The History and Evolution of Gene Therapy

The structure of DNADNA (deoxyribonucleic acid)the hereditary material in humans and almost all other organisms

See glossary for more terms > was characterized by a double helix³

The genetic code was discovered by deciphering the three bases of DNA in 1 of the 20 amino acids. The 19 remaining amino acids were deciphered soon after, paving the way for new technologies⁴

Researchers discovered a genetic engineering technique that allows genetic materialGenetic materialrefers to DNA or RNA that play a fundamental role in creating proteins critical to a cell’s structure or its function in the body

See glossary for more terms > from 1 organism to be artificially introduced, replicated, and expressed in another⁵

• DNA was spliced into a plasmid carrier (a DNA structure that can replicate without a chromosome), which then inserted genetic material into an E. coli bacterium. When the bacterium reproduced, it replicated the foreign DNA and maintained the genetic material from the original organism

One of the first times gene therapy was tested in people was done without permission from the university who provided funding for the National Institutes of Health (NIH)(US) National Institutes of Healtha federal agency in the US that conducts biomedical research in its own laboratories; supports the research of non-federal scientists in universities, medical schools, hospitals, and research institutions throughout the country and abroad; helps in the training of research investigators; and fosters communication of medical information

See glossary for more terms >. The researcher lost multiple grants and NIH warned others that human experimentation would not be tolerated⁶

• Martin Cline attempted gene therapy abroad without permission in 2 patients with beta-thalassemia, a rare inherited blood disorder, by transferring the beta-globin gene into their cells. This did not work because the cells did not replicate 

The first gene therapy clinical trial was conducted using new viral vectorViral vectora way to deliver genetic material to a cell using the blueprint of a virus as a guide; it may be used to carry genes and change mutated cells to healthy ones

See glossary for more terms > technology⁷

• 2 patients with severe combined immunodeficiency (SCID) received treatment using novel gamma retrovirusretrovirusa virus that uses RNA as its genetic material; when a retrovirus infects a host cell, the RNA is converted into DNA, which then incorporates into the genome of the host cell
  
  See glossary for more terms > vector technology. The results were mixed, with 1 modest response and 1 limited response

The first engineered nucleaseNucleasean enzyme that is capable of cleaving the bond between two bases in a nucleic acid at a specific sequence

See glossary for more terms > technology
(zinc finger nucleaseZinc finger nucleaseartificial restriction enzymes generated by fusing a zinc finger DNA-binding domain to a DNA-cleavage domain; ZFNs are used in gene editing applications

See glossary for more terms >) was studied, laying the groundwork for exploring the use of zinc finger nucleases for gene editing as a potential for gene therapy⁸

The first generation of lentiviral vectorsLentiviral vectora way to deliver genetic material to a cell using the blueprint of a lentivirus as a guide

See glossary for more terms > (LVVs) was created using 3 different plasmids (a DNA structure that can replicate without a chromosome) containing a large deactivated portion of the HIV genome, making it unlikely for HIV to replicate in human cells⁹

• Second and third generation LVVs followed a couple years later containing further reduction of the original HIV genome (less than two-thirds)

The FDA and NIH created new programs—the Gene Therapy Clinical Trial Monitoring Plan and the Gene Transfer Safety Symposia—in an effort to ensure the safety and transparency of gene therapy clinical trials. This was after the death of an 18-year-old patient during a clinical trial using an adenovirus vector. Additional patient protection caused delays in research at the time, but has led to greater emphasis on safety and data sharing in gene therapy research efforts since¹⁰ 

• Jesse Gelsinger, an 18-year-old boy with a relatively mild form of ornithine transcarbamylase (OTC) deficiency, died while participating in an adenoviral gene therapy trial due to a severe immune reaction to the vector. Investigators later found that several other patients had experienced serious side effects after being injected, but Jesse was never informed of them. This caused the FDA and NIH to enhance patient protection through 2 new programs, the Gene Therapy Clinical Trial Monitoring Plan and the Gene Transfer Safety Symposia.

A clinical trial of gene therapy using a gamma retrovirus raised concern about the safety of gene insertion^11,12

• Ten patients with X-linked severe combined immunodeficiency (SCID) were treated with gene therapy. While 9 out of 10 were treated, 4 of the 9 patients developed leukemia. This study demonstrated the need for improved viral vectors in gene therapy^11,12

The FDA approved the first clinical trial (in humans) using an LVV to test the safety and tolerability of a single infusion in patients with HIV. The phase 1 trial was successfully completed, opening the door for more LVV research including a phase 2 trial¹³ 

The National Medical Products Administration, formerly the China Food and Drug Administration, approved the world’s first commercially available gene therapy to treat squamous cell carcinoma, a form of skin cancer^14,15

In a clinical trial, a genetic eye disease was treated using an adeno-associated virus (AAV)Adeno-associated virusa single-stranded DNA virus that depends on adenoviruses for replication

See glossary for more terms > vector. Eight years later, this pivotal trial led to the FDA approval of the first gene therapy in the United States^{16, 17}

The first engineered TAL-effector nucleasesTranscription activator-like effector-based nucleases (TALEN)arrays of single-protein modules, or nucleases, where each module recognizes a single DNA base pair. These nucleases cleave DNA at a defined distance from TALEN recognition sequences. These nucleases are derived from transcription activator-like effectors

See glossary for more terms > were described with the ability to cause targeted mutagenesis¹⁸

A self-inactivating LVVSelf-inactivating lentiviral vectora vector that, through a process of deleting and manipulating lentiviral components, is no longer able to replicate

See glossary for more terms > was first used in clinical trials of gene additionGene additiona technique that adds functioning genetic material to do the work of a faulty gene

See glossary for more terms > therapy in hemoglobinopathies¹⁹

The European Medicines Agency (EMA)European Medicines Agency (EMA)the regulatory agency responsible for scientific evaluation of medicine for potential use in the European Union

See glossary for more terms > approved the first AAV-based gene addition therapy for the treatment of lipoprotein lipase deficiency (LPLD)²⁰

• This gene therapy was later removed from the market in 2017 due to its limited use²¹

Scientists developed a gene-editing technique called CRISPR/Cas9Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9a gene editing technique that uses a specially designed RNA molecule to guide a Cas9 enzyme to a specific sequence of DNA so it can change or edit that site sequence

See glossary for more terms > that can modify specific DNA sequences²²

The EMA approved the first gamma retrovirus-based gene addition therapy to treat adenosine deaminase severe combined immunodeficiency (ADA-SCID). This therapy contains CD34+ cells transduced with retroviral vector, which encodes for the human ADA cDNA sequence^23,24

The FDA approved the first in vivoIn vivoinside the body

See glossary for more terms > gene addition therapy to treat patients with a rare form of inherited blindness called biallelic RPE65 mutation-associated retinal dystrophy¹⁷⁶

The FDA approved a CAR T-cellCAR T-cell therapya treatment where a person's T-cells (a part of the immune system) are genetically modified to recognize and attack cancer cells

See glossary for more terms > therapy for the treatment of patients with relapsed/refractory large B cell lymphoma (R/R DLBCL).²⁵

• Approved by the EMA in 2019²⁶

The first clinical trial using CRISPR/Cas9Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9a gene editing technique that uses a specially designed RNA molecule to guide a Cas9 enzyme to a specific sequence of DNA so it can change or edit that site sequence

See glossary for more terms > was initiated. This study is investigating the use of CRISPR/Cas9 for gene disruption in beta hemoglobinopothies²⁷

The FDA approved an AAVAdeno-associated virusa single-stranded DNA virus that depends on adenoviruses for replication

See glossary for more terms >-based in vivo gene addition therapy for spinal muscular atrophy²⁸

• Conditionally approved by the EMA in 2020²⁹

The EMA approved an LVV-based ex vivo gene addition therapy for a genetic disease called transfusion-dependent beta-thalassemia (TDT)³⁰

FDA finalizes 6 gene therapy guidelines including draft guidelines for the research and clinical development of gene therapies³¹