Gene Therapy Overview

What is Gene Therapy?

Gene therapy is the process of introducing genetic material into patients to treat disease. Many types of gene therapy are in use and in development for conditions that often do not have other treatments or cures. The goal might be to replace or restore a malfunctioning or missing gene, remove or deactivate expression of a disease-linked gene, or even cause specific cells to be targeted by the patient’s immune system.

If the treatment takes place inside the body—via the direct introduction of a therapeutic gene—then it is termed in vivo. Conversely, ex vivo gene therapy takes place in cells outside of the body before transplanting the modified cells into patients (Figure 1). In such cases, the field of gene therapy overlaps with cell therapy.

Schematic for in vivo gene therapy and 2 types of ex vivo gene therapies

Figure 1. A summary of processes involved in in vivo and ex vivo gene therapies.

Cell therapies involve injecting, grafting, or implanting viable cells into a patient to trigger a medicinal response. When combined with gene therapy technology, cells from the patient (autologous source) or a donor (allogenic source) are extracted and genetically modified, prior to transplantation into the patient.

Both cell and gene therapies (CGTs) can be classed as Advanced Therapy Medicinal Products (ATMPs). ATMP is an EU-specific classification, ensuring that recombinant nucleic acids or cells are regulated as medicines under the ATMPs regulation (EC/1394/2007).

How are Gene Therapies Delivered?

While CGTs may or may not be mutually exclusive, they are both dependent on the use of vectors. Vectors represent genetically engineered carriers, designed to deliver a gene to a very specific location within a cell or genome. Although viruses are the most commonly used vector types in gene therapy, a number of nonviral vectors are emerging (Table 1).

Recombinant adeno-associated viral (AAV) vectors are one of the most widely used platforms in gene therapy due to their stability, efficiency, and relatively low cytotoxicity. Currently, AAV vectors have been used in multiple clinical trials to target lung, liver, eye, brain, and muscle diseases.

Table 1. Examples of viral and nonviral vectors.
Type Vector Advantages Disadvantages
Viral Adeno-associated virus (AAV) vectors
  • Ability to transduce both dividing and nondividing cells
  • Minimally cytotoxic
  • Stable and efficient
  • Small genetic capacity
  • High immunogenic potential
Adenoviral vectors
  • Efficient transduction of most cells
  • Generally high expression
  • High immunogenic potential
Retroviral vectors
  • Long term gene expression in dividing cells
  • Only transduction of dividing cells
  • Potential oncogenicity
  • High random integration risk
Lentiviral vectors
  • Ability to transduce both dividing and nondividing cells
  • Long-term gene expression in dividing cells (Non-integrating lentiviruses have potential short-term gene expression in dividing cells)
  • Effective for ex vivo transgene delivery (e.g., CAR-T cell therapy)
  • Small genetic capacity
  • Potential oncogenicity
Nonviral Nonlipid, polymeric nanoparticles (e.g., polyethylenimine [PEI] polyplexes and dendrimers)
  • Large genetic capacity
  • Easy to manufacture
  • Conjugation of various modifications can increase functionality related to toxicity, immunogenicity, stability, aggregation, and targeting
  • Lower gene-delivery efficiency with decreasing polymer length
  • Potential toxicity with increasing polymer length
  • Some are nonbiodegradable (e.g., PEI)
Lipid nanoparticles
  • Large genetic capacity
  • Use of various lipids can increase functionality related to encapsulation and bioavailability
  • Conjugation of various modifications can increase functionality related to toxicity, immunogenicity, and stability
  • Limited tissue targeting
  • Potential immunogenicity

The 3 Main Challenges with Gene Therapy Technology

  1. Safety

With any therapeutic product, the primary concern is always safety. Research and clinical trials have identified off-target effects, toxicity, and immunogenic responses as potential safety concerns when administering gene therapies (Box 1).

While improvements to genetic modification systems, downstream purification processes, and vectors are addressing these concerns, safety remains an ongoing challenge.

  1. Limited vector diversity

To truly realize the full potential of gene therapy, a diversity of vectors is essential. For example, the ability to deliver large genes is a requirement of many gene therapies. However, the widely used AAV vectors have an optimal carrying capacity of <5 kb, rendering them unsuitable for the treatment of many diseases.

Researchers are devising innovative strategies to overcome this limitation in both AAV systems and other novel vectors; however, their optimization and clinical translation will continue to be an opportunity for development.

In addition, different viral serotypes preferentially transduce different cells and tissues, known as tissue tropism. Understanding what determines tropism and how it could be optimized for specific tissues could reduce side effects and allow treatment of a wider range of diseases.

  1. Manufacturing standardization and scalability

While there are a variety of established methods to manufacture many gene therapy vectors in small quantities, creating a clinical-grade product on a larger scale is a complex process. Viral vectors, such as AAV, are produced in live cells in culture, presenting unique challenges related to improving vector yield and purification:

    1. Sufficient viral concentrations are difficult to achieve and may be further diluted by viral capsids that are either empty or mispackaged (i.e., they do not contain the correct sequences, which could include innocuous or hazardous host cell DNA).
    2. Standardized, scalable purification methods for viruses must be developed and optimized. Viral preps, especially if concentration methods are used, may contain levels of residual manufacturing or host cell components (e.g., host cell proteins, debris, media components, exogenous DNA) that may be hazardous to patients. Incorporating reliable quality control methods and standardized procedures are necessary to ensure good manufacturing procedure.
Box 1
Safety Concerns in the Gene Therapy Process

Immunogenic responses: When gene therapy is administered at high doses, the immune system can go into overdrive, leading to potentially dangerous or even deadly consequences.

Off-target effects: The consequences of nonspecific and unintended genetic modifications could be lethal genetic mutations that cause loss of gene function and cancer. The long-term effects of off-target genome editing remains unknown and consequently, it remains a key challenge.

Toxicity: Several gene therapy trials—such as those related to the overexpression of an introduced gene—have raised toxicity issues with high doses.

Examples of Approved Gene Therapies

Gene therapy has the potential to treat a wide range of diseases, such as cystic fibrosis, heart disease, diabetes, hemophilia, autoimmune diseases, AIDS, as well as many types of cancer and diseases involving neural or retinal degeneration. The number of clinical trials for gene therapies is increasing rapidly [1]. Here are 3 examples of approved treatments:

  1. AAV-based gene therapy for SMA

Spinal muscular atrophy (SMA) is a genetic condition caused by a mutation in the SMN1 gene. Loss of motor neurons leads to progressive muscle wasting and is one of the leading causes of infant mortality. By 2019, the first AAV gene therapy for SMA (Zolgensma®, Novartis AG) was approved and can now be used to treat sufferers in a single dose that delivers a functional copy of human SMN.

  1. Attenuated HSV-1-based therapy for late-stage melanoma

With over 130,000 new cases of melanoma diagnosed globally each year [2], the approval of Imlygic® (Amgen, Inc.) in 2015 has the capacity to treat a large number of people. Imlygic utilizes attenuated HSV-1 that is inserted directly into tumors and ultimately, produces a protein that stimulates an immune response to kill cancer cells.

  1. Autologous cell and gene therapy for beta thalassemia

Zyteglo (Bluebird Bio) treats a blood disorder called beta thalassemia that can cause life-threatening anemia. By introducing healthy copies of the target gene into stem cells taken from the patient, and reintroducing them into the bloodstream, healthy red blood cells that can manufacture hemoglobin are produced.

 

For more information about Teknova buffers, media, and reagents, visit our Resources for Gene Therapy.

References

  1. Lapteva L, Purohit-Sheth T, et al. (2020) Clinical development of gene therapies: The first three decades and counting. Mol Ther Methods Clin Dev 19:387–397. doi: 10.1016/j.omtm.2020.10.004
  2. World Health Organization (2017) Radiation: Ultraviolet (UV) radiation and skin cancer. [Accessed 7 May, 2021]