Gene therapy for hemoglobinopathies and other diseases

1

How does gene therapy work?

Gene therapy is a therapeutic approach that aims to add, delete, or correct genetic material to treat a disease. Modifying genetic material changes how a protein, or group of proteins, is produced by the cell. In other words, gene therapy aims to give the cell a new set of instructions to change either the amount or type of protein that is produced. By changing the instructions for the production of protein, gene therapy treats the disease at the genetic level.

2

What is the difference between gene addition and gene editing?

Overall, there are two types of gene therapy being studied: gene addition and gene editing. Gene addition treats diseases at the genetic level by adding genetic material to a person’s cells to compensate for a missing or faulty gene. Gene editing treats diseases at the genetic level by directly modifying a patient’s DNA through a number of different techniques. These techniques are gene inactivation/disruption (also called gene silencing, knockdown, or knockout) and gene correction/insertion.

3

How does gene addition work?

Gene addition therapy is a common gene technique being explored for single-gene diseases—disorders where a mutation occurs in one or both sets of a patient’s genes. This gene therapy technique usually involves the insertion of functional (or healthy) copies of a gene (otherwise known as a transgene) into a person’s cells by way of a viral vector. Vectors deliver the functional gene to the patient’s cells, either in vivo or ex vivo . Once inside the cell, the transgene provides the cell with instructions that lead to the production of functional proteins. With gene addition therapy, the mutated gene does not need to be replaced or removed. This provides the cell with the instructions that lead to the production of functional genes, while not needing to replace or remove the mutated gene.

4

What viral vectors have been used in gene therapy?

In early trials, a γ-retroviral vector was used to transduce hematopoietic stem cells (HSCs); however, this vector proved to be inadequate for the use of hemoglobinopathies. This was because of its inability to insert target genetic material into nondividing cells such as quiescent HSCs, as well as space limitations within the vector, the relatively large size of the β-globin gene, and that insertion requires promoters. Lentiviral vectors based on the human immunodeficiency virus type 1 (HIV-1) have led to significant advancements in safety and efficacy in genetic delivery into cellular genomes. These vectors have a greater capacity for larger gene sequences and beneficial modifications such as self-inactivation that decrease the risks of insertional oncogenesis. In vivo gene therapy is in the early stages of development using adenoviral vectors, which can be directly injected into the patient’s blood.

5

How does gene editing work?

Gene editing involves the creation of targeted breaks in the DNA, with or without instructions to repair them, through a number of different techniques. There are two primary techniques in gene editing: disruption/inactivation and correction/insertion.

• “Disrupting” or “inactivating” genetic material that is responsible for the genetic disease: This can be achieved by turning off genes that cause disease or disrupting a separate gene that will compensate for the disease-causing gene

• “Correcting” genetic material by creating a break in the gene and providing a corrective template or “inserting” new genetic material for the cell to use to repair the mutated gene.