What are the two approaches to gene therapy?

Gene Therapy Approaches: Replacement of Defective Genes and Selective Inhibition of Deleterious Genes

The two approaches to gene therapy are replacement of defective genes and selective inhibition of the expression of deleterious genes, making the correct answer "c. Gene therapy." 1

Understanding Gene Therapy Approaches

Gene therapy is defined by the FDA as a therapeutic process that involves:

1. Gene Replacement/Addition: Replacing a disease-causing gene with a healthy copy or adding functional genes to human cells

   • Typically achieved via vectors like retroviruses and adeno-associated viruses
   • Involves transferring nontoxic functional genes into human somatic or germline cells
   • These vectors can undergo reverse transcription and DNA integration but lack replication potential

2. Selective Inhibition/Gene Intervention: Inactivating or inhibiting the expression of disease-causing genes

   • Often achieved through RNA interference (RNAi)
   • Works via post-transcriptional gene silencing
   • Utilizes Watson-Crick base pairing interactions for targeted silencing
   • First demonstrated in mammalian cells in 2001 1

Additional Gene Therapy Mechanisms

Beyond these two primary approaches, recent advancements have expanded gene therapy to include:

• Gene Editing: Using nuclease-induced double-stranded breaks on target DNA to create variable length insertions or deletions

  • Technologies include zinc finger nucleases, meganucleases, and CRISPR-Cas9 systems 1, 2
  • Allows for precise manipulation of the human genome sequence 2

• Gene Modulation: Using artificial transcription effectors based on TALE and CRISPR technologies to regulate specific gene expression 3

Clinical Applications

Gene therapy has been applied to various conditions:

• FDA-approved therapies exist for Spinal Muscular Atrophy, cerebral adrenoleukodystrophy, β-Thalassemia, hemophilia A/B, retinal dystrophy, and Duchenne Muscular Dystrophy 4

• For Huntington's disease, researchers are working on two fronts:

  1. Correcting or deleting the mutated HTT gene at the DNA level through gene editing
  2. Suppressing the expression of mutated mHTT at the protein level 1

• Cancer represents over 60% of all gene therapy clinical trials conducted so far 3

Delivery Methods

The effectiveness of gene therapy depends on successful gene delivery systems:

• Viral Vectors: Extensively explored in clinical trials but face challenges including immunogenicity, cytotoxicity, and potential pathogenicity 1, 5

• Non-viral Vectors: Emerging alternatives including:

  • Lipid nanoparticles (LNPs)
  • Polymeric nanoparticles
  • Inorganic nanoparticles
  • Cell membrane-based nanoparticles 1, 5

• Physical, Chemical, and Biological Methods: Various approaches for transfection including electroporation, microinjection, calcium-phosphate, liposomes, and more 5

Current Limitations and Challenges

Despite promising advances, gene therapy faces several challenges:

• Immune responses to vectors and therapeutic genes
• Need for immune suppressants that may lead to secondary infections
• Unknown outcomes of gene overexpression
• Difficulties in achieving tissue-specific corrections 4
• Limited duration of therapeutic gene expression with many current vectors 6
• Safety concerns including potential oncogenicity of some vectors 6

Gene therapy represents a revolutionary approach to treating genetic and acquired diseases through either replacing defective genes or inhibiting the expression of harmful ones, with ongoing research continually expanding its therapeutic potential.