Gene Therapy - Essential Concepts

Gene Therapy Overview

  • Gene therapy involves introducing genetic material (a therapeutic gene/transgene) into a patient to treat/correct a gene defect.
  • It alters the expression of specific genes to treat inherited or acquired diseases.
  • Germ-line therapy (altering sperm or ova) is banned, while somatic cell therapy (altering single organ or tissue) is the current approach.

Gene Therapy Trials (2023)

  • The majority of gene therapy clinical trials occur in the United States, followed by China and the United Kingdom.
  • Cancer diseases are the most common indication addressed by gene therapy clinical trials, followed by monogenic diseases.

Monogenic vs Multifactorial Disorders

  • Monogenic disorders are caused by mutations within a single gene, leading to a direct loss of protein function (e.g., cystic fibrosis, hemophilia, sickle cell disease).
  • Treatment involves transfer of the correct gene and its correct expression.
  • Multifactorial disorders are caused by multiple genetic and environmental factors (e.g., coronary heart disease, cancer), with a higher incidence than monogenic diseases.

Gene Delivery Systems - Vectors

  • Naked DNA: direct introduction of plasmid DNA into target cells
  • Liposomes: spherical vesicles that encapsulate and deliver genetic material
  • Viruses: genetically modified viruses that deliver therapeutic genes
  • A therapeutic gene/transgene is placed within an expression cassette with a promoter and packaged into a vector for transportation into the target cell either ex vivo or in vivo.

Advantages & Disadvantages of Gene Delivery Systems

  • Naked DNA: easiest method but has limited cell types that take up DNA and is susceptible to degradation in vivo.
  • Liposomes: can deliver large amounts of DNA, biocompatible, and can target specific cells/tissues, but can be rapidly cleared and may cause inflammatory toxicity.
  • Viruses: most efficient system and can produce permanent correction, but size of gene carried can be limited, can elicit immune response, and has potential for random insertion.

In Vivo vs Ex Vivo Gene Therapy

  • In vivo gene therapy: direct transfer of genetic material into a patient.
  • Ex vivo gene therapy: stem cells from a patient are harvested, genetically modified, and transplanted back into the patient.

Monogenic Disorders: Cystic Fibrosis (CF)

  • Caused by inheriting two copies of a mutated CFTR gene.
  • Introducing a normal CFTR gene into lung cells may eliminate life-threatening symptoms.

Monogenic Disorders: Sickle Cell Disease (SCD)

  • Correcting the gene in the bone marrow may eliminate symptoms.
  • Lyfgenia, an SCD gene therapy, modifies hematopoietic stem cells to produce functional hemoglobin; FDA-approved in December 2023.

Single Gene Therapy

  • Best candidates are when a small amount of gene product will have a big effect and an excess will not be deleterious
  • Involves targeted modification or supplementation of one specific gene.
  • A Cautionary Tale: Jesse Gelsinger's case in 1999 highlighted risks; he died due to an immune response from a high dose of adenovirus vector in an OTC deficiency trial.

Single Gene Therapy – Severe Combined Immunodeficiency (SCID)

  • ADA deficiency: lack of adenosine deaminase leads to loss of T-lymphocytes.
  • X-linked SCID: an immunodeficiency disorder in which the body produces very few T and NK cells
  • Retroviral vectors can cause insertional oncogenesis.

Factors That Still Need to Be Considered

  • Problems with viral vectors:
    Toxicity, immune and inflammatory responses, and gene control and targeting issues (off-target effects).
  • Regulatory:
    Safety and efficacy must be ensured.
  • Economic:
    Financial incentives to compensate for the risk and costs of developing drugs.
  • Socio-political:
    Social implications of modifying the genome.