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Gene therapy is the use of DNA as a drug to treat disease by delivering therapeutic DNA into a patient's cells. The most common form of gene therapy involves using DNA that encodes a functional, therapeutic gene to replace a mutated gene. Other forms involve directly correcting a mutation, or using DNA that encodes a therapeutic protein drug (rather than a natural human gene) to provide treatment. In gene therapy, DNA that encodes a therapeutic protein is packaged within a "vector", which is used to get the DNA inside cells within the body. Once inside, the DNA becomes expressed by the cell machinery, resulting in the production of therapeutic protein, which in turn treats the patient's disease.
Gene therapy was first conceptualized in 1972, with the authors urging caution before commencing gene therapy studies in humans. The first FDA-approved gene therapy experiment in the United States occurred in 1990, when Ashanti DeSilva was treated for ADA-SCID. By January 2014, about 2,000 clinical trials had been conducted or had been approved using a number of techniques for gene therapy.
Although early clinical failures led many to dismiss gene therapy as over-hyped, clinical successes since 2006 have bolstered new optimism in the promise of gene therapy. These include successful treatment of patients with the retinal disease Leber's congenital amaurosis, X-linked SCID, ADA-SCID, adrenoleukodystrophy, chronic lymphocytic leukemia (CLL), acute lymphocytic leukemia (ALL), multiple myeloma, haemophilia and Parkinson's disease. These clinical successes have led to a renewed interest in gene therapy, with several articles in scientific and popular publications calling for continued investment in the field and between 2013 and April 2014, US companies invested over $600 million in gene therapy.
Following early advances in genetic engineering of bacteria, cells, and small animals, scientists started considering how this technique could be applied to medicine; could human chromosomes be modified to treat disease? Two main approaches have been considered - adding a gene to replace a gene that wasn't working properly, or disrupting genes that were not working properly. Scientists focused on diseases caused by single-gene defects, such as cystic fibrosis, haemophilia, muscular dystrophy, thalassemia, and sickle cell anemia. As of 2014, gene therapy was still generally an experimental technique, although in 2012 Glybera became the first gene therapy treatment to be approved for clinical use in either Europe or the United States after its endorsement by the European Commission, as a treatment for a disease caused by a defect in a single gene, lipoprotein lipase.
In gene therapy, DNA must be administered to the patient, get to the cells that need repair, enter the cell, and express a protein. Generally the DNA is incorporated into an engineered virus that serves as a vector, to get the DNA through the bloodstream, into cells, and incorporated into a chromosome. However, so-called naked DNA approaches have also been explored, especially in the context of vaccine development.
Generally, efforts have focused on administering a gene that causes a protein to be expressed, that the patient directly needs. However, with development of our understanding of the function of nucleases such as zinc finger nucleases in humans, efforts have begun to incorporate genes encoding nucleases into chromosomes; the expressed nucleases then "edit" the chromosome, disrupting genes causing disease. As of 2014 these approaches have been limited to taking cells from patients, delivering the nuclease gene to the cells, and then administering the transformed cells to patients.
There are other technologies in which nucleic acids are being developed as drugs, such as antisense, small interfering RNA, and others. To the extent that these technologies do not seek to alter the chromosome, but instead are intended to directly interact with other biomolecules such as RNA, they are generally not considered "gene therapy" per se.
Gene therapy may be classified into the two following types, only one of which has been used in humans:
As the name suggests, in somatic gene therapy, the therapeutic genes are transferred into the somatic cells (non sex-cells), or body, of a patient. Any modifications and effects will be restricted to the individual patient only, and will not be inherited by the patient's offspring or later generations. Somatic gene therapy represents the mainstream line of current basic and clinical research, where the therapeutic DNA transgene (either integrated in the genome or as an external episome or plasmid) is used to treat a disease in an individual.
Several somatic cell gene transfer experiments are currently in clinical trials with varied success. Over 600 clinical trials utilizing somatic cell therapy are underway in the United States. Most of these trials focus on treating severe genetic disorders, including immunodeficiencies, haemophilia, thalassaemia, and cystic fibrosis. These disorders are good candidates for somatic cell therapy because they are caused by single gene defects. While somatic cell therapy is promising for treatment, a complete correction of a genetic disorder or the replacement of multiple genes in somatic cells is not yet possible. Only a few of the many clinical trials are in the advanced stages.
In germline gene therapy, germ cells (sperm or eggs) are modified by the introduction of functional genes, which are integrated into their genomes. Germ cells will combine to form a zygote which will divide to produce all the other cells in an organism and therefore if a germ cell is genetically modified then all the cells in the organism will contain the modified gene. This would allow the therapy to be heritable and passed on to later generations. Although this should, in theory, be highly effective in counteracting genetic disorders and hereditary diseases, some jurisdictions, including Australia, Canada, Germany, Israel, Switzerland, and the Netherlands prohibit this for application in human beings, at least for the present, for technical and ethical reasons, including insufficient knowledge about possible risks to future generations and higher risk than somatic gene therapy (e.g. using non-integrative vectors). The USA has no federal legislation specifically addressing human germ-line or somatic genetic modification (beyond the FDA testing regulations for therapies in general).
Gene therapy utilizes the delivery of DNA into cells, which can be accomplished by a number of methods. The two major classes of methods are those that use recombinant viruses (sometimes called biological nanoparticles or viral vectors) and those that use naked DNA or DNA complexes (non-viral methods).
All viruses bind to their hosts and introduce their genetic material into the host cell as part of their replication cycle. Therefore this has been recognized as a plausible strategy for gene therapy, by removing the viral DNA and using the virus as a vehicle to deliver the therapeutic DNA.
Non-viral methods can present certain advantages over viral methods, such as large scale production and low host immunogenicity. Previously, low levels of transfection and expression of the gene held non-viral methods at a disadvantage; however, recent advances in vector technology have yielded molecules and techniques that approach the transfection efficiencies of viruses.
There are several methods for non-viral gene therapy, including the injection of naked DNA, electroporation, the gene gun, sonoporation, magnetofection, and the use of oligonucleotides, lipoplexes, dendrimers, and inorganic nanoparticles.
Some of the unsolved problems with the technology underlying gene therapy include:
Three patients' deaths have been reported in gene therapy trials, putting the field under close scrutiny. The first was that of Jesse Gelsinger in 1999, which represented a major setback in the field. One X-SCID patient died of leukemia following gene therapy treatment in 2003. In 2007, a rheumatoid arthritis patient died from an infection in a gene therapy trial; a subsequent investigation concluded that the death was not related to her gene therapy treatment.
In 1972 Friedmann and Roblin authored a paper in Science titled "Gene therapy for human genetic disease?" Rogers (1970) was cited for proposing that exogenous good DNA be used to replace the defective DNA in those who suffer from genetic defects.
The first approved gene therapy case in the United States took place on 14 September 1990, at the National Institute of Health, under the direction of Professor William French Anderson. It was performed on a four year old girl named Ashanti DeSilva. It was a treatment for a genetic defect that left her with ADA-SCID, a severe immune system deficiency. The effects were only temporary, but successful.
New gene therapy approach repairs errors in messenger RNA derived from defective genes. This technique has the potential to treat the blood disorder thalassaemia, cystic fibrosis, and some cancers. Researchers at Case Western Reserve University and Copernicus Therapeutics are able to create tiny liposomes 25 nanometers across that can carry therapeutic DNA through pores in the nuclear membrane.
Sickle-cell disease is successfully treated in mice. The mice – which have essentially the same defect that causes sickle cell disease in humans – through the use a viral vector, were made to express the production of fetal hemoglobin (HbF), which normally ceases to be produced by an individual shortly after birth. In humans, the use of hydroxyurea to stimulate the production of HbF has long been shown to temporarily alleviate the symptoms of sickle cell disease. The researchers demonstrated this method of gene therapy to be a more permanent means to increase the production of the therapeutic HbF.
In 1992 Doctor Claudio Bordignon working at the Vita-Salute San Raffaele University, Milan, Italy performed the first procedure of gene therapy using hematopoietic stem cells as vectors to deliver genes intended to correct hereditary diseases. In 2002 this work led to the publication of the first successful gene therapy treatment for adenosine deaminase-deficiency (SCID). The success of a multi-center trial for treating children with SCID (severe combined immune deficiency or "bubble boy" disease) held from 2000 and 2002 was questioned when two of the ten children treated at the trial's Paris center developed a leukemia-like condition. Clinical trials were halted temporarily in 2002, but resumed after regulatory review of the protocol in the United States, the United Kingdom, France, Italy, and Germany.
In 1993 Andrew Gobea was born with severe combined immunodeficiency (SCID). Genetic screening before birth showed that he had SCID. Blood was removed from Andrew's placenta and umbilical cord immediately after birth, containing stem cells. The allele that codes for ADA was obtained and was inserted into a retrovirus. Retroviruses and stem cells were mixed, after which the viruses entered and inserted the gene into the stem cells' chromosomes. Stem cells containing the working ADA gene were injected into Andrew's blood system via a vein. Injections of the ADA enzyme were also given weekly. For four years T cells (white blood cells), produced by stem cells, made ADA enzymes using the ADA gene. After four years more treatment was needed.
The 1999 death of Jesse Gelsinger in a gene therapy clinical trial resulted in a significant setback to gene therapy research in the United States. As a result, the U.S. FDA suspended several clinical trials pending the re-evaluation of ethical and procedural practices in the field.
In 2003 a University of California, Los Angeles research team inserted genes into the brain using liposomes coated in a polymer called polyethylene glycol. The transfer of genes into the brain is a significant achievement because viral vectors are too big to get across the blood–brain barrier. This method has potential for treating Parkinson's disease.
RNA interference or gene silencing may be a new way to treat Huntington's disease. Short pieces of double-stranded RNA (short, interfering RNAs or siRNAs) are used by cells to degrade RNA of a particular sequence. If a siRNA is designed to match the RNA copied from a faulty gene, then the abnormal protein product of that gene will not be produced.
In March 2006 an international group of scientists announced the successful use of gene therapy to treat two adult patients for X-linked chronic granulomatous disease, a disease which affects myeloid cells and which gives a defective immune system. The study, published in Nature Medicine, is believed to be the first to show that gene therapy can cure diseases of the myeloid system.
In May 2006 a team of scientists led by Dr. Luigi Naldini and Dr. Brian Brown from the San Raffaele Telethon Institute for Gene Therapy (HSR-TIGET) in Milan, Italy reported a breakthrough for gene therapy in which they developed a way to prevent the immune system from rejecting a newly delivered gene. Similar to organ transplantation, gene therapy has been plagued by the problem of immune rejection. So far, delivery of the 'normal' gene has been difficult because the immune system recognizes the new gene as foreign and rejects the cells carrying it. To overcome this problem, the HSR-TIGET group utilized a newly uncovered network of genes regulated by molecules known as microRNAs. Dr. Naldini's group reasoned that they could use this natural function of microRNA to selectively turn off the identity of their therapeutic gene in cells of the immune system and prevent the gene from being found and destroyed. The researchers injected mice with the gene containing an immune-cell microRNA target sequence, and the mice did not reject the gene, as previously occurred when vectors without the microRNA target sequence were used. This work will have important implications for the treatment of hemophilia and other genetic diseases by gene therapy.
In August 2006, scientists at the National Institutes of Health (Bethesda, Maryland) successfully treated metastatic melanoma in two patients using killer T cells genetically retargeted to attack the cancer cells. This study constitutes one of the first demonstrations that gene therapy can be effective in treating cancer.
In November 2006 Preston Nix from the University of Pennsylvania School of Medicine reported on VRX496, a gene-based immunotherapy for the treatment of human immunodeficiency virus (HIV) that uses a lentiviral vector for delivery of an antisense gene against the HIV envelope. In the Phase I trial enrolling five subjects with chronic HIV infection who had failed to respond to at least two antiretroviral regimens, a single intravenous infusion of autologous CD4 T cells genetically modified with VRX496 was safe and well tolerated. All patients had stable or decreased viral load; four of the five patients had stable or increased CD4 T cell counts. In addition, all five patients had stable or increased immune response to HIV antigens and other pathogens. This was the first evaluation of a lentiviral vector administered in U.S. Food and Drug Administration-approved human clinical trials for any disease. Data from an ongoing Phase I/II clinical trial were presented at CROI 2009.
On 1 May 2007 Moorfields Eye Hospital and University College London's Institute of Ophthalmology announced the world's first gene therapy trial for inherited retinal disease. The first operation was carried out on a 23 year-old British male, Robert Johnson, in early 2007. Leber's congenital amaurosis is an inherited blinding disease caused by mutations in the RPE65 gene. The results of a small clinical trial in children were published in New England Journal of Medicine in April 2008. They researched the safety of the subretinal delivery of recombinant adeno-associated virus (AAV) carrying RPE65 gene, and found it yielded positive results, with patients having modest increase in vision, and, perhaps more importantly, no apparent side-effects.
In May 2008, two more groups, one at the University of Florida and another at the University of Pennsylvania, reported positive results in independent clinical trials using gene therapy to treat Leber's congenital amaurosis.
In all three clinical trials, patients recovered functional vision without apparent side-effects. These studies, which used adeno-associated virus, have spawned a number of new studies investigating gene therapy for human retinal disease.
In September 2009, the journal Nature reported that researchers at the University of Washington and University of Florida were able to give trichromatic vision to squirrel monkeys using gene therapy, a hopeful precursor to a treatment for color blindness in humans. In November 2009, the journal Science reported that researchers succeeded at halting a fatal genetic disorder called adrenoleukodystrophy in two children using a lentivirus vector to deliver a functioning version of ABCD1, the gene that is mutated in the disorder.
A paper by Komáromy et al. published in April 2010, deals with gene therapy for a form of achromatopsia in dogs. Achromatopsia, or complete color blindness, is presented as an ideal model to develop gene therapy directed to cone photoreceptors. Cone function and day vision have been restored for at least 33 months in two young dogs with achromatopsia. However, the therapy was less efficient for older dogs.
In September 2010, it was announced that an 18 year old male patient in France with beta-thalassemia major had been successfully treated with gene therapy. Beta-thalassemia major is an inherited blood disease in which beta haemoglobin is missing and patients are dependent on regular lifelong blood transfusions. A team directed by Dr. Phillipe Leboulch (of the University of Paris, Bluebird Bio and Harvard Medical School) used a lentiviral vector to transduce the human ß-globin gene into purified blood and marrow cells obtained from the patient in June 2007. The patient's haemoglobin levels were stable at 9 to 10 g/dL, about a third of the hemoglobin contained the form introduced by the viral vector and blood transfusions had not been needed. Further clinical trials were planned. Bone marrow transplants are the only cure for thalassemia but 75% of patients are unable to find a matching bone marrow donor.
In 2007 and 2008, a man being treated by Gero Hütter was cured of HIV by repeated Hematopoietic stem cell transplantation (see also Allogeneic stem cell transplantation, Allogeneic bone marrow transplantation, Allotransplantation) with double-delta-32 mutation which disables the CCR5 receptor; this cure was not completely accepted by the medical community until 2011. This cure required complete ablation of existing bone marrow which is very debilitating.
In August 2011, two of three subjects of a pilot study were confirmed to have been cured from chronic lymphocytic leukemia (CLL). The study carried out by the researchers at the University of Pennsylvania used genetically modified T cells to attack cells that expressed the CD19 protein to fight the disease. In 2013, the researchers announced that 26 of 59 patients had achieved complete remission and the original patient had remained tumor-free.
Human HGF plasmid DNA therapy of cardiomyocytes is being examined as a potential treatment for coronary artery disease as well as treatment for the damage that occurs to the heart after myocardial infarction.
The FDA approves clinical trials of the use of gene therapy on thalassemia major patients in the US. Researchers at Memorial Sloan Kettering Cancer Center in New York begin to recruit 10 participants for the study in July 2012. The study is expected to end in 2014.
In July 2012, the European Medicines Agency recommended approval of a gene therapy treatment for the first time in either Europe or the United States. The treatment, called Alipogene tiparvovec (Glybera), compensates for lipoprotein lipase deficiency, which can cause severe pancreatitis. The recommendation was endorsed by the European Commission in November 2012 and commercial rollout is expected in late 2013.
In December 2012, it was reported that 10 of 13 patients with multiple myeloma were in remission "or very close to it" three months after being injected with a treatment involving genetically engineered T cells to target proteins NY-ESO-1 and LAGE-1 which exist only on cancerous myeloma cells. This procedure had been developed by a company called Adaptimmune.
In March 2013, Researchers at the Memorial Sloan-Kettering Cancer Center in New York, reported that three of five subjects who had acute lymphocytic leukemia (ALL) had been in remission for five months to two years after being treated with genetically modified T cells which attacked cells with CD19 genes on their surface, i.e. all B-cells, cancerous or not. The researchers believed that the patients immune systems would make normal T-cells and B-cells after a couple of months however they were given bone marrow to make sure. One patient had relapsed and died and one had died of a blood clot unrelated to the disease.
Following encouraging Phase 1 trials, in April 2013, researchers in the UK and the US announced they were starting Phase 2 clinical trials (called CUPID2 and SERCA-LVAD) on 250 patients at several hospitals in the US and Europe to use gene therapy to combat heart disease. These trials were designed to increase the levels of SERCA2a protein in the heart muscles and improve the function of these muscles. The FDA granted this a Breakthrough Therapy Designation which would speed up the trial and approval process in the USA.
In July 2013 the Italian San Raffaele Telethon Institute for Gene Therapy (HSR-TIGET) reported that six children with two severe hereditary diseases had been treated with a partially deactivated lentivirus to replace a faulty gene and after 7–32 months the results were promising. Three of the children had metachromatic leukodystrophy which causes children to lose cognitive and motor skills. The other children had Wiskott-Aldrich syndrome which leaves them to open to infection, autoimmune diseases and cancer due to a faulty immune system.
In October 2013, the Great Ormond Street Hospital, London reported that two children born with adenosine deaminase severe combined immunodeficiency disease (ADA-SCID) had been treated with genetically engineered stem cells 18 months previously and their immune systems were showing signs of full recovery. Another three children treated since then were also making good progress. ADA-SCID children have no functioning immune system and are sometimes known as "bubble children."
In October 2013, Amit Nathswani of the Royal Free London NHS Foundation Trust in London reported that they had treated six people with haemophilia in early 2011 using genetically engineered adeno-associated virus. Over two years later all six were still producing blood plasma clotting factor.
In January 2014, researchers at the University of Oxford reported that six people suffering from choroideremia had been treated with a genetically engineered adeno-associated virus with a copy of a gene REP1. Over a six month to two year period all had improved their sight. Choroideremia is an inherited genetic eye disease for which in the past there has been no treatment and patients eventually go blind.
In March 2014 researchers at the University of Pennsylvania reported that 12 patients with HIV had been treated since 2009 in a trial with a genetically engineered virus with a rare mutation known to protect against HIV (CCR5 deficiency). Results were promising.
Several uses for gene therapy have been speculated.
There is a risk that athletes might abuse gene therapy technologies to improve their athletic performance. This idea is known as gene doping and is as yet not known to be in use but a number of gene therapies have potential applications to athletic enhancement.
It has been speculated that genetic engineering could be used to change physical appearance, metabolism, and even improve physical capabilities and mental faculties like memory and intelligence, although for now these uses are limited to science fiction. These speculations have in turn led to ethical concerns and claims, including the belief that every fetus has an inherent right to remain genetically unmodified, the belief that parents hold the rights to modify their unborn offspring, and the belief that every child has the right to be born free from preventable diseases. On the other hand, others have made claims that many people try to improve themselves already through diet, exercise, education, cosmetics, and plastic surgery and that accomplishing these goals through genetics could be more efficient and worthwhile. This view sees the prevention of genetic diseases as a duty to humankind in preventing harm to future generations.
Policies on genetic modification tend to fall in the realm of general guidelines about human-involved biomedical research. Universal restrictions and documents have been made by international organizations to set a general standard on the issue of involving humans directly in research.
One key regulation comes from the Declaration of Helsinki (Ethical Principles for Medical Research Involving Human Subjects), last amended by the World Medical Association’s General Assembly in 2008. This document focuses on the principles physicians and researchers must consider when involving humans as the research subject. Additionally, the Statement on Gene Therapy Research initiated by the Human Genome Organization in 2001 also provides a legal baseline for all countries. HUGO’s document reiterates the organization’s common principles researchers must follow when conducting human genetic research including the recognition of human freedom and adherence to human rights, and the statement also declares recommendations for somatic gene therapy including a call for researchers and governments to attend to public concerns about the pros, cons and ethical concerns about the research.
No federal legislation specifically lays out protocol and restrictions about either germline or somatic human genetic engineering. Instead, this subject is governed by overlapping regulations from local and federal agencies. Included agencies, from the Department of Health and Human Services, are the Food and Drug Administration and the Recombinant DNA Advisory Committee of the National Institutes of Health. Additionally, researchers who wish to receive federal funds when conducting research about an investigational new drug application, which is commonly the case for somatic human genetic engineering, are required to obey international and federal guidelines dealing with the protection of human test subjects.
The National Institutes of Health (NIH) mainly serves as the gene therapy regulator for federally funded research institutions and projects. Privately funded human genetic research can only be recommended to voluntarily follow their regulations. NIH provides funding for lab research that develops or enhances devices utilized in human genetic engineering and to evaluate the ethics and quality of science present in current research labs. The NIH maintains a mandatory registry of human genetic engineering research protocols from all federally funded projects. An advisory committee to the NIH published a set of guidelines on the manipulation of genes. The document for the NIH guidelines discusses safety considerations for the lab as well as for any human patient test subject. A wide range of various experimental types which involve any type of gene transfer or alteration are discussed. Several sections specifically pertain to human genetic engineering including Section III-C-1. This section states the review process researches must undergo and the aspects that are considered when attempting to be approved to begin clinical research involving human genetic transfer into a patient. This document is an important tool required for scientists to follow in order to further scientific progress in the field of somatic cell therapy.
The United States Food and Drug Administration (FDA) regulates the quality and safety of gene therapy products and supervises how these products are implicated clinically. Therapeutic alteration of the human genome falls under the same regulatory requirements as any other medical treatment. Research involving human subjects, such as clinical trials, must be reviewed and approved by the FDA and an Institutional Review Board.
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