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  • Updated 02.19.2024
  • Released 12.16.1999
  • Expires For CME 02.19.2027

DNA- and RNA-directed gene therapies



Gene-based therapies are therapeutic molecular strategies aiming to alter the expression of a gene or its transcript involved in disease pathogenesis. They encompass a spectrum of approaches that may deliver exogenous DNA or RNA or target the cellular DNA or RNA. Therapeutic strategies include gene transfer or replacement, gene editing, RNA knockdown, and pre-mRNA splice switching. United States Food and Drug Administration and European Medicines Agency-approved gene-based therapies are in limited though growing clinical use, with many more in clinical trials for the treatment of both genetic and acquired or sporadic neurologic disorders.

Key points

• Gene-based therapies, by acting at the level of DNA or RNA, modify a disease or even cure it.

• A wide variety of technologies are available in the broad category of gene-based therapies, including gene transfer or replacement, gene editing, gene silencing, RNA knockdown, pre-mRNA splice modulation, and RNA editing.

• Cells can be transduced with genetic material either in vivo or ex vivo by viral or nonviral vectors.

• Synthetic and chemically modified oligonucleotides can be delivered into cells without the need for a vector.

• Genetically engineered cells can be transplanted to deliver therapeutic proteins or replace defective cells in vivo.

• Besides monogenic neurologic disorders, other disorders, such as cerebral vascular disease, traumatic brain and spinal cord injury, neurodegenerative diseases, neuroimmune conditions, and brain tumors, are also amenable to gene-based therapies.

Historical note and terminology

Gene therapy was originally defined as the transfer of defined genetic material to specific target cells of a patient for the ultimate purpose of preventing or altering a particular disease state. Carriers, or delivery vehicles, for therapeutic genetic material are called vectors, which are commonly viruses, but many nonviral vectors are in development and use as well. Cells can be transduced with genetic material either in vivo or ex vivo (in vitro). In addition, chemically modified oligonucleotides can be delivered to cells with or without the need for vectors. Gene therapy encompasses a range of interventions that can be directed at genes (more precisely, DNA) or their transcript products. What distinguishes it as such is the delivery aspect and the actual target, where the therapeutic agent consists, wholly or partially, of genetic material exerting its effects on DNA or RNA molecules within cells. Gene-based therapies or gene therapies can, thus, be broadly classified as follows:

Gene transfer. Somatic line gene transfer for treating genetic and nongenetic disorders. The new gene may be referred to as a “transgene.”

Gene editing. Gene editing involves the precise modification of the DNA (or RNA) sequence through molecular techniques, typically by adding, removing, or replacing specific nucleotides. This is achieved through the use of molecular tools such as CRISPR, zinc finger nucleases, or TALENs that often work in conjunction with endogenous enzymatic machinery in the cell (eg, DNA repair or adenosine deaminases), which in turn enable the targeting and editing of specific genes or transcripts within the cell.

Gene silencing. Therapeutic modification or suppression of gene function through DNA deletion or epigenetic alternations to reduce the expression of a gene or genes.

Transcript targeted therapies. Nucleic acid therapeutics that aim to reduce or modify the pre-mRNA, mRNA, or non-coding RNA molecules (eg, miRNA). Examples include antisense oligonucleotides, splice-switching oligonucleotides, and RNA interference.

Cell therapy. Implantation of genetically engineered cells for the production of therapeutic substances or replacement of defective cells in vivo.

Gene-based therapy approaches

The figure illustrates the main approaches for DNA- and RNA-directed therapies. (Contributed by Dr. Rotem Orbach. Created with

Landmarks in the historical development of gene therapy and its application to neurologic disorders are shown in Table 1. Major developments in gene therapy are taking place in both academic labs and the industrial sector, and the various approaches are reviewed elsewhere (62).

Table 1. Historical Landmarks in the Development of Gene Therapy


Discovery or Development


Identification of the double-stranded structure of the DNA (128).


Possibility of gene therapy is speculated (68).


Early attempts at the use of viral vectors (106).


Discovery of reverse transcriptase: copying of RNA into DNA (12).


Suggestion that transforming viruses be used for therapeutic gene transfer (41).


Calcium phosphate transfection (47).


First use of an oligonucleotide to act as inhibitor of translation (134).


First demonstration that antisense nucleic acids can be used to downregulate gene expression (60).


Identification of dystrophin, the protein product of Duchenne muscular dystrophy gene, which is the basis of gene therapy for this disorder (57).


The first authorized human gene therapy clinical trial for the treatment of Gaucher disease ( identifier number: NCT00001234).


Correction of adenosine deaminase deficiency in T-lymphocytes using retroviral-mediated gene transfer (06).


Use of cationic liposomes for gene transfer in experimental animals (53).


Correction of myopathy in a mouse model of Duchenne muscular dystrophy by germline gene transfer of human dystrophin using a retroviral vector (129).


First clinical trial of herpes simplex virus/thymidine kinase/ganciclovir gene therapy system in glioblastoma (94).


Treatment of amyotrophic lateral sclerosis using genetically engineered microencapsulated cells releasing neurotrophic factors (02).


RNA interference demonstrated: injection of double-stranded RNA shown to silence genes (40).


First death in a clinical trial of gene therapy: adenovirus vector-mediated transfer to replace a defect in the ornithine transcarbamylase gene causing a rare liver disorder (64). This led to a pause in the development of gene therapies due to safety concerns.


Completion of sequencing phase of the human genome project (26). Further developments in next-generation sequencing in the following years had considerable impact on personalized medicine. For neurologic disorders, it has enabled improved diagnostics, identification of gene variants, and development of therapies (133).


Definition of critical components of the CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 system, which later formed the basis for gene editing (44).


Publication of radically new gene editing method that harnessed the CRISPR-Cas9 system, invented by Doudna and Charpentier (65).


UK’s Human Fertilization and Embryo Authority approved use of CRISPR in a human embryo.


EMA approval of first ex vivo stem cell gene therapy in the world: Strimvelis for adenosine deaminase deficiency resulting in severe combined immunodeficiency.


FDA approval of splice-modulating antisense agents: eteplirsen (Exondys 51) for Duchenne muscular dystrophy and nusinersen (Spinraza®) for spinal muscular atrophy.


FDA-approved CAR (chimeric antigen receptor)-T cell CTL019, tisagenlecleucel (Kymriah®), a cell/gene therapy, for B cell acute lymphoid leukemia.


FDA-approved voretigene neparvovec (Luxturna®): first U.S. approval of an AAV vector-delivered gene therapy for treating biallelic RPE65-mediated Leber congenital amaurosis, which causes retinal degeneration and blindness.


FDA approval of patisiran (Onpattro®), a lipid nanoparticle-packaged siRNA, for polyneuropathy of hereditary transthyretin-mediated amyloidosis in adults.


FDA approval of onasemnogene abeparvovec (Zolgensma®), AAV9 delivering the human SMN1 gene to treat pediatric patients (under 2 years) with spinal muscular atrophy.


EMA approval of atidarsagene autotemcel (Libmeldy™), autologous hematopoietic stem cells transduced with a lentiviral vector to treat metachromatic leukodystrophy.


FDA approval of elivaldogene autotemcel (Skysona®) to treat cerebral adrenoleukodystrophy.


FDA approval of delandistrogene moxeparvovec (Elevidys), AAVrh74 delivering the micro-dystrophin gene to treat Duchenne muscular dystrophy.


FDA, UK, and EMA approval of exagamglogene autotemcel (Casgevy™), the first CRISPR–Cas9 gene editing therapy to treat sickle cell disease.

This drug list is not exhaustive. Examples of pertinent approaches and methodologies successfully translated into approved drugs in clinical practice are included.

Approximately 2106 clinical trials of gene therapy conducted worldwide from 1988 to 2020 from 17 clinical trial database providers were reviewed to show the clinical development of gene therapy as well as approval by regulatory authorities and acceptance by payors (04). Shifts in gene therapy clinical trial strategies over the past decade and new fields in which gene therapy has entered into clinical practice highlight its versatility and provide a valuable preview of its future use as an important therapeutic tool (08).

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