. Albinger N, Hartmann J, Ullrich E. Current status and perspective of CAR-T and CAR-NK cell therapy trials in Germany. Gene Ther. 2021;28(9):513-527. doi: 10.1038/s41434-021-00246-w. [PubMed]
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  10. Gene and cell therapy FAQ’s. American Society of Gene and Cell Therapy; 2021. https://asgct.org/education/more-resources/gene-and-cell-therapy-faqs. Accessed December 22, 2021.
  11. Phillips MI, Tang YL. Genetic modification of stem cells for transplantation. Adv Drug Deliv Rev. 2008;60(2):160-172. [PubMed]
  12. Biffi A. Gene therapy for lysosomal storage disorders: a good start. Hum Mol Genet. 2016;25(R1):R65-R75. [PubMed]
  13. Biasco L, Pellin D, Scala S, et al. In vivo tracking of human hematopoiesis reveals patterns of clonal dynamics during early and steady-state reconstitution phases. Cell Stem Cell. 2016;19:107-119. [PubMed]
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  20. Cartier N, Hacein-Bey-Abina S, Bartholomae CC, et al. Hematopoietic stem cell gene therapy with a lentiviral vector in X-linked adrenoleukodystrophy. Science. 2009;326:818-823. [PubMed]
  21. Goswami R, Subramanian G, Silayeva L, et al. Gene therapy leaves a vicious cycle. Front Oncol. 2019;24:9:297. [PubMed]
  22. Harrison C. First gene therapy for β-thalassemia approved. Nature Biotechnol. 2019;37:1102-1103. [PubMed]
  23. Libmeldy (autologous CD34+ cells encoding ARSA gene). European Medicines Agency; 2021. https://www.ema.europa.eu/en/medicines/human/EPAR/libmeldy. Accessed December 22, 2021.
  24. Skysona (elivaldogene autotemcel ). European Medicines Agency; 2021. https://www.ema.europa.eu/en/medicines/human/EPAR/skysona. Accessed December 22, 2021
  25. Zynteglo product information. European Medicines Agency; 2019. https://www.ema.europa.eu/documents/product-information/zynteglo-epar-product-information_en.pdf. Accessed December 22, 2021.
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